[Transcriber's Note:
_Text_ and =Text= represent italic and bold text respectively.
Subscripts are displayed as an underscore followed by the number
or text in braces: SiO_{2}.]
* * * * *
[Illustration: A Valley with Rocky Ledges cut in the Horizontal
Strata, Scotland]
THE ELEMENTS OF GEOLOGY
BY
WILLIAM HARMON NORTON
PROFESSOR OF GEOLOGY IN CORNELL COLLEGE
GINN & COMPANY
BOSTON * NEW YORK * CHICAGO * LONDON
Copyright, 1905, 1921, by
WILLIAM HARMON NORTON
ALL RIGHTS RESERVED
5511
The Atheneum Press
GINN & COMPANY PROPRIETORS
BOSTON * U.S.A.
PREFACE
Geology is a science of such rapid growth that no apology is expected
when from time to time a new text-book is added to those already in the
field. The present work, however, is the outcome of the need of a
text-book of very simple outline, in which causes and their consequences
should be knit together as closely as possible,--a need long felt by the
author in his teaching, and perhaps by other teachers also. The author
has ventured, therefore, to depart from the common usage which
subdivides geology into a number of departments,--dynamical, structural,
physiographic, and historical,--and to treat in immediate connection
with each geological process the land forms and the rock structures
which it has produced.
It is hoped that the facts of geology and the inferences drawn from
them have been so presented as to afford an efficient discipline in
inductive reasoning. Typical examples have been used to introduce many
topics, and it has been the author's aim to give due proportion to
both the wide generalizations of our science and to the concrete facts
on which they rest.
There have been added a number of practical exercises such as the
author has used for several years in the class room. These are not
made so numerous as to displace the problems which no doubt many
teachers prefer to have their pupils solve impromptu during the
recitation, but may, it is hoped, suggest their use.
In historical geology a broad view is given of the development of the
North American continent and the evolution of life upon the planet.
Only the leading types of plants and animals are mentioned, and
special attention is given to those which mark the lines of descent of
forms now living.
By omitting much technical detail of a mineralogical and
palæontological nature, and by confining the field of view almost
wholly to our own continent, space has been obtained to give to what
are deemed for beginners the essentials of the science a fuller
treatment than perhaps is common.
It is assumed that field work will be introduced with the commencement
of the study. The common rocks are therefore briefly described in the
opening chapters. The drift also receives early mention, and teachers
in the northern states who begin geology in the fall may prefer to
take up the chapter on the Pleistocene immediately after the chapter
on glaciers.
Simple diagrams have been used freely, not only because they are often
clearer than any verbal statement, but also because they readily lend
themselves to reproduction on the blackboard by the pupil. The text
will suggest others which the pupil may invent. It is hoped that the
photographic views may also be used for exercises in the class room.
The generous aid of many friends is recognized with special pleasure.
To Professor W. M. Davis of Harvard University there is owing a large
obligation for the broad conceptions and luminous statements of
geologic facts and principles with which he has enriched the
literature of our science, and for his stimulating influence in
education. It is hoped that both in subject-matter and in method the
book itself makes evident this debt. But besides a general obligation
shared by geologists everywhere, and in varying degrees by perhaps all
authors of recent American text-books in earth science, there is owing
a debt direct and personal. The plan of the book, with its use of
problems and treatment of land forms and rock structures in immediate
connection with the processes which produce them, was submitted to
Professor Davis, and, receiving his approval, was carried into effect,
although without the sanction of precedent at the time. Professor
Davis also kindly consented to read the manuscript throughout, and his
many helpful criticisms and suggestions are acknowledged with sincere
gratitude.
Parts of the manuscript have been reviewed by Dr. Samuel Calvin and
Dr. Frank M. Wilder of the State University of Iowa; Dr. S. W. Beyer
of the Iowa College of Agriculture and Mechanic Arts; Dr. U. S. Grant
of Northwestern University; Professor J. A. Udden of Augustana
College, Illinois; Dr. C. H. Gordon of the New Mexico State School of
Mines; Principal Maurice Ricker of the High School, Burlington, Iowa;
and the following former students of the author who are engaged in the
earth sciences: Dr. W. C. Alden of the United States Geological Survey
and the University of Chicago; Mr. Joseph Sniffen, instructor in the
Academy of the University of Chicago, Morgan Park; Professor Martin
Iorns, Fort Worth University, Texas; Professor A. M. Jayne, Dakota
University; Professor G. H. Bretnall, Monmouth College, Illinois;
Professor Howard E. Simpson, Colby College, Maine; Mr. E. J. Cable,
instructor in the Iowa State Normal College; Principal C. C. Gray of
the High School, Fargo, North Dakota; and Mr. Charles Persons of the
High School, Hannibal, Missouri. A large number of the diagrams of the
book were drawn by Mr. W. W. White of the Art School of Cornell
College. To all these friends, and to the many who have kindly
supplied the illustrations of the text, whose names are mentioned in
an appended list, the writer returns his heartfelt thanks.
WILLIAM HARMON NORTON
Cornell College, Mount Vernon, Iowa
July, 1905
INTRODUCTORY NOTE
During the preparation of this book Professor Norton has
frequently discussed its plan with me by correspondence, and we
have considered together the matters of scope, arrangement, and
presentation.
As to scope, the needs of the young student and not of the expert
have been our guide; the book is therefore a text-book, not a
reference volume.
In arrangement, the twofold division of the subject was chosen
because of its simplicity and effectiveness. The principles of
physical geology come first; the several chapters are arranged in
what is believed to be a natural order, appropriate to the
greatest part of our country, so that from a simple beginning a
logical sequence of topics leads through the whole subject. The
historical view of the science comes second, with many specific
illustrations of the physical processes previously studied, but
now set forth as part of the story of the earth, with its many
changes of aspect and its succession of inhabitants. Special
attention is here given to North America, and care is taken to
avoid overloading with details.
With respect to method of presentation, it must not be forgotten
that the text-book is only one factor in good teaching, and that
in geology, as in other sciences, the teacher, the laboratory, and
the local field are other factors, each of which should play an
appropriate part. The text suggests observational methods, but it
cannot replace observation in field or laboratory; it offers
certain exercises, but space cannot be taken to make it a
laboratory manual as well as a book for study; it explains many
problems, but its statements are necessarily more terse than the
illustrative descriptions that a good and experienced teacher
should supply. Frequent use is made of induction and inference in
order that the student may come to see how reasonable a science is
geology, and that he may avoid the too common error of thinking
that the opinions of "authorities" are reached by a private road
that is closed to him. The further extension of this method of
presentation is urged upon the teacher, so that the young
geologist may always learn the evidence that leads to a
conclusion, and not only the conclusion itself.
W. M. DAVIS
Harvard University, Cambridge, Mass.
July, 1905
ACKNOWLEDGMENT OF ILLUSTRATIONS
Adams, Professor F. D., McGill University, Canada, 241.
Alden, Dr. W. C., Washington, D.C., 353.
American Museum of Natural History, New York, 344.
Ash, H. C., Galesburg, Ill., 133.
Beyer, Dr. S. W., Iowa College of Agriculture, 363.
Calvin, Dr. Samuel, Iowa State University, 45, 295, 317, 325, 371.
Carney, Frank, Ithaca, N.Y., 356.
Clark, Dr. Wm. B., Maryland Geological Survey, 43.
Borne, Dr. Georg v. d., Jena, Germany, 5, 6.
Daly, Dr. R. A., Ottawa, Canada, 164.
Defieux, C. A., Liverpool, England, 154.
* Detroit Photographic Co., 235, 236.
* Ellis, W. M., Edna, Kan., 13.
Fairchild, Professor H. L., University of Rochester, 141, 357.
Field Columbian Museum, Chicago, 87.
Forster, Dr. A. E., University of Vienna, 32.
Gardner, J. L., Boston, 12, 140, 352.
Geological Survey of Canada, 256.
Gilbert, Dr. G. K., by courtesy of the American Book Company, 39.
* Haines, Ben, New Albany, Ind., 33.
* Haynes, F. J., St. Paul, Minn., 52, 95, 233.
Henderson, Judge Julius, Boulder, Col., 94.
James, George Wharton, Pasadena, Cal., 16, 127, 215, 229.
Johnston-Lavis, Professor H. J., Beaulieu, France, 216.
King, J. Harding, Stourbridge, England, 119.
Lawson, Dr. Andrew C., University of California, 113.
Le Conte, Professor J. N., University of California, 8.
Libbey, Dr. William, Princeton University, 92.
* McAllister, T. H., New York, 242.
* Meyers, H. C., Boise, Id., 19.
Mills, Professor H. A., Cornell College, 208, 304.
Norton, Professor W. H., Cornell College, 14, 35, 59, 88, 128,
183, 226, 234, 255, 340, 364, 367.
* Notman, Wm. & Son, Montreal, Canada, 98, 181.
Obrutschew, Dr. W., Tomsk Technological Institute, Siberia, 73.
Oldham, Dr. R. D., Geological Survey of India, 120.
* Peabody, H. C., Pasadena, Cal., 54.
* Pierce, C. C. & Co., Los Angeles, Cal., 15.
Pillsbury, Arthur, San Francisco, Cal., .115.
* Rau, Wm., Philadelphia, 18, 21, 122, 123, 218.
Reusch, Dr. Hans, Geological Survey of Norway, 112.
Reynolds, Professor S. H., University College, Bristol, England, 202.
Ricker, Principal Maurice, Burlington, Iowa, 48, 89.
* Shepard, E. A., Minneapolis, Minn., 105.
Smith, W. S. Tangier, Los Gatos, Cal., 186.
* Soule Photographic Co., Boston, 131.
U. S. Geological Survey, 3, 4, 23, 25, 34, 41, 63, 69, 78, 79,
80, 110, 111, 114, 125, 126, 129, 130, 142, 151, 153, 169,
172,177, 178, 188, 211, 212, 214, 228, 237, 238, 239, 243, 244,
254, 257, 340, 341, 353, 355.
U. S. National Museum, 149, 220, 221, 222, 225, 332.
* Valentine & Sons, Dundee, Scotland, 40, 136, 227.
Vroman, A. C., Pasadena, Cal., 17.
* Ward's Natural Science Establishment, Rochester, N.Y., 152.
* Welch, R., Belfast, Ireland, 1, 37.
* Westgate, Dr. L. G., Ohio Wesleyan University, 66.
Whymper, Edward, London, England, 106.
* Wilcox, W. D., Washington, D.C., 20.
* Wilson, Dr. A. W. G., McGill University, Canada, 68.
* Wilson, G. W., & Co., Aberdeen, Scotland, 82, 213.
* Worsley-Benison, F. H., Cheapstow, England, 170.
* Dealer in photographs or lantern slides.
CONTENTS
Page
Introduction.--The Scope And Aim Of Geology 1
PART I
EXTERNAL GEOLOGICAL AGENCIES
Chapter
I. The Work Of The Weather 5
II. The Work Of Ground Water 39
III. Rivers And Valleys 54
IV. River Deposits 93
V. The Work Of Glaciers 113
VI. The Work Of The Wind 144
VII. The Sea And Its Shores 155
VIII. Offshore And Deep-Sea Deposits 174
PART II
INTERNAL GEOLOGICAL AGENCIES
IX. Movements Of The Earth's Crust 195
X. Earthquakes 233
XI. Volcanoes 238
XII. Underground Structures Of Igneous Origin 265
XIII. Metamorphism And Mineral Veins 281
PART III
HISTORICAL GEOLOGY
XIV. The Geological Record 291
XV. The Pre-Cambrian Systems 304
XVI. The Cambrian 315
XVII. The Ordovician And Silurian 327
XVIII. The Devonian 341
XIX. The Carboniferous 350
XX. The Mesozoic 368
XXI. The Tertiary 394
XXII. The Quaternary 416
INDEX 451
THE ELEMENTS OF GEOLOGY
INTRODUCTION
THE SCOPE AND AIM OF GEOLOGY
Geology deals with the rocks of the earth's crust. It learns from
their composition and structure how the rocks were made and how they
have been modified. It ascertains how they have been brought to their
present places and wrought to their various topographic forms, such as
hills and valleys, plains and mountains. It studies the vestiges which
the rocks preserve of ancient organisms which once inhabited our
planet. Geology is the history of the earth and its inhabitants, as
read in the rocks of the earth's crust.
To obtain a general idea of the nature and method of our science
before beginning its study in detail, we may visit some valley, such
as that illustrated in the frontispiece, on whose sides are rocky
ledges. Here the rocks lie in horizontal layers. Although only their
edges are exposed, we may infer that these layers run into the upland
on either side and underlie the entire district; they are part of the
foundation of solid rock which everywhere is found beneath the loose
materials of the surface.
The ledges of the valley of our illustration are of sandstone. Looking
closely at the rock we see that it is composed of myriads of grains of
sand cemented together. These grains have been worn and rounded. They
are sorted also, those of each layer being about of a size. By some
means they have been brought hither from some more ancient source.
Surely these grains have had a history before they here found a
resting place,--a history which we are to learn to read.
The successive layers of the rock suggest that they were built one
after another from the bottom upward. We may be as sure that each
layer was formed before those above it as that the bottom courses of
stone in a wall were laid before the courses which rest upon them.
We have no reason to believe that the lowest layers which we see here
were the earliest ever formed. Indeed, some deep boring in the
vicinity may prove that the ledges rest upon other layers of rock
which extend downward for many hundreds of feet below the valley
floor. Nor may we conclude that the highest layers here were the
latest ever laid; for elsewhere we may find still later layers lying
upon them.
A short search may find in the rock relics of animals, such as the
imprints of shells, which lived when it was deposited; and as these
are of kinds whose nearest living relatives now have their home in the
sea, we infer that it was on the flat sea floor that the sandstone was
laid. Its present position hundreds of feet above sea level proves
that it has since emerged to form part of the land; while the flatness
of the beds shows that the movement was so uniform and gentle as not
to break or strongly bend them from their original attitude.
The surface of some of these layers is ripple-marked. Hence the sand
must once have been as loose as that of shallow sea bottoms and sea
beaches to-day, which is thrown into similar ripples by movements of
the water. In some way the grains have since become cemented into firm
rock.
Note that the layers on one side of the valley agree with those on the
other, each matching the one opposite at the same level. Once they
were continuous across the valley. Where the valley now is was once a
continuous upland built of horizontal layers; the layers now show
their edges, or _outcrop_, on the valley sides because they have been
cut by the valley trench.
The rock of the ledges is crumbling away. At the foot of each step of
rock lie fragments which have fallen. Thus the valley is slowly
widening. It has been narrower in the past; it will be wider in the
future.
Through the valley runs a stream. The waters of rains which have
fallen on the upper parts of the stream's basin are now on their way
to the river and the sea. Rock fragments and grains of sand creeping
down the valley slopes come within reach of the stream and are washed
along by the running water. Here and there they lodge for a time in
banks of sand and gravel, but sooner or later they are taken up again
and carried on. The grains of sand which were brought from some
ancient source to form these rocks are on their way to some new goal.
As they are washed along the rocky bed of the stream they slowly rasp
and wear it deeper. The valley will be deeper in the future; it has
been less deep in the past.
In this little valley we see slow changes now in progress. We find
also in the composition, the structure, and the attitude of the rocks,
and the land forms to which they have been sculptured, the record of a
long succession of past changes involving the origin of sand grains
and their gathering and deposit upon the bottom of some ancient sea,
the cementation of their layers into solid rock, the uplift of the
rocks to form a land surface, and, last of all, the carving of a
valley in the upland.
Everywhere, in the fields, along the river, among the mountains, by the
seashore, and in the desert, we may discover slow changes now in
progress and the record of similar changes in the past. Everywhere we
may catch glimpses of a process of gradual change, which stretches
backward into the past and forward into the future, by which the forms
and structures of the face of the earth are continually built and
continually destroyed. The science which deals with this long process is
geology. Geology treats of the natural changes now taking place upon the
earth and within it, the agencies which produce them, and the land forms
and rock structures which result. It studies the changes of the present
in order to be able to read the history of the earth's changes in the
past.
The various agencies which have fashioned the face of the earth may.
be divided into two general classes. In Part I we shall consider those
which work upon the earth from without, such as the weather, running
water, glaciers, the wind, and the sea. In Part II we shall treat of
those agencies whose sources are within the earth, and among whose
manifestations are volcanoes and earthquakes and the various movements
of the earth's crust. As we study each agency we shall notice not only
how it does its work, but also the records which it leaves in the rock
structures and the land forms which it produces. With this preparation
we shall be able in Part III to read in the records of the rocks the
history of our planet and the successive forms of life which have
dwelt upon it.
Part I
EXTERNAL GEOLOGICAL AGENCIES
CHAPTER I
THE WORK OF THE WEATHER
In our excursion to the valley with sandstone ledges we witnessed a
process which is going forward in all lands. Everywhere the rocks are
crumbling away; their fragments are creeping down hillsides to the
stream ways and are carried by the streams to the sea, where they are
rebuilt into rocky layers. When again the rocks are lifted to form
land the process will begin anew; again they will crumble and creep
down slopes and be washed by streams to the sea. Let us begin our
study of this long cycle of change at the point where rocks
disintegrate and decay under the action of the weather. In studying
now a few outcrops and quarries we shall learn a little of some common
rocks and how they weather away.
=Stratification and jointing.= At the sandstone ledges we saw that the
rock was divided into parallel layers. The thicker layers are known as
_strata_, and the thin leaves into which each stratum may sometimes be
split are termed _laminæ_. To a greater or less degree these layers
differ from each other in fineness of grain, showing that the material
has been sorted. The planes which divide them are called _bedding
planes_.
Besides the bedding planes there are other division planes, which cut
across the strata from top to bottom. These are found in all rocks and
are known as _joints_ (Fig. 1). Two sets of joints, running at about
right angles to each other, together with the bedding planes, divide
the sandstone into quadrangular blocks.
[Illustration: Fig. 1. Cliff of Sandstone, Ireland]
=Sandstone.= Examining a piece of sandstone we find it composed of
grains quite like those of river sand or of sea beaches. Most of the
grains are of a clear glassy mineral called quartz. These quartz
grains are very hard and will scratch the steel of a knife blade. They
are not affected by acid, and their broken surfaces are irregular like
those of broken glass.
The grains of sandstone are held together by some cement. This may be
_calcareous_, consisting of soluble carbonate of lime. In brown
sandstones the cement is commonly _ferruginous_,--hydrated iron oxide,
or iron rust, forming the bond, somewhat as in the case of iron nails
which have rusted together. The strongest and most lasting cement is
_siliceous_, and sand rocks whose grains are closely cemented by
silica, the chemical substance of which quartz is made, are known as
quartzites.
We are now prepared to understand how sandstone is affected by the
action of the weather. On ledges where the rock is exposed to view its
surface is more or less discolored and the grains are loose and may be
rubbed off with the finger. On gentle slopes the rock is covered with
a soil composed of sand, which evidently is crumbled sandstone, and
dark carbonaceous matter derived from the decay of vegetation. Clearly
it is by the dissolving of the cement that the rock thus breaks down
to loose sand. A piece of sandstone with calcareous cement, or a bit
of old mortar, which is really an artificial stone also made of sand
cemented by lime, may be treated in a test tube with hydrochloric acid
to illustrate the process.
[Illustration: Fig. 2. Section of Limestone Quarry
Scale, 1 in. = 30 ft. _a_, red residual clay; _mn_, pitted
surface of rotted limestone; _bb_, limestone divided into thin
layers; _c_, thick layers of laminated limestone, the laminæ
being firmly cemented together; _j_, _j_, _j_, joints. Is _bb_
thin-layered because originally so laid, or because it has been
broken up by weathering, although once like _c_ thick-layered?]
A limestone quarry. Here also we find the rock stratified and jointed
(Fig. 2). On the quarry face the rock is distinctly seen to be altered
for some distance from its upper surface. Below the altered zone the
rock is sound and is quarried for building; but the altered upper
layers are too soft and broken to be used for this purpose. If the
limestone is laminated, the laminae here have split apart, although
below they hold fast together. Near the surface the stone has become
rotten and crumbles at the touch, while on the top it has completely
broken down to a thin layer of limestone meal, on which rests a fine
reddish clay.
Limestone is made of minute grains of carbonate of lime all firmly
held together by a calcareous cement. A piece of the stone placed in a
test tube with hydrochloric acid dissolves with brisk effervescence,
leaving the insoluble impurities, which were disseminated through it,
at the bottom of the tube as a little clay.
We can now understand the changes in the upper layers of the quarry.
At the surface of the rock the limestone has completely dissolved,
leaving the insoluble residue as a layer of reddish clay. Immediately
below the clay the rock has disintegrated into meal where the cement
between the limestone grains has been removed, while beneath this the
laminae are split apart where the cement has been dissolved only along
the planes of lamination where the stone is more porous. As these
changes in the rock are greatest at the surface and diminish downward,
we infer that they have been caused by agents working downward from
the surface.
At certain points these agencies have been more effective than
elsewhere. The upper rock surface is pitted. Joints are widened as
they approach the surface, and along these seams we may find that the
rock is altered even down to the quarry floor.
=A shale pit.= Let us now visit some pit where shale--a laminated and
somewhat hardened clay--is quarried for the manufacture of brick. The
laminae of this fine-grained rock may be as thin as cardboard in
places, and close joints may break the rock into small rhombic blocks.
On the upper surface we note that the shale has weathered to a clayey
soil in which all traces of structure have been destroyed. The clay
and the upper layers of the shale beneath it are reddish or yellow,
while in many cases the color of the unaltered rock beneath is blue.
=The sedimentary rocks.= The three kinds of layered rocks whose
acquaintance we have made--sandstone, limestone, and shale--are the
leading types of the great group of stratified, or sedimentary, rocks.
This group includes all rocks made of sediments, their materials
having settled either in water upon the bottoms of rivers, lakes, or
seas, or on dry land, as in the case of deposits made by the wind and
by glaciers. Sedimentary rocks are divided into the fragmental
rocks--which are made of fragments, either coarse or fine--and the far
less common rocks which are constituted of chemical precipitates.
[Illustration: Fig. 3. Conglomerate]
The sedimentary rocks are divided according to their composition into
the following classes:
1. The arenaceous, or quartz rocks, including beds of loose sand and
gravel, sandstone, quartzite, and conglomerate (a rock made of
cemented rounded gravel or pebbles).
2. The calcareous, or lime rocks, including limestone and a soft white
rock formed of calcareous powder known as chalk.
3. The argillaceous, or clay rocks, including muds, clays, and shales.
These three classes pass by mixture into one another. Thus there are
limy and clayey sandstones, sandy and clayey limestones, and sandy and
limy shales.
=Granite.= This familiar rock may be studied as an example of the
second great group of rocks,--_the unstratified_, or _igneous rocks_.
These are not made of cemented sedimentary grains, but of interlocking
crystals which have crystallized from a molten mass. Examining a piece
of granite, the most conspicuous crystals which meet the eye are those
of feldspar. They are commonly pink, white, or yellow, and break along
smooth cleavage planes which reflect the light like tiny panes of
glass. Mica may be recognized by its glittering plates, which split
into thin elastic scales. A third mineral, harder than steel, breaking
along irregular surfaces like broken glass, we identify as quartz.
How granite alters under the action of the weather may be seen in
outcrops where it forms the bed rock, or country rock, underlying the
loose formations of the surface, and in many parts of the northern
states where granite bowlders and pebbles more or less decayed may be
found in a surface sheet of stony clay called the drift. Of the
different minerals composing granite, quartz alone remains unaltered.
Mica weathers to detached flakes which have lost their elasticity. The
feldspar crystals have lost their luster and hardness, and even have
decayed to clay. Where long-weathered granite forms the country rock,
it often may be cut with spade or trowel for several feet from the
surface, so rotten is the feldspar, and here the rock is seen to break
down to a clayey soil containing grains of quartz and flakes of mica.
These are a few simple illustrations of the surface changes which some
of the common kinds of rocks undergo. The agencies by which these
changes are brought about we will now take up under two
divisions,--_chemical agencies_ producing rock decay and _mechanical
agencies_ producing rock disintegration.
The Chemical Work Of Water
As water falls on the earth in rain it has already absorbed from the
air carbon dioxide (carbonic acid gas) and oxygen. As it sinks into
the ground and becomes what is termed ground water, it takes into
solution from the soil humus acids and carbon dioxide, both of which
are constantly being generated there by the decay of organic matter.
So both rain and ground water are charged with active chemical agents,
by the help of which they corrode and rust and decompose all rocks to
a greater or less degree. We notice now three of the chief chemical
processes concerned in weathering,--solution, the formation of
carbonates, and oxidation.
=Solution.= Limestone, although so little affected by pure water that
five thousand gallons would be needed to dissolve a single pound, is
easily dissolved in water charged with carbon dioxide. In limestone
regions well water is therefore "hard." On boiling the water for some
time the carbon dioxide gas is expelled, the whole of the lime
carbonate can no longer be held in solution, and much of it is thrown
down to form a crust or "scale" in the kettle or in the tubes of the
steam boiler. All waters which flow over limestone rocks or soak
through them are constantly engaged in dissolving them away, and in
the course of time destroy beds of vast extent and great thickness.
[Illustration: Fig. 4. Surface of Limestone furrowed by
Weathering, Montana]
The upper surface of limestone rocks becomes deeply pitted, as we saw
in the limestone quarry, and where the mantle of waste has been
removed it may be found so intricately furrowed that it is difficult
to traverse (Fig. 4).
Beds of _rock salt_ buried among the strata are dissolved by seeping
water, which issues in salt springs. _Gypsum_, a mineral composed of
hydrated sulphate of lime, and so soft that it may be scratched with
the finger nail, is readily taken up by water, giving to the water of
wells and springs a peculiar hardness difficult to remove.
The dissolving action of moisture may be noted on marble tombstones of
some age, marble being a limestone altered by heat and pressure and
composed of crystalline grains. By assuming that the date on each
monument marks the year of its erection, one may estimate how many
years on the average it has taken for weathering to loosen fine grains
on the polished surface, so that they may be rubbed off with the
finger, to destroy the polish, to round the sharp edges of tool marks
in the lettering, and at last to open cracks and seams and break down
the stone. We may notice also whether the gravestones weather more
rapidly on the sunny or the shady side, and on the sides or on the
top.
The weathered surface of granular limestone containing shells shows
them standing in relief. As the shells are made of crystalline
carbonate of lime, we may infer whether the carbonate of lime is less
soluble in its granular or in its crystalline condition.
=The formation of carbonates.= In attacking minerals water does more
than merely take them into solution. It decomposes them, forming new
chemical compounds of which the carbonates are among the most
important. Thus feldspar consists of the insoluble silicate of
alumina, together with certain alkaline silicates which are broken up
by the action of water containing carbon dioxide, forming alkaline
carbonates. These carbonates are freely soluble and contribute potash
and soda to soils and river waters. By the removal of the soluble
ingredients of feldspar there is left the silicate of alumina, united
with water or hydrated, in the condition of a fine plastic clay which,
when white and pure, is known as _kaolin_ and is used in the
manufacture of porcelain. Feldspathic rocks which contain no iron
compounds thus weather to whitish crusts, and even apparently sound
crystals of feldspar, when ground to thin slices and placed under the
microscope, may be seen to be milky in color throughout because an
internal change to kaolin has begun.
[Illustration: Fig. 5. Bowlder split by Heat and Cold,
Western Texas]
=Oxidation.= Rocks containing compounds of iron weather to reddish
crusts, and the seams of these rocks are often lined with rusty films.
Oxygen and water have here united with the iron, forming hydrated iron
oxide. The effects of oxidation may be seen in the alteration of many
kinds of rocks and in red and yellow colors of soils and subsoils.
_Pyrite_ is a very hard mineral of a pale brass color, found in
scattered crystals in many rocks, and is composed of iron and sulphur
(iron sulphide). Under the attack of the weather it takes up oxygen,
forming iron sulphate (green vitriol), a soluble compound, and
insoluble hydrated iron oxide, which as a mineral is known as
limonite. Several large masses of iron sulphide were placed some years
ago on the lawn in front of the National Museum at Washington. The
mineral changed so rapidly to green vitriol that enough of this
poisonous compound was washed into the ground to kill the roots of the
surrounding grass.
Agents Of Mechanical Disintegration
=Heat and cold.= Rocks exposed to the direct rays of the sun become
strongly heated by day and expand. After sunset they rapidly cool and
contract. When the difference in temperature between day and night is
considerable, the repeated strains of sudden expansion and contraction
at last become greater than the rocks can bear, and they break, for
the same reason that a glass cracks when plunged into boiling water
(Fig. 5).
Rocks are poor conductors of heat, and hence their surfaces may become
painfully hot under the full blaze of the sun, while the interior
remains comparatively cool. By day the surface shell expands and tends
to break loose from the mass of the stone. In cooling in the evening
the surface shell suddenly contracts on the unyielding interior and in
time is forced off in scales (Fig. 6).
[Illustration: Fig. 6. Bowlders scaling off under Heat and Cold,
Western Texas]
Many rocks, such as granite, are made up of grains of various minerals
which differ in color and in their capacity to absorb heat, and which
therefore contract and expand in different ratios. In heating and
cooling these grains crowd against their neighbors and tear loose from
them, so that finally the rock disintegrates into sand.
The conditions for the destructive action of heat and cold are most
fully met in arid regions when vegetation is wanting for lack of
sufficient rain. The soil not being held together by the roots of
plants is blown away over large areas, leaving the rocks bare to the
blazing sun in a cloudless sky. The air is dry, and the heat received
by the earth by day is therefore rapidly radiated at night into space.
There is a sharp and sudden fall of temperature after sunset, and the
rocks, strongly heated by day, are now chilled perhaps even to the
freezing point.
In the Sahara the thermometer has been known to fall 131° F. within a
few hours. In the light air of the Pamir plateau in central Asia a
rise of 90° F. has been recorded from seven o'clock in the morning to
one o'clock in the afternoon. On the mountains of southwestern Texas
there are frequently heard crackling noises as the rocks of that arid
region throw off scales from a fraction of an inch to four inches in
thickness, and loud reports are made as huge bowlders split apart.
Desert pebbles weakened by long exposure to heat and cold have been
shivered to fine sharp-pointed fragments on being placed in sand
heated to 180 degrees F. Beds half a foot thick, forming the floor of
limestone quarries in Wisconsin, have been known to buckle and arch
and break to fragments under the heat of the summer sun.
=Frost.= By this term is meant the freezing and thawing of water
contained in the pores and crevices of rocks. All rocks are more or
less porous and all contain more or less water in their pores. Workers
in stone call this "quarry water," and speak of a stone as "green"
before the quarry water has dried out. Water also seeps along joints
and bedding planes and gathers in all seams and crevices. Water
expands in freezing, ten cubic inches of water freezing to about
eleven cubic inches of ice. As water freezes in the rifts and pores of
rocks it expands with the irresistible force illustrated in the
freezing and breaking of water pipes in winter. The first rift in the
rock, perhaps too narrow to be seen, is widened little by little by
the wedges of successive frosts, and finally the rock is broken into
detached blocks, and these into angular chip-stone by the same
process.
It is on mountain tops and in high latitudes that the effects of frost
are most plainly seen. "Every summit" says Whymper, "amongst the rock
summits upon which I have stood has been nothing but a piled-up heap
of fragments" (Fig. 7). In Iceland, in Spitzbergen, in Kamchatka, and
in other frigid lands large areas are thickly strewn with sharp-edged
fragments into which the rock has been shattered by frost.
[Illustration: Fig. 7. Rocks broken by Frost, Summit of the
Eggischhorn, Switzerland]
=Organic agents.= We must reckon the roots of plants and trees among
the agents which break rocks into pieces. The tiny rootlet in its
search for food and moisture inserts itself into some minute rift, and
as it grows slowly wedges the rock apart. Moreover, the acids of the
root corrode the rocks with which they are in contact. One may
sometimes find in the soil a block of limestone wrapped in a mesh of
roots, each of which lies in a little furrow where it has eaten into
the stone.
Rootless plants called _lichens_ often cover and corrode rocks as yet
bare of soil; but where lichens are destroying the rock less rapidly
than does the weather, they serve in a way as a protection.
=Conditions favoring disintegration and decay.= The disintegration of
rocks under frost and temperature changes goes on most rapidly in cold
and arid climates, and where vegetation is scant or absent. On the
contrary, the decay of rocks under the chemical action of water is
favored by a warm, moist climate and abundant vegetation. Frost and
heat and cold can only act within the few feet from the surface to
which the necessary temperature changes are limited, while water
penetrates and alters the rocks to great depths.
The pupil may explain.
In what ways the presence of joints and bedding planes assists in the
breaking up and decay of rocks under the action of the weather.
Why it is a good rule of stone masons never to lay stones on edge, but
always on their natural bedding planes.
Why stones fresh from the quarry sometimes go to pieces in early
winter, when stones which have been quarried for some months remain
uninjured.
Why quarrymen in the northern states often keep their quarry floors
flooded during winter.
Why laminated limestone should not be used for curbstone.
Why rocks composed of layers differing in fineness of grain and in
ratios of expansion do not make good building stone.
Fine-grained rocks with pores so small that capillary attraction keeps
the water which they contain from readily draining away are more apt
to hold their pores ten elevenths full of water than are rocks whose
pores are larger. Which, therefore, are more likely to be injured by
frost?
Which is subject to greater temperature changes, a dark rock or one of
a light color? the north side or the south side of a valley?
The Mantle of Rock Waste
We have seen that rocks are everywhere slowly wasting away. They are
broken in pieces by frost, by tree roots, and by heat and cold. They
dissolve and decompose under the chemical action of water and the
various corrosive substances which it contains, leaving their
insoluble residues as residual clays and sands upon the surface. As a
result there is everywhere forming a mantle of rock waste which covers
the land. It is well to imagine how the country would appear were this
mantle with its soil and vegetation all scraped away or had it never
been formed. The surface of the land would then be everywhere of bare
rock as unbroken as a quarry floor.
=The thickness of the mantle.= In any locality the thickness of the
mantle of rock waste depends as much on the rate at which it is
constantly being removed as on the rate at which it is forming. On the
face of cliffs it is absent, for here waste is removed as fast as it
is made. Where waste is carried away more slowly than it is produced,
it accumulates in time to great depth.
The granite of Pikes Peak is disintegrated to a depth of twenty feet.
In the city of Washington granite rock is so softened to a depth of
eighty feet that it can be removed with pick and shovel. About
Atlanta, Georgia, the rocks are completely rotted for one hundred feet
from the surface, while the beginnings of decay may be noticed at
thrice that depth. In places in southern Brazil the rock is decomposed
to a depth of four hundred feet.
In southwestern Wisconsin a reddish residual clay has an average depth
of thirteen feet on broad uplands, where it has been removed to the
least extent. The country rock on which it rests is a limestone with
about ten per cent of insoluble impurities. At least how thick, then,
was that portion of the limestone which has rotted down to the clay?
=Distinguishing characteristics of residual waste.= We must learn to
distinguish waste formed in place by the action of the weather from
the products of other geological agencies. Residual waste is
unstratified. It contains no substances which have not been derived
from the weathering of the parent rock. There is a gradual transition
from residual waste into the unweathered rock beneath. Waste resting
on sound rock evidently has been shifted and was not formed in place.
In certain regions of southern Missouri the land is covered with a
layer of broken flints and red clay, while the country rock is
limestone. The limestone contains nodules of flint, and we may infer
that it has been by the decay and removal of thick masses of limestone
that the residual layer of clay and flints has been left upon the
surface. Flint is a form of quartz, dull-lustered, usually gray or
blackish in color, and opaque except on thinnest edges, where it is
translucent.
Over much of the northern states there is spread an unstratified stony
clay called the _drift_. It often rests on sound rocks. It contains
grains of sand, pebbles, and bowlders composed of many different
minerals and rocks that the country rock cannot furnish. Hence the
drift cannot have been formed by the decay of the rock of the region.
A shale or limestone, for example, cannot waste to a clay containing
granite pebbles. The origin of the drift will be explained in
subsequent chapters.
The differences in rocks are due more to their soluble than to their
insoluble constituents. The latter are few in number and are much the
same in rocks of widely different nature, being chiefly quartz,
silicate of alumina, and iron oxide. By the removal of their soluble
parts very many and widely different rocks rot down to a residual clay
gritty with particles of quartz and colored red or yellow with iron
oxide.
In a broad way the changes which rocks undergo in weathering are an
adaptation to the environment in which they find themselves at the
earth's surface,--an environment different from that in which they
were formed under sea or under ground. In open air, where they are
attacked by various destructive agents, few of the rock-making
minerals are stable compounds except quartz, the iron oxides, and the
silicate of alumina; and so it is to one or more of these
comparatively insoluble substances that most rocks are reduced by long
decay.
Which produces a mantle of finer waste, frost or chemical decay? which
a thicker mantle? In what respects would you expect that the mantle of
waste would differ in warm humid lands like India, in frozen countries
like Alaska, and in deserts such as the Sahara?
=The soil.= The same agencies which produce the mantle of waste are
continually at work upon it, breaking it up into finer and finer
particles and causing its more complete decay. Thus on the surface,
where the waste has weathered longest, it is gradually made fine
enough to support the growth of plants, and is then known as _soil_.
The coarser waste beneath is sometimes spoken of as subsoil. Soil
usually contains more or less dark, carbonaceous, decaying organic
matter, called humus, and is then often termed the _humus layer_. Soil
forms not only on waste produced in place from the rock beneath, but
also on materials which have been transported, such as sheets of
glacial drift and river deposits.
Until rocks are reduced to residual clays the work of the weather is
more rapid and effective on the fragments of the mantle of waste than
on the rocks from which waste is being formed. Why?
Any fresh excavation of cellar or cistern, or cut for road or railway,
will show the characteristics of the humus layer. It may form only a
gray film on the surface, or we may find it a layer a foot or more
thick, dark, or even black, above, and growing gradually lighter in
color as it passes by insensible gradations into the subsoil. In some
way the decaying vegetable matter continually forming on the surface
has become mingled with the material beneath it.
=How humus and the subsoil are mingled.= The mingling of humus and the
subsoil is brought about by several means. The roots of plants
penetrate the waste, and when they die leave their decaying substance
to fertilize it. Leaves and stems falling on the surface are turned
under by several agents. Earthworms and other animals whose home is in
the waste drag them into their burrows either for food or to line
their nests. Trees overthrown by the wind, roots and all, turn over
the soil and subsoil and mingle them together. Bacteria also work in
the waste and contribute to its enrichment. The animals living in the
mantle do much in other ways toward the making of soil. They bring the
coarser fragments from beneath to the surface, where the waste
weathers more rapidly. Their burrows allow air and water to penetrate
the waste more freely and to affect it to greater depths.
=Ants.= In the tropics the mantle of waste is worked over chiefly by
ants. They excavate underground galleries and chambers, extending
sometimes as much as fourteen feet below the surface, and build mounds
which may reach as high above it. In some parts of Paraguay and
southern Brazil these mounds, like gigantic potato hills, cover tracts
of considerable area.
In search for its food--the dead wood of trees--the so-called white
ant constructs runways of earth about the size of gas pipes, reaching
from the base of the tree to the topmost branches. On the plateaus of
central Africa explorers have walked for miles through forests every
tree of which was plastered with these galleries of mud. Each grain of
earth used in their construction is moistened and cemented by slime as
it is laid in place by the ant, and is thus acted on by organic
chemical agents. Sooner or later these galleries are beaten down by
heavy rains, and their fertilizing substances are scattered widely by
the winds.
=Earthworms.= In temperate regions the waste is worked over largely by
earthworms. In making their burrows worms swallow earth in order to
extract from it any nutritive organic matter which it may contain.
They treat it with their digestive acids, grind it in their stony
gizzards, and void it in castings on the surface of the ground. It was
estimated by Darwin that in many parts of England each year, on every
acre, more than ten tons of earth pass through the bodies of
earthworms and are brought to the surface, and that every few years
the entire soil layer is thus worked over by them.
In all these ways the waste is made fine and stirred and enriched.
Grain by grain the subsoil with its fresh mineral ingredients is
brought to the surface, and the rich organic matter which plants and
animals have taken from the atmosphere is plowed under. Thus Nature
plows and harrows on "the great world's farm" to make ready and ever
to renew a soil fit for the endless succession of her crops.
The world processes by which rocks are continually wasting away are
thus indispensable to the life of plants and animals. The organic
world is built on the ruins of the inorganic, and because the solid
rocks have been broken down into soil men are able to live upon the
earth.
=Solar energy.= The source of the energy which accomplishes all this
necessary work is the sun. It is the radiant energy of the sun which
causes the disintegration of rocks, which lifts vapor into the
atmosphere to fall as rain, which gives life to plants and animals.
Considering the earth in a broad way, we may view it as a globe of
solid rock,--_the lithosphere_,--surrounded by two mobile envelopes:
the envelope of air,--_the atmosphere_; and the envelope of
water,--_the hydrosphere_. Under the action of solar energy these
envelopes are in constant motion. Water from the hydrosphere is
continually rising in vapor into the atmosphere, the air of the
atmosphere penetrates the hydrosphere,--for its gases are dissolved in
all waters,--and both air and water enter and work upon the solid
earth. By their action upon the lithosphere they have produced a third
envelope,--the mantle of rock waste.
This envelope also is in movement, not indeed as a whole, but particle
by particle. The causes which set its particles in motion, and the
different forms which the mantle comes to assume, we will now proceed
to study.
Movements of the Mantle of Rock Waste
At the sandstone ledges which we first visited we saw not only that
the rocks were crumbling away, but also that grains and fragments of
them were creeping down the slopes of the valley to the stream and
were carried by it onward toward the sea. This process is going on
everywhere. Slowly it may be, and with many interruptions, but surely,
the waste of the land moves downward to the sea. We may divide its
course into two parts,--the path to the stream, which we will now
consider, and its carriage onward by the stream, which we will defer
to a later chapter.
=Gravity.= The chief agent concerned in the movement of waste is
gravity. Each particle of waste feels the unceasing downward pull of
the earth's mass and follows it when free to do so. All agencies which
produce waste tend to set its particles free and in motion, and
therefore coöperate with gravity. On cliffs, rocks fall when wedged
off by frost or by roots of trees, and when detached by any other
agency. On slopes of waste, water freezes in chinks between stones,
and in pores between particles of soil, and wedges them apart. Animals
and plants stir the waste, heat expands it, cold contracts it, the
strokes of the raindrops drive loose particles down the slope and the
wind lifts and lets them fall. Of all these movements, gravity assists
those which are downhill and retards those which are uphill. On the
whole, therefore, the downhill movements prevail, and the mantle of
waste, block by block and grain by grain, creeps along the downhill
path.
A slab of sandstone laid on another of the same kind at an angle of
17° and left in the open air was found to creep down the slope at the
rate of a little more than a millimeter a month. Explain why it did
so.
=Rain.= The most efficient agent in the carriage of waste to the
streams is the rain. It moves particles of soil by the force of the
blows of the falling drops, and washes them down all slopes to within
reach of permanent streams. On surfaces unprotected by vegetation, as
on plowed fields and in arid regions, the rain wears furrows and
gullies both in the mantle of waste and in exposures of unaltered rock
(Fig. 17).
At the foot of a hill we may find that the soil has accumulated by
creep and wash to the depth of several feet; while where the hillside
is steepest the soil may be exceedingly thin, or quite absent, because
removed about as fast as formed. Against the walls of an abbey built
on a slope in Wales seven hundred years ago, the creeping waste has
gathered on the uphill side to a depth of seven feet. The slow-flowing
sheet of waste is often dammed by fences and walls, whose uphill side
gathers waste in a few years so as to show a distinctly higher surface
than the downhill side, especially in plowed fields where the movement
is least checked by vegetation.
=Talus.= At the foot of cliffs there is usually to be found a slope of
rock fragments which clearly have fallen from above (Fig. 8). Such a
heap of waste is known as _talus_. The amount of talus in any place
depends both on the rate of its formation and the rate of its removal.
Talus forms rapidly in climates where mechanical disintegration is
most effective, where rocks are readily broken into blocks because
closely jointed and thinly bedded rather than massive, and where they
are firm enough to be detached in fragments of some size instead of in
fine grains. Talus is removed slowly where it decays slowly, either
because of the climate or the resistance of the rock. It may be
rapidly removed by a stream flowing along its base.
[Illustration: Fig. 8. Talus at Foot of Granite Cliffs, Sierra
Nevada Mountains]
In a moist climate a soluble rock, such as massive limestone, may form
talus little if any faster than the talus weathers away. A
loose-textured sandstone breaks down into incoherent sand grains,
which in dry climates, where unprotected by vegetation, may be blown
away as fast as they fall, leaving the cliff bare to the base. Cliffs
of such slow-decaying rocks as quartzite and granite when closely
jointed accumulate talus in large amounts.
[Illustration: Fig. 9. Diagram Illustrating Retreat of Cliff,
_c_, and Talus, _t_]
Talus slopes may be so steep as to reach _the angle of repose_, i.e.
the steepest angle at which the material will lie. This angle varies
with different materials, being greater with coarse and angular
fragments than with fine rounded grains. Sooner or later a talus
reaches that equilibrium where the amount removed from its surface
just equals that supplied from the cliff above. As the talus is
removed and weathers away its slope retreats together with the retreat
of the cliff, as seen in Figure 9.
=Graded slopes.= Where rocks weather faster than their waste is
carried away, the waste comes at last to cover all rocky ledges. On
the steeper slopes it is coarser and in more rapid movement than on
slopes more gentle, but mountain sides and hills and plains alike come
to be mantled with sheets of waste which everywhere is creeping toward
the streams. Such unbroken slopes, worn or built to the least
inclination at which the waste supplied by weathering can be urged
onward, are known as _graded slopes_.
Of far less importance than the silent, gradual creep of waste, which
is going on at all times everywhere about us, are the startling local
and spasmodic movements which we are now to describe.
=Avalanches.= On steep mountain sides the accumulated snows of winter
often slip and slide in avalanches to the valleys below. These rushing
torrents of snow sweep their tracks clean of waste and are one of
Nature's normal methods of moving it along the downhill path.
[Illustration: Fig. 10. A Landslide, Quebec]
=Landslides.= Another common and abrupt method of delivering waste to
streams is by slips of the waste mantle in large masses. After long
rains and after winter frosts the cohesion between the waste and the
sound rock beneath is loosened by seeping water underground. The waste
slips on the rock surface thus lubricated and plunges down the
mountain side in a swift roaring torrent of mud and stones.
[Illustration: Fig. 11. Diagram Illustrating Conditions favorable
to a Landslide
_lm_, limestone dipping toward valley of river, _r_; _sh_, shale]
We may conveniently mention here a second type of landslide, where
masses of solid rock as well as the mantle of waste are involved in
the sudden movement. Such slips occur when valleys have been rapidly
deepened by streams or glaciers and their sides have not yet been
graded. A favorable condition is where the strata dip (i.e. incline
downwards) towards the valley (Fig. 11), or are broken by joint planes
dipping in the same direction. The upper layers, including perhaps the
entire mountain side, have been cut across by the valley trench and
are left supported only on the inclined surface of the underlying
rocks. Water may percolate underground along this surface and loosen
the cohesion between the upper and the underlying strata by converting
the upper surface of a shale to soft wet clay, by dissolving layers of
a limestone, or by removing the cement of a sandstone and converting
it into loose sand. When the inclined surface is thus lubricated the
overlying masses may be launched into the valley below. The solid
rocks are broken and crushed in sliding and converted into waste
consisting, like that of talus, of angular unsorted fragments, blocks
of all sizes being mingled pell-mell with rock meal and dust. The
principal effects of landslides may be gathered from the following
examples.
At Gohna, India, in 1893, the face of a spur four thousand feet high,
of the lower ranges of the Himalayas, slipped into the gorge of the
headwaters of the Ganges River in successive rock falls which lasted
for three days. Blocks of stone were projected for a mile, and clouds
of limestone dust were spread over the surrounding country. The débris
formed a dam one thousand feet high, extending for two miles along the
valley. A lake gathered behind this barrier, gradually rising until it
overtopped it in a little less than a year. The upper portion of the
dam then broke, and a terrific rush of water swept down the valley in
a wave which, twenty miles away, rose one hundred and sixty feet in
height. A narrow lake is still held by the strong base of the dam.
In 1896, after forty days of incessant rain, a cliff of sandstone
slipped into the Yangtse River in China, reducing the width of the
channel to eighty yards and causing formidable rapids.
[Illustration: Fig. 12. Bowlders of Weathering, Granite Quarry,
Cape Ann, Massachusetts]
At Flims, in Switzerland, a prehistoric landslip flung a dam eighteen
hundred feet high across the headwaters of the Rhine. If spread evenly
over a surface of twenty-eight square miles, the material would cover
it to a depth of six hundred and sixty feet. The barrier is not yet
entirely cut away, and several lakes are held in shallow basins on its
hummocky surface.
A slide from the precipitous river front of the citadel hill of
Quebec, in 1889, dashed across Champlain Street, wrecking a number of
houses and causing the death of forty-five persons. The strata here
are composed of steeply dipping slate (Fig. 10).
In lofty mountain ranges there may not be a single valley without its
traces of landslides, so common there is this method of the movement
of waste, and of building to grade over-steepened slopes.
Rock Sculpture By Weathering
We are now to consider a few of the forms into which rock masses are
carved by the weather.
[Illustration: Fig. 13. Differential Weathering on a Monument,
Colorado]
=Bowlders of weathering.= In many quarries and outcrops we may see
that the blocks into which one or more of the uppermost layers have
been broken along their joints and bedding planes are no longer
angular, as are those of the layers below. The edges and corners of
these blocks have been worn away by the weather. Such rounded cores,
known as bowlders of weathering, are often left to strew the surface.
=Differential weathering.= This term covers all cases in which a rock
mass weathers differently in different portions. Any weaker spots or
layers are etched out on the surface, leaving the more resistant in
relief. Thus massive limestones become pitted where the weather drills
out the weaker portions. In these pits, when once they are formed,
moisture gathers, a little soil collects, vegetation takes root, and
thus they are further enlarged until the limestone may be deeply
honeycombed.
[Illustration: Fig. 14. Honeycombed Limestone, Iowa]
[Illustration: Fig. 15. Cliffs and Slopes on North Wall of the
Grand Canyon of the Colorado River, Arizona]
On the sides of canyons, and elsewhere where the edges of strata are
exposed, the harder layers project as cliffs, while the softer weather
back to slopes covered with the talus of the harder layers above them.
It is convenient to call the former _cliff makers_ and the latter
_slope makers_ (Fig. 15).
Differential weathering plays a large part in the sculpture of the
land. Areas of weak rock are wasted to plains, while areas of hard
rock adjacent are still left as hills and mountain ridges, as in the
valleys and mountains of eastern Pennsylvania. But in such instances
the lowering of the surface of the weaker rock is also due to the wear
of streams, and especially to the removal by them from the land of the
waste which covers and protects the rocks beneath.
[Illustration: Fig. 16. Taverlone Mesa, New Mexico]
Rocks owe their weakness to several different causes. Some, such as
beds of loose sand, are soft and easily worn by rains; some, as
limestone and gypsum for example, are soluble. Even hard insoluble
rocks are weak under the attack of the weather when they are closely
divided by joints and bedding planes and are thus readily broken up
into blocks by mechanical agencies.
[Illustration: Fig. 17. Monuments, Arizona
Note the rain furrows on the slope at the foot of the monuments.
In the foreground are seen fragments of petrified trunks of trees,
composed of silica and extremely resistant to the weather. On the
removal of the rock layers in which these fragments were imbedded
they are left to strew the surface in the same way as are the
residual flints of southern Missouri.]
=Outliers and monuments.= As cliffs retreat under the attack of the
weather, portions are left behind where the rock is more resistant or
where the attack for any reason is less severe. Such remnant masses,
if large, are known as outliers. When flat-topped, because of the
protection of a resistant horizontal capping layer, they are termed
_mesas_ (Fig. 16),--a term applied also to the flat-topped portions of
dissected plateaus (Fig. 129). Retreating cliffs may fall back a
number of miles behind their outliers before the latter are finally
consumed.
[Illustration: Fig. 18. Undercut Monuments, Colorado]
Monuments are smaller masses and may be but partially detached from
the cliff face. In the breaking down of sheets of horizontal strata,
outliers grow smaller and smaller and are reduced to massive
rectangular monuments resembling castles (Fig. 17). The rock castle
falls into ruin, leaving here and there an isolated tower; the tower
crumbles to a lonely pillar, soon to be overthrown. The various and
often picturesque shapes of monuments depend on the kind of rock, the
attitude of the strata, and the agent by which they are chiefly
carved. Thus pillars may have a capital formed of a resistant stratum.
Monuments may be undercut and come to rest on narrow pedestals,
wherever they weather more rapidly near the ground, either because of
the greater moisture there, or--in arid climates--because worn at
their base by drifting sands.
Stony clays disintegrating under the rain often contain bowlders
which protect the softer material beneath from the vertical blows
of raindrops, and thus come to stand on pedestals of some height
(Fig. 19). One may sometimes see on the ground beneath dripping eaves
pebbles left in the same way, protecting tiny pedestals of sand.
=Mountain peaks and ridges.= Most mountains have been carved out of
great broadly uplifted folds and blocks of the earth's crust. Running
water and glacier ice have cut these folds and blocks into masses
divided by deep valleys; but it is by the weather, for the most part,
that the masses thus separated have been sculptured to the present
forms of the individual peaks and ridges.
[Illustration: Fig. 19. Roosevelt Column, Idaho
An erosion pillar 70 feet high. How was it produced? Why
quadrangular? What does it show as to the recent height of the
hillside surface?]
Frost and heat and cold sculpture high mountains to sharp, tusklike
peaks and ragged, serrate crests, where their waste is readily removed
(Fig. 8).
The Matterhorn of the Alps is a famous example of a mountain peak
whose carving by the frost and other agents is in active progress. On
its face "scarcely a rock anywhere is firmly attached," and the fall
of loosened stones is incessant. Mountain climbers who have camped at
its base tell how huge rocks from time to time come leaping down its
precipices, followed by trains of dislodged smaller fragments and rock
dust; and how at night one may trace the course of the bowlders by the
sparks which they strike from the mountain walls. Mount Assiniboine,
Canada (Fig. 20), resembles the Matterhorn in form and has been carved
by the same agencies.
"The Needles" of Arizona are examples of sharp mountain peaks in a
warm arid region sculptured chiefly by temperature changes.
Chemical decay, especially when carried on beneath a cover of waste
and vegetation, favors the production of rounded knobs and dome-shaped
mountains.
=The weather curve.= We have seen that weathering reduces the angular
block quarried by the frost to a rounded bowlder by chipping off its
corners and smoothing away its edges. In much the same way weathering
at last reduces to rounded hills the earth blocks cut by streams or
formed in any other way. High mountains may at first be sculptured by
the weather to savage peaks (Fig. 181), but toward the end of their
life history they wear down to rounded hills (Fig. 182). The weather
curve, which may be seen on the summits of low hills (Fig. 21), is
convex upward.
[Illustration: Fig. 20. Mount Assiniboine, Canada]
[Illustration: Fig. 21. Big Round Top and Little Round Top,
Gettysburg, Pennsylvania]
In Figure 22, representing a cubic block of stone whose faces are a
yard square, how many square feet of surface are exposed to the
weather by a cubic foot at a corner _a_; by one situated in the middle
of an edge _b_; by one in the center of a side _c_? How much faster
will _a_ and _b_ weather than _c_, and what will be the effect on the
shape of the block?
[Illustration: Fig. 22]
=The coöperation of various agencies in rock sculpture.= For the sake
of clearness it is necessary to describe the work of each geological
agent separately. We must not forget, however, that in Nature no agent
works independently and alone; that every result is the outcome of a
long chain of causes. Thus, in order that the mountain peak may be
carved by the agents of disintegration, the waste must be rapidly
removed,--a work done by many agents, including some which we are yet
to study; and in order that the waste may be removed as fast as
formed, the region must first have been raised well above the level of
the sea, so that the agents of transportation could do their work
effectively. The sculpture of the rocks is accomplished only by the
coöperation of many forces.
The constant removal of waste from the surface by creep and wash and
carriage by streams is of the highest importance, because it allows
the destruction of the land by means of weathering to go on as long as
any land remains above sea level. If waste were not removed, it would
grow to be so thick as to protect the rock beneath from further
weathering, and the processes of destruction which we have studied
would be brought to an end. The very presence of the mantle of waste
over the land proves that on the whole rocks weather more rapidly than
their waste is removed. The destruction of the land is going on as
fast as the waste can be carried away.
We have now learned to see in the mantle of waste the record of the
destructive action of the agencies of weathering on the rocks of the
land surface. Similar records we shall find buried deeply among the
rocks of the crust in old soils and in rocks pitted and decayed,
telling of old land surfaces long wasted by the weather. Ever since
the dry land appeared these agencies have been as now quietly and
unceasingly at work upon it, and have ever been the chief means of the
destruction of its rocks. The vast bulk of the stratified rocks of the
earth's crust is made up almost wholly of the waste thus worn from
ancient lands.
[Illustration: Fig. 23. Mount Sneffels, Colorado
Describe and account for what you see in this view. What
changes may the mountain be expected to undergo in the future
from the agencies now at work upon it?]
In studying the various geological agencies we must remember the
almost inconceivable times in which they work. The slowest process
when multiplied by the immense time in which it is carried on produces
great results. The geologist looks upon the land forms of the earth's
surface as monuments which record the slow action of weathering and
other agents during the ages of the past. The mountain peak, the
rounded hill, the wide plain which lies where hills and mountains once
stood, tell clearly of the great results which slow processes will
reach when given long time in which to do their work. We should
accustom ourselves also to think of the results which weathering will
sooner or later bring to pass. The tombstone and the bowlder of the
field, which each year lose from their surfaces a few crystalline
grains, must in time be wholly destroyed. The hill whose rocks are
slowly rotting underneath a cover of waste must become lower and lower
as the centuries and millenniums come and go, and will finally
disappear. Even the mountains are crumbling away continually, and
therefore are but fleeting features of the landscape.
CHAPTER II
THE WORK OF GROUND WATER
=Land waters.= We have seen how large is the part that water plays at
and near the surface of the land in the processes of weathering and in
the slow movement of waste down all slopes to the stream ways. We now
take up the work of water as it descends beneath the ground,--a
corrosive agent still, and carrying in solution as its load the
invisible waste of rocks derived from their soluble parts.
Land waters have their immediate source in the rainfall. By the heat
of the sun water is evaporated from the reservoir of the ocean and
from moist surfaces everywhere. Mingled as vapor with the air, it is
carried by the winds over sea and land, and condensed it returns to
the earth as rain or snow. That part of the rainfall which descends on
the ocean does not concern us, but that which falls on the land
accomplishes, as it returns to the sea, the most important work of all
surface geological agencies.
The rainfall may be divided into three parts: the first _dries up_,
being discharged into the air by evaporation either directly from the
soil or through vegetation; the second _runs off_ over the surface to
flood the streams; the third _soaks in_ the ground and is henceforth
known as _ground_ or _underground water_.
=The descent of ground water.= Seeping through the mantle of waste,
ground water soaks into the pores and crevices of the underlying rock.
All rocks of the upper crust of the earth are more or less porous, and
all drink in water. _Impervious rocks_, such as granite, clay, and
shale, have pores so minute that the water which they take in is held
fast within them by capillary attraction, and none drains through.
_Pervious rocks_, on the other hand, such as many sandstones, have
pore spaces so large that water filters through them more or less
freely. Besides its seepage through the pores of pervious rocks, water
passes to lower levels through the joints and cracks by which all
rocks, near the surface are broken.
Even the closest-grained granite has a pore space of 1 in 400, while
sandstone may have a pore space of 1 in 4. Sand is so porous that it
may absorb a third of its volume of water, and a loose loam even as
much as one half.
[Illustration: Fig. 24. Diagram Illustrating the Relation of the
Ground-Water Surface to the Surface of the Ground
The dotted line represents the ground-water surface, and the
arrows indicate the direction of the movements of ground-water.
_m_, marsh; _w_, well; _r_, river]
=The ground-water surface= is the name given the upper surface of
ground water, the level below which all rocks are saturated. In dry
seasons the ground-water surface sinks. For ground water is constantly
seeping downward under gravity, it is evaporated in the waste and its
moisture is carried upward by capillarity and the roots of plants to
the surface to be evaporated in the air. In wet seasons these constant
losses are more than made good by fresh supplies from that part of the
rainfall which soaks into the ground, and the ground-water surface
rises.
In moist climates the ground-water surface (Fig. 24) lies, as a rule,
within a few feet of the land surface and conforms to it in a general
way, although with slopes of less inclination than those of the hills
and valleys. In dry climates permanent ground water may be found only
at depths of hundreds of feet. Ground water is held at its height by
the fact that its circulation is constantly impeded by capillarity and
friction. If it were as free to drain away as are surface streams, it
would sink soon after a rain to the level of the deepest valleys of
the region.
=Wells and springs.= Excavations made in permeable rocks below the
ground-water surface fill to its level and are known as wells. Where
valleys cut this surface permanent streams are formed, the water
either oozing forth along ill-defined areas or issuing at definite
points called springs, where it is concentrated by the structure of
the rocks. A level tract where the ground-water surface coincides with
the surface of the ground is a swamp or marsh.
By studying a spring one may learn much of the ways and work of ground
water. Spring water differs from that of the stream into which it
flows in several respects. If we test the spring with a thermometer
during successive months, we shall find that its temperature remains
much the same the year round. In summer it is markedly cooler than the
stream; in winter it is warmer and remains unfrozen while the latter
perhaps is locked in ice. This means that its underground path must
lie at such a distance from the surface that it is little affected by
summer's heat and winter's cold.
While the stream is often turbid with surface waste washed into it by
rains, the spring remains clear; its water has been filtered during
its slow movement through many small underground passages and the
pores of rocks. Commonly the spring differs from the stream in that it
carries a far larger load of dissolved rock. Chemical analysis proves
that streams contain various minerals in solution, but these are
usually in quantities so small that they are not perceptible to the
taste or feel. But the water of springs is often well charged with
soluble minerals; in its slow, long journey underground it has
searched out the soluble parts of the rocks through which it seeps and
has dissolved as much of them as it could. When spring water is boiled
away, the invisible load which it has carried is left behind, and in
composition is found to be practically identical with that of the
soluble ingredients of the country rock. Although to some extent the
soluble waste of rocks is washed down surface slopes by the rain, by
far the larger part is carried downward by ground water and is
delivered to streams by springs.
In limestone regions springs are charged with calcium carbonate (the
carbonate of lime), and where the limestone is magnesian they contain
magnesium carbonate also. Such waters are "hard"; when used in
washing, the minerals which they contain combine with the fatty acids
of soap to form insoluble curdy compounds. When springs rise from
rocks containing gypsum they are hard with calcium sulphate. In
granite regions they contain more or less soda and potash from the
decay of feldspar.
The flow of springs varies much less during the different seasons of
the year than does that of surface streams. So slow is the movement of
ground water through the rocks that even during long droughts large
amounts remain stored above the levels of surface drainage.
=Movements of ground water.= Ground water is in constant movement
toward its outlets. Its rate varies according to many conditions, but
always is extremely slow. Even through loose sands beneath the beds of
rivers it sometimes does not exceed a fifth of a mile a year.
[Illustration: Fig. 26. Geological Conditions favorable to
Strong Springs
_a_, limestone; _b_, shale; _c_, coarse sandstone; _d_,
limestone; _e_, sandstone; _f_, fissure. The strata dip toward
the South, _S_. Redraw the diagram, marking the points at which
strong springs (_ss_) may be expected.]
In any region two zones of flow may be distinguished. The _upper zone
of flow_ extends from the ground-water surface downward through the
waste mantle and any permeable rocks on which the mantle rests, as far
as the first impermeable layer, where the descending movement of the
water is stopped. The =deep zones of flow= occupy any pervious rocks
which may be found below the impervious layer which lies nearest to
the surface. The upper zone is a vast sheet of water saturating the
soil and rocks and slowly seeping downward through their pores and
interstices along the slopes to the valleys, where in part it
discharges in springs and often unites also in a wide underflowing
stream which supports and feeds the river (Fig. 24).
[Illustration: Fig. 27. Diagram of Well which goes dry in
Drought, _a_, and of of Unfailing Well, _b_
Redraw the diagram, showing by dotted line the normal
ground-water surface and by broken line the ground-water
surface at times of drought]
[Illustration: Fig. 28. Diagram of Wet Weather Stream, _a_, and
of Permanent Stream, _b_
Redraw the diagram, showing ground-water surface by dotted line]
A city in a region of copious rains, built on the narrow flood plain
of a river, overlooked by hills, depends for its water supply on
driven wells, within the city limits, sunk in the sand a few yards
from the edge of the stream. Are these wells fed by water from the
river percolating through the sand, or by ground water on its way to
the stream and possibly contaminated with the sewage of the town?
At what height does underground water stand in the wells of your
region? Does it vary with the season? Have you ever known wells to go
dry? It may be possible to get data from different wells and to draw a
diagram showing the ground-water surface as compared with the surface
of the ground.
=Fissure springs and artesian wells.= The _deeper zones of flow_ lie
in pervious strata which are overlain by some impervious stratum. Such
layers are often carried by their dip to great depths, and water may
circulate in them to far below the level of the surface streams and
even of the sea. When a fissure crosses a water-bearing stratum, or
_aquifer, water is forced upward by the pressure of the weight of
the water contained in the higher parts of the stratum, and may reach
the surface as a fissure spring. A boring which taps such an aquifer
is known as an artesian well, a name derived from a province in France
where wells of this kind have been long in use. The rise of the water
in artesian wells, and in fissure springs also, depends on the
following conditions illustrated in Figure 29. The aquifer dips toward
the region of the wells from higher ground, where it outcrops and
receives its water. It is inclosed between an impervious layer above
and water-tight or water-logged layers beneath. The weight of the
column of water thus inclosed in the aquifer causes water to rise in
the well, precisely as the weight of the water in a standpipe forces
it in connected pipes to the upper stories of buildings.
[Illustration: Fig. 29. Section across South Dakota from the
Black Hills to Sioux Falls (S), Illustrating the Conditions
of Artesian Wells
_a_, crystalline impervious rocks; _b_, sedimentary rocks,
shales, limestones, and sandstones; _c_, pervious sandstone,
the aquifer; _d_, impervious shales; _w_, _w_, _w_, artesian wells.]
Which will supply the larger region with artesian wells, an aquifer
whose dip is steep or one whose dip is gentle? Which of the two
aquifers, their thickness being equal, will have the larger outcrop
and therefore be able to draw upon the larger amount of water from the
rainfall? Illustrate with diagrams.
=The zone of solution.= Near the surface, where the circulation of
ground water is most active, it oxidizes, corrodes, and dissolves the
rocks through which it passes. It leaches soils and subsoils of their
lime and other soluble minerals upon which plants depend for their
food. It takes away the soluble cements of rocks; it widens fissures
and joints and opens winding passages along the bedding planes; it may
even remove whole beds of soluble rocks, such as rock salt, limestone,
or gypsum. The work of ground water in producing landslides has
already been noticed. The zone in which the work of ground water is
thus for the most part destructive we may call the zone of solution.
[Illustration: Fig. 30. Diagram of Caverns and Sink Holes]
=Caves.= In massive limestone rocks, ground water dissolves channels
which sometimes form large caves (Fig. 30). The necessary conditions
for the excavation of caves of great size are well shown in central
Kentucky, where an upland is built throughout of thick horizontal beds
of limestone. The absence of layers of insoluble or impervious rock in
its structure allows a free circulation of ground water within it by
the way of all natural openings in the rock. These water ways have
been gradually enlarged by solution and wear until the upland is
honeycombed with caves. Five hundred open caverns are known in one
county.
Mammoth Cave, the largest of these caverns, consists of a labyrinth of
chambers and winding galleries whose total length is said to be as
much as thirty miles. One passage four miles long has an average width
of about sixty feet and an average height of forty feet. One of the
great halls is three hundred feet in width and is overhung by a solid
arch of limestone one hundred feet above the floor. Galleries at
different levels are connected by well-like pits, some of which
measure two hundred and twenty-five feet from top to bottom. Through
some of the lowest of these tunnels flows Echo River, still at work
dissolving and wearing away the rock while on its dark way to appear
at the surface as a great spring.
=Natural bridges.= As a cavern enlarges and the surface of the land
above it is lowered by weathering, the roof at last breaks down and
the cave becomes an open ravine. A portion of the roof may for a while
remain, forming a "natural bridge."
=Sink holes.= In limestone regions channels under ground may become so
well developed that the water of rains rapidly drains away through
them. Ground water stands low and wells must be sunk deep to find it.
Little or no surface water is left to form brooks.
[Illustration: Fig. 31. Sink Holes in the Karst, Austria]
Thus across the limestone upland of central Kentucky one meets but
three surface streams in a hundred miles. Between their valleys
surface water finds its way underground by means of sink holes. These
are pits, commonly funnel shaped, formed by the enlargement of crevice
or joint by percolating water, or by the breakdown of some portion of
the roof of a cave. By clogging of the outlet a sink hole may come to
be filled by a pond.
Central Florida is a limestone region with its drainage largely
subterranean and in part below the level even of the sea. Sink holes
are common, and many of them are occupied by lakelets. Great springs
mark the point of issue of underground streams, while some rise from
beneath the sea. Silver Spring, one of the largest, discharges from a
basin eight hundred feet wide and thirty feet deep a little river
navigable for small steamers to its source. About the spring there are
no surface streams for sixty miles.
[Illustration: Fig. 32. Underground Stream Issuing from Base of
Cliff, the Karst, Austria]
=The Karst.= Along the eastern coast of the Adriatic, as far south as
Montenegro, lies a belt of limestone mountains singularly worn and
honeycombed by the solvent action of water. Where forests have been
cut from the mountain sides and the red soil has washed away, the
surface of the white limestone forms a pathless desert of rock where
each square rod has been corroded into an intricate branch work of
shallow furrows and sharp ridges. Great sink holes, some of them six
hundred feet deep and more, pockmark the surface of the land. The
drainage is chiefly subterranean. Surface streams are rare and a
portion of their courses is often under ground. Fragmentary valleys
come suddenly to an end at walls of rock where the rivers which occupy
the valleys plunge into dark tunnels to reappear some miles away.
Ground water stands so far below the surface that it cannot be reached
by wells, and the inhabitants depend on rain water stored for
household uses. The finest cavern of Europe, the Adelsberg Grotto, is
in this region. Karst, the name of a part of this country, is now used
to designate any region or landscape thus sculptured by the chemical
action of surface and ground water. We must remember that Karst
regions are rare, and striking as is the work of their subterranean
streams, it is far less important than the work done by the sheets of
underground water slowly seeping through all subsoils and porous rocks
in other regions.
Even when gathered into definite channels, ground water does not have
the erosive power of surface streams, since it carries with it little
or no rock waste. Regions whose underground drainage is so perfect
that the development of surface streams has been retarded or prevented
escape to a large extent the leveling action of surface running
waters, and may therefore stand higher than the surrounding country.
The hill honeycombed by Luray Cavern, Virginia, has been attributed to
this cause.
[Illustration: Fig. 33. Stalactites and Stalagmites, Marengo
Cavern, Indiana]
=Cavern deposits.= Even in the zone of solution water may under
certain circumstances deposit as well as erode. As it trickles from
the roof of caverns, the lime carbonate which it has taken into
solution from the layers of limestone above is deposited by
evaporation in the air in icicle-like pendants called _stalactites_.
As the drops splash on the floor there are built up in the same way
thicker masses called _stalagmites_, which may grow to join the
stalactites above, forming pillars. A stalagmitic crust often seals
with rock the earth which accumulates in caverns, together with
whatever relics of cave dwellers, either animals or men, it may
contain.
Can you explain why slender stalactites formed by the drip of single
drops are often hollow pipes?
=The zone of cementation.= With increasing depth subterranean water
becomes more and more sluggish in its movements and more and more
highly charged with minerals dissolved from the rocks above. At such
depths it deposits these minerals in the pores of rocks, cementing
their grains together, and in crevices and fissures, forming mineral
veins. Thus below the zone of solution where the work of water is to
dissolve, lies the zone of cementation where its work is chemical
deposit. A part of the invisible load of waste is thus transferred
from rocks near the surface to those at greater depths.
As the land surface is gradually lowered by weathering and the work of
rain and streams, rocks which have lain deep within the zone of
cementation are brought within the zone of solution. Thus there are
exposed to view limestones, whose cracks were filled with calcite
(crystallized carbonate of lime), with quartz or other minerals, and
sandstones whose grains were well cemented many feet below the
surface.
=Cavity filling.= Small cavities in the rocks are often found more or
less completely filled with minerals deposited from solution by water
in its constant circulation underground. The process may be
illustrated by the deposit of salt crystals in a cup of evaporating
brine, but in the latter instance the solution is not renewed as in
the case of cavities in the rocks. A cavity thus lined with
inward-pointing crystals is called a _geode_.
=Concretions.= Ground water seeping through the pores of rocks may
gather minerals disseminated throughout them into nodular masses
called concretions. Thus silica disseminated through limestone is
gathered into nodules of flint. While geodes grow from the outside
inwards, concretions grow outwards from the center. Nor are they
formed in already existing cavities as are geodes. In soft clays
concretions may, as they grow, press the clay aside. In many other
rocks concretions are made by the process of _replacement_. Molecule
by molecule the rock is removed and the mineral of the concretion
substituted in its place. The concretion may in this way preserve
intact the lamination lines or other structures of the rock (Fig. 34).
Clays and shales often contain concretions of lime carbonate, of iron
carbonate, or of iron sulphide. Some fossil, such as a leaf or shell,
frequently forms the nucleus around which the concretion grows.
Why are building stones more easily worked when "green" than after
their quarry water has dried out?
[Illustration: Fig. 34. Concretions in Sandstone, Wyoming]
=Deposits of ground water in arid regions.= In arid lands where ground
water is drawn by capillarity to the surface and there evaporates, it
leaves as surface incrustations the minerals held in solution. White
limy incrustations of this nature cover considerable tracts in
northern Mexico. Evaporating beneath the surface, ground water may
deposit a limy cement in beds of loose sand and gravel. Such firmly
cemented layers are not uncommon in western Kansas and Nebraska, where
they are known as "mortar beds."
=Thermal springs.= While the lower limit of surface drainage is sea
level, subterranean water circulates much below that depth, and is
brought again to the surface by hydrostatic pressure. In many
instances springs have a higher temperature than the average annual
temperature of the region, and are then known as thermal springs. In
regions of present or recent volcanic activity, such as the
Yellowstone National Park, we may believe that the heat of thermal
springs is derived from uncooled lavas, perhaps not far below the
surface. But when hot springs occur at a distance of hundreds of miles
from any volcano, as in the case of the hot springs of Bath, England,
it is probable that their waters have risen from the heated rocks
of the earth's interior. The springs of Bath have a temperature of
120° F., 70° above the average annual temperature of the place. If
we assume that the rate of increase in the earth's internal heat is
here the average rate, 1° F. to every sixty feet of descent, we may
conclude that the springs of Bath rise from at least a depth of
forty-two hundred feet.
Water may descend to depths from which it can never be brought back by
hydrostatic pressure. It is absorbed by highly heated rocks deep below
the surface. From time to time some of this deep-seated water may be
returned to open air in the steam of volcanic eruptions.
[Illustration: Fig. 35. Calcareous Deposits from Hot Springs,
Yellowstone National Park]
=Surface deposits of springs.= Where subterranean water returns to the
surface highly charged with minerals in solution, on exposure to the
air it is commonly compelled to lay down much of its invisible load in
chemical deposits about the spring. These are thrown down from
solution either because of cooling, evaporation, the loss of carbon
dioxide, or the work of algae.
Many springs have been charged under pressure with carbon dioxide from
subterranean sources and are able therefore to take up large
quantities of lime carbonate from the limestone rocks through which
they pass. On reaching the surface the pressure is relieved, the gas
escapes, and the lime carbonate is thrown down in deposits called
_travertine_. The gas is sometimes withdrawn and the deposit produced
in large part by the action of algae and other humble forms of plant
life.
At the Mammoth Hot Springs in the valley of the Gardiner River,
Yellowstone National Park, beautiful terraces and basins of travertine
(Fig. 35) are now building, chiefly by means of algae which cover the
bottoms, rims, and sides of the basins and deposit lime carbonate upon
them in successive sheets. The rock, snow-white where dry, is coated
with red and orange gelatinous mats where the algae thrive in the
over-flowing waters.
Similar terraces of travertine are found to a height of fourteen
hundred feet up the valley side. We may infer that the springs which
formed these ancient deposits discharged near what was then the bottom
of the valley, and that as the valley has been deepened by the river
the ground water of the region has found lower and lower points of
issue.
In many parts of the country calcareous springs occur which coat with
lime carbonate mosses, twigs, and other objects over which their
waters flow. Such are popularly known as petrifying springs, although
they merely incrust the objects and do not convert them into stone.
Silica is soluble in alkaline waters, especially when these are hot.
Hot springs rising through alkaline siliceous rocks, such as lavas,
often deposit silica in a white spongy formation known as _siliceous
sinter_, both by evaporation and by the action of algae which secrete
silica from the waters. It is in this way that the cones and mounds of
the geysers in the Yellowstone National Park and in Iceland have been
formed (Fig. 234).
Where water oozes from the earth one may sometimes see a rusty deposit
on the ground, and perhaps an iridescent scum upon the water. The scum
is often mistaken for oil, but at a touch it cracks and breaks, as oil
would not do. It is a film of hydrated iron oxide, or _limonite_, and
the spring is an iron, or chalybeate, spring. Compounds of iron have
been taken into solution by ground water from soil and rocks, and are
now changed to the insoluble oxide on exposure to the oxygen of the
air.
In wet ground iron compounds leached by ground water from the soil
often collect in reddish deposits a few feet below the surface, where
their downward progress is arrested by some impervious clay. At the
bottom of bogs and shallow lakes iron ores sometimes accumulate to a
depth of several feet.
Decaying organic matter plays a large part in these changes. In its
presence the insoluble iron oxides which give color to most red and
yellow rocks are decomposed, leaving the rocks of a gray or bluish
color, and the soluble iron compounds which result are readily leached
out,--effects seen where red or yellow clays have been bleached about
some decaying tree root.
The iron thus dissolved is laid down as limonite when oxidized, as
about a chalybeate spring; but out of contact with the air and in the
presence of carbon dioxide supplied by decaying vegetation, as in a
peat bog, it may be deposited as iron carbonate, or _siderite_.
=Total amount of underground waters.= In order to realize the vast work
in solution and cementation which underground waters are now doing and
have done in all geological ages, we must gain some conception of their
amount. At a certain depth, estimated at about six miles, the weight of
the crust becomes greater than the rocks can bear, and all cavities and
pores in them must be completely closed by the enormous pressure which
they sustain. Below a depth, therefore, water cannot go. Above it all
rocks are water-soaked, up to the limit of their capacity, to within a
few feet of the surface. Estimating the average pore space of the rocks
above a depth of six miles at from two and a half per cent to five per
cent of their volume, it is found that the total amount of ground water
may be great enough to cover the entire surface of the earth to a depth
of from eight hundred to sixteen hundred feet.
CHAPTER III
RIVERS AND VALLEYS
=The run-off.= We have traced the history of that portion of the
rainfall which soaks into the ground; let us now return to that part
which washes along the surface and is known as the _run-off_. Fed by
rains and melting snows, the run-off gathers into courses, perhaps but
faintly marked at first, which join more definite and deeply cut
channels, as twigs their stems. In a humid climate the larger ravines
through which the run-off flows soon descend below the ground-water
surface. Here springs discharge along the sides of the little valleys
and permanent streams begin. The water supplied by the run-off here
joins that part of the rainfall which had soaked into the soil, and
both now proceed together by way of the stream to the sea.
=River floods.= Streams vary greatly in volume during the year. At
stages of flood they fill their immediate banks, or overrun them and
inundate any low lands adjacent to the channel; at stages of low water
they diminish to but a fraction of their volume when at flood.
At times of flood, rivers are fed chiefly by the run-off; at times of
low water, largely or even wholly by springs.
How, then, will the water of streams differ at these times in
turbidity and in the relative amount of solids carried in solution?
In parts of England streams have been known to continue flowing after
eighteen months of local drought, so great is the volume of water
which in humid climates is stored in the rocks above the drainage
level, and so slowly is it given off in springs.
In Illinois and the states adjacent, rivers remain low in winter and a
"spring freshet" follows the melting of the winter's snows. A "June
rise" is produced by the heavy rains of early summer. Low water
follows in July and August, and streams are again swollen to a
moderate degree under the rains of autumn.
=The discharge of streams.= The per cent of rainfall discharged by
rivers varies with the amount of rainfall, the slope of the drainage
area, the texture of the rocks, and other factors. With an annual
rainfall of fifty inches in an open country, about fifty per cent is
discharged; while with a rainfall of twenty inches only fifteen per
cent is discharged, part of the remainder being evaporated and part
passing underground beyond the drainage area. Thus the Ohio discharges
thirty per cent of the rainfall of its basin, while the Missouri
carries away but fifteen per cent. A number of the streams of the
semi-arid lands of the West do not discharge more than five per cent
of the rainfall.
Other things being equal, which will afford the larger proportion of
run-off, a region underlain with granite rock or with coarse
sandstone? grass land or forest? steep slopes or level land? a
well-drained region or one abounding in marshes and ponds? frozen or
unfrozen ground? Will there be a larger proportion of run-off after
long rains or after a season of drought? after long and gentle rains,
or after the same amount of precipitation in a violent rain? during
the months of growing vegetation, from June to August, or during the
autumn months?
[Illustration: Fig. 36. Rise of Ground-Water Surface (broken
line) beneath Valley (_V_) in Arid Region]
=Desert streams.= In arid regions the ground-water surface lies so low
that for the most part stream ways do not intersect it. Streams
therefore are not fed by springs, but instead lose volume as their
waters soak into the thirsty rocks over which they flow. They
contribute to the ground water of the region instead of being
increased by it. Being supplied chiefly by the run-off, they wither at
times of drought to a mere trickle of water, to a chain of pools, or
go wholly dry, while at long intervals rains fill their dusty beds
with sudden raging torrents. Desert rivers therefore periodically
shorten and lengthen their courses, withering back at times of drought
for scores of miles, or even for a hundred miles from the point
reached by their waters during seasons of rain.
=The geological work of streams.= The work of streams is of three
kinds,--transportation, erosion, and deposition. Streams _transport_
the waste of the land; they wear, or _erode_, their channels both on
bed and banks; and they _deposit_ portions of their load from time to
time along their courses, finally laying it down in the sea. Most of
the work of streams is done at times of flood.
Transportation
=The invisible load of streams.= Of the waste which a river transports
we may consider first the invisible load which it carries in solution,
supplied chiefly by springs but also in part by the run-off and from
the solution of the rocks of its bed. More than half the dissolved
solids in the water of the average river consists of the carbonates of
lime and magnesia; other substances are gypsum, sodium sulphate
(Glauber's salts), magnesium sulphate (Epsom salts), sodium chloride
(common salt), and even silica, the least soluble of the common
rock-making minerals. The amount of this invisible load is
surprisingly large. The Mississippi, for example, transports each year
113,000,000 tons of dissolved rock to the Gulf.
=The visible load of streams.= This consists of the silt which the
stream carries in suspension, and the sand and gravel and larger
stones which it pushes along its bed. Especially in times of flood one
may note the muddy water, its silt being kept from settling by the
rolling, eddying currents; and often by placing his ear close to the
bottom of a boat one may hear the clatter of pebbles as they are
hurried along. In mountain torrents the rumble of bowlders as they
clash together may be heard some distance away. The amount of the load
which a stream can transport depends on its velocity. A current of two
thirds of a mile per hour can move fine sand, while one of four miles
per hour sweeps along pebbles as large as hen's eggs. The transporting
power of a stream varies as the sixth power of its velocity. If its
velocity is multiplied by two, its transporting power is multiplied by
the sixth power of two: it can now move stones sixty-four times as
large as it could before.
Stones weigh from two to three times as much as water, and in water
lose the weight of the volume of water which they displace. What
proportion, then, of their weight in air do stones lose when
submerged?
=Measurement of stream loads.= To obtain the total amount of waste
transported by a river is an important but difficult matter. The
amount of water discharged must first be found by multiplying the
number of square feet in the average cross section of the stream by
its velocity per second, giving the discharge per second in cubic
feet. The amount of silt to a cubic foot of water is found by
filtering samples of the water taken from different parts of the
stream and at different times in the year, and drying and weighing the
residues. The average amount of silt to the cubic foot of water,
multiplied by the number of cubic feet of water discharged per year,
gives the total load carried in suspension during that time. Adding to
this the estimated amount of sand and gravel rolled along the bed,
which in many swift rivers greatly exceeds the lighter material held
in suspension, and adding also the total amount of dissolved solids,
we reach the exceedingly important result of the total load of waste
discharged by the river. Dividing the volume of this load by the area
of the river basin gives another result of the greatest geological
interest,--the rate at which the region is being lowered by the
combined action of weathering and erosion, or the rate of denudation.
=The rate of denudation of river basins.= This rate varies widely. The
Mississippi basin may be taken as a representative land surface
because of the varieties of surface, altitude and slope, climate, and
underlying rocks which are included in its great extent. Careful
measurements show that the Mississippi basin is now being lowered at a
rate of one four-thousandth of a foot a year, or one foot in four
thousand years. Taking this as the average rate of denudation for the
land surfaces of the globe, estimates have been made of the length of
time required at this rate to wash and wear the continents to the
level of the sea. As the average elevation of the lands of the globe
is reckoned at 2411 feet, this result would occur in nine or ten
million years, if the present rate of denudation should remain
unchanged. But even if no movements of the earth's crust should lift
or depress the continents, the rate of wear and the removal of waste
from their surfaces will not remain the same. It must constantly
decrease as the lands are worn nearer to sea level and their slopes
become more gentle. The length of time required to wear them away is
therefore far in excess of that just stated.
The drainage area of the Potomac is 11,000 square miles. The silt
brought down in suspension in a year would cover a square mile to the
depth of four feet. At what rate is the Potomac basin being lowered
from this cause alone?
It is estimated that the Upper Ganges is lowering its basin at the
rate of one foot in 823 years, and the Po one foot in 720 years. Why
so much faster than the Potomac and the Mississippi?
=How streams get their loads.= The load of streams is derived from a
number of sources, the larger part being supplied by the weathering of
valley slopes. We have noticed how the mantle of waste creeps and
washes to the stream ways. Watching the run-off during a rain, as it
hurries muddy with waste along the gutter or washes down the hillside,
we may see the beginning of the route by which the larger part of
their load is delivered to rivers. Streams also secure some of their
load by wearing it from their beds and banks,--a process called
erosion.
Erosion
Streams erode their beds chiefly by means of their bottom load,--the
stones of various sizes and the sand and even the fine mud which they
sweep along. With these tools they smooth, grind, and rasp the rock of
their beds, using them in much the fashion of sandpaper or a file.
[Illustration: Fig. 37. Pothole in Bed of Stream, Ireland]
=Weathering of river beds.= The erosion of stream beds is greatly
helped by the work of the weather. Especially at low water more or
less of the bed is exposed to the action of frost and heat and cold,
joints are opened, rocks are pried loose and broken up and made ready
to be swept away by the stream at time of flood.
=Potholes.= In rapids streams also drill out their rocky beds. Where
some slight depression gives rise to an eddy, the pebbles which gather
in it are whirled round and round, and, acting like the bit of an
auger, bore out a cylindrical pit called a pothole. Potholes sometimes
reach a depth of a score of feet. Where they are numerous they aid
materially in deepening the channel, as the walls between them are
worn away and they coalesce.
=Waterfalls.= One of the most effective means of erosion which the
river possesses is the waterfall. The plunging water dislodges stones
from the face of the ledge over which it pours, and often undermines
it by excavating a deep pit at its base. Slice after slice is thus
thrown down from the front of the cliff, and the cataract cuts its way
upstream leaving a gorge behind it.
[Illustration: Fig. 38. Map of the Gorge of the Niagara River]
=Niagara Falls.= The Niagara River flows from Lake Erie at Buffalo in
a broad channel which it has cut but a few feet below the level of the
region. Some thirteen miles from the outlet it plunges over a ledge
one hundred and seventy feet high into the head of a narrow gorge
which extends for seven miles to the escarpment of the upland in which
the gorge is cut. The strata which compose the upland dip gently
upstream and consist at top of a massive limestone, at the Falls about
eighty feet thick, and below of soft and easily weathered shale.
Beneath the Falls the underlying shale is cut and washed away by the
descending water and retreats also because of weathering, while the
overhanging limestone breaks down in huge blocks from time to time.
Niagara is divided by Goat Island into the Horseshoe Falls and the
American Falls. The former is supplied by the main current of the
river, and from the semicircular sweep of its rim a sheet of water in
places at least fifteen or twenty feet deep plunges into a pool a
little less than two hundred feet in depth. Here the force of the
falling water is sufficient to move about the fallen blocks of
limestone and use them in the excavation of the shale of the bed. At
the American Falls the lesser branch of the river, which flows along
the American side of Goat Island, pours over the side of the gorge and
breaks upon a high talus of limestone blocks which its smaller volume
of water is unable to grind to pieces and remove.
A series of surveys have determined that from 1842 to 1890 the
Horseshoe Falls retreated at the rate of 2.18 feet per year, while the
American Falls retreated at the rate of 0.64 feet in the same period.
We cannot doubt that the same agency which is now lengthening the
gorge at this rapid rate has cut it back its entire length of seven
miles.
While Niagara Falls have been cutting back a gorge seven miles long
and from two hundred to three hundred feet deep, the river above the
Falls has eroded its bed scarcely below the level of the upland on
which it flows. Like all streams which are the outlets of lakes, the
Niagara flows out of Lake Erie clear of sediment, as from a settling
basin, and carries no tools with which to abrade its bed. We may infer
from this instance how slight is the erosive power of clear water on
hard rock.
[Illustration: Fig. 39. Longitudinal Section of Niagara Gorge
Black, water; _F_, falls; _R_, rapids; _W_, whirlpool;
_E_, escarpment; _N_, north; _S_, south]
Assuming that the rate of recession of the combined volumes of the
American and Horseshoe Falls was three feet a year below Goat Island,
and _assuming that this rate has been uniform in the past_, how long
is it since the Niagara River fell over the edge of the escarpment
where now is the mouth of the present gorge?
The profile of the bed of the Niagara along the gorge (Fig. 39) shows
alternating deeps and shallows which cannot be accounted for, except
in a single instance, by the relative hardness of the rocks of the
river bed. The deeps do not exceed that at the foot of the Horseshoe
Falls at the present time. When the gorge was being cut along the
shallows, how did the Falls compare in excavating power, in force, and
volume with the Niagara of to-day? How did the rate of recession at
those times compare with the present rate? Is the assumption made
above that the rate of recession has been uniform correct?
The first stretch of shallows below the Falls causes a tumultuous
rapid impossible to sound. Its depth has been estimated at thirty-five
feet. From what data could such an estimate be made?
Suggest a reason why the Horseshoe Falls are convex upstream.
At the present rate of recession which will reach the head of Goat
Island the sooner, the American or the Horseshoe Falls? What will be
the fate of the Falls left behind when the other has passed beyond the
head of the island?
The rate at which a stream erodes its bed depends in part upon the
nature of the rocks over which it flows. Will a stream deepen its
channel more rapidly on massive or on thin-bedded and close-jointed
rocks? on horizontal strata or on strata steeply inclined?
[Illustration: Fig. 40. A Stream in Scotland
In what ways is the bed now being deepened?]
Deposition
While the river carries its invisible load of dissolved rock on
without stop to the sea, its load of visible waste is subject to many
delays en route. Now and again it is laid aside, to be picked up later
and carried some distance farther on its way. One of the most striking
features of the river therefore is the waste accumulated along its
course, in bars and islands in the channel, beneath its bed, and in
flood plains along its banks. All this _alluvium_, to use a general
term for river deposits, with which the valley is cumbered is really
en route to the sea; it is only temporarily laid aside to resume its
journey later on. Constantly the river is destroying and rebuilding
its alluvial deposits, here cutting and there depositing along its
banks, here eroding and there building a bar, here excavating its bed
and there filling it up, and at all times carrying the material picked
up at one point some distance on downstream before depositing it at
another.
[Illustration: Fig. 41. Sand Bar deposited by Stream, showing
Cross Bedding]
These deposits are laid down by slackening currents where the velocity
of the stream is checked, as on the inner side of curves, and where
the slope of the bed is diminished, and in the lee of islands, bridge
piers and projecting points of land. How slight is the check required
to cause a current to drop a large part of its load may be inferred
from the law of the relation of the transporting power to the
velocity. If the velocity is decreased one half, the current can move
fragments but one sixty-fourth the size of those which it could move
before, and must drop all those of larger size.
Will a river deposit more at low water or at flood? when rising or
when falling?
=Stratification.= River deposits are stratified, as may be seen in any
fresh cut in banks or bars. The waste of which they are built has been
sorted and deposited in layers, one above another; some of finer and
some of coarser material. The sorting action of running water depends
on the fact that its transporting power varies with the velocity. A
current whose diminishing velocity compels it to drop coarse gravel,
for example, is still able to move all the finer waste of its load,
and separating it from the gravel, carries it on downstream; while at
a later time slower currents may deposit on the gravel bed layers of
sand, and, still later, slack water may leave on these a layer of mud.
In case of materials lighter than water the transporting power does
not depend on the velocity, and logs of wood, for instance, are
floated on to the sea on the slowest as well as on the most rapid
currents.
[Illustration: Fig. 42. Longitudinal Section of a River Bar]
=Cross bedding.= A section of a bar exposed at low water may show that
it is formed of layers of sand, or coarser stuff, inclined downstream
as steeply often as the angle of repose of the material. From a boat
anchored over the lower end of a submerged sand bar we may observe the
way in which this structure, called cross bedding, is produced. Sand
is continually pushed over the edge of the bar at _b_ (Fig. 42) and
comes to rest in successive layers on the sloping surface. At the same
time the bar may be worn away at the upper end, _a_, and thus slowly
advance down stream. While the deposit is thus cross bedded, it
constitutes as a whole a stratum whose upper and lower surfaces are
about horizontal. In sections of river banks one may often see a
vertical succession of cross-bedded strata, each built in the way
described.
=Water wear.= The coarser material of river deposits, such as
cobblestones, gravel, and the larger grains of sand, are _water worn_,
or rounded, except when near their source. Rolling along the bottom
they have been worn round by impact and friction as they rubbed
against one another and the rocky bed of the stream.
Experiments have shown that angular fragments of granite lose nearly
half their weight and become well rounded after traveling fifteen
miles in rotating cylinders partly filled with water. Marbles are
cheaply made in Germany out of small limestone cubes set revolving in
a current of water between a rotating bed of stone and a block of oak,
the process requiring but about fifteen minutes. It has been found
that in the upper reaches of mountain streams a descent of less than a
mile is sufficient to round pebbles of granite.
[Illustration: Fig. 43. Water-Worn Pebbles, Upper Potomac River,
Maryland]
Land Forms Due To River Erosion
=River valleys.= In their courses to the sea, rivers follow valleys of
various forms, some shallow and some deep, some narrow and some wide.
Since rivers are known to erode their beds and banks, it is a fair
presumption that, aided by the weather, they have excavated the
valleys in which they flow.
Moreover, a bird's-eye view or a map of a region shows the significant
fact that the valleys of a system unite with one another in a branch
work, as twigs meet their stems and the branches of a tree its trunk.
Each valley, from that of the smallest rivulet to that of the master
stream, is proportionate to the size of the stream which occupies it.
With a few explainable exceptions the valleys of tributaries join that
of the trunk stream at a level; there is no sudden descent or break in
the bed at the point of juncture. These are the natural consequences
which must follow if the land has long been worked upon by streams,
and no other process has ever been suggested which is competent to
produce them. We must conclude that valley systems have been formed by
the river systems which drain them, aided by the work of the weather;
they are not gaping fissures in the earth's crust, as early observers
imagined, but are the furrows which running water has drawn upon the
land.
As valleys are made by the slow wear of streams and the action of the
weather, they pass in their development through successive stages,
each of which has its own characteristic features. We may therefore
classify rivers and valleys according to the stage which they have
reached in their life history from infancy to old age.
Young River Valleys
=Infancy.= The Red River of the North. A region in northwestern
Minnesota and the adjacent portions of North Dakota and Manitoba was
so recently covered by the waters of an extinct lake, known as Lake
Agassiz, that the surface remains much as it was left when the lake
was drained away. The flat floor, spread smooth with lake-laid silts,
is still a plain, to the eye as level as the sea. Across it the Red
River of the North and its branches run in narrow, ditch-like
channels, steep-sided and shallow, not exceeding sixty feet in depth,
their gradients differing little from the general slopes of the
region. The trunk streams have but few tributaries; the river system,
like a sapling with few limbs, is still undeveloped. Along the banks
of the trunk streams short gullies are slowly lengthening headwards,
like growing twigs which are sometime to become large branches.
[Illustration: Fig. 44. A Young Lacustrine Plain; the Red River
of the North
Scale 5 inches = about 11 miles. Contour interval, 20 feet]
The flat interstream areas are as yet but little scored by drainage
lines, and in wet weather water lingers in ponds in any initial
depressions on the plain.
[Illustration: Fig. 45. A Young River, Iowa
Note that it has hardly begun to cut in the plain of glacial
drift on which it flows]
=Contours.= In order to read the topographic maps of the text-book and
the laboratory the student should know that contours are lines drawn
on maps to represent relief, all points on any given contour being of
equal height above sea level. The _contour interval_ is the uniform
vertical distance between two adjacent contours and varies on
different maps. To express regions of faint relief a contour interval
of ten or twenty feet is commonly selected; while in mountainous
regions a contour interval of two hundred and fifty, five hundred, or
even one thousand feet may be necessary in order that the contours may
not be too crowded for easy reading.
Whether a river begins its life on a lake plain, as in the example
just cited, or upon a coastal plain lifted from beneath the sea or on
a spread of glacial drift left by the retreat of continental ice
sheets, such as covers much of Canada and the northeastern parts of
the United States, its infantile stage presents the same
characteristic features,--a narrow and shallow valley, with
undeveloped tributaries and undrained interstream areas. Ground water
stands high, and, exuding in the undrained initial depressions, forms
marshes and lakes.
[Illustration: Fig. 46. A Young Drift Region in Wisconsin
Describe this area. How high are the hills? Are they such in form
and position as would be left by stream erosion? Consult a map of
the entire state and notice that the Fox River finds its way to Lake
Michigan, while the Wisconsin empties into the Mississippi. Describe
that portion of the divide here shown between the Mississippi and
the St. Lawrence systems. Which is the larger river, the Wisconsin
or the Fox? Other things being equal, which may be expected to
deepen its bed the more rapidly? What changes are likely to occur
when one of these rivers comes to flow at a lower level than the
other? Why have not these changes occurred already?]
=Lakes.= Lakes are perhaps the most obvious of these fleeting features
of infancy. They are short-lived, for their destruction is soon
accomplished by several means. As a river system advances toward
maturity the deepening and extending valleys of the tributaries lower
the ground-water surface and invade the undrained depressions of the
region. Lakes having outlets are drained away as their basin rims are
cut down by the outflowing streams,--a slow process where the rim is
of hard rock, but a rapid one where it is of soft material such as
glacial drift.
Lakes are effaced also by the filling of their basins. Inflowing
streams and the wash of rains bring in waste. Waves abrade the shore
and strew the débris worn from it over the lake bed. Shallow lakes are
often filled with organic matter from decaying vegetation.
Does the outflowing stream, from a lake carry sediment? How does this
fact affect its erosive power on hard rock? on loose material?
Lake Geneva is a well-known example of a lake in process of
obliteration. The inflowing Rhone has already displaced the waters of
the lake for a length of twenty miles with the waste brought down from
the high Alps. For this distance there extends up the Rhone Valley an
alluvial plain, which has grown lakeward at the rate of a mile and a
half since Roman times, as proved by the distance inland at which a
Roman port now stands.
[Illustration: Fig. 47. A Small Lake being broadened and shoaled
by Wave Wear
_ls_, lake surface; dotted line, initial shore;
_b_, fill made of material taken from _a_]
How rapidly a lake may be silted up under exceptionally favorable
conditions is illustrated by the fact that over the bottom of the
artificial lake, of thirty-five square miles, formed behind the great
dam across the Colorado River at Austin, Texas, sediments thirty-nine
feet deep gathered in seven years.
Lake Mendota, one of the many beautiful lakes of southern Wisconsin,
is rapidly cutting back the soft glacial drift of its shores by means
of the abrasion of its waves. While the shallow basin is thus
broadened, it is also being filled with the waste; and the time is
brought nearer when it will be so shoaled that vegetation can complete
the work of its effacement.
[Illustration: Fig. 48. A Lake well-nigh effaced, Montana
By what means is the lake bed being filled?]
Along the margin of a shallow lake mosses, water lilies, grasses, and
other water-loving plants grow luxuriantly. As their decaying remains
accumulate on the bottom, the ring of marsh broadens inwards, the lake
narrows gradually to a small pond set in the midst of a wide bog, and
finally disappears. All stages in this process of extinction may be
seen among the countless lakelets which occupy sags in the recent
sheets of glacial drift in the northern states; and more numerous than
the lakes which still remain are those already thus filled with
carbonaceous matter derived from the carbon dioxide of the atmosphere.
Such fossil lakes are marked by swamps or level meadows underlain with
muck.
[Illustration: Fig. 49. A Level Meadow, Scotland
Explain its origin. What will be its future?]
=The advance to maturity.= The infantile stage is brief. As a river
advances toward maturity the initial depressions, the lake basins of
its area, are gradually effaced. By the furrowing action of the rain
wash and the head ward lengthening, of tributaries a branchwork of
drainage channels grows until it covers the entire area, and not an
acre is left on which the fallen raindrop does not find already cut
for it an uninterrupted downward path which leads it on by way of
gully, brook, and river to the sea. The initial surface of the land,
by whatever agency it was modeled, is now wholly destroyed; the region
is all reduced to valley slopes.
[Illustration: Fig. 50. Drainage Maps
_A_, an area in its infancy, Buena Vista County, Iowa;
_B_, an area in its maturity, Ringgold County, Iowa]
[Illustration: Fig. 51. Successive Longitudinal Profiles of a
Stream
_am_, initial profile, with waterfall at _w_, and basins at _l_
and _l´_, which at first are occupied by lakes and later are
filled or drained; _b_, _c_, _d_, and _e_, profiles established
in succession as the stream advances from infancy toward old
age. Note that these profiles are concave toward the sky. This
is the _erosion curve_. What contrasting form has the weather
weather curve (p. 34)?]
=The longitudinal profile of a stream.= This at first corresponds with
the initial surface of the region on which the stream begins to flow,
although its way may lead through basins and down steep descents. The
successive profiles to which it reduces its bed are illustrated in
Figure 51. As the gradient, or rate of descent of its bed, is lowered,
the velocity of the river is decreased until its lessening energy is
wholly consumed in carrying its load and it can no longer erode its
bed. The river is now _at grade_, and its capacity is just equal to
its load. If now its load is increased the stream deposits, and thus
builds up, or _aggrades_, its bed. On the other hand, if its load is
diminished it has energy to spare, and resuming its work of erosion,
_degrades_ its bed. In either case the stream continues aggrading or
degrading until a new gradient is found where the velocity is just
sufficient to move the load, and here again it reaches grade.
[Illustration: Fig. 52. A V-Valley,--the Canyon of the
Yellowstone
Note the steep sides. What processes are at work upon them? How
wide is the valley at the base compared with the width of the
stream? Do you see any river deposits along the banks? Is the
stream flowing swiftly over a rock bed, or quietly over a bed
which it has built up? Is it graded or ungraded? Note that the
canyon walls project in interlocking spurs]
=V-Valleys.= Vigorous rivers well armed with waste make short work of
cutting their beds to grade, and thus erode narrow, steep-sided gorges
only wide enough at the base to accommodate the stream. The steepness
of the valley slopes depends on the relative rates at which the bed is
cut down by the stream and the sides are worn back by the weather. In
resistant rock a swift, well-laden stream may saw out a gorge whose
sides are nearly or even quite vertical, but as a rule young valleys
whose streams have not yet reached grade are V-shaped; their sides
flare at the top because here the rocks have longest been opened up to
the action of the weather. Some of the deepest canyons may be found
where a rising land mass, either mountain range or plateau, has long
maintained by its continued uplift the rivers of the region above
grade.
[Illustration: Fig. 53. Section of the Yellowstone Canyon
This canyon is 100 feet deep, 2500 feet wide at the top, and
about 250 feet wide at the bottom. Neglecting any cutting of the
river against the banks, estimate what part of the excavation
of the canyon is due to the vertical erosion of its bed by the
river and what to weathering and rain wash on the canyon sides]
In the northern hemisphere the north sides of river valleys are
sometimes of more gentle slope than the south sides. Can you suggest a
reason?
=The Grand Canyon of the Colorado River in Arizona.= The Colorado
River trenches the high plateau of northern Arizona with a colossal
canyon two hundred and eighteen miles long and more than a mile in
greatest depth (Fig. 15). The rocks in which the canyon is cut are for
the most part flat-lying, massive beds of limestones and sandstones,
with some shales, beneath which in places harder crystalline rocks are
disclosed. Where the canyon is deepest its walls have been profoundly
dissected. Lateral ravines have widened into immense amphitheaters,
leaving between them long ridges of mountain height, buttressed
and rebuttressed with flanking spurs and carved into majestic
architectural forms. From the extremity of one of these promontories
it is two miles or more across the gulf to the point of the one
opposite, and the heads of the amphitheaters are thirteen miles apart.
[Illustration: Fig. 54. Grand Canyon of the Colorado River,
Arizona]
The lower portion of the canyon is much narrower (Fig. 54) and its
walls of dark crystalline rock sink steeply to the edge of the river,
a swift, powerful stream a few hundred feet wide, turbid with reddish
silt, by means of which it continually rasps its rocky bed as it
hurries on. The Colorado is still deepening its gorge. In the Grand
Canyon its gradient is seven and one half feet to the mile, but, as in
all ungraded rivers, the descent is far from uniform. Graded reaches
in soft rock alternate with steeper declivities in hard rock, forming
rapids such as, for example, a stretch of ten miles where the fall
averages twenty-one feet to the mile. Because of these dangerous
rapids the few exploring parties who have traversed the Colorado
canyon have done so at the hazard of their lives.
The canyon has been shaped by several agencies. Its depth is due to
the river which has sawed its way far toward the base of a lofty
rising plateau. Acting alone this would have produced a slitlike gorge
little wider than the breadth of the stream. The impressive width of
the canyon and the magnificent architectural masses which fill it are
owing to two causes. Running water has gulched the walls and
weathering has everywhere attacked and driven them back. The
horizontal harder beds stand out in long lines of vertical cliffs,
often hundreds of feet in height, at whose feet talus slopes conceal
the outcrop of the weaker strata (Fig. 15). As the upper cliffs have
been sapped and driven back by the weather, broad platforms are left
at their bases and the sides of the canyon descend to the river by
gigantic steps. Far up and down the canyon the eye traces these
horizontal layers, like the flutings of an elaborate molding,
distinguishing each by its contour as well as by its color and
thickness.
[Illustration: Fig. 55. Diagrams illustrating Conditions which
produce Falls or Rapids
_A_, vertical succession of harder and softer rocks;
_B_, horizontal succession of the same. In _A_ the stream _ab_
in sinking its bed through a mass of strata of different degrees
of hardness has discovered the weak layer _s_ beneath the hard
layer _h_. It rapidly cuts its way in _s_, while in _A_ its
work is delayed. Thus the profile _afb´_ is soon reached, with
falls at _f_. In _B_ the initial profile is shown by dotted
line.]
The Grand Canyon of the Colorado is often and rightly cited as an
example of the stupendous erosion which may be accomplished by a
river. And yet the Colorado is a young stream and its work is no more
than well begun. It has not yet wholly reached grade, and the great
task of the river and its tributaries--the task of leveling the lofty
plateau to a low plain and of transporting it grain by grain to the
sea--still lies almost entirely in the future.
[Illustration: Fig. 56. Longitudinal Section of Yellowstone
River at Lower Fall, _F_, and Upper Fall, _F´_, Yellowstone
National Park
_la_, lava deeply decayed through action of thermal waters; _m_
and _m´_, masses of decayed lavas to whose hardness the falls
are due. Which fall will be worn away the sooner? How far
upstream will each fall migrate? Draw profile of the river when
one fall has disappeared]
[Illustration: Fig. 57. Diagram illustrating Migration of a
Fall due to a Hard Layer _H_, in the Midst of Soft Layers
_S_ and _S_, all dipping upstream
_a_, _b_, _c_, _d_, and _e_, successive positions of the fall;
_r_, rapid to which the fall is reduced. Draw diagram showing
migration of fall in strata dipping _downstream_. Under what
conditions of inclination of the strata will a fall migrate the
farthest and have the longest life? Under what conditions will
it migrate the least distance and soonest be destroyed?]
=Waterfalls and rapids.= Before the bed of a stream is reduced to
grade it may be broken by abrupt descents which give rise to
waterfalls and rapids. Such breaks in a river's bed may belong to the
initial surface over which it began its course; still more commonly
are they developed in the rock mass through which it is cutting its
valley. Thus, wherever a stream leaves harder rocks to flow over
softer ones the latter are quickly worn below the level of the former,
and a sharp change in slope, with a waterfall or rapid, results.
At time of flood young tributaries with steeper courses than that of
the trunk stream may bring down stones and finer waste, which the
gentler current cannot move along, and throw them as a dam across its
way. The rapids thus formed are also ephemeral, for as the gradient of
the tributaries is lowered the main stream becomes able to handle the
smaller and finer load which they discharge.
A rare class of falls is produced where the minor tributaries of a
young river are not able to keep pace with their master stream in the
erosion of their beds because of their smaller volume, and thus join
it by plunging over the side of its gorge. But as the river approaches
grade and slackens its down cutting, the tributaries sooner or later
overtake it, and effacing their falls, unite with it on a level.
[Illustration: Fig. 58. Maturely Dissected Plateau near
Charleston, West Virginia
Compare the number of streams in any given number of square
miles with the number on an area of the same size in the Red
River Valley (Fig. 44). What is the shape of the ridges? Are
their summits broad or narrow? Are their crests even or broken
by knobs and cols (the depressions on the crest line)? If the
latter, how deeply have the cols been worn beneath the summits
of the knobs?]
Waterfalls and rapids of all kinds are evanescent features of a
river's youth. Like lakes they are soon destroyed, and if any long
time had already elapsed since their formation they would have been
obliterated already.
=Local baselevels.= That balanced condition called grade, where a
river neither degrades its bed by erosion nor aggrades it by
deposition, is first attained along reaches of soft rocks, ungraded
outcrops of hard rocks remaining as barriers which give rise to rapids
or falls. Until these barriers are worn away they constitute local
baselevels, below which level the stream, up valley from them, cannot
cut. They are eroded to grade one after another, beginning with the
least strong, or the one nearest the mouth of the stream. In a similar
way the surface of a lake in a river's course constitutes for all
inflowing streams a local baselevel, which disappears when the basin
is filled or drained.
[Illustration: Fig. 59. A Maturity Dissected Region of Slight
Relief, Iowa]
Mature And Old Rivers
Maturity is the stage of a river's complete development and most
effective work. The river system now has well under way its great task
of wearing down the land mass which it drains and carrying it particle
by particle to the sea. The relief of the land is now at its greatest;
for the main channels have been sunk to grade, while the divides
remain but little worn below their initial altitudes. Ground water now
stands low. The run-off washes directly to the streams, with the least
delay and loss by evaporation in ponds and marches; the discharge of
the river is therefore at its height. The entire region is dissected
by stream ways. The area of valley slopes is now largest and sheds to
the streams a heavier load of waste than ever before. At maturity the
river system is doing its greatest amount of work both in erosion and
in the carriage of water and of waste to the sea.
[Illustration: Fig. 60. Successive Stages, _A_, _B_, _C_, and
_D_, in Valley-Widening by Planation
Describe valley _A_. What changes have taken place in _B_, _C_,
and _D_? Do the river bends remain stationary or move up or
down valley? With what effect on the projecting spurs of the
valley sides? Draw diagrams showing a still later stage than _D_]
=Lateral erosion.= On reaching grade a river ceases to scour its bed,
and it does not again begin to do so until some change in load or
volume enables it to find grade at a lower level. On the other hand, a
stream erodes its banks at all stages in its history, and with graded
rivers this process, called lateral erosion, or _planation_, is
specially important. The current of a stream follows the outer side of
all curves or bends in the channel, and on this side it excavates its
bed the deepest and continually wears and saps its banks. On the inner
side deposition takes place in the more shallow and slower-moving
water. The inner bank of bends is thus built out while the outer bank
is worn away. By swinging its curves against the valley sides a graded
river continually cuts a wider and wider floor. The V-valley of youth
is thus changed by planation to a flat-floored valley with flaring
sides which gradually become subdued by the weather to gentle slopes.
While widening their valleys streams maintain a constant width of
channel, so that a wide-floored valley does not signify that it ever
was occupied by a river of equal width.
=The gradient.= The gradients of graded rivers differ widely. A large
river with a light load reaches grade on a faint slope, while a
smaller stream heavily burdened with waste requires a steep slope to
give it velocity sufficient to move the load.
The Platte, a graded river of Nebraska with its headwaters in the
Rocky Mountains, is enfeebled by the semi-arid climate of the Great
Plains and surcharged with the waste brought down both by its branches
in the mountains and by those whose tracks lie over the soft rocks of
the plains. It is compelled to maintain a gradient of eight feet to
the mile in western Nebraska. The Ohio reaches grade with a slope of
less than four inches to the mile from Cincinnati to its mouth, and
the powerful Mississippi washes along its load with a fall of but
three inches per mile from Cairo to the Gulf.
Other things being equal, which of graded streams will have the
steeper gradient, a trunk stream or its tributaries? a stream supplied
with gravel or one with silt?
Other factors remaining the same, what changes would occur if the
Platte should increase in volume? What changes would occur if the load
should be increased in amount or in coarseness?
[Illustration: Fig. 61. Successive Cross Sections of a Region as
it advances from Infancy _a_, to Old Age _e_]
_The old age of rivers._ As rivers pass their prime, as denudation
lowers the relief of the region, less waste and finer is washed over
the gentler slopes of the lowering hills. With smaller loads to carry,
the rivers now deepen their valleys and find grade with fainter
declivities nearer the level of the sea. This limit of the level of
the sea beneath which they cannot erode is known as _baselevel_.[1] As
streams grow old they approach more and more closely to baselevel,
although they are never able to attain it. Some slight slope is needed
that water may flow and waste be transported over the land. Meanwhile
the relief of the land has ever lessened. The master streams and their
main tributaries now wander with sluggish currents over the broad
valley floors which they have planed away; while under the erosion of
their innumerable branches and the wear of the weather the divides
everywhere are lowered and subdued to more and more gentle slopes.
Mountains and high plateaus are thus reduced to rolling hills, and at
last to plains, surmounted only by such hills as may still be
unreduced to the common level, because of the harder rocks of which
they are composed or because of their distance from the main erosion
channels. Such regions of faint relief, worn down to near base level
by subaërial agencies, are known as _peneplains_ (almost plains).
Any residual masses which rise above them are called _monadnocks_,
from the name of a conical peak of New Hampshire which overlooks the
now uplifted peneplain of southern New England.
[1] The term "baselevel" is also used to designate the close
approximation to sea level to which streams are able to
subdue the land.
In its old age a region becomes mantled with thick sheets of fine and
weathered waste, slowly moving over the faint slopes toward the water
ways and unbroken by ledges of bare rock. In other words, the waste
mantle also is now graded, and as waterfalls have been effaced in the
river beds, so now any ledges in the wide streams of waste are worn
away and covered beneath smooth slopes of fine soil. Ground water
stands high and may exude in areas of swamp. In youth the land mass
was roughhewn and cut deep by stream erosion. In old age the faint
reliefs of the land dissolve away, chiefly under the action of the
weather, beneath their cloak of waste.
[Illustration: Fig. 62. Peneplain surrounded by Monadnocks,
Piedmont Belt, Virginia
From Davis' _Elementary Physical Geography]
=The cycle of erosion.= The successive stages through which a land
mass passes while it is being leveled to the sea constitute together a
cycle of erosion. Each stage of the cycle from infancy to old age
leaves, as we have seen, its characteristic records in the forms
sculptured on the land, such as the shapes of valleys and the contours
of hills and plains. The geologist is thus able to determine by the
land forms of any region the stage in the erosion cycle to which it
now belongs, and knowing what are the earlier stages of the cycle, to
read something of the geological history of the region.
=Interrupted cycles.= So long a time is needed to reduce a land mass
to baselevel that the process is seldom if ever completed during a
single uninterrupted cycle of erosion. Of all the various
interruptions which may occur the most important are gradual movements
of the earth's crust, by which a region is either depressed or
elevated relative to sea level.
[Illustration: Fig. 63. Young Inner Gorge in Wide Older Valley,
Alaska]
The _depression_ of a region hastens its old age by decreasing the
gradient of streams, by destroying their power to excavate their beds
and carry their loads to a degree corresponding to the amount of the
depression, and by lessening the amount of work they have to do. The
slackened river currents deposit their waste in Hood plains which
increase in height as the subsidence continues. The lower courses of
the rivers are invaded by the sea and become estuaries, while the
lower tributaries are cut off from the trunk stream.
_Elevation_, on the other hand, increases the activity of all agencies
of weathering, erosion, and transportation, restores the region to its
youth, and inaugurates a new cycle of erosion. Streams are given a
steeper gradient, greater velocity, and increased energy to carry
their loads and wear their beds. They cut through the alluvium of
their flood plains, leaving it on either bank as successive terraces,
and intrench themselves in the underlying rock. In their older and
wider valleys they cut narrow, steep-walled inner gorges, in which
they flow swiftly over rocky floors, broken here and there by falls
and rapids where a harder layer of rock has been discovered. Winding
streams on plains may thus incise their meanders in solid rock as the
plains are gradually uplifted. Streams which are thus restored to
their youth are said to be _revived_.
[Illustration: Fig. 64. Incised Meanders of Oneota River, Iowa]
As streams cut deeper and the valley slopes are steepened, the mantle
of waste of the region undergoing elevation is set in more rapid
movement. It is now removed particle by particle faster than it forms.
As the waste mantle thins, weathering attacks the rocks of the region
more energetically until an equilibrium is reached again; the rocks
waste rapidly and their waste is as rapidly removed.
=Dissected peneplains.= When a rise of the land brings one cycle to an
end and begins another, the characteristic land forms of each cycle
are found together and the topography of the region is composite until
the second cycle is so far advanced that the land forms of the first
cycle are entirely destroyed. The contrast between the land surfaces
of the later and the earlier cycles is most striking when the earlier
had advanced to age and the later is still in youth. Thus many
peneplains which have been elevated and dissected have been recognized
by the remnants of their ancient erosion surfaces, and the length of
time which has elapsed since their uplift has been measured by the
stage to which the new cycle has advanced.
[Illustration: Fig. 65.
Describe the valley of stream _a_. Is it young or old? How does
the valley of _b_ differ from that of _a_? Compare as to form
and age the inner valley of _b_ with the outer valley and with
the valley of _a_. Account for the inner valley. Why does it
not extend to the upper portion of the course of _b_? Will it
ever do so? Draw longitudinal profile of _b_, showing the
different gradient of upper and lower portions of its course
not here seen. As the inner valley of tributary _c_ extends
headward it may invade the valley of _a_ before the inner
valley of _a_ has worked upstream to the area seen in the
diagram. With what results?]
=The Piedmont Belt.= As an example of an ancient peneplain uplifted
and dissected we may cite the Piedmont Belt, a broad upland lying
between the Appalachian Mountains and the Atlantic coastal plain. The
surface of the Piedmont is gently rolling. The divides, which are
often smooth areas of considerable width, rise to a common plane, and
from them one sees in every direction an even sky line except where in
places some lone hill or ridge may lift itself above the general level
(Fig. 62). The surface is an ancient one, for the mantle of residual
waste lies deep upon it, soils are reddened by long oxidation, and the
rocks are rotted to a depth of scores of feet.
At present, however, the waste mantle is not forming so rapidly as it
is being removed. The streams of the upland are actively engaged in
its destruction. They flow swiftly in narrow, rock-walled valleys over
rocky beds. This contrast between the young streams and the aged
surface which they are now so vigorously dissecting can only be
explained by the theory that the region once stood lower than at
present and has recently been upraised. If now we imagine the valleys
refilled with the waste which the streams have swept away, and the
upland lowered, we restore the Piedmont region to the condition in
which it stood before its uplift and dissection,--a gently rolling
plain, surmounted here and there by isolated hills and ridges.
[Illustration: Fig. 66. Dissected Peneplain of Southern New
England]
The surface of the ancient Piedmont plain, as it may be restored from
the remnants of it found on the divides, is not in accordance with the
structures of the country rocks. Where these are exposed to view they
are seen to be far from horizontal. On the walls of river gorges they
dip steeply and in various directions and the streams flow over their
upturned edges. As shown in Figure 67, the rocks of the Piedmont have
been folded and broken and tilted.
[Illustration: Fig. 67. Section in Piedmont Belt
_M_, a monadnock]
It is not reasonable to believe that when the rocks of the Piedmont
were thus folded and otherwise deformed the surface of the region was
a plain. The upturned layers have not always stopped abruptly at the
even surface of the Piedmont plain which now cuts across them. They
are the bases of great folds and tilted blocks which must once have
risen high in air. The complex and disorderly structures of the
Piedmont rocks are those seen in great mountain ranges, and there is
every reason to believe that these rocks after their deformation rose
to mountain height.
[Illustration: Fig. 68. The area of the Laurentian Peneplain
(shaded)]
The ancient Piedmont plain cuts across these upturned rocks as
independently of their structure as the even surface of the sawed
stump of some great tree is independent of the direction of its
fibers. Hence the Piedmont plain as it was before its uplift was not a
coastal plain formed of strata spread in horizontal sheets beneath the
sea and then uplifted; nor was it a structural plain, due to the
resistance to erosion of some hard, flat-lying layer of rock. Even
surfaces developed on rocks of discordant structure, such as the
Piedmont shows, are produced by long denudation, and we may consider
the Piedmont as a peneplain formed by the wearing down of mountain
ranges, and recently uplifted.
=The Laurentian peneplain.= This is the name given to a denuded
surface on very ancient rocks which extends from the Arctic Ocean to
the St. Lawrence River and Lake Superior, with small areas also in
northern Wisconsin and New York. Throughout this U-shaped area, which
incloses Hudson Bay within its arms, the country rocks have the
complicated and contorted structures which characterize mountain
ranges (see Fig. 179, P. 211). But the surface of the area is by no
means mountainous. The sky line when viewed from the divides is
unbroken by mountain peaks or rugged hills. The surface of the arm
west of Hudson Bay is gently undulating and that of the eastern arm
has been roughened to low-rolling hills and dissected in places by
such deep river gorges as those of the Ottawa and Saguenay. This
immense area may be regarded as an ancient peneplain truncating the
bases of long-vanished mountains and dissected after elevation.
In the examples cited the uplift has been a broad one and to
comparatively little height. Where peneplains have been uplifted to
great height and have since been well dissected, and where they have
been upfolded and broken and uptilted, their recognition becomes more
difficult. Yet recent observers have found evidences of ancient
lowland surfaces of erosion on the summits of the Allegheny ridges,
the Cascade Mountains (Fig. 69), and the western slope of the Sierra
Nevadas.
[Illustration: Fig. 69. View in the Cascade Mountains, Washington
The general level to which these ridges rise may be accounted
for by the uplift and dissection of a once low-lying peneplain]
=The southern Appalachian region.= We have here an example of an area
the latter part of whose geological history may be deciphered by means
of its land forms. The generalized section of Figure 70, which passes
from west to east across a portion of the region in eastern Tennessee,
shows on the west a part of the broad Cumberland plateau. On the east
is a roughened upland platform, from which rise in the distance the
peaks of the Great Smoky Mountains. The plateau, consisting of strata
but little changed from their original flat-lying attitude, and the
platform, developed on rocks of disordered structure made crystalline
by heat and pressure, both stand at the common level of the line AB.
They are separated by the Appalachian valley, forty miles wide, cut in
strata which have been folded and broken into long narrow blocks. The
valley is traversed lengthwise by long, low ridges, the outcropping
edges of the harder strata, which rise to about the same level,--that
of the line _cd_. Between these ridges stretch valley lowlands at the
level _ef_ excavated in the weaker rocks, while somewhat below them lie
the channels of the present streams now busily engaged in deepening
their beds.
_The valley lowlands._ Were they planed by graded or ungraded streams?
Have the present streams reached grade? Why did the streams cease
widening the floors of the valley lowlands? How long since? When will
they begin anew the work of lateral planation? What effect will this
have on the ridges if the present cycle of erosion continues long
uninterrupted?
[Illustration: Fig. 70. Generalized Section of the Southern
Appalachian Region in Eastern Tennessee]
_The ridges of the Appalachian valley._ Why do they stand above the
valley lowlands? Why do their summits lie in about the same plane?
Refilling the valleys intervening between these ridges with the
material removed by the streams, what is the nature of the surface
thus restored? Does this surface _cd_ accord with the rock structures
on which it has been developed? How may it have been made? At what
height did the land stand then, compared with its present height? What
elevations stood above the surface _cd_? Why? What name may you use to
designate them? How does the length of time needed to develop the
surface _cd_ compare with that needed to develop the valley lowlands?
_The Platform And Plateau._ Why do they stand at a common level ab? Of
what surface may they be remnants? Is it accordant with the rock
structure? How was it produced? What unconsumed masses overlooked it?
Did the rocks of the Appalachian valley stand above this surface when
it was produced? Did they then stand below it? Compare the time needed
to develop this surface with that needed to develop _cd_. Which surface
is the older?
How many cycles of erosion are represented here? Give the erosion
history of the region by cycles, beginning with the oldest, the work
done in each and the work left undone, what brought each cycle to a
close, and how long relatively it continued.
CHAPTER IV
RIVER DEPOSITS
The characteristic features of river deposits and the forms which they
assume may be treated under three heads: (1) valley deposits, (2)
basin deposits, and (3) deltas.
Valley Deposits
=Flood plains.= The deposits which streams build along their courses
at times of flood are known as flood plains. A swift current then
sweeps along the channel, while a shallow sheet of water moves slowly
over the flood plain, spreading upon it a thin layer of sediment. It
has been estimated that each inundation of the Nile leaves a layer of
fertilizing silt three hundredths of an inch thick over the flood
plain of Egypt.
Flood plains may consist of a thin spread of alluvium over the flat
rock floor of a valley which is being widened by the lateral erosion
of a graded stream (Fig. 60). Flood-plain deposits of great thickness
may be built by aggrading rivers even in valleys whose rock floors
have never been thus widened (Fig. 368).
[Illustration: Fig. 71. Cross Section of a Flood Plain]
A cross section of a flood plain (Fig. 71) shows that it is highest
next the river, sloping gradually thence to the valley sides. These
wide natural embankments are due to the fact that the river deposit is
heavier near the bank, where the velocity of the silt-laden channel
current is first checked by contact with the slower-moving overflow.
[Illustration: Fig. 72. Waste-filled Valley and Braided
Channels of the Upper Mississippi]
Thus banked off from the stream, the outer portions of a flood plain
are often ill-drained and swampy, and here vegetal deposits, such as
peat, may be interbedded with river silts.
A map of a wide flood plain, such as that of the Mississippi or the
Missouri (Fig. 77), shows that the courses of the tributaries on
entering it are deflected downstream. Why?
The aggrading streams by which flood plains are constructed gradually
build their immediate banks and beds to higher and higher levels, and
therefore find it easy at times of great floods to break their natural
embankments and take new courses over the plain. In this way they
aggrade each portion of it in turn by means of their shifting
channels.
=Braided channels.= A river actively engaged in aggrading its valley
with coarse waste builds a flood plain of comparatively steep gradient
and often flows down it in a fairly direct course and through a
network of braided channels. From time to time a channel becomes
choked with waste, and the water no longer finding room in it breaks
out and cuts and builds itself a new way which reunites down valley
with the other channels. Thus there becomes established a network of
ever-changing channels inclosing low islands of sand and gravel.
[Illustration: Fig. 73. Terraced Valley of River in Central Asia]
[Illustration: Fig. 74. Terraces carved in Alluvial Deposits]
Which is older, the rock floor of the valley or the river
deposits which fill it? What are the relative ages of terraces
_a_, _b_, _c_, and _e_? It will be noted that the remnants of
the higher flood plains have not been swept away by the
meandering river, as it swung from side to side of the valley
at lower levels, because they have been defended by ledges of
hard rock in the projecting spurs of the initial valley. The
stream has encountered such defending ledges at the point
marked _d_]
[Illustration: Fig. 75. River Terraces of Rock covered with
Alluvium
_c_, recent flood plain of the river. To what processes is it
due? Account for the alluvium at _a_ and _b_ and on the
opposite side of the valley at the same levels. Which is the
older? Account for the flat rock floors on which these deposits
of alluvium rest. Give the entire history which may be read in
the section]
=Terraces.= While aggrading streams thus tend to shift their channels,
degrading streams, on the contrary, become more and more deeply
intrenched in their valleys. It often occurs that a stream, after
having built a flood plain, ceases to aggrade its bed because of a
lessened load or for other reasons, such as an uplift of the region,
and begins instead to degrade it. It leaves the original flood plain
out of reach of even the highest floods. When again it reaches grade
at a lower level it produces a new flood plain by lateral erosion in
the older deposits, remnants of which stand as terraces on one or both
sides of the valley. In this way a valley may be lined with a
succession of terraces at different levels, each level representing an
abandoned flood plain.
[Illustration: Fig. 76. Development of a Meander
The dotted line in _a_, _b_, and _c_ shows the stage preceding that indicate by the unbroken line]
=Meanders.= Valleys aggraded with fine waste form well-nigh level
plains over which streams wind from side to side of a direct course in
symmetric bends known as meanders, from the name of a winding river of
Asia Minor. The giant Mississippi has developed meanders with a radius
of one and one half miles, but a little creek may display on its
meadow as perfect curves only a rod or so in radius. On the flood
plain of either river or creek we may find examples of the successive
stages in the development of the meander, from its beginning in the
slight initial bend sufficient to deflect the current against the
outer side. Eroding here and depositing on the inner side of the bend,
it gradually reaches first the open bend (Fig. 76, _a_) whose width
and length are not far from equal, and later that of the horseshoe
meander (Fig. 76, _b_) whose diameter transverse to the course of the
stream is much greater than that parallel with it. Little by little
the neck of land projecting into the bend is narrowed, until at last
it is cut through and a "cut-off" is established. The old channel is
now silted up at both ends and becomes a crescentic lagoon (Fig. 76,
_c_), or oxbow lake, which fills gradually to an arc-shaped shallow
depression.
[Illustration: Fig. 77. Map of a portion of the Flood Plain of
the Missouri River
Each small square represents one square mile. How wide is the
flood plain of the Missouri? How wide is the flood plain of the
Big Sioux? Why is the latter river deflected down valley on
entering the flood plain of the master stream? How do the
meanders of the two rivers compare in size? How does the width
of each flood plain compare with the width of the belt occupied
by the meanders of the river? Do you find traces of any former
channels?]
=Flood plains characteristic of mature rivers.= On reaching grade a
stream planes a flat floor for its continually widening valley. Ever
cutting on the outer bank of its curves, it deposits on the inner bank
scroll-like flood-plain patches (Fig 60). For a while the valley bluffs
do not give its growing meanders room to develop to their normal size,
but as planation goes on, the bluffs are driven back to the full width
of the meander belt and still later to a width which gives room for
broad stretches of flood plain on either side (Fig. 77).
Usually a river first attains grade near its mouth, and here first sinks
its bed to near baselevel. Extending its graded course upstream by
cutting away barrier after barrier, it comes to have a widened and
mature valley over its lower course, while its young headwaters are
still busily eroding their beds. Its ungraded branches may thus bring
down to its lower course more waste than it is competent to carry on to
the sea, and here it aggrades its bed and builds a flood plain in order
to gain a steeper gradient and velocity enough to transport its load.
As maturity is past and the relief of the land is lessened, a smaller
and smaller load of waste is delivered to the river. It now has energy
to spare and again degrades its valley, excavating its former flood
plains and leaving them in terraces on either side, and at last in its
old age sweeping them away.
[Illustration: Fig. 78. Alluvial Cones, Wyoming]
=Alluvial cones and fans.= In hilly and mountainous countries one often
sees on a valley side a conical or fan-shaped deposit of waste at the
mouth of a lateral stream. The cause is obvious: the young branch has
not been able as yet to wear its bed to accordant level with the already
deepened valley of the master stream. It therefore builds its bed to
grade at the point of juncture by depositing here its load of waste,--a
load too heavy to be carried along the more gentle profile of the trunk
valley.
[Illustration: Fig. 79. Tributaries and Distributaries of a
Fan-Building Stream]
Where rivers descend from a mountainous region upon the plain they may
build alluvial fans of exceedingly gentle slope. Thus the rivers of
the western side of the Sierra Nevada Mountains have spread fans with
a radius of as much as forty miles and a slope too slight to be
detected without instruments, where they leave the rock-cut canyons in
the mountains and descend upon the broad central valley of California.
As a river flows over its fan it commonly divides into a branchwork of
shifting channels called _distributaries_, since they lead off the
water from the main stream. In this way each part of the fan is
aggraded and its symmetric form is preserved.
=Piedmont plains.= Mountain streams may build their confluent fans
into widespread piedmont (foot of the mountain) alluvial plains. These
are especially characteristic of arid lands, where the streams wither
as they flow out upon the thirsty lowlands and are therefore compelled
to lay down a large portion of their load. In humid climates
mountain-born streams are usually competent to carry their loads of
waste on to the sea, and have energy to spare to cut the lower
mountain slopes into foothills. In arid regions foothills are commonly
absent and the ranges rise, as from pedestals, above broad, sloping
plains of stream-laid waste.
[Illustration: Fig. 80. Section from the Rocky Mountains Eastward
River deposits dotted]
=The High Plains.= The rivers which flow eastward from the Rocky
Mountains have united their fans in a continuous sheet of waste which
stretches forward from the base of the mountains for hundreds of miles
and in places is five hundred feet thick (Fig. 80). That the deposit
was made in ancient times on land and not in the sea is proved by the
remains which it contains of land animals and plants of species now
extinct. That it was laid by rivers and not by fresh-water lakes is
shown by its structure. Wide stretches of flat-lying, clays and sands
are interrupted by long, narrow belts of gravel which mark the
channels of the ancient streams. Gravels, and sands are often cross
bedded, and their well worn pebbles may be identified with the rocks
of the mountains. After building this sheet of waste the streams
ceased to aggrade and began the work of destruction. Large uneroded
remnants, their surfaces flat as a floor, remain as the High Plains of
western Kansas and Nebraska.
=River deposits in subsiding troughs.= To a geologist the most
important river deposits are those which gather in areas of gradual
subsidence; they are often of vast extent and immense thickness, and
such deposits of past geological ages have not infrequently been
preserved, with all their records of the times in which they were
built, by being carried below the level of the sea, to be brought to
light by a later uplift. On the other hand, river deposits which
remain above baselevels of erosion are swept away comparatively soon.
=The Great Valley Of California= is a monotonously level plain of
great fertility, four hundred miles in length and fifty miles in
average width, built of waste swept down by streams from the mountain
ranges which inclose it,--the Sierra Nevada on the east and the Coast
Range on the west. On the waste slopes at the foot of the bordering
hills coarse gravels and even bowlders are left, while over the
interior the slow-flowing streams at times of flood spread wide sheets
of silt. Organic deposits are now forming by the decay of vegetation
in swampy tule (reed) lands and in shallow lakes which occupy
depressions left by the aggrading streams.
Deep borings show that this great trough is filled to a depth of at
least two thousand feet below sea level with recent unconsolidated
sands and silts containing logs of wood and fresh-water shells. These
are land deposits, and the absence of any marine deposits among them
proves that the region has not been invaded by the sea since the
accumulation began. It has therefore been slowly subsiding and its
streams, although continually carried below grade, have yet been able
to aggrade the surface as rapidly as the region sank, and have
maintained it, as at present, slightly above sea level.
=The Indo-Gangetic Plain=, spread by the Brahmaputra, the Ganges, and
the Indus river systems, stretches for sixteen hundred miles along the
southern base of the Himalaya Mountains and occupies an area of three
hundred thousand square miles (Fig. 342). It consists of the flood
plains of the master streams and the confluent fans of the tributaries
which issue from the mountains on the north. Large areas are subject
to overflow each season of flood, and still larger tracts mark
abandoned flood plains below which the rivers have now cut their beds.
The plain is built of far-stretching beds of clay, penetrated by
streaks of sand, and also of gravel near the mountains. Beds of impure
peat occur in it, and it contains fresh-water shells and the bones of
land animals of species now living in northern India. At Lucknow an
artesian well was sunk to one thousand feet below sea level without
reaching the bottom of these river-laid sands and silts, proving a
slow subsidence with which the aggrading rivers have kept pace.
=Warped valleys.= It is not necessary that an area should sink below
sea level in order to be filled with stream-swept waste. High valleys
among growing mountain ranges may suffer warping, or may be blockaded
by rising mountain folds athwart them. Where the deformation is rapid
enough, the river may be ponded and the valley filled with lake-laid
sediments. Even when the river is able to maintain its right of way it
may yet have its declivity so lessened that it is compelled to aggrade
its course continually, filling the valley with river deposits which
may grow to an enormous thickness.
Behind the outer ranges of the Himalaya Mountains lie several
waste-filled valleys, the largest of which are Kashmir and Nepal, the
former being an alluvial plain about as large as the state of
Delaware. The rivers which drain these plains have already cut down
their outlet gorges sufficiently to begin the task of the removal of
the broad accumulations which they have brought in from the
surrounding mountains. Their present flood plains lie as much as some
hundreds of feet below wide alluvial terraces which mark their former
levels. Indeed, the horizontal beds of the Hundes Valley have been
trenched to the depth of nearly three thousand feet by the Sutlej
River. These deposits are recent or subrecent, for there have been
found at various levels the remains of land plants and land and
fresh-water shells, and in some the bones of such animals as the hyena
and the goat, of species or of genera now living. Such soft deposits
cannot be expected to endure through any considerable length of future
time the rapid erosion to which their great height above the level of
the sea will subject them.
[Illustration: Fig. 81. Cross Section of Aggraded Valley,
showing Structure of River Deposits]
=Characteristics of river deposits.= The examples just cited teach
clearly the characteristic features of extensive river deposits. These
deposits consist of broad, flat-lying sheets of clay and fine sand
left by the overflow at time of flood, and traversed here and there by
long, narrow strips of coarse, cross-bedded sands and gravels thrown
down by the swifter currents of the shifting channels. Occasional beds
of muck mark the sites of shallow lakelets or fresh-water swamps. The
various strata also contain some remains of the countless myriads of
animals and plants which live upon the surface of the plain as it is
in process of building. River shells such as the mussel, land shells
such as those of snails, the bones of fishes and of such land animals
as suffer drowning at times of flood or are mired in swampy places,
logs of wood, and the stems and leaves of plants are examples of the
variety of the remains of land and fresh-water organisms which are
entombed in river deposits and sealed away as a record of the life of
the time, and as proof that the deposits were laid by streams and not
beneath the sea.
Basin Deposits
=Deposits in dry basins.= On desert areas without outlet to the sea,
as on the Great Basin of the United States and the deserts of central
Asia, stream-swept waste accumulates indefinitely. The rivers of the
surrounding mountains, fed by the rains and melting snows of these
comparatively moist elevations, dry and soak away as they come down
upon the arid plains. They are compelled to lay aside their entire
load of waste eroded from the mountain valleys, in fans which grow to
enormous size, reaching in some instances thousands of feet in
thickness.
The monotonous levels of Turkestan include vast alluvial tracts now in
process of building by the floods of the frequently shifting channels
of the Oxus and other rivers of the region. For about seven hundred
miles from its mouth in Aral Lake the Oxus receives no tributaries,
since even the larger branches of its system are lost in a network of
distributaries and choked with desert sands before they reach their
master stream. These aggrading rivers, which have channels but no
valleys, spread their muddy floods--which in the case of the Oxus
sometimes equal the average volume of the Mississippi--far and wide
over the plain, washing the bases of the desert dunes.
=Playas.= In arid interior basins the central depressions may be
occupied by playas,--plains of fine mud washed forward from the
margins. In the wet season the playa is covered with a thin sheet of
muddy water, a playa lake, supplied usually by some stream at flood.
In the dry season the lake evaporates, the river which fed it
retreats, and there is left to view a hard, smooth, level floor of
sun-baked and sun-cracked yellow clay utterly devoid of vegetation.
In the Black Rock desert of Nevada a playa lake spreads over an area
fifty miles long and twenty miles wide. In summer it disappears; the
Quinn River, which feeds it, shrinks back one hundred miles toward its
source, leaving an absolutely barren floor of clay, level as the sea.
=Lake deposits.= Regarding lakes as parts of river systems, we may now
notice the characteristic features of the deposits in lake basins.
Soundings in lakes of considerable size and depth show that their
bottoms are being covered with tine clays. Sand and gravel are found
along; their margins, being brought in by streams and worn by waves
from the shore, but there are no tidal or other strong currents to
sweep coarse waste out from shore to any considerable distance. Where
fine clays are now found on the land in even, horizontal layers
containing the remains of fresh-water animals and plants, uncut by
channels tilled with cross-bedded gravels and sands and bordered by
beach deposits of coarse waste, we may safely infer the existence of
ancient lakes.
=Marl.= Marl is a soft, whitish deposit of carbonate of lime, mingled
often with more or less of clay, accumulated in small lakes whose
feeding springs are charged with carbonate of lime and into which
little waste is washed from the land. Such lakelets are not infrequent
on the surface of the younger drift sheets of Michigan and northern
Indiana, where their beds of marl--sometimes as much as forty feet
thick--are utilized in the manufacture of Portland cement. The deposit
results from the decay of certain aquatic plants which secrete lime
carbonate from the water, from the decomposition of the calcareous
shells of tiny mollusks which live in countless numbers on the lake
floor, and in some cases apparently from chemical precipitation.
=Peat.= We have seen how lakelets are extinguished by the decaying
remains of the vegetation which they support. A section of such a
fossil lake shows that below the growing mosses and other plants of
the surface of the bog lies a spongy mass composed of dead vegetable
tissue, which passes downward gradually into _peat_,--a dense, dark
brown carbonaceous deposit in which, to the unaided eye, little or no
trace of vegetable structure remains. When dried, peat forms a fuel of
some value and is used either cut into slabs and dried or pressed into
bricks by machinery.
[Illustration: Fig. 82. Digging Peat, Scotland]
When vegetation decays in open air the carbon of its tissues, taken
from the atmosphere by the leaves, is oxidized and returned to it in
its original form of carbon dioxide. But decomposing in the presence
of water, as in a bog, where the oxygen of the air is excluded, the
carbonaceous matter of plants accumulates in deposits of peat.
Peat bogs are numerous in regions lately abandoned by glacier ice,
where river systems are so immature that the initial depressions left
in the sheet of drift spread over the country have not yet been
drained. One tenth of the surface of Ireland is said to be covered
with peat, and small bogs abound in the drift-covered area of New
England and the states lying as far west as the Missouri River. In
Massachusetts alone it has been reckoned that there are fifteen
billion cubic feet of peat, the largest bog occupying several thousand
acres.
Much larger swamps occur on the young coastal plain of the Atlantic
from New Jersey to Florida. The Dismal Swamp, for example, in Virginia
and North Carolina is forty miles across. It is covered with a dense
growth of water-loving trees such as the cypress and black gum. The
center of the swamp is occupied by Lake Drummond, a shallow lake seven
miles in diameter, with banks of pure-peat, and still narrowing from
the encroachment of vegetation along its borders.
=Salt lakes.= In arid climates a lake rarely receives sufficient
inflow to enable it to rise to the basin rim and find an outlet.
Before this height is reached its surface becomes large enough to
discharge by evaporation into the dry air the amount of water that is
supplied by streams. As such a lake has no outlet, the minerals in
solution brought into it by its streams cannot escape from the basin.
The lake water becomes more and more heavily charged with such
substances as common salt and the sulphates and carbonates of lime, of
soda, and of potash, and these are thrown down from solution one after
another as the point of saturation for each mineral is reached.
Carbonate of lime, the least soluble and often the most abundant
mineral brought in, is the first to be precipitated. As concentration
goes on, gypsum, which is insoluble in a strong brine, is deposited,
and afterwards common salt. As the saltness of the lake varies with
the seasons and with climatic changes, gypsum and salt are laid in
alternate beds and are interleaved with sedimentary clays spread from
the waste brought in by streams at times of flood. Few forms of life
can live in bodies of salt water so concentrated that chemical
deposits take place, and hence the beds of salt, gypsum, and silt of
such lakes are quite barren of the remains of life. Similar deposits
are precipitated by the concentration of sea water in lagoons and arms
of the sea cut off from the ocean.
[Illustration: Fig. 83. Map of Lake Bonneville and Lahontan
From Davis' _Physical Geography_]
=Lakes Bonneville and Lahontan.= These names are given to extinct
lakes which once occupied large areas in the Great Basin, the former
in Utah, the latter in northwestern Nevada. Their records remain in
old horizontal beach lines which they drew along their mountainous
shores at the different levels at which they stood, and in the
deposits of their beds. At its highest stage Lake Bonneville, then one
thousand feet deep, overflowed to the north and was a fresh-water
lake. As it shrank below the outlet it became more and more salty, and
the Great Salt Lake, its withered residue, is now depositing salt
along its shores. In its strong brine lime carbonate is insoluble, and
that brought in by streams is thrown down at once in the form of
travertine.
[Illustration: Fig. 84. Section of Deposits in Beds of Lakes
Bonneville and Lahontan]
Lake Lahontan never had an outlet. The first chemical deposits to be
made along its shores were deposits of travertine, in places eighty
feet thick. Its floor is spread with fine clays, which must have been
laid in deep, still water, and which are charged with the salts
absorbed by them as the briny water of the lake dried away. These
sedimentary clays are in two divisions, the upper and lower, each
being about one hundred feet thick (_a_ and _c_, Fig. 84). They are
separated by heavy deposits of well-rounded, cross-bedded gravels and
sands (_b_, Fig. 84), similar to those spread at the present time by
the intermittent streams of arid regions. A similar record is shown in
the old floors of Lake Bonneville. What conclusions do you draw from
these facts as to the history of these ancient lakes?
Deltas
In the river deposits which are left above sea level particles of
waste are allowed to linger only for a time. From alluvial fans and
flood plains they are constantly being taken up and swept farther on
downstream. Although these land forms may long persist, the particles
which compose them are ever changing. We may therefore think of the
alluvial deposits of a valley as a stream of waste fed by the waste
mantle as it creeps and washes down the valley sides, and slowly
moving onwards to the sea.
In basins waste finds a longer rest, but sooner or later lakes and dry
basins are drained or filled, and their deposits, if above sea level,
resume their journey to their final goal. It is only when carried
below the level of the sea that they are indefinitely preserved.
On reaching this terminus, rivers deliver their load to the ocean. In
some cases the ocean is able to take it up by means of strong tidal
and other currents, and to dispose of it in ways which we shall study
later. But often the load is so large, or the tides are so weak, that
much of the waste which the river brings in settles at its mouth,
there building up a deposit called the _delta_, from the Greek letter
(D) of that name, whose shape it sometimes resembles.
Deltas and alluvial fans have many common characteristics. Both owe
their origin to a sudden check in the velocity of the river,
compelling a deposit of the load; both are triangular in outline, the
apex pointing upstream; and both are traversed by distributaries which
build up all parts in turn.
In a delta we may distinguish deposits of two distinct kinds,--the
submarine and the subaërial. In part a delta is built of waste
brought down by the river and redistributed and spread by waves and
tides over the sea bottom adjacent to the river's mouth. The origin of
these deposits is recorded in the remains of marine animals and plants
which they contain.
[Illustration: Fig. 85. Delta of the Mississippi River]
As the submarine delta grows near to the level of the sea the
distributaries of the river cover it with subaërial deposits
altogether similar to those of the flood plain, of which indeed the
subaërial delta is the prolongation. Here extended deposits of peat
may accumulate in swamps, and the remains of land and fresh-water
animals and plants swept down by the stream are imbedded in the silts
laid at times of flood.
Borings made in the deltas of great rivers such as the Mississippi,
the Ganges, and the Nile, show that the subaërial portion often
reaches a surprising thickness. Layers of peat, old soils, and forest
grounds with the stumps of trees are discovered hundreds of feet below
sea level. In the Nile delta some eight layers of coarse gravel were
found interbedded with river silts, and in the Ganges delta at
Calcutta a boring nearly five hundred feet in depth stopped in such a
layer.
The Mississippi has built a delta of twelve thousand three hundred
square miles, and is pushing the natural embankments of its chief
distributaries into the Gulf at a maximum rate of a mile in sixteen
years. Muddy shoals surround its front, shallow lakes, e.g. lakes
Pontchartrain and Borgne, are formed between the growing delta and the
old shore line, and elongate lakes and swamps are inclosed between the
natural embankments of the distributaries.
The delta of the Indus River, India, lies so low along shore that a
broad tract of country is overflowed by the highest tides. The
submarine portion of the delta has been built to near sea level over
so wide a belt offshore that in many places large vessels cannot come
even within sight of land because of the shallow water.
[Illustration: Fig. 86. Radial Section of a Delta
This section of a delta illustrates the structure of the
platform which swift streams well loaded with coarse waste
build in the water bodies into which they empty. Three members
may be distinguished: the _bottom set beds_, _a_: the _fore set
beds_, _b_; and the _top set beds_, _c_. Account for the slope
of each of these. Why are the bottom set beds of the finer
material and why do they extend beyond the others? How does the
profile of this delta differ from that of an alluvial cone and
why?]
A former arm of the sea, the Rann of Cutch, adjoining the delta on the
east has been silted up and is now an immense barren flat of sandy mud
two hundred miles in length and one hundred miles in greatest breadth.
Each summer it is flooded with salt water when the sea is brought in
by strong southwesterly monsoon winds, and the climate during the
remainder of the year is hot and dry. By the evaporation of sea water
the soil is thus left so salty that no vegetation can grow upon it,
and in places beds of salt several feet in thickness have accumulated.
Under like conditions salt beds of great thickness have been formed in
the past and are now found buried among the deposits of ancient
deltas.
=Subsidence of great deltas.= As a rule great deltas are slowly
sinking. In some instances upbuilding by river deposits has gone on as
rapidly as the region has subsided. The entire thickness of the Ganges
delta, for example, so far as it has been sounded, consists of
deposits laid in open air. In other cases interbedded limestones and
other sedimentary rocks containing marine fossils prove that at times
subsidence has gained on the upbuilding and the delta has been covered
with the sea.
It is by gradual depression that delta deposits attain enormous
thickness, and, being lowered beneath the level of the sea, are safely
preserved from erosion until a movement of the earth's crust in the
opposite direction lifts them to form part of the land. We shall read
later in the hard rocks of our continent the records of such ancient
deltas, and we shall not be surprised to find them as thick as are
those now building at the mouths of great rivers.
=Lake deltas.= Deltas are also formed where streams lose their
velocity on entering the still waters of lakes. The shore lines of
extinct lakes, such as Lake Agassiz and Lakes Bonneville and Lahontan,
may be traced by the heavy deposits at the mouths of their tributary
streams.
* * * * *
We have seen that the work of streams is to drain the lands of the
water poured upon them by the rainfall, to wear them down, and to
carry their waste away to the sea, there to be rebuilt by other agents
into sedimentary rocks. The ancient strata of which the continents are
largely made are composed chiefly of material thus worn from still
more ancient lands--lands with their hills and valleys like those of
to-day--and carried by their rivers to the ocean. In all geological
times, as at the present, the work of streams has been to destroy the
lands, and in so doing to furnish to the ocean the materials from
which the lands of future ages were to be made. Before we consider how
the waste of the land brought in by streams is rebuilt upon the ocean
floor, we must proceed to study the work of two agents, glacier ice
and the wind, which coöperate with rivers in the denudation of the
land.
[Illustration: Fig. 87. Section of Undifferentiated Drift near
Chicago]
CHAPTER V
THE WORK OF GLACIERS
=The drift.= The surface of northeastern North America, as far south
as the Ohio and Missouri rivers, is generally covered by the drift,--a
formation which is quite unlike any which we have so far studied. A
section of it, such as that illustrated in Figure 87, shows that for
the most part it is unstratified, consisting of clay, sand, pebbles,
and even large bowlders, all mingled pell-mell together. The agent
which laid the drift is one which can carry a load of material of all
sizes, from the largest bowlder to the finest clay, and deposit it
without sorting.
[Illustration: Fig. 88. Characteristic Pebbles from the Drift
No. 1 has six facets; No. 4, originally a rounded river
pebble, has been nibbled down to one flat face; Nos. 3
and 5 are battered subangular fragments on one side only]
The stones of the drift are of many kinds. The region from which it
was gathered may well have been large in order to supply these many
different varieties of rocks. Pebbles and bowlders have been left far
from their original homes, as may be seen in southern Iowa, where the
drift contains nuggets of copper brought from the region about Lake
Superior. The agent which laid the drift is one able to gather its
load over a large area and carry it a long way.
[Illustration: Fig. 89. Smoothed and Scored Rock Surface exposed
to View by the Removal of Overlying Drift, Iowa]
The pebbles of the drift are unlike those rounded by running water or
by waves. They are marked with scratches. Some are angular, many have
had their edges blunted, while others have been ground flat and smooth
on one or more sides, like gems which have been faceted by being held
firmly against the lapidary's wheel (Fig. 88). In many places the
upper surface of the country rock beneath the drift has been swept
clean of residual clays and other waste. All rock rotten has been
planed away, and the ledges of sound rock to which the surface has
been cut down have been rubbed smooth and scratched with long,
straight, parallel lines (Fig. 89). The agent which laid the drift can
hold sand and pebbles firmly in its grasp and can grind them against
the rock beneath, thus planing it down and scoring it, while faceting
the pebbles also.
Neither water nor wind can do these things. Indeed, nothing like the
drift is being formed by any process now at work anywhere in the
eastern United States. To find the agent which has laid this extensive
formation we must go to a region of different climatic conditions.
[Illustration: Fig. 90. Map of Greenland
Glacier ice covers all but the areas shaded]
=The inland ice of Greenland.= Greenland is about fifteen hundred
miles long and nearly seven hundred miles in greatest width. With the
exception of a narrow fringe of mountainous coast land, it is
completely buried beneath a sheet of ice, in shape like a vast white
shield, whose convex surface rises to a height of nine thousand feet
above the sea. The few explorers who have crossed the ice cap found it
a trackless desert destitute of all life save such lowly forms as the
microscopic plant which produces the so-called "red snow." On the
smooth plain of the interior no rock waste relieves the snow's
dazzling whiteness; no streams of running water are seen; the silence
is broken only by howling storm winds and the rustle of the surface
snow which they drive before them. Sounding with long poles, explorers
find that below the powdery snow of the latest snowfall lie successive
layers of earlier snows, which grow more and more compact downward,
and at last have altered to impenetrable ice. The ice cap formed by
the accumulated snows of uncounted centuries may well be more than a
mile in depth. Ice thus formed by the compacting of snow is
distinguished when in motion as _glacier ice_.
[Illustration: Fig. 91. Hypothetical Cross Section of Greenland]
The inland ice of Greenland moves. It flows with imperceptible
slowness under its own weight, like, a mass of some viscous or plastic
substance, such as pitch or molasses candy, in all directions outward
toward the sea. Near the edge it has so thinned that mountain peaks
are laid bare, these islands in the sea of ice being known as
_nunataks_. Down the valleys of the coastal belt it drains in separate
streams of ice, or _glaciers_. The largest of these reach the sea at
the head of inlets, and are therefore called _tide glaciers_. Their
fronts stand so deep in sea water that there is visible seldom more
than three hundred feet of the wall of ice, which in many glaciers
must be two thousand and more feet high. From the sea walls of tide
glaciers great fragments break off and float away as icebergs. Thus
snows which fell in the interior of this northern land, perhaps many
thousands of years ago, are carried in the form of icebergs to melt at
last in the North Atlantic.
Greenland, then, is being modeled over the vast extent of its interior
not by streams of running water, as are regions in warm and humid
climates, nor by currents of air, as are deserts to a large extent,
but by a sheet of flowing ice. What the ice sheet is doing in the
interior we may infer from a study of the separate glaciers into which
it breaks at its edge.
=The smaller Greenland glaciers.= Many of the smaller glaciers of
Greenland do not reach the sea, but deploy on plains of sand and
gravel. The edges of these ice tongues are often as abrupt as if
sliced away with a knife (Fig. 92), and their structure is thus
readily seen. They are stratified, their layers representing in part
the successive snowfalls of the interior of the country. The upper
layers are commonly white and free from stones; but the lower layers,
to the height of a hundred feet or more, are dark with debris which is
being slowly carried on. So thickly studded with stones is the base of
the ice that it is sometimes difficult to distinguish it from the rock
waste which has been slowly dragged beneath the glacier or left about
its edges. The waste beneath and about the glacier is unsorted. The
stones are of many kinds, and numbers of them have been ground to flat
faces. Where the front of the ice has retreated the rock surface is
seen to be planed and scored in places by the stones frozen fast in
the sole of the glacier.
[Illustration: Fig. 92. A Greenland Glacier]
We have now found in glacier ice an agent able to produce the drift of
North America. The ice sheet of Greenland is now doing what we have
seen was done in the recent past in our own land. It is carrying for
long distances rocks of many kinds gathered, we may infer, over a
large extent of country. It is laying down its load without assortment
in unstratified deposits. It grinds down and scores the rock over
which it moves, and in the process many of the pebbles of its load are
themselves also ground smooth and scratched. Since this work can be
done by no other agent, we must conclude that the northeastern part of
our own continent was covered in the recent past by glacier ice, as
Greenland is to-day.
Valley Glaciers
The work of glacier ice can be most conveniently studied in the
separate ice streams which creep down mountain valleys in many regions
such as Alaska, the western mountains of the United States and Canada,
the Himalayas, and the Alps. As the glaciers of the Alps have been
studied longer and more thoroughly than any others, we shall describe
them in some detail as examples of valley glaciers in all parts of the
world.
=Conditions of glacier formation.= The condition of the great
accumulation of snow to which glaciers are due--that more or less of
each winter's snow should be left over unmelted and unevaporated to
the next--is fully met in the Alps. There is abundant moisture brought
by the winds from neighboring seas. The currents of moist air driven
up the mountain slopes are cooled by their own expansion as they rise,
and the moisture which they contain is condensed at a temperature at
or below 32° F., and therefore is precipitated in the form of snow.
The summers are cool and their heat does not suffice to completely
melt the heavy snow of the preceding winter. On the Alps the _snow
line_--the lower limit of permanent snow--is drawn at about eight
thousand five hundred feet above sea level. Above the snow line on the
slopes and crests, where these are not too steep, the snow lies the
year round and gathers in valley heads to a depth of hundreds of feet.
[Illustration: Fig. 93. Glaciers heading in Snow-filled
Amphitheaters, the Alps]
[Illustration: Fig. 94. Bergschrund of a Glacier in Colorado]
This is but a small fraction of the thickness to which snow would be
piled on the Alps were it not constantly being drained away. Below the
snow fields which mantle the heights the mountain valleys are occupied
by glaciers which extend as much as a vertical mile below the snow
line. The presence in the midst of forests and meadows and cultivated
fields of these tongues of ice, ever melting and yet from year to year
losing none of their bulk, proves that their loss is made good in the
only possible way. They are fed by snow fields above, whose surplus of
snow they drain away in the form of ice. The presence of glaciers
below the snow line is a clear proof that, rigid and motionless as
they appear, glaciers really are in constant motion down valley.
=The névé field.= The head of an Alpine valley occupied by a glacier
is commonly a broad amphitheater deeply filled with snow (Fig. 93).
Great peaks tower above it, and snowy slopes rise on either side on
the flanks of mountain spurs. From these heights fierce winds drift
the snows into the amphitheater, and avalanches pour in their torrents
of snow and waste. The snow of the amphitheater is like that of drifts
in late winter after many successive thaws and freezings. It is made
of hard grains and pellets and is called _névé_. Beneath the surface
of the névé field and at its outlet the granular névé has been
compacted to a mass of porous crystalline ice. Snow has been changed
to névé, and névé to glacial ice, both by pressure, which drives the
air from the interspaces of the snowflakes, and also by successive
meltings and freezings, much as a snowball is packed in the warm hand
and becomes frozen to a ball of ice.
[Illustration: Fig. 95. Sea Wall of the Muir Glacier, Alaska]
=The bergschrund.= The névé is in slow motion. It breaks itself loose
from the thinner snows about it, too shallow to share its motion, and
from the rock rim which surrounds it, forming a deep fissure called
the bergschrund, sometimes a score and more feet wide (Fig. 94).
=Size of glaciers.= The ice streams of the Alps vary in size according
to the amount of precipitation and the area of the névé fields which
they drain. The largest of Alpine glaciers, the Aletsch, is nearly ten
miles long and has an average width of about a mile. The thickness of
some of the glaciers of the Alps is as much as a thousand feet. Giant
glaciers more than twice the length of the longest in the Alps occur
on the south slope of the Himalaya Mountains, which receive frequent
precipitations of snow from moist winds from the Indian Ocean. The
best known of the many immense glaciers of Alaska, the Muir, has an
area of about eight hundred square miles (Fig. 95).
[Illustration: Fig. 96. Diagram showing Movement of Row of
Stakes _a_, set in a direct line across the surface of a glacier;
_b_, _c_, and _d_, successive later positions of the stakes]
[Illustration: Fig. 97. Diagram showing Movement of Vertical
Row of Stakes _a_, set on side of glacier]
=Glacier motion.= The motion of the glaciers of the Alps seldom
exceeds one or two feet a day. Large glaciers, because of the enormous
pressure of their weight and because of less marginal resistance, move
faster than small ones. The Muir advances at the rate of seven feet a
day, and some of the larger tide glaciers of Greenland are reported to
move at the exceptional rate of fifty feet and more in the same time.
Glaciers move faster by day than by night, and in summer than in
winter. Other laws of glacier motion may be discovered by a study of
Figures 96 and 97. It is important to remember that glaciers do not
slide bodily over their beds, but urged by gravity move slowly down
valley in somewhat the same way as would a stream of thick mud.
Although small pieces of ice are brittle, the large mass of granular
ice which composes a glacier acts as a viscous substance.
[Illustration: Fig. 98. Crevasses of a Glacier, Canada]
=Crevasses.= Slight changes of slope in the glacier bed, and the
different rates of motion in different parts, produce tensions under
which the ice cracks and opens in great fissures called crevasses. At
an abrupt descent in the bed the ice is shattered into great
fragments, which unite again below the icefall. Crevasses are opened
on lines at right angles to the direction of the tension. _Transverse
crevasses_ are due to a convexity in the bed which stretches the ice
lengthwise (Fig. 99). _Marginal crevasses_ are directed upstream and
inwards; _radial crevasses_ are found where the ice stream deploys
from some narrow valley and spreads upon some more open space. What is
the direction of the tension which causes each and to what is it due?
(Figs. 100 and 101).
[Illustration: Fig. 99. Longitudinal Section of a Portion of a
Glacier, showing Traverse Crevasses]
[Illustration: Fig. 100. Map view of Marginal Crevasses]
[Illustration: Fig. 101. The Rhone Glacier, showing Radial
Crevasses, the Alps]
[Illustration: Fig. 102. Map View of the Junction of Two
Branches of a Glacier
The moraines are represented by broken lines]
=Lateral and medial moraines.= The surface of a glacier is striped
lengthwise by long dark bands of rock debris. Those in the center are
called the medial moraines. The one on either margin is a lateral
moraine, and is clearly formed of waste which has fallen on the edge
of the ice from the valley slopes. A medial moraine cannot be formed
in this way, since no rock fragments can fall so far out from the
sides. But following it up the glacial stream, one finds that a medial
moraine takes its beginning at the junction of the glacier and some
tributary and is formed by the union of their two adjacent lateral
moraines (Fig. 102). Each branch thus adds a medial moraine, and by
counting the number of medial moraines of a trunk stream one may learn
of how many branches it is composed.
[Illustration: Fig. 103. Cross Section of a Glacier showing
Lateral Moraines
_l_, _l_, and Medial Moraines _m_, _m_]
Surface moraines appear in the lower course of the glacier as ridges,
which may reach the exceptional height of one hundred feet. The bulk
of such a ridge is ice. It has been protected from the sun by the
veneer of moraine stuff; while the glacier surface on either side has
melted down at least the distance of the height of the ridge. In
summer the lowering of the glacial surface by melting goes on rapidly.
In Swiss glaciers it has been estimated that the average lowering of
the surface by melting and evaporation amounts to ten feet a year. As
a moraine ridge grows higher and more steep by the lowering of the
surface of the surrounding ice, the stones of its cover tend to slip
down its sides. Thus moraines broaden, until near the terminus of a
glacier they may coalesce in a wide field of stony waste.
[Illustration: Fig. 104. Glacier with Medial Moraines, the Alps
Is the ice moving from or towards the observer?]
=Englacial drift.= This name is applied to whatever debris is carried
within the glacier. It consists of rock waste fallen on the névé and
there buried by accumulations of snow, and of that engulfed in the
glacier where crevasses have opened beneath a surface moraine. As the
surface of the glacier is lowered by melting, more or less englacial
drift is brought again to open air, and near the terminus it may help
to bury the ice from view beneath a sheet of debris.
=The ground moraine.= The drift dragged along at the glacier's base
and lodged beneath it is known as the ground moraine. Part of the
material of it has fallen down deep crevasses and part has been torn
and worn from the glacier's bed and banks. While the stones of the
surface moraines remain as angular as when they lodged on the ice,
many of those of the ground moraine have been blunted on the edges and
faceted and scratched by being ground against one another and the
rocky bed.
In glaciers such as those of Greenland, whose basal layers are well
loaded with drift and whose surface layers are nearly clean, different
layers have different rates of motion, according to the amount of
drift with which they are clogged. One layer glides over another, and
the stones inset in each are ground and smoothed and scratched.
Usually the sides of glaciated pebbles are more worn than the ends,
and the scratches upon them run with the longer axis of the stone.
Why?
=The terminal moraine.= As a glacier is in constant motion, it brings
to its end all of its load except such parts of the ground moraine as
may find permanent lodgment beneath the ice. Where the glacier front
remains for some time at one place, there is formed an accumulation of
drift known as the terminal moraine. In valley glaciers it is shaped
by the ice front to a crescent whose convex side is downstream. Some
of the pebbles of the terminal moraine are angular, and some are
faceted and scored, the latter having come by the hard road of the
ground moraine. The material of the dump is for the most part
unsorted, though the water of the melting ice may find opportunity to
leave patches of stratified sands and gravels in the midst of the
unstratified mass of drift, and the finer material is in places washed
away.
[Illustration: Fig. 105. Terminal Moraine of a Glacier in Montana
The ice has melted back from the morainic ridge on the left and
is building another on the right. The hollow between the ridges
is occupied by a lakelet.]
=Glacier drainage.= The terminal moraine is commonly breached by a
considerable stream, which issues from beneath the ice by a tunnel
whose portal has been enlarged to a beautiful archway by melting
in the sun and the warm air (Fig. 107). The stream is gray with
silt and loaded with sand and gravel washed from the ground
moraine. "Glacier milk" the Swiss call this muddy water, the gray
color of whose silt proves it rock flour freshly ground by the ice
from the unoxidized sound rock of its bed, the mud of streams
being yellowish when it is washed from the oxidized mantle of
waste. Since glacial streams are well loaded with waste due to
vigorous ice erosion, the valley in front of the glacier is
commonly aggraded to a broad, flat floor. These outwash deposits
are known as _valley drift_.
[Illustration: Fig. 106. Heavy Moraine about the Terminus of a
Glacier in the Rocky Mountains of Canada
Account for the fact that the morainic ridge rises considerably
above the surface of the ice]
The sand brought out by streams from beneath a glacier differs from
river sand in that it consists of freshly broken angular grains. Why?
The stream derives its water chiefly from the surface melting of the
glacier. As the ice is touched by the rays of the morning sun in
summer, water gathers in pools, and rills trickle and unite in
brooklets which melt and cut shallow channels in the blue ice. The
course of these streams is short. Soon they plunge into deep wells cut
by their whirling waters where some crevasse has begun to open across
their path. These wells lead into chambers and tunnels by which sooner
or later their waters find way to the rock floor of the valley and
there unite in a subglacial stream.
[Illustration: Fig. 107. Subglacial Stream Issuing from Tunnel
in the Ice, Norway]
=The lower limit of glaciers.= The glaciers of a region do not by any
means end at a uniform height above sea level. Each terminates where
its supply is balanced by melting. Those therefore which are fed by
the largest and deepest névés and those also which are best protected
from the sun by a northward exposure or by the depth of their
inclosing valleys flow to lower levels than those whose supply is less
and whose exposure to the sun is greater.
A series of cold, moist years, with an abundant snowfall, causes
glaciers to thicken and advance; a series of warm, dry years causes
them to wither and melt back. The variation in glaciers is now
carefully observed in many parts of the world. The Muir glacier has
retreated two miles in twenty years. The glaciers of the Swiss Alps
are now for the most part melting back, although a well-known glacier
of the eastern Alps, the Vernagt, advanced five hundred feet in the
year 1900, and was then plowing up its terminal moraine.
How soon would you expect a glacier to advance after its névé fields
have been swollen with unusually heavy snows, as compared with the
time needed for the flood of a large river to reach its mouth after
heavy rains upon its headwaters?
[Illustration: Fig. 108. A Glacier Table]
On the surface of glaciers in summer time one may often see large
stones supported by pillars of ice several feet in height (Fig. 108).
These "glacier tables" commonly slope more or less strongly to the
south, and thus may be used to indicate roughly the points of the
compass. Can you explain their formation and the direction of their
slope? On the other hand, a small and thin stone, or a patch of dust,
lying on the ice, tends to sink a few inches into it. Why?
In what respects is a valley glacier like a mountain stream which
flows out upon desert plains?
Two confluent glaciers do not mingle their currents as do two
confluent rivers. What characteristics of surface moraines prove this
fact?
What effect would you expect the laws of glacier motion to have on the
slant of the sides of transverse crevasses?
[Illustration: Fig. 109. Map of Malaspina Glacier, Alaska]
A trunk glacier has four medial moraines. Of how many tributaries is
it composed? Illustrate by diagram.
State all the evidences which you have found that glaciers move.
If a glacier melts back with occasional pauses up a valley, what
records are left of its retreat?
[Illustration: Fig. 110. Outwash Plain, the Delta of the Yahtse
River, Alaska]
Piedmont Glaciers
=The Malaspina glacier.= Piedmont (foot of the mountain) glaciers are,
as the name implies, ice fields formed at the foot of mountains by the
confluence of valley glaciers. The Malaspina glacier of Alaska, the
typical glacier of this kind, is seventy miles wide and stretches for
thirty miles from the foot of the Mount Saint Elias range to the shore
of the Pacific Ocean. The valley glaciers which unite and spread to
form this lake of ice lie above the snow line and their moraines are
concealed beneath névé. The central area of the Malaspina is also
free from debris; but on the outer edge large quantities of englacial
drift are exposed by surface melting and form a belt of morainic waste
a few feet thick and several miles wide, covered in part with a
luxuriant forest, beneath which the ice is in places one thousand feet
in depth. The glacier here is practically stagnant, and lakes a few
hundred yards across, which could not exist were the ice in motion and
broken with crevasses, gather on their beds sorted waste from the
moraine. The streams which drain the glacier have cut their courses in
englacial and subglacial tunnels; none flow for any distance on the
surface. The largest, the Yahtse River, issues from a high archway in
the ice,--a muddy torrent one hundred feet wide and twenty feet deep,
loaded with sand and stones which it deposits in a broad outwash plain
(Fig. 110). Where the ice has retreated from the sea there is left a
hummocky drift sheet with hollows filled with lakelets. These deposits
help to explain similar hummocky regions of drift and similar plains
of coarse, water-laid material often found in the drift-covered area
of the northeastern United States.
The Geological Work Of Glacier Ice
The sluggish glacier must do its work in a different way from the
agile river. The mountain stream is swift and small, and its channel
occupies but a small portion of the valley. The glacier is slow and
big; its rate of motion may be less than a millionth of that of
running water over the same declivity, and its bulk is proportionately
large and fills the valley to great depth. Moreover, glacier ice is a
solid body plastic under slowly applied stresses, while the water of
rivers is a nimble fluid.
=Transportation.= Valley glaciers differ from rivers as carriers in
that they float the major part of their load upon their surface,
transporting the heaviest bowlder as easily as a grain of sand; while
streams push and roll much of their load along their beds, and their
power of transporting waste depends solely upon their velocity. The
amount of the surface load of glaciers is limited only by the amount
of waste received from the mountain slopes above them. The moving
floor of ice stretched high across a valley sweeps along as lateral
moraines much of the waste which a mountain stream would let
accumulate in talus and alluvial cones.
While a valley glacier carries much of its load on top, an ice sheet,
such as that of Greenland, is free from surface debris, except where
moraines trail away from some nunatak. If at its edge it breaks into
separate glaciers which drain down mountain valleys, these tongues of
ice will carry the selvages of waste common to valley glaciers. Both
ice sheets and valley glaciers drag on large quantities of rock waste
in their ground moraines.
Stones transported by glaciers are sometimes called erratics. Such are
the bowlders of the drift of our northern states. Erratics may be set
down in an insecure position on the melting of the ice.
=Deposit.= Little need be added here to what has already been said of
ground and terminal moraines. All strictly glacial deposits are
unstratified. The load laid down at the end of a glacier in the
terminal moraine is loose in texture, while the drift lodged beneath
the glacier as ground moraine is often an extremely dense, stony clay,
having been compacted under the pressure of the overriding ice.
=Erosion.= A glacier erodes its bed and banks in two ways,--by
abrasion and by plucking.
The rock bed over which a glacier has moved is seen in places to have
been abraded, or ground away, to smooth surfaces which are marked by
long, straight, parallel scorings aligned with the line of movement of
the ice and varying in size from hair lines and coarse scratches to
exceptional furrows several feet deep. Clearly this work has been
accomplished by means of the sharp sand, the pebbles, and the larger
stones with which the base of the glacier is inset, and which it holds
in a firm grasp as running water cannot. Hard and fine-grained rocks,
such as granite and quartzite, are often not only ground down to a
smooth surface but are also highly polished by means of fine rock
flour worn from the glacier bed.
In other places the bed of the glacier is rough and torn. The rocks
have been disrupted and their fragments have been carried away,--a
process known as _plucking_. Moving under immense pressure the ice
shatters the rock, breaks off projections, presses into crevices and
wedges the rocks apart, dislodges the blocks into which the rock is
divided by joints and bedding planes, and freezing fast to the
fragments drags them on. In this work the freezing and thawing of
subglacial waters in any cracks and crevices of the rock no doubt play
an important part. Plucking occurs especially where the bed rock is
weak because of close jointing. The product of plucking is bowlders,
while the product of abrasion is fine rock flour and sand.
Is the ground moraine of Figure 87 due chiefly to abrasion or to
plucking?
[Illustration: Fig. 111. Roches Moutonnés, Bronx Park, New York]
=Roches moutonnées and rounded hills.= The prominences left between
the hollows due to plucking are commonly ground down and rounded on
the stoss side,--the side from which the ice advances,--and sometimes
on the opposite, the lee side, as well. In this way the bed rock often
comes to have a billowy surface known as roches moutonnées (sheep
rocks). Hills overridden by an ice sheet often have similarly rounded
contours on the stoss side, while on the lee side they may be craggy,
either because of plucking or because here they have been less worn
from their initial profile (Fig. 112).
=The direction of glacier movement.= The direction of the flow of
vanished glaciers and ice sheets is recorded both in the differences
just mentioned in the profiles of overridden hills and also in the
minute details of the glacier trail.
Flint nodules or other small prominences in the bed rock are found
more worn on the stoss than on the lee side, where indeed they may
have a low cone of rock protected by them from abrasion. Cavities, on
the other hand, have their edges worn on the lee side and left sharp
upon the stoss.
Surfaces worn and torn in the ways which we have mentioned are said to
be glaciated. But it must not be supposed that a glacier everywhere
glaciates its bed. Although in places it acts as a rasp or as a pick,
in others, and especially where its pressure is least, as near the
terminus, it moves over its bed in the manner of a sled. Instances are
known where glaciers have advanced over deposits of sand and gravel
without disturbing them to any notable degree. Like a river, a glacier
does not everywhere erode. In places it leaves its bed undisturbed and
in places aggrades it by deposits of the ground moraine.
[Illustration: Fig. 112. A Glaciated Hill, Norway. Sharp
Weathered Mountain Peaks in the Distance]
=Cirques.= Valley glaciers commonly head as we have seen, in broad
amphitheaters deeply filled with snow and ice. On mountains now
destitute of glaciers, but whose glaciation shows that they have
supported glaciers in the past, there are found similar crescentic
hollows with high, precipitous walls and glaciated floors. Their
floors are often basined and hold lakelets whose deep and quiet waters
reflect the sheltering ramparts of rugged rock which tower far above
them. Such mountain hollows are termed _cirques_. As a powerful spring
wears back a recess in the valley side where it discharges, so the
fountain head of a glacier gradually wears back a cirque. In its slow
movement the névé field broadly scours its bed to a flat or basined
floor. Meanwhile the sides of the valley head are steepened and driven
back to precipitous walls. For in winter the crevasse of the
bergschrund which surrounds the névé field is filled with snow and the
névé is frozen fast to the rocky sides of the valley. In early summer
the névé tears itself free, dislodging and removing any loosened
blocks, and the open fissure of the bergschrund allows frost and other
agencies of weathering to attack the unprotected rock. As cirques are
thus formed and enlarged the peaks beneath which they lie are
sharpened, and the mountain crests are scalloped and cut back from
either side to knife-edged ridges (Figs. 113 and 93).
[Illustration: Fig. 113. Cirques, Sierra Nevada Mountains]
In the western mountains of the United States many cirques, now empty
of névé and glacier ice, and known locally as "basins," testify to the
fact that in recent times the snow line stood beneath the levels of
their floors, and thus far below its present altitude.
[Illustration: Fig. 114. A Glacier Trough, Montana]
=Glacier troughs.= The channel worn to accommodate the big and clumsy
glacier differs markedly from the river valley cut as with a saw by
the narrow and flexible stream and widened by the weather and the wash
of rains. The valley glacier may easily be from one thousand to three
thousand feet deep and from one to three miles wide. Such a ponderous
bulk of slowly moving ice does not readily adapt itself to sharp turns
and a narrow bed. By scouring and plucking all resisting edges it
develops a fitting channel with a wide, flat floor, and steep, smooth
sides, above which are seen the weathered slopes of stream-worn
mountain valleys. Since the trunk glacier requires a deeper channel
than do its branches, the bed of a branch glacier enters the main
trough at some distance above the floor of the latter, although the
surface of the two ice streams may be accordant. Glacier troughs can
be studied best where large glaciers have recently melted completely
away, as is the case in many valleys of the mountains of the western
United States and of central and northern Europe (Fig. 114). The
typical glacier trough, as shown in such examples, is U-shaped, with a
broad, flat floor, and high, steep walls. Its walls are little broken
by projecting spurs and lateral ravines. It is as if a V-valley cut by
a river had afterwards been gouged deeper with a gigantic chisel,
widening the floor to the width of the chisel blade, cutting back the
spurs, and smoothing and steepening the sides. A river valley could
only be as wide-floored as this after it had long been worn down to
grade.
[Illustration: Fig. 115 Lynn Canal, Alaska, a Fjord]
But the floor of a glacier trough may not be graded; it is often
interrupted by irregular steps perhaps hundreds and even a thousand
feet in height, over which the stream that now drains the valley
tumbles in waterfalls. Reaches between the steps are often basined.
Lakelets may occupy hollows excavated in solid rock, and other lakes
may be held behind terminal moraines left as dams across the valley at
pauses in the retreat of the glacier.
=Fjords= are glacier troughs now occupied in part or wholly by the
sea, either because they were excavated by a tide glacier to their
present depth below sea level, or because of a submergence of the
land. Their characteristic form is that of a long, deep, narrow bay
with steep rock walls and basined floor (Fig. 115). Fjords are found
only in regions which have suffered glaciation, such as Norway and
Alaska.
[Illustration: Fig. 116. _A_, V-River Valley, with Valley of
Tributary joining it a Accordant Level; _B_, the Same changed
after Long Glaciation to a Trough with Hanging Valley]
=Hanging valleys.= These are lateral valleys which open on their main
valley some distance above its floor. They are conspicuous features of
glacier troughs from which the ice has vanished; for the trunk glacier
in widening and deepening its channel cut its bed below the bottoms of
the lateral valleys (Fig. 116).
Since the mouths of hanging valleys are suspended on the walls of the
glacier trough, their streams are compelled to plunge down its steep,
high sides in waterfalls. Some of the loftiest and most beautiful
waterfalls of the world leap from hanging valleys,--among them the
celebrated Staubbach of the Lauterbrunnen valley of Switzerland, and
those of the fjords of Norway and Alaska (Fig. 117).
[Illustration: Fig. 117. Hanging Valley on the Wall of a Fjord,
Norway]
Hanging valleys are found also in river gorges where the smaller
tributaries have not been able to keep pace with a strong master
stream in cutting down their beds. In this case, however, they are a
mark of extreme youth; for, as the trunk stream approaches grade and
its velocity and power to erode its bed decrease, the side streams
soon cut back their falls and wear their beds at their mouths to a
common level with that of the main river. The Grand Canyon of the
Colorado must be reckoned a young valley. At its base it narrows to
scarcely more than the width of the river, and yet its tributaries,
except the very smallest, enter it at a common level.
Why could not a wide-floored valley, such as a glacier trough, with
hanging valleys opening upon it, be produced in the normal development
of a river valley?
=The troughs of young and of mature glaciers.= The features of a
glacier trough depend much on the length of time the preexisting
valley was occupied with ice. During the infancy of a glacier, we may
believe, the spurs of the valley which it fills are but little blunted
and its bed is but little broken by steps. In youth the glacier
develops icefalls, as a river in youth develops waterfalls, and its
bed becomes terraced with great stairs. The mature glacier, like the
mature river, has effaced its falls and smoothed its bed to grade. It
has also worn back the projecting spurs of its valley, making itself a
wide channel with smooth sides. The bed of a mature glacier may form a
long basin, since it abrades most in its upper and middle course,
where its weight and motion are the greatest. Near the terminus, where
weight and motion are the least, it erodes least, and may instead
deposit a sheet of ground moraine, much as a river builds a flood
plain in the same part of its course as it approaches maturity. The
bed of a mature glacier thus tends to take the form of a long,
relatively narrow basin, across whose lower end may be stretched the
dam of the terminal moraine. On the disappearance of the ice the basin
is rilled with a long, narrow lake, such as Lake Chelan in Washington
and many of the lakes in the Highlands of Scotland.
Piedmont glaciers apparently erode but little. Beneath their lake-like
expanse of sluggish or stagnant ice a broad sheet of ground moraine is
probably being deposited.
Cirques and glaciated valleys rapidly lose their characteristic forms
after the ice has withdrawn. The weather destroys all smoothed,
polished, and scored surfaces which are not protected beneath glacial
deposits. The over-steepened sides of the trough are graded by
landslips, by talus slopes, and by alluvial cones. Morainic heaps of
drift are dissected and carried away. Hanging valleys and the
irregular bed of the trough are both worn down to grade by the streams
which now occupy them. The length of time since the retreat of the ice
from a mountain valley may thus be estimated by the degree to which
the destruction of the characteristic features of the glacier trough
has been carried.
In Figure 104 what characteristics of a glacier trough do you notice?
What inference do you draw as to the former thickness of the glacier?
Name all the evidences you would expect to find to prove the fact that
in the recent geological past the valleys of the Alps contained far
larger glaciers than at present, and that on the north of the Alps the
ice streams united in a piedmont glacier which extended across the
plains of Switzerland to the sides of the Jura Mountains.
=The relative importance of glaciers and of rivers.= Powerful as
glaciers are, and marked as are the land forms which they produce, it
is easy to exaggerate their geological importance as compared with
rivers. Under present climatic conditions they are confined to lofty
mountains or polar lands. Polar ice sheets are permanent only so long
as the lands remain on which they rest. Mountain glaciers can stay
only the brief time during which the ranges continue young and high.
As lofty mountains, such as the Selkirks and the Alps, are lowered by
frost and glacier ice, the snowfall will decrease, the line of
permanent snow will rise, and as the mountain hollows in which snow
may gather are worn beneath the snow line, the glaciers must
disappear. Under present climatic conditions the work of glaciers is
therefore both local and of short duration.
[Illustration: Fig. 118. Longitudinal Section of a Tide Glacier
occupying a Fjord and discharging Icebergs
Dotted Line, sea level]
Even the glacial epoch, during which vast ice sheets deposited drift
over northeastern North America, must have been brief as well as
recent, for many lofty mountains, such as the Rockies and the Alps,
still bear the marks of great glaciers which then filled their
valleys. Had the glacial epoch been long, as the earth counts time,
these mountains would have been worn low by ice; had the epoch been
remote, the marks of glaciation would already have been largely
destroyed by other agencies.
On the other hand, rivers are well-nigh universally at work over the
land surfaces of the globe, and ever since the dry land appeared they
have been constantly engaged in leveling the continents and in
delivering to the seas the waste which there is built into the
stratified rocks.
=Icebergs.= Tide glaciers, such as those of Greenland and Alaska, are
able to excavate their beds to a considerable distance below sea
level. From their fronts the buoyancy of sea water raises and breaks
away great masses of ice which float out to sea as icebergs. Only
about one seventh of a mass of glacier ice floats above the surface,
and a berg three hundred feet high may be estimated to have been
detached from a glacier not less than two thousand feet thick where it
met the sea.
Icebergs transport on their long journeys whatever drift they may have
carried when part of the glacier, and scatter it, as they melt, over
the ocean floor. In this way pebbles torn by the inland ice from the
rocks of the interior of Greenland and glaciated during their carriage
in the ground moraine are dropped at last among the oozes of the
bottom of the North Atlantic.
CHAPTER VI
THE WORK OF THE WIND
[Illustration: Fig. 119. A sandy Region in a Desert, the Sahara]
We are now to study the geological work of the currents of the
atmosphere, and to learn how they erode, and transport and deposit
waste as they sweep over the land. Illustrations of the wind's work
are at hand in dry weather on any windy day.
Clouds of dust are raised from the street and driven along by the
gale. Here the roadway is swept bare; and there, in sheltered places,
the dust settles in little windrows. The erosive power of waste-laden
currents of air is suggested as the sharp grains of flying sand sting
one's face or clatter against the window. In the country one sometimes
sees the dust whirled in clouds from dry, plowed fields in spring and
left in the lee of fences in small drifts resembling in form those of
snow in winter.
=The essential conditions= for the wind's conspicuous work are
illustrated in these simple examples; they are aridity and the absence
of vegetation. In humid climates these conditions are only rarely and
locally met; for the most part a thick growth of vegetation protects
the moist soil from the wind with a cover of leaves and stems and a
mattress of interlacing roots. But in arid regions either vegetation
is wholly lacking, or scant growths are found huddled in detached
clumps, leaving interspaces of unprotected ground (Fig. 119). Here,
too, the mantle of waste, which is formed chiefly under the action of
temperature changes, remains dry and loose for long periods. Little or
no moisture is present to cause its particles to cohere, and they are
therefore readily lifted and drifted by the wind.
Transportation By The Wind
In the desert the finer waste is continually swept to and fro by the
ever-shifting wind. Even in quiet weather the air heated by contact
with the hot sands rises in whirls, and the dust is lifted in stately
columns, sometimes as much as one thousand feet in height, which march
slowly across the plain. In storms the sand is driven along the ground
in a continuous sheet, while the air is tilled with dust. Explorers
tell of sand storms in the deserts of central Asia and Africa, in
which the air grows murky and suffocating. Even at midday it may
become dark as night, and nothing can be heard except the roar of the
blast and the whir of myriads of grains of sand as they fly past the
ear.
Sand storms are by no means uncommon in the arid regions of the
western United States. In a recent year, six were reported from Yuma,
Arizona. Trains on transcontinental railways are occasionally
blockaded by drifting sand, and the dust sifts into closed passenger
coaches, covering the seats and floors. After such a storm thirteen
car loads of sand were removed from the platform of a station on a
western railway.
=Dust falls.= Dust launched by upward-whirling winds on the swift
currents of the upper air is often blown for hundreds of miles beyond
the arid region from which it was taken. Dust falls from western
storms are not unknown even as far east as the Great Lakes. In 1896 a
"black snow" fell in Chicago, and in another dust storm in the same
decade the amount of dust carried in the air over Rock Island, Ill.,
was estimated at more than one thousand tons to the cubic mile.
[Illustration: Fig. 120. A Tract of Rocky Desert, Arabia
By what process have these rocks been broken up?
Why is finer waste here absent?]
In March, 1901, a cyclonic storm carried vast quantities of dust from
the Sahara northward across the Mediterranean to fall over southern
and central Europe. On March 8th dust storms raged in southern
Algeria; two days later the dust fell in Italy; and on the 11th it had
reached central Germany and Denmark. It is estimated that in these few
days one million eight hundred thousand tons of waste were carried
from northern Africa and deposited on European soil.
We may see from these examples the importance of the wind as an agent
of transportation, and how vast in the aggregate are the loads which
it carries. There are striking differences between air and water as
carriers of waste. Rivers flow in fixed and narrow channels to
definite goals. The channelless streams of the air sweep across broad
areas, and, shifting about continually, carry their loads back and
forth, now in one direction and now in another.
Wind Deposits
The mantle of waste of deserts is rapidly sorted by the wind. The
coarser rubbish, too heavy to be lifted into the air, is left to strew
wide tracts with residual gravels (Fig. 120). The sand derived from
the disintegration of desert rocks gathers in vast fields. About one
eighth of the surface of the Sahara is said to be thus covered with
drifting sand. In desert mountains, as those of Sinai, it lies like
fields of snow in the high valleys below the sharp peaks. On more
level tracts it accumulates in seas of sand, sometimes, as in the
deserts of Arabia, two hundred and more feet deep.
[Illustration: Fig. 121. Longitudinal Dunes, Desert of
Northwestern India
Scale, 1 in = 3 miles]
=Dunes.= The sand thus accumulated by the wind is heaped in wavelike
hills called dunes. In the desert of northwestern India, where the
prevalent wind is of great strength, the sand is laid in longitudinal
dunes, i.e. in stripes running parallel with the direction of the
wind; but commonly dunes lie, like ripple marks, transverse to the
wind current. On the windward side they show a long, gentle slope, up
which grains of sand can readily be moved; while to the lee their
slope is frequently as great as the angle of repose (Fig. 122). Dunes
whose sands are not fixed by vegetation travel slowly with the wind;
for their material is ever shifted forward as the grains are driven up
the windward slope and, falling over the crest, are deposited in
slanting layers in the quiet of the lee.
[Illustration: Fig. 122. A Transverse Dune, Seven Mile Beach,
New Jersey
Account for the difference of slope in the two sides of the
dune. Is the dune marching? In what direction? With what
effect? Do the ridges of the ripple marks upon the dune extend
along it or athwart it? Why?]
Like river deposits, wind-blown sands are stratified, since they are
laid by currents of air varying in intensity, and therefore in
transporting power, which carry now finer and now coarser materials
and lay them down where their velocity is checked (Fig. 123). Since
the wind varies in direction, the strata dip in various directions.
They also dip at various angles, according to the inclination of the
surface on which they were laid.
[Illustration: Fig. 123. Stratified Wind-Blown Sands, Bermuda
Islands
These islands are made wholly of limestone, the product of
reef-building corals, and of lime from the sea water. The
limestone sand of the beaches has been blown up into great
dunes, some more than two hundred feet in height. Much of the
loose dune sand has been changed to firm rock by percolating
waters, which have dissolved some of the limestone and
deposited it again as a cement between the grains]
Dunes occur not only in arid regions, but also wherever loose sand
lies unprotected by vegetation from the wind. From the beaches of sea
and lake shores the wind drives inland the surface sand left dry
between tides and after storms, piling it in dunes which may invade
forests and fields and bury villages beneath their slowly advancing
waves. On flood plains during summer droughts river deposits are often
worked over by the wind; the sand is heaped in hummocks and much of
the fine silt is caught and held by the forests and grassy fields of
the bordering hills.
[Illustration: Fig. 124. Cross Section of Transverse Dune after
Reversal of Wind
Redraw diagram, showing by dotted line the original outline of
the dune]
The sand of shore dunes differs little in composition and the shape of
its grains from that of the beach from which it was derived. But in
deserts, by the long wear of grain on grain as they are blown hither
and thither by the wind, all soft minerals are ground to powder and
the sand comes to consist almost wholly of smooth round grams of hard
quartz.
[Illustration: Fig. 125. Dune Sands, Shore of Lake Michigan
Account for the dead forest, for its leaning tree trunks. Is
the lake shore to the right or left? What has been the history
of the landscape?]
Some marine sandstones, such as the St. Peter sandstone of the upper
Mississippi valley, are composed so entirely of polished spherules of
quartz that it has been believed by some that their grains were long
blown about in ancient deserts before they were deposited in the sea.
[Illustration: Fig. 126. Crescentic Sand Dunes, Valley of the
Columbia River
Did the wind which shaped them blow from the left or from the
right?]
=Dust deposits.= As desert sands are composed almost wholly of quartz,
we may ask what has become of the softer minerals of which the rocks
whose disintegration has supplied the sand were in part, and often in
large part, composed. The softer minerals have been ground to powder,
and little by little the quartz sand also is worn by attrition to fine
dust. Yet dust deposits are scant and few in great deserts such as the
Sahara. The finer waste is blown beyond its limits and laid in
adjacent oceans, where it adds to the muds and oozes of their floors,
and on bordering steppes and forest lands, where it is bound fast by
vegetation and slowly accumulates in deposits of unstratified loose
yellow earth. The fine waste of the Sahara has been identified in
dredgings from the bottom of the Atlantic Ocean, taken hundreds of
miles from the coast of Africa.
=Loess.= In northern China an area as large as France is deeply
covered with a yellow pulverulent earth called loess (German, loose),
which many consider a dust deposit blown from the great Mongolian
desert lying to the west. Loess mantles the recently uplifted
mountains to the height of eight thousand feet and descends on the
plains nearly to sea level. Its texture and lack of stratification
give it a vertical cleavage; hence it stands in steep cliffs on the
sides of the deep and narrow trenches which have been cut in it by
streams.
On loess hillsides in China are thousands of villages whose eavelike
dwellings have been excavated in this soft, yet firm, dry loam. While
dust falls are common at the present time in this region, the loess is
now being rapidly denuded by streams, and its yellow silt gives name
to the muddy Hwang-ho (Yellow River), and to the Yellow Sea, whose
waters it discolors for scores of miles from shore.
Wind deposits both of dust and of sand may be expected to contain the
remains of land shells, bits of wood, and bones of land animals,
testifying to the fact that they were accumulated in open air and not
in the sea or in bodies of fresh water.
Wind Erosion
[Illustration: Fig. 127. Wind-Carved Rocks, Arizona]
Sand-laden currents of air abrade and smooth and polish exposed rock
surfaces, acting in much the same way as does the jet of steam fed
with sharp sand, which is used in the manufacture of ground glass.
Indeed, in a single storm at Cape Cod a plate glass of a lighthouse
was so ground by flying sand that its transparency was destroyed and
its removal made necessary.
[Illustration: Fig. 128. A Wind-Carved Pebble, Cape Cod]
Telegraph poles and wires whetted by wind-blown sands are destroyed
within a few years. In rocks of unequal resistance the harder parts
are left in relief, while the softer are etched away. Thus in the pass
of San Bernardino, Cal., through which strong winds stream from the
west, crystals of garnet are left projecting on delicate rock fingers
from the softer rock in which they were imbedded.
Wind-carved pebbles are characteristically planed, the facets meeting
along a summit ridge or at a point like that of a pyramid. We may
suppose that these facets were ground by prevalent winds from certain
directions, or that from time to time the stone was undermined and
rolled over as the sand beneath it was blown away on the windward
side, thus exposing fresh surfaces to the driving sand. Such
wind-carved pebbles are sometimes found in ancient rocks and may be
accepted as evidence that the sands of which the rocks are composed
were blown about by the wind.
=Deflation.= In the denudation of an arid region, wind erosion is
comparatively ineffective as compared with deflation (Latin, _de_,
from; _flare_, to blow),--a term by which is meant the constant
removal of waste by the wind, leaving the rocks bare to the continuous
attack of the weather. In moist climates denudation is continually
impeded by the mantle of waste and its cover of vegetation, and the
land surface can be lowered no faster than the waste is removed by
running water. Deep residual soils come to protect all regions of
moderate slope, concealing from view the rock structure, and the
various forms of the land are due more to the agencies of erosion and
transportation than to differences in the resistance of the underlying
rocks.
[Illustration: Fig. 129. Mesa Verde, Colorado
In the distance on the left are high volcanic mountains. On the
extreme right are seen outliers of strata which once covered
the region of the mesa]
But in arid regions the mantle is rapidly removed, even from well-nigh
level plains and plateaus, by the sweep of the wind and the wash of
occasional rains. The geological structure of these regions of naked
rock can be read as far as the eye can see, and it is to this
structure that the forms of the land are there largely due. In a land
mass of horizontal strata, for example, any softer surface rocks wear
down to some underlying, resistant stratum, and this for a while forms
the surface of a level plateau (Fig. 129). The edges of the capping
layer, together with those of any softer layers beneath it, wear back
in steep cliffs, dissected by the valleys of wet-weather streams and
often swept bare to the base by the wind. As they are little protected
by talus, which commonly is removed about as fast as formed, these
escarpments and the walls of the valleys retreat indefinitely,
exposing some hard stratum beneath which forms the floor of a widening
terrace.
The high plateaus of northern Arizona and southern Utah (Fig. 130),
north of the Grand Canyon of the Colorado River, are composed of
stratified rocks more than ten thousand feet thick and of very gentle
inclination northward. From the broad plat form in which the canyon
has been cut rises a series of gigantic stairs, which are often more
than one thousand feet high and a score or more of miles in breadth.
The retreating escarpments, the cliffs of the mesas and buttes which
they have left behind as outliers, and the walls of the ravines are
carved into noble architectural forms--into cathedrals, pyramids,
amphitheaters, towers, arches, and colonnades--by the processes of
weathering aided by deflation. It is thus by the help of the action of
the wind that great plateaus in arid regions are dissected and at last
are smoothed away to waterless plains, either composed of naked rock,
or strewed with residual gravels, or covered with drifting residual
sand.
[Illustration: Fig. 130. North-South Section, Eighty-Five Miles
Long, across the Plateau North of the Grand Canyon of the
Colorado River, Arizona, showing Retreating Escarpments
_O_, outliers; _V_, canyon of the Colorado; _A-H_, rock systems
from the Archean to the Tertiary; _P_, platform of the plateau
from which the once overlying rocks have been stripped; dotted
lines indicate probable former extension of the strata. How
thick is the mass of strata which has been removed from over
the platform? Has this work been accomplished while the
Colorado River has been cutting its present canyon?]
The specific gravity of air is 1/823 that of water. How does this fact
affect the weight of the material which each can carry at the same
velocity?
If the rainfall should lessen in your own state to from five to ten
inches a year, what changes would take place in the vegetation of the
country? in the soil? in the streams? in the erosion of valleys? in
the agencies chiefly at work in denuding the land?
In what way can a wind-carved pebble be distinguished from a
river-worn pebble? from a glaciated pebble?
CHAPTER VII
THE SEA AND ITS SHORES
[Illustration: Fig. 131. Sea Cliff and Rock Bench Cut in Chalk,
Dover, England]
We have already seen that the ocean is the goal at which the waste of
the land arrives. The mantle of rock waste, creeping down slopes, is
washed to the sea by streams, together with the material which the
streams have worn from their beds and that dissolved by underground
waters. In arid regions the winds sweep waste either into bordering
oceans or into more humid regions where rivers take it up and carry it
on to the sea. Glaciers deliver the load of their moraines either
directly to the sea or leave it for streams to transport to the same
goal. All deposits made on the land, such as the flood plains of
rivers, the silts of lake beds, dune sands, and sheets of glacial
drift, mark but pauses in the process which is to bring all the
materials of the land now above sea level to rest upon the ocean bed.
But the sea is also at work along all its shores as an agent of
destruction, and we must first take up its work in erosion before we
consider how it transports and deposits the waste of the land.
Sea Erosion
=The sea cliff and the rock bench.= On many coasts the land fronts the
ocean in a line of cliffs (Fig. 131). To the edge of the cliffs there
lead down valleys and ridges, carved by running water, which, if
extended, would meet the water surface some way out from shore.
Evidently they are now abruptly cut short at the present shore line
because the land has been cut back.
[Illustration: Fig. 132. Diagram of Sea Cliff _sc_, and Rock Bench _rb_
The broken line indicates the former extent of the land.]
Along the foot of the cliff lies a gently shelving bench of rock, more
or less thickly veneered with sand and shingle. At low tide its inner
margin is laid bare, but at high tide it is covered wholly, and the
sea washes the base of the cliffs. A notch, of which the _sea cliff_
and the _rock bench_ are the two sides, has been cut along the shore
(Fig. 132).
=Waves.= The position of the rock bench, with its inner margin
slightly above low tide, shows that it has been cut by some agent
which acts like a horizontal saw set at about sea level. This agent is
clearly the surface agitation of the water; it is the wind-raised
wave.
As a wave comes up the shelving bench the crest topples forward and
the wave "breaks," striking a blow whose force is measured by the
momentum of all its tons of falling water (Fig. 133). On the coast of
Scotland the force of the blows struck by the waves of the heaviest
storms has sometimes exceeded three tons to the square foot. But even
a calm sea constantly chafes the shore. It heaves in gentle
undulations known as the ground swell, the result of storms perhaps a
thousand miles distant, and breaks on the shore in surf.
[Illustration: Fig. 133. Breaking Wave, Lake Superior]
The blows of the waves are not struck with clear water only, else they
would have little effect on cliffs of solid rock. Storm waves arm
themselves with the sand and gravel, the cobbles, and even the large
bowlders which lie at the base of the cliff, and beat against it with
these hammers of stone.
Where a precipice descends sheer into deep water, waves swash up and
down the face of the rocks but cannot break and strike effective
blows. They therefore erode but little until the talus fallen from the
cliff is gradually built up beneath the sea to the level at which the
waves drag bottom upon it and break.
Compare the ways in which different agents abrade. The wind lightly
brushes sand and dust over exposed surfaces of rock. Running water
sweeps fragments of various sizes along its channels, holding them
with a loose hand. Glacial ice grinds the stones of its ground moraine
against the underlying rock with the pressure of its enormous weight.
The wave hurls fragments of rock against the sea cliff, bruising and
battering it by the blow. It also rasps the bench as it drags sand and
gravel to and fro upon it.
=Weathering of sea cliffs.= The sea cliff furnishes the weapons for
its own destruction. They are broken from it not only by the wave but
also by the weather. Indeed the sea cliff weathers more rapidly, as a
rule, than do rock ledges inland. It is abundantly wet with spray.
Along its base the ground water of the neighboring land finds its
natural outlet in springs which under mine it. Moreover, it is
unprotected by any shield of talus. Fragments of rock as they fall
from its face are battered to pieces by the waves and swept out to
sea. The cliff is thus left exposed to the attack of the weather, and
its retreat would be comparatively rapid for this reason alone.
[Illustration: Fig. 134. Sea Caves, La Jolla, California
Copyright, 1899, by the Detroit Photography Company]
Sea cliffs seldom overhang, but commonly, as in Figure 134, slope
seaward, showing that the upper portion has retreated at a more rapid
rate than has the base. Which do you infer is on the whole the more
destructive agent, weathering or the wave?
Draw a section of a sea cliff cut in well jointed rocks whose joints
dip toward the land. Draw a diagram of a sea cliff where the joints
dip toward the sea.
=Sea caves.= The wave does not merely batter the face of the cliff.
Like a skillful quarryman it inserts wedges in all natural fissures,
such as joints, and uses explosive forces. As a wave flaps against a
crevice it compresses the air within with the sudden stroke; as it
falls back the air as suddenly expands. On lighthouses heavily barred
doors have been burst outward by the explosive force of the air
within, as it was released from pressure when a partial vacuum was
formed by the refluence of the wave. Where a crevice is filled with
water the entire force of the blow of the wave is transmitted by
hydraulic pressure to the sides of the fissure. Thus storm waves
little by little pry and suck the rock loose, and in this way, and by
the blows which they strike with the stones of the beach, they quarry
out about a joint, or wherever the rock may be weak, a recess known as
a _sea cave_, provided that the rock above is coherent enough to form
a roof. Otherwise an open chasm results.
[Illustration: Fig. 135. A Sea Arch, California
Copyright, 1899, by the Detroit Photography Company]
=Blowholes and sea arches.= As a sea cave is drilled back into the
rock, it may encounter a joint or crevice opened to the surface by
percolating water. The shock of the waves soon enlarges this to a
blowhole, which one may find on the breezy upland, perhaps a hundred
yards and more back from the cliff's edge. In quiet weather the
blowhole is a deep well; in storm it plays a fountain as the waves
drive through the long tunnel below and spout their spray high in air
in successive jets. As the roof of the cave thus breaks down in the
rear, there may remain in front for a while a sea arch, similar to the
natural bridges of land caverns (Fig. 135).
[Illustration: Fig. 136. Chasms worn by Waves, Coast of Scotland]
=Stacks and wave-cut islands.= As the sea drives its tunnels and open
drifts into the cliff, it breaks through behind the intervening
portions and leaves them isolated as stacks, much as monuments are
detached from inland escarpments by the weather; and as the sea cliff
retreats, these remnant masses may be left behind as rocky islets.
Thus the rock bench is often set with stacks, islets in all stages of
destruction, and sunken reefs,--all wrecks of the land testifying to
its retreat before the incessant attack of the waves.
[Illustration: Fig. 137. A Stack, Scotland]
[Illustration: Fig. 138. Wave-Cut Islands, Scotland
How far did the land once extend?]
=Coves.= Where zones of soft or closely jointed rock outcrop along a
shore, or where minor water courses conic down to the sea and aid in
erosion, the shore is worn back in curved reëntrants called coves;
while the more resistant rocks on either hand are left projecting as
headlands (Fig. 139). After coves are cut back a short distance by the
waves, the headlands come to protect them, as with breakwaters, and
prevent their indefinite retreat. The shore takes a curve of
equilibrium, along which the hard rock of the exposed headland and the
weak rock of the protected cove wear back at an equal rate.
[Illustration: Fig. 139. Coves formed in Softer Strata _S_, _S_;
while the Harder Strata _H_, _H_, are left as Headlands]
=Rate of recession.= The rate at which a shore recedes depends on
several factors. In soft or incoherent rocks exposed to violent storms
the retreat is so rapid as to be easily measured. The coast of
Yorkshire, England, whose cliffs are cut in glacial drift, loses seven
feet a year on the average, and since the Norman conquest a strip a
mile wide, with farmsteads and villages and historic seaports, has
been devoured by the sea. The sandy south shore of Martha's Vineyard
wears back three feet a year. But hard rocks retreat so slowly that
their recession has seldom been measured by the records of history.
[Illustration: Fig. 140. A Pebble Beach, Cape Ann, Massachusetts]
Shore Drift
=Bowlder and pebble beaches.= About as fast as formed the waste of the
sea cliff is swept both along the shore and out to sea. The road of
waste along shore is the _beach_. We may also define the beach as the
exposed edge of the sheet of sediment formed by the carriage of land
waste out to sea. At the foot of sea cliffs, where the waves are
pounding hardest, one commonly finds the rock bench strewn on its
inner margin with large stones, dislodged by the waves and by the
weather and somewhat worn on their corners and edges. From this
_bowlder beach_ the smaller fragments of waste from the cliff and the
fragments into which the bowlders are at last broken drift on to more
sheltered places and there accumulate in a _pebble beach_, made of
pebbles well rounded by the wear which they have suffered. Such
beaches form a mill whose raw material is constantly supplied by the
cliff. The breakers of storms set it in motion to a depth of several
feet, grinding the pebbles together with a clatter to be heard above
the roar of the surf. In such a rock crusher the life of a pebble is
short. Where ships have stranded on our Atlantic coast with cargoes of
hard-burned brick or of coal, a year of time and a drift of five miles
along the shore have proved enough to wear brick and coal to powder.
At no great distance from their source, therefore, pebble beaches give
place to beaches of sand, which occupy the more sheltered reaches of
the shore.
=Sand beaches.= The angular sand grains of various minerals into which
pebbles are broken by the waves are ground together under the beating
surf and rounded, and those of the softer minerals are crushed to
powder. The process, however, is a slow one, and if we study these
sand grains under a lens we may be surprised to see that, though their
corners and edges have been blunted, they are yet far from the
spherical form of the pebbles from which they were derived. The grains
are small, and in water they have lost about half their weight in
air; the blows which they strike one another are therefore weak.
Besides, each grain of sand of the wet beach is protected by a cushion
of water from the blows of its neighbors.
The shape and size of these grains and the relative proportion of
grains of the softer minerals which still remain give a rough measure
of the distance in space and time which they have traveled from their
source. The sand of many beaches, derived from the rocks of adjacent
cliffs or brought in by torrential streams from neighboring highlands,
is dark with grains of a number of minerals softer than quartz. The
white sand of other beaches, as those of the east coast of Florida, is
almost wholly composed of quartz grains; for in its long travel down
the Atlantic coast the weaker minerals have been worn to powder and
the hardest alone survive.
How does the absence of cleavage in quartz affect the durability of
quartz sand?
=How shore drift migrates.= It is under the action of waves and
currents that shore drift migrates slowly along a coast. Where waves
strike a coast obliquely they drive the waste before them little by
little along the shore. Thus on a north-south coast, where the
predominant storms are from the northeast, there will be a migration
of shore drift southwards.
All shores are swept also by currents produced by winds and tides.
These are usually far too gentle to transport of themselves the coarse
materials of which beaches are made. But while the wave stirs the
grains of sand and gravel, and for a moment lifts them from the
bottom, the current carries them a step forward on their way. The
current cannot lift and the wave cannot carry, but together the two
transport the waste along the shore. The road of shore drift is
therefore the zone of the breaking waves.
[Illustration: Fig. 141. A Bay Bar, Lake Ontario]
=The bay-head beach.= As the waste derived from the wear of waves and
that brought in by streams is trailed along a coast it assumes, under
varying conditions, a number of distinct forms. When swept into the
head of a sheltered bay it constitutes the bay-head beach. By the
highest storm waves the beach is often built higher than the ground
immediately behind it, and forms a dam inclosing a shallow pond or
marsh.
=The bay bar.= As the stream of shore drift reaches the mouth of a bay
of some size it often occurs that, instead of turning in, it sets
directly across toward the opposite headland. The waste is carried out
from shore into the deeper waters of the bay mouth; where it is no
longer supported by the breaking waves, and sinks to the bottom. The
dump is gradually built to the surface as a stubby spur, pointing
across the bay, and as it reaches the zone of wave action current and
wave can now combine to carry shore drift along it, depositing their
load continually at the point of the spur. An embankment is thus
constructed in much the same manner as a railway fill, which, while it
is building, serves as a roadway along which the dirt from an adjacent
cut is carted to be dumped at the end. When the embankment is
completed it bridges the bay with a highway along which shore drift
now moves without interruption, and becomes a bay bar.
[Illustration: Fig. 142. A Hook, Lake Michigan]
=Incomplete bay bars.= Under certain conditions the sea cannot carry
out its intention to bridge a bay. Rivers discharging in bays demand
open way to the ocean. Strong tidal currents also are able to keep
open channels scoured by their ebb and flow. In such cases the most
that land waste can do is to build spits and shoals, narrowing and
shoaling the channel as much as possible. Incomplete bay bars
sometimes have their points recurved by currents setting at right
angles to the stream of shore drift and are then classified as _hooks_
(Fig. 142).
[Illustration: Fig. 143. Cross Section of Sand Reef _sr_, and
Lagoon; _sl_, Sea Level]
=Sand reefs.= On low coasts where shallow water extends some distance
out, the highway of shore drift lies along a low, narrow ridge, termed
the sand reef, separated from the land by a narrow stretch of shallow
water called the _lagoon_ (Fig. 143). At intervals the reef is held
open by _inlets_,--gaps through which the tide flows and ebbs, and by
which the water of streams finds way to the sea.
[Illustration: Fig. 144. Sand Reef and Lagoon, Texas]
No finer example of this kind of shore line is to be found in the
world than the coast of Texas. From near the mouth of the Rio Grande a
continuous sand reef draws its even curve for a hundred miles to
Corpus Christi Pass, and the reefs are but seldom interrupted by
inlets as far north as Galveston Harbor. On this coast the tides are
variable and exceptionally weak, being less than one foot in height,
while the amount of waste swept along the shore is large. The lagoon
is extremely shallow, and much of it is a mud flat too shoal for even
small boats. On the coast of New Jersey strong tides are able to keep
open inlets at intervals of from two to twenty miles in spite of a
heavy alongshore drift.
Sand reefs are formed where the water is so shallow near shore that
storm waves cannot run in it and therefore break some distance out
from land. Where storm waves first drag bottom they erode and deepen
the sea floor, and sweep in sediment as far as the line where they
break. Here, where they lose their force, they drop their load and
beat up the ridge which is known as the sand reef when it reaches the
surface.
Shores of Elevation and Depression
Our studies have already brought to our notice two distinct forms of
strand lines,--one the high, rocky coast cut back to cliffs by the
attack of the waves, and the other the low, sandy coast where the
waves break usually upon the sand reef. To understand the origin of
these two types we must know that the meeting place of sea and land is
determined primarily by movements of the earth's crust. Where a coast
land emerges the--shore line moves seaward; where it is being
submerged the shore line advances on the land.
=Shores of elevation.= The retreat of the sea, either because of a
local uplift of the land or for any other reason, such as the lowering
of any portion of ocean bottom, lays bare the inner margin of the sea
floor. Where the sea floor has long received the waste of the land it
has been built up to a smooth, subaqueous plain, gently shelving from
the land. Since the new shore line is drawn across this even surface
it is simple and regular, and is bordered on the one side by shallow
water gradually deepening seaward, and on the other by low land
composed of material which has not yet thoroughly consolidated to firm
rock. A sand reef is soon beaten up by the waves, and for some time
conditions will favor its growth. The loss of sand driven into the
lagoon beyond, and of that ground to powder by the surf and carried
out to sea, is more than made up by the stream of alongshore drift,
and especially by the drag of sediments to the reef by the waves as
they deepen the sea floor on its seaward side.
Meanwhile the lagoon gradually fills with waste from the reef and from
the land. It is invaded by various grasses and reeds which have
learned to grow in salt and brackish water; the marsh, laid bare only
at low tide, is built above high tide by wind drift and vegetable
deposits, and becomes a meadow, soldering the sand reef to the
mainland.
While the lagoon has been filling, the waves have been so deepening
the sea floor off the sand reef that at last they are able to attack
it vigorously. They now wear it back, and, driving the shore line
across the lagoon or meadow, cut a line of low cliffs on the mainland.
Such a shore is that of Gascony in southwestern France,--a low,
straight, sandy shore, bordered by dunes and unprotected by reefs from
the attack of the waves of the Bay of Biscay.
[Illustration: Fig. 145. Map of New Jersey, with that Portion of
the State one Hundred Feet and more above Sea Level shaded
Describe the coast line which the state would have if depressed
one hundred feet. Compare it with the present coastline]
We may say, then, that on shores of elevation the presence of sand
reefs and lagoons indicates the stage of youth, while the absence of
these features and the vigorous and unimpeded attack by the sea upon
the mainland indicate the stage of maturity. Where much waste is
brought in by rivers the maturity of such a coast may be long delayed.
The waste from the land keeps the sea shallow offshore and constantly
renews the sand reef. The energy of the waves is consumed in handling
shore drift, and no energy is left for an effective attack upon the
land. Indeed, with an excessive amount of waste brought down by
streams the land may be built out and encroach temporarily upon the
sea; and not until long denudation has lowered the land, and thus
decreased the amount of waste from it, may the waves be able to cut
through the sand reef and thus the coast reach maturity.
Shores of Depression
Where a coastal region is undergoing submergence the shore line moves
landward. The horizontal plane of the sea now intersects an old land
surface roughened by subaërial denudation. The shore line is irregular
and indented in proportion to the relief of the land and the amount of
the submergence which the land has suffered. It follows up partially
submerged valleys, forming bays, and bends round the divides, leaving
them to project as promontories and peninsulas. The outlines of shores
of depression are as varied as are the forms of the land partially
submerged. We give a few typical illustrations.
[Illustration: Fig. 146. Chesapeake Bay
Draw a sketch of this area before its depression]
The characteristics of the coast of Maine are due chiefly to the fact
that a mountainous region of hard rocks, once worn to a peneplain, and
after a subsequent elevation deeply dissected by north-south valleys,
has subsided, the depression amounting on its southern margin to as
much as six hundred feet below sea level. Drowned valleys penetrate
the land in long, narrow bays, and rugged divides project in long,
narrow land arms prolonged seaward by islands representing the high
portions of their extremities. Of this exceedingly ragged shore there
are said to be two thousand miles from the New Brunswick boundary as
far west as Portland,--a straight-line distance of but two hundred
miles. Since the time of its greatest depression the land is known to
have risen some three hundred feet; for the bays have been shortened,
and the waste with which their floors were strewn is now in part laid
bare as clay plains about the bay heads and in narrow selvages about
the peninsulas and islands.
The coast of Dalmatia, on the Adriatic Sea, is characterized by long
land arms and chains of long and narrow islands, all parallel to the
trend of the coast. A region of parallel mountain ranges has been
depressed, and the longitudinal valleys which lie between them are
occupied by arms of the sea.
Chesapeake Bay is a branching bay due to the depression of an ancient
coastal plain which, after having emerged from the sea, was channeled
with broad, shallow valleys. The sea has invaded the valley of the
trunk stream and those of its tributaries, forming a shallow bay whose
many branches are all directed toward its axis (Fig. 146).
Hudson Bay, and the North, the Baltic, and the Yellow seas are
examples where the sinking of the land has brought the sea in over low
plains of large extent, thus deeply indenting the continental
outline. The rise of a few hundred feet would restore these submerged
plains to the land.
=The cycle of shores of depression.= In its _infantile stage_ the
outline of a shore of depression depends almost wholly on the previous
relief of the land, and but little on erosion by the sea. Sea cliffs
and narrow benches appear where headlands and outlying islands have
been nipped by the waves. As yet, little shore waste has been formed.
The coast of Maine is an example of this stage.
In _early youth_ all promontories have been strongly cliffed, and under
a vigorous attack of the sea the shore of open bays may be cut back
also. Sea stacks and rocky islets, caves and coves, make the shore
minutely ragged. The irregularity of the coast, due to depression, is
for a while increased by differential wave wear on harder and softer
rocks. The rock bench is still narrow. Shore waste, though being
produced in large amounts, is for the most part swept into deeper
water and buried out of sight. Examples of this stage are the east
coast of Scotland and the California coast near San Francisco.
_Later youth_ is characterized by a large accumulation of shore waste.
The rock bench has been cut back so that it now furnishes a good
roadway for shore drift. The stream of alongshore drift grows larger
and larger, filling the heads of the smaller bays with beaches,
building spits and hooks, and tying islands with sand bars to the
mainland. It bridges the larger bays with bay bars, while their length
is being reduced as their inclosing promontories are cut back by the
waves. Thus there comes to be a straight, continuous, and easy road,
no longer interrupted by headlands and bays, for the transportation of
waste alongshore. The Baltic coast of Germany is in this stage.
[Illustration: Fig. 147. Portion of the Northwest Coast of France]
All this while streams have been busy filling with delta deposits the
bays into which they empty. By these steps a coast gradually advances
to _maturity_, the stage when the irregularities due to depression
have been effaced, when outlying islands formed by subsidence have
been planed away, and when the shore line has been driven back behind
the former bay heads. The sea now attacks the land most effectively
along a continuous and fairly straight line of cliffs. Although the
first effect of wave wear was to increase the irregularities of the
shore, it sooner or later rectifies it, making it simple and smooth.
The northwest coast of France is often cited as an example of a coast
which has reached this stage of development (Fig. 147).
In the _old age_ of coasts the rock bench is cut back so far that the
waves can no longer exert their full effect upon the shore. Their
energy is dissipated in moving shore drift hither and thither and in
abrading the bench when they drag bottom upon it. Little by little the
bench is deepened by tidal currents and the drag of waves; but this
process is so slow that meanwhile the sea cliffs melt down under the
weather, and the bench becomes a broad shoal where waves and tides
gradually work over the waste from the land to greater fineness and
sweep it out to sea.
[Illustration: Fig. 148. The South Shore of Martha's Vineyard
The land is shaded. To what class of coasts does this belong?
What stage has it reached, and by what process? What changes
will take place in the future?]
=Plains of marine abrasion.= While subaërial denudation reduces the
land to baselevel, the sea is sawing its edges to _wave base_, i.e.
the lowest limit of the wave's effective wear. The widened rock bench
forms when uplifted a plain of marine abrasion, which like the
peneplain bevels across strata regardless of their various
inclinations and various degrees of hardness.
How may a plain of marine abrasion be expected to differ from a
peneplain in its mantle of waste?
Compared with subaërial denudation, marine abrasion is a comparatively
feeble agent. At the rate of five feet per century--a higher rate than
obtains on the youthful rocky, coast of Britain--it would require more
than ten million years to pare a strip one hundred miles wide from the
margin of a continent, a time sufficient, at the rate at which the
Mississippi valley is now being worn away, for subaërial denudation to
lower the lands of the globe to the level of the sea.
Slow submergence favors the cutting of a wide rock bench. The water
continually deepens upon the bench; storm waves can therefore always
ride in to the base of the cliffs and attack them with full force;
shore waste cannot impede the onset of the waves, for it is
continually washed out in deeper water below wave base.
=Basal conglomerates.= As the sea marches across the land during a
slow submergence, the platform is covered with sheets of sea-laid
sediments. Lowest of these is a conglomerate,--the bowlder and pebble
beach, widened indefinitely by the retreat of the cliffs at whose base
it was formed, and preserved by the finer deposits laid upon it in
the constantly deepening water as the land subsides. Such basal
conglomerates are not uncommon among the ancient rocks of the land,
and we may know them by their rounded pebbles and larger stones,
composed of the same kind of rock as that of the abraded and evened
surface on which they lie.
CHAPTER VIII
OFFSHORE AND DEEP-SEA DEPOSITS
The alongshore deposits which we have now studied are the exposed edge
of a vast subaqueous sheet of waste which borders the continents and
extends often for as much as two or three hundred miles from land.
Soundings show that offshore deposits are laid in belts parallel to
the coast, the coarsest materials lying nearest to the land and the
finest farthest out. The pebbles and gravel and the clean, coarse sand
of beaches give place to broad stretches of sand, which grows finer
and finer until it is succeeded by sheets of mud. Clearly there is an
offshore movement of waste by which it is sorted, the coarser being
sooner dropped and the finer being carried farther out.
Offshore Deposits
The debris torn by waves from rocky shores is far less in amount than
the waste of the land brought down to the sea by rivers, being only
one thirty-third as great, according to a conservative estimate. Both
mingle alongshore in all the forms of beach and bar that have been
described, and both are together slowly carried out to sea. On the
shelving ocean floor waste is agitated by various movements of the
unquiet water,--by the undertow (an outward-running bottom current
near the shore), by the ebb and flow of tides, by ocean currents where
they approach the land, and by waves and ground swells, whose effects
are sometimes felt to a depth of six hundred feet. By all these means
the waste is slowly washed to and fro, and as it is thus ground finer
and finer and its soluble parts are more and more dissolved, it drifts
farther and farther out from land. It is by no steady and rapid
movement that waste is swept from the shore to its final resting
place. Day after day and century after century the grains of sand and
particles of mud are shifted to and fro, winnowed and spread in
layers, which are destroyed and rebuilt again and again before they
are buried safe from further disturbance.
These processes which are hidden from the eye are among the most
important of those with which our science has to do; for it is they
which have given shape to by far the largest part of the stratified
rocks of which the land is made.
=The continental delta.= This fitting term has been recently suggested
for the sheet of waste slowly accumulating along the borders of the
continents. Within a narrow belt, which rarely exceeds two or three
hundred miles, except near the mouths of muddy rivers such as the
Amazon and Congo, nearly all the waste of the continent, whether worn
from its surface by the weather, by streams, by glaciers, or by the
wind, or from its edge by the chafing of the waves, comes at last to
its final resting place. The agencies which spread the material of the
continental delta grow more and more feeble as they pass into deeper
and more quiet water away from shore. Coarse materials are therefore
soon dropped along narrow belts near land. Gravels and coarse sands
lie in thick, wedge-shaped masses which thin out seaward rapidly and
give place to sheets of finer sand.
=Sea muds.= Outermost of the sediments derived from the waste of the
continents is a wide belt of mud; for fine clays settle so slowly,
even in sea water,--whose saltness causes them to sink much faster
than they would in fresh water,--that they are wafted far before they
reach a bottom where they may remain undisturbed. Muds are also found
near shore, carpeting the floors of estuaries, and among stretches of
sandy deposits in hollows where the more quiet water has permitted the
finer silt to rest.
Sea muds are commonly bluish and consolidate to bluish shales; the red
coloring matter brought from land waste--iron oxide--is altered to
other iron compounds by decomposing organic matter in the presence of
sea water. Yellow and red muds occur where the amount of iron oxide in
the silt brought down to the sea by rivers is too great to be reduced,
or decomposed, by the organic matter present.
Green muds and green sand owe their color to certain chemical changes
which take place where waste from the land accumulates on the sea
floor with extreme slowness. A greenish mineral called _glauconite_--a
silicate of iron and alumina--is then formed. Such deposits, known as
_green sand_, are now in process of making in several patches off the
Atlantic coast, and are found on the coastal plain of New Jersey among
the offshore deposits of earlier geological ages.
=Organic deposits.= Living creatures swarm along the shore and on the
shallows out from land as nowhere else in the ocean. Seaweed often
mantles the rock of the sea cliff between the levels of high and low
tide, protecting it to some degree from the blows of waves. On the
rock bench each little pool left by the ebbing tide is an aquarium
abounding in the lowly forms of marine life. Below low-tide level
occur beds of molluscous shells, such as the oyster, with countless
numbers of other humble organisms. Their harder parts--the shells of
mollusks, the white framework of corals, the carapaces of crabs and
other crustaceans, the shells of sea urchins, the bones and teeth of
fishes--are gradually buried within the accumulating sheets of
sediment, either whole or, far more often, broken into fragments by
the waves.
By means of these organic remains each layer of beach deposits and
those of the continental delta may contain a record of the life of the
time when it was laid. Such a record has been made ever since living
creatures with hard parts appeared upon the globe. We shall find it
sealed away in the stratified rocks of the continents,--parts of
ancient sea deposits now raised to form the dry land. Thus we have in
the traces of living creatures found in the rocks, i.e. in fossils, a
history of the progress of life upon the planet.
[Illustration: Fig. 149. Coquina, Florida]
=Molluscous shell deposits.= The forms of marine life of importance in
rock making thrive best in clear water, where little sediment is being
laid, and where at the same time the depth is not so great as to
deprive them of needed light, heat, and of sufficient oxygen absorbed
by sea water from the air. In such clear and comparatively shallow
water there often grow countless myriads of animals, such as mollusks
and corals, whose shells and skeletons of carbonate of lime gradually
accumulate in beds of limestone.
A shell limestone made of broken fragments cemented together is
sometimes called _coquina_, a local term applied to such beds recently
uplifted from the sea along the coast of Florida (Fig. 149).
_Oölitic_ limestone (_öon_, an egg; _lithos_, a stone) is so named
from the likeness of the tiny spherules which compose it to the roe of
fish. Corals and shells have been pounded by the waves to calcareous
sand, and each grain has been covered with successive concentric
coatings of lime carbonate deposited about it from solution.
The impalpable powder to which calcareous sand is ground by the waves
settles at some distance from shore in deeper and quieter water as a
limy silt, and hardens into a dense, fine-grained limestone in which
perhaps no trace of fossil is found to suggest the fact that it is of
organic origin.
From Florida Keys there extends south to the trough of Florida Straits
a limestone bank covered by from five hundred and forty to eighteen
hundred feet of water. The rocky bottom consists of limestone now
slowly building from the accumulation of the remains of mollusks,
small corals, sea urchins, worms with calcareous tubes, and
lime-secreting seaweed, which live upon its surface.
Where sponges and other silica-secreting organisms abound on limestone
banks, silica forms part of the accumulated deposit, either in its
original condition, as, for example, the spicules of sponges, or
gathered into concretions and layers of flint.
Where considerable mud is being deposited along with carbonate of lime
there is in process of making a clayey limestone or a limy shale;
where considerable sand, a sandy limestone or a limy sandstone.
=Consolidation of offshore deposits.= We cannot doubt that all these
loose sediments of the sea floor are being slowly consolidated to
solid rock. They are soaked with water which carries in solution lime
carbonate and other cementing substances. These cements are deposited
between the fragments of shells and corals, the grains of sand and
the particles of mud, binding them together into firm rock. Where
sediments have accumulated to great thickness the lower portions tend
also to consolidate under the weight of the overlying beds. Except in
the case of limestones, recent sea deposits uplifted to form land are
seldom so well cemented as are the older strata, which have long been
acted upon by underground waters deep below the surface within the
zone of cementation, and have been exposed to view by great erosion.
[Illustration: Fig. 150. Ripple Marks on Layers of Ancient
Sandstone, Wisconsin]
=Ripple marks, sun cracks, etc.= The pulse of waves and tidal currents
agitates the loose material of offshore deposits, throwing it into
fine parallel ridges called ripple marks. One may see this beautiful
ribbing imprinted on beach sands uncovered by the outgoing tide, and
it is also produced where the water is of considerable depth. While
the tide is out the surface of shore deposits may be marked by the
footprints of birds and other animals, or by the raindrops of a
passing shower (Fig. 153). The mud of flats, thus exposed to the sun
and dried, cracks in a characteristic way (Figs. 151 and 152). Such
markings may be covered over with a thin layer of sediment at the next
flood tide and sealed away as a lasting record of the manner and place
in which the strata were laid. In Figure 150 we have an illustration
of a very ancient ripple-marked sand consolidated to hard stone,
uplifted and set on edge by movements of the earth's crust, and
exposed to open air after long erosion.
[Illustration: Fig. 151. Sun Cracks]
=Stratification.= For the most part the sheet of sea-laid waste is
hidden from our sight. Where its edge is exposed along the shore we
may see the surface markings which have just been noticed. Soundings
also, and the observations made in shallow waters by divers, tell
something of its surface; but to learn more of its structures we must
study those ancient sediments which have been lifted from the sea and
dissected by subaërial agencies. From them we ascertain that sea
deposits are stratified. They lie in distinct layers which often
differ from one another in thickness, in size of particles, and
perhaps in color. They are parted by bedding planes, each of which
represents either a change in material or a pause during which
deposition ceased and the material of one layer had time to settle and
become somewhat consolidated before the material of the next was laid
upon it. Stratification is thus due to intermittently acting forces,
such as the agitation of the water during storms, the flow and ebb of
the tide, and the shifting channels of tidal currents. Off the mouths
of rivers, stratification is also caused by the coarser and more
abundant material brought down at time of floods being laid on the
finer silt which is discharged during ordinary stages.
[Illustration: Fig. 152. The Under Side of a Layer deposited
upon a Sun-Cracked Surface, showing Casts of the Cracks]
[Illustration: Fig. 153. Rain Prints]
How stratified deposits are built up is well illustrated in the flats
which border estuaries, such as the Bay of Fundy. Each advance of the
tide spreads a film of mud, which dries and hardens in the air during
low water before another film is laid upon it by the next incoming
tidal flood. In this way the flats have been covered by a clay which
splits into leaves as thin as sheets of paper.
It is in fine material, such as clays and shales and limestones, that
the thinnest and most uniform layers, as well as those of widest
extent, occur. On the other hand, coarse materials are commonly laid
in thick beds, which soon thin out seaward and give place to deposits
of finer stuff. In a general way strata are laid in well-nigh
horizontal sheets, for the surface on which they are laid is generally
of very gentle inclination. Each stratum, however, is lenticular, or
lenslike, in form, having an area where it is thickest, and thinning
out thence to its edges, where it is overlapped by strata similar in
shape.
[Illustration: Fig. 154. Cross Bedding in Sandstone, England]
=Cross bedding.= There is an apparent exception to this rule where
strata whose upper and lower surfaces may be about horizontal are made
up of layers inclined at angles which may be as high as the angle of
repose. In this case each stratum grew by the addition along its edge
of successive layers of sediment, precisely as does a sand bar in a
river, the sand being pushed continuously over the edge and coming to
rest on a sloping surface. Shoals built by strong and shifting tidal
currents often show successive strata in which the cross bedding is
inclined in different directions.
=Thickness of sea deposits.= Remembering the vast amount of material
denuded from the land and deposited offshore, we should expect that
with the lapse of time sea deposits would have grown to an enormous
thickness. It is a suggestive fact that, as a rule, the profile of the
ocean bed is that of a soup plate,--a basin surrounded by a flaring
rim. On the _continental shelf_, as the rim is called, the water is
seldom more than six hundred feet in depth at the outer edge, and
shallows gradually towards shore. Along the eastern coast of the
United States the continental shelf is from fifty to one hundred and
more miles in width; on the Pacific coast it is much narrower. So far
as it is due to upbuilding, a wide continental shelf, such as that of
the Atlantic coast, implies a massive continental delta thousands of
feet in thickness. The coastal plain of the Atlantic states may be
regarded as the emerged inner margin of this shelf, and borings made
along the coast probe it to the depth of as much as three thousand
feet without finding the bottom of ancient offshore deposits.
Continental shelves may also be due in part to a submergence of the
outer margin of a continental plateau and to marine abrasion.
=Deposition of sediments and subsidence.= The stratified rocks of the
land show in many places ancient sediments which reach a thickness
which is measured in miles, and which are yet the product of well-nigh
continuous deposition. Such strata may prove by their fossils and by
their composition and structure that they were all laid offshore in
shallow water. We must infer that, during the vast length of time
recorded by the enormous pile, the floor of the sea along the coast
was slowly sinking, and that the trough was constantly being filled,
foot by foot, as fast as it was depressed. Such gradual, quiet
movements of the earth's crust not only modify the outline of coasts,
as we have seen, but are of far greater geological importance in that
they permit the making of immense deposits of stratified rock.
A slow subsidence continued during long time is recorded also in the
succession of the various kinds of rock that come to be deposited in
the same area. As the sea transgresses the land, i.e. encroaches upon
it, any given part of the sea bottom is brought farther and farther
from the shore. The basal conglomerate formed by bowlder and pebble
beaches comes to be covered with sheets of sand, and these with layers
of mud as the sea becomes deeper and the shore more remote; while
deposits of limestone are made when at last no waste is brought to the
place from the now distant land, and the water is left clear for the
growth of mollusks and other lime-secreting organisms.
[Illustration: Fig. 155. Succession of Deposits recording a
Transgressing Sea
_c_, conglomerate; _ss_, sandstone; _sh_, shale; _lm_, limestone]
=Rate of deposition.= As deposition in the sea corresponds to
denudation on the land, we are able to make a general estimate of the
rate at which the former process is going on. Leaving out of account
the soluble matter removed, the Mississippi is lowering its basin at
the rate of one foot in five thousand years, and we may assume this as
the average rate at which the earth's land surface of fifty-seven
million square miles is now being denuded by the removal of its
mechanical waste. But sediments from the land are spread within a zone
but two or three hundred miles in width along the margin of the
continents, a line one hundred thousand miles long. As the area of
deposition--about twenty-five million square miles--is about one half
the area of denudation, the average rate of deposition must be twice
the average rate of denudation, i.e. about one foot in twenty-five
hundred years. If some deposits are made much more rapidly than this,
others are made much more slowly. If they were laid no faster than the
present average rate, the strata of ancient sea deposits exposed in a
quarry fifty feet deep represent a lapse of at least one hundred and
twenty-five thousand years, and those of a formation five hundred feet
thick required for their accumulation one million two hundred and
fifty thousand years.
[Illustration: Fig. 156. Thick Offshore Deposits of Coarse Waste
recording the Presence of a Young Mountain Range near Shore]
=The sedimentary record and the denudation cycle.= We have seen that
the successive stages in a cycle of denudation, such as that by which
a land mass of lofty mountains is worn to low plains, are marked each
by its own peculiar land forms, and that the forms of the earlier
stages are more or less completely effaced as the cycle draws toward
an end. Far more lasting records of each stage are left in the
sedimentary deposits of the continental delta. Thus, in the youth of
such a land mass as we have mentioned, torrential streams flowing down
the steep mountain sides deliver to the adjacent sea their heavy loads
of coarse waste, and thick offshore deposits of sand and gravel (Fig.
156) record the high elevation of the bordering land. As the land is
worn to lower levels, the amount and coarseness of the waste brought
to the sea diminishes, until the sluggish streams carry only a fine
silt which settles on the ocean floor near to land in wide sheets of
mud which harden into shale. At last, in the old age of the region
(Fig. 157), its low plains contribute little to the sea except the
soluble elements of the rocks, and in the clear waters near the land
lime-secreting organisms flourish and their remains accumulate in beds
of limestone. When long-weathered lands mantled with deep,
well-oxidized waste are uplifted by a gradual movement of the earth's
crust, and the mantle is rapidly stripped off by the revived streams,
the uprise is recorded in wide deposits of red and yellow clays and
sands upon the adjacent ocean floor.
Where the waste brought in is more than the waves can easily
distribute, as off the mouths of turbid rivers which drain highlands
near the sea, deposits are little winnowed, and are laid in rapidly
alternating, shaly sandstones and sandy shales.
[Illustration: Fig. 157. Offshore Deposits recording Old Age of
the Adjacent Land
_ss_, sandstone; _sh_, shale; _lm_, limestone]
Where the highlands are of igneous rock, such as granite, and
mechanical disintegration is going on more rapidly than chemical
decay, these conditions are recorded in the nature of the deposits
laid offshore. The waste swept in by streams contains much feldspar
and other minerals softer and more soluble than quartz, and where the
waves have little opportunity to wear and winnow it, it comes to rest
in beds of sandstone in which grains of feldspar and other soft
minerals are abundant. Such feldspathic sandstones are known as
_arkose_.
On the other hand, where the waste supplied to the sea comes chiefly
from wide, sandy, coastal plains, there are deposited offshore clean
sandstones of well-worn grains of quartz alone. In such coastal plains
the waste of the land is stored for ages. Again and again they are
abandoned and invaded by the sea as from time to time the land slowly
emerges and is again submerged. Their deposits are long exposed to the
weather, and sorted over by the streams, and winnowed and worked over
again and again by the waves. In the course of long ages such deposits
thus become thoroughly sorted, and the grains of all minerals softer
than quartz are ground to mud.
[Illustration: Fig. 158. Globigerina Ooze under the Microscope]
Deep-Sea Oozes and Clays
=Globigerina ooze.= Beyond the reach of waste from the land the bottom
of the deep sea is carpeted for the most part with either chalky ooze
or a fine red clay. The surface waters of the warm seas swarm with
minute and lowly animals belonging to the order of the _Foraminifera_,
which secrete shells of carbonate of lime. At death these tiny white
shells fall through the sea water like snowflakes in the air, and,
slowly dissolving, seem to melt quite away before they can reach
depths greater than about three miles. Near shore they reach bottom,
but are masked by the rapid deposit of waste derived from the land. At
intermediate depths they mantle the ocean floor with a white, soft
lime deposit known as _Globigerina ooze_, from a genus of the
Foraminifera which contributes largely to its formation.
=Red clay.= Below depths of from fifteen to eighteen thousand feet the
ocean bottom is sheeted with red or chocolate colored clay. It is the
insoluble residue of seashells, of the debris of submarine volcanic
eruptions, of volcanic dust wafted by the winds, and of pieces of
pumice drifted by ocean currents far from the volcanoes from which
they were hurled. The red clay builds up with such inconceivable
slowness that the teeth of sharks and the hard ear bones of whales may
be dredged in large numbers from the deep ocean bed, where they have
lain unburied for thousands of years; and an appreciable part of the
clay is also formed by the dust of meteorites consumed in the
atmosphere,--a dust which falls everywhere on sea and land, but which
elsewhere is wholly masked by other deposits.
The dark, cold abysses of the ocean are far less affected by change
than any other portion of the surface of the lithosphere. These vast,
silent plains of ooze lie far below the reach of storms. They know no
succession of summer and winter, or of night and day. A mantle of deep
and quiet water protects them from the agents of erosion which
continually attack, furrow, and destroy the surface of the land. While
the land is the area of erosion, the sea is the area of deposition.
The sheets of sediment which are slowly spread there tend to efface
any inequalities, and to form a smooth and featureless subaqueous
plain.
With few exceptions, the stratified rocks of the land are proved by
their fossils and composition to have been laid in the sea; but in the
same way they are proved to be offshore, shallow-water deposits, akin
to those now making on continental shelves. Deep-sea deposits are
absent from the rocks of the land, and we may therefore infer that the
deep sea has never held sway where the continents now are,--that the
continents have ever been, as now, the elevated portions of the
lithosphere, and that the deep seas of the present have ever been its
most depressed portions.
The Reef-Building Corals
In warm seas the most conspicuous of rock-making organisms are the
corals known as the reef builders. Floating in a boat over a coral
reef, as, for example, off the south coast of Florida or among the
Bahamas, one looks down through clear water on thickets of branching
coral shrubs perhaps as much as eight feet high, and hemispherical
masses three or four feet thick, all abloom with countless minute
flowerlike coral polyps, gorgeous in their colors of yellow, orange,
green, and red. In structure each tiny polyp is little more than a
fleshy sac whose mouth is surrounded with petal-like tentacles, or
feelers. From the sea water the polyps secrete calcium carbonate and
build it up into the stony framework which supports their colonies.
Boring mollusks, worms, and sponges perforate and honeycomb this
framework even while its surface is covered with myriads of living
polyps. It is thus easily broken by the waves, and white fragments of
coral trees strew the ground beneath. Brilliantly colored fishes live
in these coral groves, and countless mollusks, sea urchins, and other
forms of marine life make here their home. With the debris from all
these sources the reef is constantly built up until it rises to
low-tide level. Higher than this the corals cannot grow, since they
are killed by a few hours' exposure to the air.
[Illustration: Fig. 159. Patch of Growing Corals exposed at an
Exceptionally Low Tide, Great Barrier Reef, Australia]
When the reef has risen to wave base, the waves abrade it on the
windward side and pile to leeward coral blocks torn from their
foundation, filling the interstices with finer fragments. Thus they
heap up along the reef low, narrow islands (Fig. 160).
Reef building is a comparatively rapid progress. It has been estimated
that off Florida a reef could be built up to the surface from a depth
of fifty feet in about fifteen hundred years.
[Illustration: Fig. 160. Wave-Built Island on Coral Reef
_r_, reef; _sl_, sea level]
=Coral limestones.= Limestones of various kinds are due to the reef
builders. The reef rock is made of corals in place and broken
fragments of all sizes, cemented together with calcium carbonate from
solution by infiltrating waters. On the island beaches coral sand is
forming oolitic limestone, and the white coral mud with which the sea
is milky for miles about the reef in times of storm settles and
concretes into a compact limestone of finest grain. Corals have been
among the most important limestone builders of the sea ever since they
made their appearance in the early geological ages.
The areas on which coral limestone is now forming are large. The Great
Barrier Reef of Australia, which lies off the northeastern coast, is
twelve hundred and fifty miles long, and has a width of from ten to
ninety miles. Most of the islands of the tropics are either skirted
with coral reefs or are themselves of coral formation.
=Conditions of coral growth.= Reef-building corals cannot live except
in clear salt water less, as a rule, than one hundred and fifty feet
in depth, with a winter temperature not lower than 68° F. An important
condition also is an abundant food supply, and this is best secured in
the path of the warm oceanic currents.
Coral reefs may be grouped in three classes,--fringing reefs, barrier
reefs, and atolls.
=Fringing reefs.= These take their name from the fact that they are
attached as narrow fringes to the shore. An example is the reef which
forms a selvage about a mile wide along the northeastern coast of
Cuba. The outer margin, indicated by the line of white surf, where the
corals are in vigorous growth, rises from about forty feet of water.
Between this and the shore lies a stretch of shoal across which one
can wade at low water, composed of coral sand with here and there a
clump of growing coral.
=Barrier reefs.= Reefs separated from the shore by a ship channel of
quiet water, often several miles in width and sometimes as much as
three hundred feet in depth, are known as barrier reefs. The seaward
face rises abruptly from water too deep for coral growth. Low islands
are cast up by the waves upon the reef, and inlets give place for the
ebb and flow of the tides. Along the west coast of the island of New
Caledonia a barrier reef extends for four hundred miles, and for a
length of many leagues seldom approaches within eight miles of the
shore.
=Atolls.= These are ring-shaped or irregular coral islands, or
island-studded reefs, inclosing a central lagoon. The narrow zone of
land, like the rim of a great bowl sunken to the water's edge, rises
hardly more than twenty feet at most above the sea, and is covered
with a forest of trees such as the cocoanut, whose seeds can be
drifted to it uninjured from long distances. The white beach of coral
sand leads down to the growing reef, on whose outer margin the surf is
constantly breaking. The sea face of the reef falls off abruptly,
often to depths of thousands of feet, while the lagoon varies in depth
from a few feet to one hundred and fifty or two hundred, and
exceptionally measures as much as three hundred and fifty feet.
=Theories of coral reefs.= Fringing reefs require no explanation,
since the depth of water about them is not greater than that at which
coral can grow; but barrier reefs and atolls, which may rise from
depths too great for coral growth demand a theory of their origin.
[Illustration: Fig. 161. Diagram illustrating the Subsidence
Theory of Coral Reefs]
Darwin's theory holds that barrier reefs and atolls are formed from
fringing reefs by _subsidence_. The rate of sinking cannot be greater
than that of the upbuilding of the reef, since otherwise the corals
would be carried below their depth and drowned. The process is
illustrated in Figure 161, where v represents a volcanic island in mid
ocean undergoing slow depression, and _ss_ the sea level before the
sinking began, when the island was surrounded by a fringing reef. As
the island slowly sinks, the reef builds up with equal pace. It rears
its seaward face more steep than the island slope, and thus the
intervening space between the sinking, narrowing land and the outer
margin of the reef constantly widens. In this intervening space the
corals are more or less smothered with silt from the outer reef and
from the land, and are also deprived in large measure of the needful
supply of food and oxygen by the vigorous growth of the corals on the
outer rim. The outer rim thus becomes a barrier reef and the inner
belt of retarded growth is deepened by subsidence to a ship channel,
_s´s´_ representing sea level at this time. The final stage, where the
island has been carried completely beneath the sea and overgrown by
the contracting reef, whose outer ring now forms an atoll, is
represented by _s´´s´´_.
[Illustration: Fig. 162. Barrier Reef formed without Subsidence
_a_, zone of coral growth; _f_, former fringing reef; _t_,
talus; _b_, barrier reef]
In very many instances, however, atolls and barrier reefs may be
explained without subsidence. Thus a barrier reef may be formed by the
seaward growth of a fringing reef upon the talus of its sea face. In
Figure 162, _f_ is a fringing reef whose outer wall rises from about
one hundred and fifty feet, the lower limit of the reef-building
species. At the foot of this submarine cliff a talus of fallen blocks
t accumulates, and as it reaches the zone of coral growth becomes the
foundation on which the reef is steadily extended seaward. As the reef
widens, the polyps of the circumference flourish, while those of the
inner belt are retarded in their growth and at last perish. The coral
rock of the inner belt is now dissolved by sea water and scoured out
by tidal currents until it gives place to a gradually deepening ship
channel, while the outer margin is left as a barrier reef.
[Illustration: Fig. 163. Section of Atoll on a Shoal which has
been built up to near the Surface by Organic Deposits upon a
Submarine Volcanic Peak
_v_, volcano; _f_, foraminiferal deposits; _m_, molluscous shell
deposits; _c_, coral reef; _sl_, sea level]
In much the same way atolls may be built on any shoal which lies
within the zone of coral growth. Such shoals may be produced when
volcanic islands are leveled by waves and ocean currents, and when
submarine plateaus, ridges, and peaks are built up by various organic
agencies, such as molluscous and foraminiferal shell deposits (Fig.
163). The reef-building corals, whose eggs are drifted widely over the
tropic seas by ocean currents, colonize such submarine foundations
wherever the conditions are favorable for their growth. As the reef
approaches the surface the corals of the inner area are smothered by
silt and starved, and their Submarine Volcanic Peak hard parts are
dissolved and scoured away; while those of the circumference, with
abundant food supply, nourish and build the ring of the atoll. Atolls
may be produced also by the backward drift of sand from either end of
a crescentic coral reef or island, the spits uniting in the quiet
water of the lee to inclose a lagoon. In the Maldive Archipelago all
gradations between crescent-shaped islets and complete atoll rings
have been observed.
In a number of instances where coral reefs have been raised by
movements of the earth's crust, the reef formation is found to be a
thin veneer built upon a foundation of other deposits. Thus Christmas
Island, in the Indian Ocean, is a volcanic pile rising eleven hundred
feet above sea level and fifteen thousand five hundred feet above the
bottom of the sea. The summit is a plateau surrounded by a rim of
hills of reef formation, which represent the ring of islets of an
ancient atoll. Beneath the reef are thick beds of limestone, composed
largely of the remains of foraminifers, which cover the lavas and
fragmental materials of the old submarine volcano.
Among the ancient sediments which now form the stratified rocks of the
land there occur many thin reef deposits, but none are known of the
immense thickness which modern reefs are supposed to reach according
to the theory of subsidence.
Barrier and fringing reefs are commonly interrupted off the mouths of
rivers. Why?
=Summary.= We have seen that the ocean bed is the goal to which the
waste of the rocks of the land at last arrives. Their soluble parts,
dissolved by underground waters and carried to the sea by rivers, are
largely built up by living creatures into vast sheets of limestone.
The less soluble portions--the waste brought in by streams and the
waste of the shore--form the muds and sands of continental deltas. All
of these sea deposits consolidate and harden, and the coherent rocks
of the land are thus reconstructed on the ocean floor. But the
destination is not a final one. The stratified rocks of the land are
for the most part ancient deposits of the sea, which have been lifted
above sea level; and we may believe that the sediments now being laid
offshore are the "dust of continents to be," and will some time emerge
to form additions to the land. We are now to study the movements of
the earth's crust which restore the sediments of the sea to the light
of day, and to whose beneficence we owe the habitable lands of the
present.
PART II
INTERNAL GEOLOGICAL AGENCIES
CHAPTER IX
MOVEMENTS OF THE EARTH'S CRUST
The geological agencies which we have so far studied--weathering,
streams, underground waters, glaciers, winds, and the ocean--all work
upon the earth from without, and all are set in motion by an energy
external to the earth, namely, the radiant energy of the sun. All,
too, have a common tendency to reduce the inequalities of the earth's
surface by leveling the lands and strewing their waste beneath the
sea.
But despite the unceasing efforts of these external agencies, they
have not destroyed the continents, which still rear their broad plains
and great plateaus and mountain ranges above the sea. Either, then,
the earth is very young and the agents of denudation have not yet had
time to do their work, or they have been opposed successfully by other
forces.
We enter now upon a department of our science which treats of forces
which work upon the earth from within, and increase the inequalities
of its surface. It is they which uplift and recreate the lands which
the agents of denudation are continually destroying; it is they which
deepen the ocean bed and thus withdraw its waters from the shores. At
times also these forces have aided in the destruction of the lands by
gradually lowering them and bringing in the sea. Under the action of
forces resident within the earth the crust slowly rises or sinks; from
time to time it has been folded and broken; while vast quantities of
molten rock have been pressed up into it from beneath and outpoured
upon its surface. We shall take up these phenomena in the following
chapters, which treat of upheavals and depressions of the crust,
foldings and fractures of the crust, earthquakes, volcanoes, the
interior conditions of the earth, mineral veins, and metamorphism.
Oscillations of the Crust
Of the various movements of the crust due to internal agencies we will
consider first those called oscillations, which lift or depress large
areas so slowly that a long time is needed to produce perceptible
changes of level, and which leave the strata in nearly their original
horizontal attitude. These movements are most conspicuous along
coasts, where they can be referred to the datum plane of sea level; we
will therefore take our first illustrations from rising and sinking
shores.
=New Jersey.= Along the coasts of New Jersey one may find awash at
high tide ancient shell heaps, the remains of tribal feasts of
aborigines. Meadows and old forest grounds, with the stumps still
standing, are now overflowed by the sea, and fragments of their turf
and wood are brought to shore by waves. Assuming that the sea level
remains constant, it is clear that the New Jersey coast is now
gradually sinking. The rate of submergence has been estimated at about
two feet per century.
On the other hand, the wide coastal plain of New Jersey is made of
stratified sands and clays, which, as their marine fossils show, were
outspread beneath the sea. Their present position above sea level
proves that the land now subsiding emerged in the recent past.
The coast of New Jersey is an example of the slow and tranquil
oscillations of the earth's unstable crust now in progress along many
shores. Some are emerging from the sea, some are sinking beneath it;
and no part of the land seems to have been exempt from these changes
in the past.
=Evidences of changes of level.= Taking the surface of the sea as a
level of reference, we may accept as proofs of relative upheaval
whatever is now found in place above sea level and could have been
formed only at or beneath it, and as proofs of relative subsidence
whatever is now found beneath the sea and could only have been formed
above it.
Thus old strand lines with sea cliffs, wave-cut rock benches, and
beaches of wave-worn pebbles or sand, are striking proofs of recent
emergence to the amount of their present height above tide. No less
conclusive is the presence of sea-laid rocks which we may find in the
neighboring quarry or outcrop, although it may have been long ages
since they were lifted from the sea to form part of the dry land.
Among common proofs of subsidence are roads and buildings and other
works of man, and vegetal growths and deposits, such as forest grounds
and peat beds, now submerged beneath the sea. In the deltas of many
large rivers, such as the Po, the Nile, the Ganges, and the
Mississippi, buried soils prove subsidences of hundreds of feet; and
in several cases, as in the Mississippi delta, the depression seems to
be now in progress.
Other proofs of the same movement are drowned land forms which are
modeled only in open air. Since rivers cannot cut their valleys
farther below the baselevel of the sea than the depths of their
channels, _drowned valleys_ are among the plainest proofs of
depression. To this class belong Narragansett, Delaware, Chesapeake,
Mobile, and San Francisco bays, and many other similar drowned valleys
along the coasts of the United States. Less conspicuous are the
_submarine channels_ which, as soundings show, extend from the mouths
of a number of rivers some distance out to sea. Such is the submerged
channel which reaches from New York Bay southeast to the edge of the
continental shelf, and which is supposed to have been cut by the
Hudson River when this part of the shelf was a coastal plain.
=Warping.= In a region undergoing changes of level the rate of
movement commonly varies in different parts. Portions of an area may
be rising or sinking, while adjacent portions are stationary or moving
in the opposite direction. In this way a land surface becomes
_warped_. Thus, while Nova Scotia and New Brunswick are now rising
from the level of the sea, Prince Edward Island and Cape Breton Island
are sinking, and the sea now flows over the site of the famous old
town of Louisburg destroyed in 1758.
Since the close of the glacial epoch the coasts of Newfoundland and
Labrador have risen hundreds of feet, but the rate of emergence has
not been uniform. The old strand line, which stands at five hundred
and seventy-five feet above tide at St. John's, Newfoundland, declines
to two hundred and fifty feet near the northern point of Labrador
(Fig. 164).
[Illustration: Fig. 164. Warped Strand Line from St. John's,
Newfoundland, to Nachvak, Labrador]
=The Great Lakes= is now undergoing perceptible warping. Rivers enter
the lakes from the south and west with sluggish currents and deep
channels resembling the estuaries of drowned rivers; while those that
enter from opposite directions are swift and shallow. At the western
end of Lake Erie are found submerged caves containing stalactites, and
old meadows and forest grounds are now under water. It is thus seen
that the water of the lakes is rising along their southwestern shores,
while from their northeastern shores it is being withdrawn. The
region of the Great Lakes is therefore warping; it is rising in the
northeast as compared with the southwest.
From old bench marks and records of lake levels it has been estimated
that _the rate of warping_ amounts to five inches a century for every
one hundred miles. It is calculated that the water of Lake Michigan is
rising at Chicago at the rate of nine or ten inches per century. The
divide at this point between the tributaries of the Mississippi and
Lake Michigan is but eight feet above the mean stage of the lake. If
the canting of the region continues at its present rate, in a thousand
years the waters of the lake will here overflow the divide. In three
thousand five hundred years all the lakes except Ontario will
discharge by this outlet, via the Illinois and Mississippi rivers,
into the Gulf of Mexico. The present outlet by the Niagara River will
be left dry, and the divide between the St. Lawrence and the
Mississippi systems will have shifted from Chicago to the vicinity of
Buffalo.
=Physiographic effects of oscillations.= We have already mentioned
several of the most important effects of movements of elevation and
depression, such as their effects on rivers, the mantle of waste (pp.
85, 86), and the forms of coasts (p. 166). Movements of
elevation--including uplifts by folding and fracture of the crust to
be noticed later--are the necessary conditions for erosion by whatever
agent. They determine the various agencies which are to be chiefly
concerned m the wear of any land,--whether streams or glaciers,
weathering or the wind,--and the degree of their efficiency. The lands
must be uplifted before they can be eroded, and since they must be
eroded before their waste can be deposited, movements of elevation are
a prerequisite condition for sedimentation also. Subsidence is a
necessary condition for deposits of great thickness, such as those of
the Great Valley of California and the Indo-Gangetic plain (p. 101),
the Mississippi delta (p. 109), and the still more important
formations of the continental delta in gradually sinking troughs (p.
183). It is not too much to say that the character and thickness of
each formation of the stratified rocks depend primarily on these
crustal movements.
Along the Baltic coast of Sweden, bench marks show that the sea is
withdrawing from the land at a rate which at the north amounts to
between three and four feet per century; Towards the south the rate
decreases. South of Stockholm, until recent years, the sea has gained
upon the land, and here in several seaboard towns streets by the shore
are still submerged. The rate of oscillation increases also from the
coast inland. On the other hand, along the German coast of the Baltic
the only historic fluctuations of sea level are those which may be
accounted for by variations due to changes in rainfall. In 1730
Celsius explained the changes of level of the Swedish coast as due to
a lowering of the Baltic instead of to an elevation of the land. Are
the facts just stated consistent with his theory?
[Illustration: Fig. 165. Old Strand Lines, Tadousac, Quebec]
At the little town of Tadousac--where the Saguenay River empties into
the St. Lawrence--there are terraces of old sea beaches, some almost
as fresh as recent railway fills, the highest standing two hundred and
thirty feet above the river (Fig. 165). Here the Saguenay is eight
hundred and forty feet in depth, and the tide ebbs and flows far up
its stream. Was its channel cut to this depth by the river when the
land was at its present height? What oscillations are here recorded,
and to what amount?
[Illustration: Fig. 166. Diagram showing Ruins of Temple, North
of Naples
_C_, ancient sea cliff; _m_, marble pillars, dotted where bored
by mollusks; _sl_, sea level]
A few miles north of Naples, Italy, the ruins of an ancient Roman
temple lie by the edge of the sea, on a narrow plain which is
overlooked in the rear by an old sea cliff (Fig. 166). Three marble
pillars are still standing. For eleven feet above their bases these
columns are uninjured, for to this height they were protected by an
accumulation of volcanic ashes; but from eleven to nineteen feet they
are closely pitted with the holes of boring marine mollusks. From
these facts trace the history of the oscillations of the region.
[Illustration: Fig. 167. Section in a Region of Folded Rocks]
Foldings of the Crust
The oscillations which we have just described leave the strata not far
from their original horizontal attitude. Figure 167 represents a
region in which movements of a very different nature have taken place.
Here, on either side of the valley _v_, we find outcrops of layers
tilted at high angles. Sections along the ridge _r_ show that it is
composed of layers which slant inward from either side. In places the
outcropping strata stand nearly on edge, and on the right of the
valley they are quite overturned; a shale _sh_ has come to overlie a
limestone _lm_ although the shale is the older rock, whose original
position was beneath the limestone.
[Illustration: Fig. 168. Dip and Strike]
It is not reasonable to suppose that these rocks were deposited in the
attitude in which we find them now; we must believe that, like other
stratified rocks, they were outspread in nearly level sheets upon the
ocean floor. Since that time they must have been deformed. Layers of
solid rock several miles in thickness have been crumpled and folded
like soft wax in the hand, and a vast denudation has worn away the
upper portions of the folds, in part represented in our section by
dotted lines.
=Dip and strike.= In districts where the strata have been disturbed it
is desirable to record their attitude. This is most easily done by
taking the angle at which the strata are inclined and the compass
direction in which they slant. It is also convenient to record the
direction in which the outcrop of the strata trends across the
country.
[Illustration: Fig. 169. An Anticline, Maryland]
The inclination of a bed of rocks to the horizon is its _dip_ (Fig.
168). The amount of the dip is the angle made with a horizontal plane.
The dip of a horizontal layer is zero, and that of a vertical layer
is 90°. The direction of the dip is taken with the compass. Thus a
geologist's notebook in describing the attitude of outcropping strata
contains many such entries as these: dip 32° north, or dip 8° south 20°
west,--meaning in the latter case that the amount of the dip is 8° and
the direction of the dip bears 20° west of south.
The line of intersection of a layer with the horizontal plane is the
_strike_. The strike always runs at right angles to the dip.
Dip and strike may be illustrated by a book set aslant on a shelf. The
dip is the acute angle made with the shelf by the side of the book,
while the strike is represented by a line running along the book's
upper edge. If the dip is north or south, the strike runs east and
west.
[Illustration: Fig. 170. Folded Strata, Coast of England
A syncline in the center, with an anticline on either side]
=Folded structures.= An upfold, in which the strata dip away from a
line drawn along the crest and called the axis of the fold, is known
as an _anticline_ (Fig. 169). A downfold, where the strata dip from
either side toward the axis of the trough, is called a _syncline_
(Fig. 170). There is sometimes seen a downward bend in horizontal or
gently inclined strata, by which they descend to a lower level. Such a
single flexure is a _monocline_ (Fig. 171).
[Illustration: Fig. 171. A Monocline]
=Degrees of folding.= Folds vary in degree from broad, low swells,
which can hardly be detected, to the most highly contorted and
complicated structures. In _symmetric_ folds (Figs. 169 and 180) the
dips of the rocks on each side the axis of the fold are equal. In
_unsymmetrical_ folds one limb is steeper than the other, as in the
anticline in Figure 167. In _overturned_ folds (Figs. 167 and 172) one
limb is inclined beyond the perpendicular. _Fan folds_ have been so
pinched that the original anticlines are left broader at the top than
at the bottom (Fig. 173).
[Illustration: Fig. 172. Overturned Fold, Vermont]
In folds where the compression has been great the layers are often
found thickened at the crest and thinned along the limbs (174). Where
strong rocks such as heavy limestones are folded together with weak
rocks such as shales, the strong rocks are often bent into great
simple folds, while the weak rocks are minutely crumpled.
[Illustration: Fig. 173. Fan Folds, the Alps]
=Systems of folds.= As a rule, folds occur in systems. Over the
Appalachian mountain belt, for example, extending from northeastern
Pennsylvania to northern Alabama and Georgia, the earth's crust has
been thrown into a series of parallel folds whose axes run from
northeast to southwest (Fig. 175). In Pennsylvania one may count a
score or more of these earth waves,--some but from ten to twenty miles
in length, and some extending as much as two hundred miles before they
die away. On the eastern part of this belt the folds are steeper and
more numerous than on the western side.
[Illustration: Fig. 174. Folds with Layers thickened at the
Crest and thinned along the Limbs]
=Cause and conditions of folding.= The sections which we have studied
suggest that rocks are folded by lateral pressure. While a single,
simple fold might be produced by a heave, a series of folds, including
overturns, fan folds, and folds thickened on their crests at the
expense of their limbs, could only be made in one way,--by pressure
from the side. Experiment has reproduced all forms of folds by
subjecting to lateral thrust layers of plastic material such as wax.
Vast as the force must have been which could fold the solid rocks of
the crust as one may crumple the leaves of a magazine in the fingers,
it is only under certain conditions that it could have produced the
results which we see. Rocks are brittle, and it is only when under a
_heavy load_ and by _great pressure slowly applied_, that they can
thus be folded and bent instead of being crushed to pieces. Under
these conditions, experiments prove that not only metals such as
steel, but also brittle rocks such as marble, can be deformed and
molded and made to flow like plastic clay.
[Illustration: Fig. 175. Relief Map of the Northern Appalachian
Region
From Bingham's _Geographic Influences in American History_]
=Zone of flow, zone of flow and fracture, and zone of fracture.= We
may believe that at depths which must be reckoned in tens of thousands
of feet the load of overlying rocks is so great that rocks of all
kinds yield by folding to lateral pressure, and flow instead of
breaking. Indeed, at such profound depths and under such inconceivable
weight no cavity can form, and any fractures would be healed at once
by the welding of grain to grain. At less depths there exists a zone
where soft rocks fold and flow under stress, and hard rocks are
fractured; while at and near the surface hard and soft rocks alike
yield by fracture to strong pressure.
Structures developed in Compressed Rocks
Deformed rocks show the effects of the stresses to which they have
yielded, not only in the immense folds into which they have been
thrown but in their smallest parts as well. A hand specimen of slate,
or even a particle under the microscope, may show plications similar
in form and origin to the foldings which have produced ranges of
mountains. A tiny flake of mica in the rocks of the Alps may be
puckered by the same resistless forces which have folded miles of
solid rock to form that lofty range.
=Slaty cleavage.= Rocks which have yielded to pressure often split
easily in a certain direction across the bedding planes. This cleavage
is known as slaty cleavage, since it is most perfectly developed in
fine-grained, homogeneous rocks, such as slates, which cleave to the
thin, smooth-surfaced plates with which we are familiar in the slates
used in roofing and for ciphering and blackboards. In coarse-grained
rocks, pressure develops more distant partings which separate the
rocks into blocks.
Slaty cleavage cannot be due to lamination, since it commonly crosses
bedding planes at an angle, while these planes have been often
well-nigh or quite obliterated. Examining slate with a microscope, we
find that its cleavage is due to the grain of the rock. Its particles
are flattened and lie with their broad faces in parallel planes, along
which the rock naturally splits more easily than in any other
direction. The irregular grains of the mud which has been altered to
slate have been squeezed flat by a pressure exerted at right angles to
the plane of cleavage. Cleavage is found only in folded rocks, and, as
we may see in Figure 176, the strike of the cleavage runs parallel to
the strike of the strata and the axis of the folds. The dip of the
cleavage is generally steep, hence the pressure was nearly horizontal.
The pressure which has acted at right angles to the cleavage, and to
which it is due, is the same lateral pressure which has thrown the
strata into folds.
[Illustration: Fig. 176. Slaty Cleavage]
We find additional proof that slates have undergone compression at
right angles to their cleavage in the fact that any inclusions in
them, such as nodules and fossils, have been squeezed out of shape and
have their long diameters lying in the planes of cleavage.
That pressure is competent to cause cleavage is shown by experiment.
Homogeneous material of fine grain, such as beeswax, when subjected to
heavy pressure cleaves at right angles to the direction of the
compressing force.
=Rate of folding.= All the facts known with regard to rock deformation
agree that it is a secular process, taking place so slowly that, like
the deepening of valleys by erosion, it escapes the notice of the
inhabitants of the region. It is only under stresses slowly applied
that rocks bend without breaking. The folds of some of the highest
mountains have risen so gradually that strong, well-intrenched rivers
which had the right of way across the region were able to hold to
their courses, and as a circular saw cuts its way through the log
which is steadily driven against it, so these rivers sawed their
gorges through the fold as fast as it rose beneath them. Streams which
thus maintain the course which they had antecedent to a deformation of
the region are known as _antecedent_ streams. Examples of such are the
Sutlej and other rivers of India, whose valleys trench the outer
ranges of the Himalayas and whose earlier river deposits have been
upturned by the rising ridges. On the other hand, mountain crests are
usually divides, parting the head waters of different drainage
systems. In these cases the original streams of the region have been
broken or destroyed by the uplift of the mountain mass across their
paths.
On the whole, which have worked more rapidly, processes of deformation
or of denudation?
[Illustration: Fig. 177. An Unroofed Anticline]
Land Forms due to Folding
As folding goes on so slowly, it is never left to form surface
features unmodified by the action of other agencies. An anticlinal
fold is attacked by erosion as soon as it begins to rise above the
original level, and the higher it is uplifted, and the stronger are
its slopes, the faster is it worn away. Even while rising, a young
upfold is often thus unroofed, and instead of appearing as a long,
Smooth, boat-shaped ridge, it commonly has had opened along the rocks
of the axis, when these are weak, a valley which is overlooked by the
infacing escarpments of the hard layers of the sides of the fold (Fig.
177). Under long-continued erosion, anticlines may be degraded to
valleys, while the synclines of the same system may be left in relief
as ridges (Fig. 167).
=Folded mountains.= The vastness of the forces which wrinkle the crust
is best realized in the presence of some lofty mountain range. All
mountains, indeed, are not the result of folding. Some, as we shall
see, are due to upwarps or to fractures of the crust; some are piles
of volcanic material; some are swellings caused by the intrusion of
molten matter beneath the surface; some are the relicts left after the
long denudation of high plateaus.
[Illustration: Fig. 178. Mountain Peaks carved in Folded
Strata, Rocky Mountains, Montana]
But most of the mountain ranges of the earth, and some of the
greatest, such as the Alps and the Himalayas, were originally
mountains of folding. The earth's crust has wrinkled into a fold; or
into a series of folds, forming a series of parallel ridges and
intervening valleys; or a number of folds have been mashed together
into a vast upswelling of the crust, in which the layers have been so
crumpled and twisted, overturned and crushed, that it is exceedingly
difficult to make out the original structure.
The close and intricate folds seen in great mountain ranges were
formed, as we have seen, deep below the surface, within the zone of
folding. Hence they may never have found expression in any individual
surface features. As the result of these deformations deep under
ground the surface was broadly lifted to mountain height, and the
crumpled and twisted mountain structures are now to be seen only
because erosion has swept away the heavy cover of surface rocks under
whose load they were developed.
[Illustration: Fig. 179. Section of a Portion of the Alps]
When the structure of mountains has been deciphered it is possible to
estimate roughly the amount of horizontal compression which the region
has suffered. If the strata of the folds of the Alps were smoothed
out, they would occupy a belt seventy-four miles wider than that to
which they have been compressed, or twice their present width. A
section across the Appalachian folds in Pennsylvania shows a
compression to about two thirds the original width; the belt has been
shortened thirty-five miles in every hundred.
Considering the thickness of their strata, the compression which
mountains have undergone accounts fully for their height, with enough
to spare for all that has been lost by denudation.
The Appalachian folds involve strata thirty thousand feet in
thickness. Assuming that the folded strata rested on an unyielding
foundation, and that what was lost in width was gained in height, what
elevation would the range have reached had not denudation worn it as
it rose?
=The life history of mountains.= While the disturbance and uplift of
mountain masses are due to deformation, their sculpture into ridges
and peaks, valleys and deep ravines, and all the forms which meet the
eye in mountain scenery, excepting in the very youngest ranges, is due
solely to erosion. We may therefore classify mountains according to
the degree to which they have been dissected. The Juras are an example
of the stage of early youth, in which the anticlines still persist as
ridges and the synclines coincide with the valleys; this they owe as
much to the slight height of their uplift as to the recency of its
date (Fig. 180).
[Illustration: Fig. 180. Section of a Portion of the Jura
Mountains]
The Alps were upheaved at various times (Fig. 399), the last uplift
being later than the uplift of the Juras, but to so much greater
height that erosion has already advanced them well on towards
maturity. The mountain mass has been cut to the core, revealing
strange contortions of strata which could never have found expression
at the surface. Sharp peaks, knife-edged crests, deep valleys with
ungraded slopes subject to frequent landslides, are all features of
Alpine scenery typical of a mountain range at this stage in its life
history. They represent the survival of the hardest rocks and the
strongest structures, and the destruction of the weaker in their long
struggle for existence against the agents of erosion. Although miles
of rock have been removed from such ranges as the Alps, we need not
suppose that they ever stood much, if any, higher than at present. All
this vast denudation may easily have been accomplished while their
slow upheaval was going on; in several mountain ranges we have
evidence that elevation has not yet ceased.
[Illustration: Fig. 181. Young Mountains, Rocky Mountains of
Canada]
Under long denudation mountains are subdued to the forms
characteristic of old age. The lofty peaks and jagged crests of their
earlier life are smoothed down to low domes and rounded crests. The
southern Appalachians and portions of the Hartz Mountains in Germany
(Fig. 182) are examples of mountains which have reached this stage.
[Illustration: Fig. 182. Subdued Mountains, the Hartz
Mountains, Germany]
There are numerous regions of upland and plains in which the rocks are
found to have the same structure that we have seen in folded
mountains; they are tilted, crumpled, and overturned, and have clearly
suffered intense compression. We may infer that their folds were once
lifted to the height of mountains and have since been wasted to
low-lying lands. Such a section as that of Figure 67 illustrates how
ancient mountains may be leveled to their roots, and represents the
final stage to which even the Alps and the Himalayas must sometime
arrive. Mountains, perhaps of Alpine height, once stood about Lake
Superior; a lofty range once extended from New England and New Jersey
southwestward to Georgia along the Piedmont belt. In our study of
historic geology we shall see more clearly how short is the life of
mountains as the earth counts time, and how great ranges have been
lifted, worn away, and again upheaved into a new cycle of erosion.
=The sedimentary history of folded mountains.= We may mention here
some of the conditions which have commonly been antecedent to great
foldings of the crust.
1. Mountain ranges are made of belts of enormously and exceptionally
thick sediments. The strata of the Appalachians are thirty thousand
feet thick, while the same formations thin out to five thousand feet
in the Mississippi valley. The folds of the Wasatch Mountains involve
strata thirty thousand feet thick, which thin to two thousand feet in
the region of the Plains.
2. The sedimentary strata of which mountains are made are for the most
part the shallow-water deposits of continental deltas. Mountain ranges
have been upfolded along the margins of continents.
3. Shallow-water deposits of the immense thickness found in mountain
ranges can be laid only in a gradually sinking area. A profound
subsidence, often to be reckoned in tens of thousands of feet,
precedes the upfolding of a mountain range.
Thus the history of mountains of folding is as follows: For long ages
the sea bottom off the coast of a continent slowly subsides, and the
great trough, as fast as it forms, is filled with sediments, which at
last come to be many thousands of feet thick. The downward movement
finally ceases. A slow but resistless pressure sets in, and gradually,
and with a long series of many intermittent movements, the vast mass
of accumulated sediments is crumpled and uplifted into a mountain
range.
Fractures and Dislocations of the Crust
Considering the immense stresses to which the rocks of the crust are
subjected, it is not surprising to find that they often yield by
fracture, like brittle bodies, instead of by folding and flowing, like
plastic solids. Whether rocks bend or break depends on the character
and condition of the rocks, the load of overlying rocks which they
bear, and the amount of the force and the slowness with which it is
applied.
=Joints.= At the surface, where their load is least, we find rocks
universally broken into blocks of greater or less size by partings
known as joints. Under this name are included many division planes
caused by cooling and drying; but it is now generally believed that
the larger and more regular joints, especially those which run
parallel to the dip and strike of the strata, are fractures due to
up-and-down movements and foldings and twistings of the rocks.
[Illustration: Fig. 183. Joints utilized by a River in widening
its Valley, Iowa]
Joints are used to great advantage in quarrying, and we have seen how
they are utilized by the weather in breaking up rock masses, by rivers
in widening their valleys, by the sea in driving back its cliffs, by
glaciers in plucking their beds, and how they are enlarged in soluble
rocks to form natural passageways for underground waters. The ends of
the parted strata match along both sides of joint planes; in. joints
there has been little or no displacement of the broken rocks.
[Illustration: Fig. 184. A Normal Fault]
=Faults.= In Figure 184 the rocks have been both broken and dislocated
along the plane _ff´_. One side must have been moved up or down past
the other. Such a dislocation is called a fault. The amount of the
displacement, as measured by the vertical distance between the ends of
a parted layer, is the _throw_ (_cd_). The angle (_ff´v_) which the
fault plane makes with the vertical is the _hade_. In Figure 184 the
right side has gone down relatively to the left; the right is the side
of the downthrow, while the left is the side of the upthrow. Where the
fault plane is not vertical the surfaces on the two sides may be
distinguished as the _hanging wall_ (that on the right of Figure 184)
and the _foot wall_ (that on the left of the same figure). Faults
differ in throw from a fraction of an inch to many thousands of feet.
=Slickensides.= If we examine the walls of a fault, we may find
further evidence of movement in the fact that the surfaces are
polished and grooved by the enormous friction which they have suffered
as they have ground one upon the other. These appearances, called
slickensides, have sometimes been mistaken for the results of glacial
action.
=Normal faults.= Faults are of two kinds,--normal faults and thrust
faults. Normal faults, of which Figure 184 is an example, hade to the
downthrow; the hanging wall has gone down. The total length of the
strata has been increased by the displacement. It seems that the
strata have been stretched and broken, and that the blocks have
readjusted themselves under the action of gravity as they settled.
=Thrust faults.= Thrust faults hade to the upthrow; the hanging wall
has gone up. Clearly such faults, where the strata occupy less space
than before, are due to lateral thrust. Folds and thrust faults are
closely associated. Under lateral pressure strata may fold to a
certain point and then tear apart and fault along the surface of least
resistance. Under immense pressure strata also break by shear without
folding. Thus, in Figure 185, the rigid earth block under lateral
thrust has found it easier to break along the fault plane than to
fold. Where such faults are nearly horizontal they are distinguished
as _thrust planes_.
[Illustration: Fig. 185. A Thrust Fault]
In all thrust faults one mass has been pushed over another, so as to
bring the underlying and older strata upon younger beds; and when the
fault planes are nearly horizontal, and especially when the rocks have
been broken into many slices which have slidden far one upon another,
the true succession of strata is extremely hard to decipher.
In the Selkirk Mountains of Canada the basement rocks of the region
have been driven east for seven miles on a thrust plane, over rocks
which originally lay thousands of feet above them.
Along the western Appalachians, from Virginia to Georgia, the mountain
folds are broken by more than fifteen parallel thrust planes, running
from northeast to southwest, along which the older strata have been
pushed westward over the younger. The longest continuous fault has
been traced three hundred and seventy-five miles, and the greatest
horizontal displacement has been estimated at not less than eleven
miles.
=Crush breccia.= Rocks often do not fault with a clean and simple
fracture, but along a zone, sometimes several yards in width, in which
they are broken to fragments. It may occur also that strata which as a
whole yield to lateral thrust by folding include beds of brittle
rocks, such as thin-layered limestones, which are crushed to pieces by
the strain. In either case the fragments when recemented by
percolating waters form a rock known as a _crush breccia_ (pronounced
_bretcha_)(Fig. 186).
[Illustration: Fig. 186. Breccia]
Breccia is a term applied to any rock formed of cemented _angular_
fragments. This rock may be made by the consolidation of volcanic
cinders, of angular waste at the foot of cliffs, or of fragments of
coral torn by the waves from coral reefs, as well as of strata crushed
by crustal movements.
Surface Features due to Dislocations
=Fault scarps.= A fault of recent date may be marked at surface by a
scarp, because the face of the upthrown block has not yet been worn to
the level of the downthrow side.
After the upthrown block has been worn down to this level,
differential erosion produces fault scarps wherever weak rocks and
resistant rocks are brought in contact along the fault plane; and the
harder rocks, whether on the upthrow or the downthrow side, emerge in
a line of cliffs. Where a fault is so old that no abrupt scarps
appear, its general course is sometimes marked by the line of division
between highland and lowland or hill and plain. Great faults have
sometimes brought ancient crystalline rocks in contact with weaker and
younger sedimentary rocks, and long after erosion has destroyed all
fault scarps the harder crystallines rise in an upland of rugged or
mountainous country which meets the lowland along the line of
faulting.
[Illustration: Fig. 187. A Concealed Fault
This fault may be inferred from the changes in strata in
passing along the strike, as from _b_ to _a´_ and from
_c_ to _b´_]
The vast majority of faults give rise to no surface features. The
faulted region may be old enough to have been baseleveled, or the
rocks on both sides of the line of dislocation may be alike in their
resistance to erosion and therefore have been worn down to a common
slope. The fault may be entirely concealed by the mantle of waste, and
in such cases it can be inferred from abrupt changes in the character
or the strike and dip of the strata where they may outcrop near it
(Fig. 187).
[Illustration: Fig. 188. East-West Section across the Broken
Plateau north of the Grand Canyon of the Colorado River,
Arizona]
The plateau trenched by the Grand Canyon of the Colorado River
exhibits a series of magnificent fault scarps whose general course is
from north to south, marking the edges of the great crust blocks into
which the country has been broken. The highest part of the plateau is
a crust block ninety miles long and thirty-five miles in maximum
width, which has been hoisted to nine thousand three hundred feet
above, sea level. On the east it descends four thousand feet by a
monoclinal fold, which passes into a fault towards the north. On the
west it breaks down by a succession of terraces faced by fault scarps.
The throw of these faults varies from seven hundred feet to more than
a mile. The escarpments, however, are due in a large degree to the
erosion of weaker rock on the downthrow side.
[Illustration: Fig. 189. The Fault separating the Highlands and
the Lowlands, Scotland]
The Highlands of Scotland (Fig. 189) meet the Lowlands on the south
with a bold front of rugged hills along a line of dislocation which
runs across the country from sea to sea. On the one side are hills of
ancient crystalline rocks whose crumpled structures prove that they
are but the roots of once lofty mountains; on the other lies a lowland
of sandstone and other stratified rocks formed from the waste of those
long-vanished mountain ranges. Remnants of sandstone occur in places
on the north of the great fault, and are here seen to rest on the worn
and fairly even surface of the crystallines. We may infer that these
ancient mountains were reduced along their margins to low plains,
which were slowly lowered beneath the sea to receive a cover of
sedimentary rocks. Still later came an uplift and dislocation. On the
one side erosion has since stripped off the sandstones for the most
part, but the hard crystalline rocks yet stand in bold relief. On the
other side the weak sedimentary rocks have been worn down to lowlands.
=Rift valleys.= In a broken region undergoing uplift or the unequal
settling which may follow, a slice inclosed between two fissures may
sink below the level of the crust blocks on either side, thus forming
a linear depression known as a rift valley, or valley of fracture.
[Illustration: Fig. 190. Section from the Mountains of
Palestine to the Mountains of Moab across the Dead Sea
_a_, ancient schists; _b_, Carboniferous strata; _c_, _d_, and
_e_, Cretaceous strata]
One of the most striking examples of this rare type of valley is the
long trough which runs straight from the Lebanon Mountains of Syria on
the north to the Red Sea on the south, and whose central portion is
occupied by the Jordan valley and the Dead Sea. The plateau which it
gashes has been lifted more than three thousand feet above sea level,
and the bottom of the trough reaches a depth of two thousand six
hundred feet below that level in parts of the Dead Sea. South of the
Dead Sea the floor of the trough rises somewhat above sea level, and
in the Gulf of Akabah again sinks below it. This uneven floor could be
accounted for either by the profound warping of a valley of erosion or
by the unequal depression of the floor of a rift valley. But that the
trough is a true valley of fracture is proved by the fact that on
either side it is bounded by fault scarps and monoclinal folds. The
keystone of the arch has subsided. Many geologists believe that the
Jordan-Akabah trough, the long narrow basin of the Red Sea, and the
chain of down-faulted valleys which in Africa extends from the strait
of Bab-el-Mandeb as far south as Lake Nyassa--valleys which contain
more than thirty lakes--belong to a single system of dislocation.
Should you expect the lateral valleys of a rift valley at the time of
its formation to enter it as hanging valleys or at a common level?
=Block mountains.= Dislocations take place on so grand a scale that by
the upheaval of blocks of the earth's crust or the downfaulting of
the blocks about one which is relatively stationary, mountains known
as block mountains are produced. A tilted crust block may present a
steep slope on the side upheaved and a more gentle descent on the side
depressed.
[Illustration: Fig. 191. Block Mountains, Southern Oregon]
=The Basin ranges.= The plateaus of the United States bounded by the
Rocky Mountains on the east, and on the west by the ranges which
front the Pacific, have been profoundly fractured and faulted. The
system of great fissures by which they are broken extends north and
south, and the long, narrow, tilted crust blocks intercepted between
the fissures give rise to the numerous north-south ranges of the
region. Some of the tilted blocks, as those of southern Oregon, are as
yet but moderately carved by erosion, and shallow lakes lie on the
waste that has been washed into the depressions between them (Fig.
191). We may therefore conclude that their displacement is somewhat
recent. Others, as those of Nevada, are so old that they have been
deeply dissected; their original form has been destroyed by erosion,
and the intermontane depressions are occupied by wide plains of waste.
=Dislocations and river valleys.= Before geologists had proved that
rivers can by their own unaided efforts cut deep canyons, it was
common to consider any narrow gorge as a gaping fissure of the crust.
This crude view has long since been set aside. A map of the plateaus
of northern Arizona shows how independent of the immense faults of the
region is the course of the Colorado River. In the Alps the tunnels on
the Saint Gotthard railway pass six times beneath the gorge of the
Reuss, but at no point do the rocks show the slightest trace of a
fault.
[Illustration: Fig. 192. Fault crossing Valley in Japan]
=Rate of dislocation.= So far as human experience goes, the earth
movements which we have just studied, some of which have produced
deep-sunk valleys and lofty mountain ranges, and faults whose throw is
to be measured in thousands of feet, are slow and gradual. They are
not accomplished by a single paroxysmal effort, but by slow creep and
a series of slight slips continued for vast lengths of time.
In the Aspen mining district in Colorado faulting is now going on at a
comparatively rapid rate. Although no sudden slips take place, the
creep of the rock along certain planes of faulting gradually bends out
of shape the square-set timbers in horizontal drifts and has closed
some vertical shafts by shifting the upper portion across the lower.
Along one of the faults of this region it is estimated that there has
been a movement of at least four hundred feet since the Glacial epoch.
More conspicuous are the instances of active faulting by means of
sudden slips. In 1891 there occurred along an old fault plane in Japan
a slip which produced an earth rent traced for fifty miles (Fig. 192).
The country on one side was depressed in places twenty feet below that
on the other, and also shifted as much as thirteen feet horizontally
in the direction of the fault line.
In 1872 a slip occurred for forty miles on the great line of
dislocation which runs along the eastern base of the Sierra Nevada
Mountains. In the Owens valley, California, the throw amounted to
twenty-five feet in places, with a horizontal movement along the fault
line of as much as eighteen feet. Both this slip and that in Japan
just mentioned caused severe earthquakes.
For the sake of clearness we have described oscillations, foldings,
and fractures of the crust as separate processes, each giving rise to
its own peculiar surface features, but in nature earth movements are
by no means so simple,--they are often implicated with one another:
folds pass into faults; in a deformed region certain rocks have bent,
while others under the same strain, but under different conditions of
plasticity and load, have broken; folded mountains have been worn to
their roots, and the peneplains to which they have been denuded have
been upwarped to mountain height and afterwards dissected,--as in the
case of the Allegheny ridges, the southern Carpathians, and other
ranges,--or, as in the case of the Sierra Nevada Mountains, have been
broken and uplifted as mountains of fracture.
Draw the following diagrams, being careful to show the direction
in which the faulted blocks have moved, by the position of the two
parts of some well-defined layer of limestone, sandstone, or
shale, which occurs on each side of the fault plane, as in Figure
184.
1. A normal fault with a hade of 15°, the original fault
scarp remaining.
2. A normal fault with a hade of 50°, the original fault
scarp worn away, showing cliffs caused by harder strata on the
downthrow side.
3. A thrust fault with a hade of 30°, showing cliffs due to
harder strata outcropping on the downthrow.
4. A thrust fault with a hade of 80°, with surface
baseleveled.
5. In a region of normal faults a coal mine is being worked along
the seam of coal _AB_ (Fig. 193). At _B_ it is found broken by a fault
f which hades toward _A_. To find the seam again, should you advise
tunneling up or down from _B_?
[Illustration: Fig. 193]
6. In a vertical shaft of a coal mine the same bed of coal is
pierced twice at different levels because of a fault. Draw a
diagram to show whether the fault is normal or a thrust.
[Illustration: Fig. 194. Ridges to be explained by Faulting]
7. Copy the diagram in Figure 194, showing how the two ridges may
be accounted for by a single resistant stratum dislocated by a
fault. Is the fault a _strike fault_, i.e. one running parallel with
the strike of the strata, or a _dip fault_, one running parallel
with the direction of the dip?
[Illustration: Fig. 195. Earth Block of Tilted Strata, with
Included Seam of Coal _cc_]
8. Draw a diagram of the block in Figure 195 as it would appear if
dislocated along the plane _efg_ by a normal fault whose throw equals
one fourth the height of the block. Is the fault a strike or a dip
fault? Draw a second diagram showing the same block after denudation
has worn it down below the center of the upthrown side. Note that the
outcrop of the coal seam is now deceptively repeated. This exercise
may be done in blocks of wood instead of drawings.
[Illustration: Fig. 196. _A_ and _B_. Repeated Outcrops of Same
Strata]
9. Draw diagrams showing by dotted lines the conditions both of _A_
and _B_, Figure 196, after deformation had given the strata their
present attitude.
[Illustration: Fig. 197. A Block Mountain]
10. What is the attitude of the strata of this earth block, Figure
197? What has taken place along the plane _baf_? When did the
dislocation occur compared with the folding of the strata? With the
erosion of the valleys on the right-hand side of the mountain? With
the deposition of the sediments _efg_? Do you find any remnants of the
original surface _baf_ produced by the dislocation? From the left-hand
side of the mountain infer what was the relief of the region before
the dislocation. Give the complete history recorded in the diagram
from the deposition of the strata to the present.
[Illustration: Fig. 198. A Faulted Lava Flow _aa´_]
11. Which is the older fault, in Figure 198, _f_ or _f´_? When did the
lava flow occur? How long a time elapsed between the formation of the
two faults as measured in the work done in the interval? How long a
time since the formation of the later fault?
[Illustration: Fig. 199. Measurement of the Thickness of
Inclined Strata]
12. Measure by the scale the thickness _bc_ of the coal-bearing strata
outcropping from _a_ to _b_ in Figure 199. On any convenient scale
draw a similar section of strata with a dip of 30° outcropping along a
horizontal line normal to the strike one thousand feet in length, and
measure the thickness of the strata by the scale employed. The
thickness may also be calculated by trigonometry.
[Illustration: Fig. 200. Unconformity between Parallel Strata]
[Illustration: Fig. 201. Unconformity between Non-parallel
Strata]
Unconformity
Strata deposited one upon, another in an unbroken succession are said
to be _conformable_. But the continuous deposition of strata is often
interrupted by movements of the earth's crust, Old sea floors are
lifted to form land and are again depressed beneath the sea to receive
a cover of sediments only after an interval during which they were
carved by subaërial erosion. An erosion surface which thus parts older
from younger strata is known as an _unconformity_, and the strata
above it are said to be _unconformable_ with the rocks below, or to
rest unconformably upon them. An unconformity thus records movements
of the crust and a consequent break in the deposition of the strata.
It denotes a period of land erosion of greater or less length, which
may sometimes be roughly measured by the stage in the erosion cycle
which the land surface had attained before its burial. Unconformable
strata may be _parallel_, as in Figure 200, where the record includes
the deposition of strata _a_, their emergence, the erosion of the land
surface _ss_, a submergence and the deposit of the strata _b_, and
lastly, emergence and the erosion of the present surface _s´s´_.
[Illustration: Fig. 202. Carboniferous Limestone resting
unconformably on Early Silurian Slates, Yorkshire, England]
Often the earth movements to which the uplift or depression was due
involved tilting or folding of the earlier strata, so that the strata
are now nonparallel as well as unconformable. In Figure 201, for
example, the record includes deposition, uplift, and _tilting_ of _a_;
erosion, depression, the deposit of _b_; and finally the uplift which
has brought the rocks to open air and permitted the dissection by
which the unconformity is revealed.
From this section we infer that during early Silurian times the area
was sea, and thick sea muds were laid upon it. These were later
altered to hard slates by pressure and upfolded into mountains. During
the later Silurian and the Devonian the area was land and suffered
vast denudation. In the Carboniferous period it was lowered beneath
the sea and received a cover of limestone.
[Illustration: Fig. 203. Diagram Illustrating how the Age of
Mountains is determined]
=The age of mountains.= It is largely by means of unconformities that
we read the history of mountain making and other deformations and
movements of the crust. In Figure 203, for example, the deformation
which upfolded the range of mountains took place after the deposit of
the series of strata a of which the mountains are composed, and before
the deposit of the stratified rocks, which rest unconformably on a and
have not shared their uplift.
[Illustration: Fig. 204. Section of Mountain Range showing
repeated Uplifts
_a_, strata whose folding formed a mountain range; on,
baseleveled surface produced by long denudation of the
mountains; _b_, tilted strata resting unconformably on _a_;
_c_, horizontal strata parted from _b_ by the unconformity
_u´u´_. The first uplift of the range preceded the period of
time when _b_ was deposited. The and uplift, to which the
present mountains owe their height, was later than this period
but earlier than the period when strata _c_ were laid]
Most great mountain ranges, like the Sierra Nevada and the Alps, mark
lines of weakness along which the earth's crust has yielded again and
again during the long ages of geological time. The strata deposited at
various times about their flanks have been infolded by later
crumplings with the original mountain mass, and have been repeatedly
crushed, inverted, faulted, intruded with igneous rocks, and denuded.
The structure of great mountain ranges thus becomes exceedingly
complex and difficult to read. A comparatively simple case of repeated
uplift is shown in Figure 204. In the section of a portion of the Alps
shown in Figure 179 a far more complicated history may be deciphered.
[Illustration: Fig. 205. Unconformity showing Buried Valleys
_lm_, limestone; _sh_, shale; _r_, _r´_, and _r´´_, river silts
filling eroded valleys in the limestone. The upper surface of
the limestone is evidently a land surface developed by erosion.
The valleys which trench it are narrow and steep-sided; hence
the land surface had not reached maturity. The sands and muds,
now hardened to firm rock, which fill these valleys, _r_, _r´_,
and _r´´_, contain no relics of the sea, but Instead the remains
of land animals and plants. They are river deposits, and we may
infer that owing to a subsidence the young rivers ceased to
degrade their channels and slowly filled their gorges with
sands and silts. The overlying shale records a further
depression which brought the lanes below the level of the sea.
A section similar to this is to be seen in the coal mines of
Bernissant, Belgium, where a gorge twice as deep as that of
Niagara was discovered within whose ancient river deposits were
found entombed the skeletons of more than a score of the huge
reptiles characteristic of the age when the gorge was cut and
filled]
[Illustration: Fig. 206. Unconformity showing Buried Mountains,
Scotland
_gn_, ancient crystalline rocks; _ss_, marine sandstones. The
surface _bb_ of the ancient crystalline rocks is mountainous,
with peaks rising to a height of as much as three thousand
feet. It is one of the most ancient land surfaces on the planet
and is covered unconformably with pre-Cambrian sandstones
thousands of feet in thickness, in which the Torridonian
Mountains of Scotland have been carved. What has been the
history of the region since the mountainous surface _bb_ was
produced by erosion?]
=Unconformities in the Colorado Canyon, Arizona.= How geological
history may be read in unconformities is further illustrated in
Figures 207 and 208>. The dark crystalline rocks _a_ at the bottom of
the canyon are among the most ancient known, and are overlain
unconformably by a mass of tilted coarse marine sandstones _b_, whose
total thickness is not seen in the diagram and measures twelve
thousand feet perpendicularly to the dip. Both _a_ and _b_ rise to a
common level _nn´_ and upon them rest the horizontal sea-laid strata
_c_, in which the upper portion of the canyon has been cut.
[Illustration: Fig. 207. Diagram of Wall of the Colorado
Canyon, Arizona, showing Unconformities]
Note that the crystalline rocks a have been crumpled and crushed.
Comparing their structure with that of folded mountains, what do you
infer as to their relief after their deformation? To which surface
were they first worn down, _mm´_ or _nm_? Describe and account for the
surface _mm´_. How does it differ from the surface of the crystalline
rocks seen in the Torridonian Mountains (Fig. 206), and why? This
surface _mm´_ is one of the oldest land surfaces of which any vestige
remains. It is a bit of fossil geography buried from view since the
earliest geological ages and recently brought to light by the erosion
of the canyon.
[Illustration: Fig. 208. View of the North Wall of the Grand
Canyon of the Colorado River, Arizona, showing the
Unconformities illustrated in Figure 207]
How did the surface _mm´_ come to receive its cover of sandstones _b_?
From the thickness and coarseness of these sediments draw inferences
as to the land mass from which they were derived. Was it rising or
subsiding? high or low? Were its streams slow or swift? Was the amount
of erosion small or great?
Note the strong dip of these sandstones _b_. Was the surface _mm´_
tilted as now when the sandstones were deposited upon it? When was it
tilted? Draw a diagram showing the attitude of the rocks after this
tilting occurred, and their height relative to sea level.
The surface _nn´_ is remarkably even, although diversified by some low
hills which rise into the bedded rocks of _c_, and it may be traced
for long distances up and down the canyon. Were the layers of _b_ and
the surface _mm´_ always thus cut short by _nn´_ as now? What has made
the surface _nn´_ so even? How does it come to cross the hard
crystalline rocks a and the weaker sandstones _b_ at the same
impartial level? How did the sediments of _c_ come to be laid upon it?
Give now the entire history recorded in the section, and in addition
that involved in the production of the platform _P_, shown in Figure
130, and that of the cutting of the canyon. How does the time involved
in the cutting of the canyon compare with that required for the
production of the surfaces _mm´_, _nn´_, and _P_?
CHAPTER X
EARTHQUAKES
Any sudden movement of the rocks of the crust, as when they tear apart
when a fissure is formed or extended, or slip from time to time along
a growing fault, produces a jar called an earthquake, which spreads in
all directions from the place of disturbance.
=The Charleston earthquake.= On the evening of August 31, 1886, the
city of Charleston, S.C., was shaken by one of the greatest
earthquakes which has occurred in the United States. A slight tremor
which rattled the windows was followed a few seconds later by a roar,
as of subterranean thunder, as the main shock passed beneath the city.
Houses swayed to and fro, and their heaving floors overturned
furniture and threw persons off their feet as, dizzy and nauseated,
they rushed to the doors for safety. In sixty seconds a number of
houses were completely wrecked, fourteen thousand chimneys were
toppled over, and in all the city scarcely a building was left without
serious injury. In the vicinity of Charleston railways were twisted
and trains derailed. Fissures opened in the loose superficial
deposits, and in places spouted water mingled with sand from shallow
underlying aquifers.
The point of origin, or _focus_, of the earthquake was inferred from
subsequent investigations to be a rent in the rocks about twelve miles
beneath the surface. From the center of greatest disturbance, which
lay above the focus, a few miles northwest of the city, the surface
shock traveled outward in every direction, with decreasing effects, at
the rate of nearly two hundred miles per minute. It was felt from
Boston to Cuba, and from eastern Iowa to the Bermudas, over a circular
area whose diameter was a thousand miles.
An earthquake is transmitted from the focus through the elastic rocks
of the crust, as a wave, or series of waves, of compression and
rarefaction, much as a sound wave is transmitted through the elastic
medium of the air. Each earth particle vibrates with exceeding
swiftness, but over a very short path. The swing of a particle in firm
rock seldom exceeds one tenth of an inch in ordinary earthquakes, and
when it reaches one half an inch and an inch, the movement becomes
dangerous and destructive.
[Illustration: Fig. 210. Block of the Earth's Crust shaken by
an Earthquake
_x_, focus; _a_, _b_, _c_, _d_, successive spheroidal waves in
the crust; _a´_, _b´_, _c´_, _d´_, successive surface waves
produced by the outcropping of _a_, _b_, _c_, and _d_]
The velocity of earthquake waves, like that of all elastic waves,
varies with the temperature and elasticity of the medium. In the deep,
hot, elastic rocks they speed faster than in the cold and broken rocks
near the surface. The deeper the point of origin and the more violent
the initial shock, the faster and farther do the vibrations run.
Great earthquakes, caused by some sudden displacement or some violent
rending of the rocks, shake the entire planet. Their waves run through
the body of the earth at the rate of about three hundred and fifty
miles a minute, and more slowly round its circumference, registering
their arrival at opposite sides of the globe on the exceedingly
delicate instruments of modern earthquake observatories.
=Geological effects.= Even great earthquakes seldom produce geological
effects of much importance. Landslides may be shaken down from the
sides of mountains and hills, and cracks may be opened in the surface
deposits of plains; but the transient shiver, which may overturn
cities and destroy thousands of human lives, runs through the crust
and leaves it much the same as before.
=Earthquakes attending great displacements.= Great earthquakes
frequently attend the displacement of large masses of the rocks of the
crust. In 1822 the coast of Chile was suddenly raised three or four
feet, and the rise was five or six feet a mile inland. In 1835 the
same region was again upheaved from two to ten feet. In each instance
a destructive earthquake was felt for one thousand miles along the
coast.
The great California earthquake of 1906.= A sudden dislocation
occurred in 1906 along an ancient fault plane which extends for 300
miles through western California. The vertical displacement did not
exceed four feet, while the horizontal shifting reached a maximum of
twenty feet. Fences, rows of trees, and roads which crossed the fault
were broken and offset. The latitude and longitude of all points over
thousands of square miles were changed. On each side of the fault the
earth blocks moved in opposite directions, the block on the east
moving southward and that on the west moving northward and to twice
the distance. East and west of the fault the movements lessened with
increasing distance from it.
This sudden slip set up an earthquake lasting sixty-five seconds,
followed by minor shocks recurring for many days. In places the jar
shook down the waste on steep hillsides, snapped off or uprooted
trees, and rocked houses from their foundations or threw down their
walls or chimneys. The water mains of San Francisco were broken, and
the city was thus left defenseless against a conflagration which
destroyed $500,000,000 worth of property. The destructive effects
varied with the nature of the ground. Buildings on firm rock suffered
least, while those on deep alluvium were severely shaken by the
undulations, like water waves, into which the loose material was
thrown. Well-braced steel structures, even of the largest size, were
earthquake proof, and buildings of other materials, when honestly
built and intelligently designed to withstand earthquake shocks,
usually suffered little injury. The length of the intervals between
severe earthquakes in western California shows that a great
dislocation so relieves the stresses of the adjacent earth blocks that
scores of years may elapse before the stresses again accumulate and
cause another dislocation.
Perhaps the most violent earthquake which ever visited the United
States attended the depression, in 1812, of a region seventy-five
miles long and thirty miles wide, near New Madrid, Mo. Much of the
area was converted into swamps and some into shallow lakes, while a
region twenty miles in diameter was bulged up athwart the channel of
the Mississippi. Slight quakes are still felt in this region from time
to time, showing that the strains to which the dislocation was due
have not yet been fully relieved.
=Earthquakes originating beneath the sea.= Many earthquakes originate
beneath the sea, and in a number of examples they seem to have been
accompanied, as soundings indicate, by local subsidences of the ocean
bottom. There have been instances where the displacement has been
sufficient to set the entire Pacific Ocean pulsating for many hours.
In mid ocean the wave thus produced has a height of only a few feet,
while it may be two hundred miles in width. On shores near the point
of origin destructive waves two or three score feet in height roll in,
and on coasts thousands of miles distant the expiring undulations may
be still able to record themselves on tidal gauges.
=Distribution of earthquakes.= Every half hour some considerable area
of the earth's surface is sensibly shaken by an earthquake, but
earthquakes are by no means uniformly distributed over the globe. As
we might infer from what we know as to their causes, earthquakes are
most frequent in regions now undergoing deformation. Such are young
rising mountain ranges, fault lines where readjustments recur from
time to time, and the slopes of suboceanic depressions whose steepness
suggests that subsidence may there be in progress.
Earthquakes, often of extreme severity, frequently visit the lofty and
young ranges of the Andes, while they are little known in the subdued
old mountains of Brazil. The Highlands of Scotland are crossed by a
deep and singularly straight depression called the Great Glen, which
has been excavated along a very ancient line of dislocation. The
earthquakes which occur from time to time in this region, such as the
Inverness earthquake in 1891, are referred to slight slips along this
fault plane.
In Japan, earthquakes are very frequent. More than a thousand are
recorded every year, and twenty-nine world-shaking earthquakes
occurred in the three years ending with 1901. They originate, for the
most part, well down on the eastern flank of the earth fold whose
summit is the mountainous crest of the islands, and which plunges
steeply beneath the sea to the abyss of the Tuscarora Deep.
=Minor causes of earthquakes.= Since any concussion within the crust
sets up an earth jar, there are several minor causes of earthquakes,
such as volcanic explosions and even the collapse of the roofs of
caves. The earthquakes which attend the eruption of volcanoes are
local, even in the case of the most violent volcanic paroxysms known.
When the top of a volcano has been blown to fragments, the
accompanying earth shock has sometimes not been felt more than
twenty-five miles away.
=Depth of focus.= The focus of the Charleston earthquake, estimated at
about twelve miles below the surface, was exceptionally deep. Volcanic
earthquakes are particularly shallow, and probably no earthquakes
known have started at a greater depth than fifteen or twenty miles.
This distance is so slight compared with the earth's radius that we
may say that earthquakes are but skin-deep.
Should you expect the velocity of an earthquake to be greater in a
peneplain or in a river delta?
After an earthquake, piles on which buildings rested were found driven
into the ground, and chimneys crushed at base. From what direction did
the shock come?
Chimneys standing on the south walls of houses toppled over on the
roof. Should you infer that the shock in this case came from the north
or south?
How should you expect a shock from the east to affect pictures hanging
on the east and the west walls of a room? how the pictures hanging on
the north and the south walls?
In parts of the country, as in southwestern Wisconsin, slender erosion
pillars, or "monuments," are common. What inference could you draw as
to the occurrence in such regions of severe earthquakes in the recent
past?
CHAPTER XI
VOLCANOES
Connected with movements of the earth's crust which take place so
slowly that they can be inferred only from their effects is one of the
most rapid and impressive of all geological processes,--the extrusion
of molten rock from beneath the surface of the earth, giving rise to
all the various phenomena of volcanoes.
In a volcano, molten rock from a region deep below, which we may call
its reservoir, ascends through a pipe or fissure to the surface. The
materials erupted may be spread over vast areas, or, as is commonly
the case, may accumulate about the opening, forming a conical pile
known as the volcanic cone. It is to this cone that popular usage
refers the word _volcano_; but the cone is simply a conspicuous part
of the volcanic mechanism whose still more important parts, the
reservoir and the pipe, are hidden from view.
Volcanic eruptions are of two types,--_effusive_ eruptions, in which
molten rock wells up from below and flows forth in streams of _lava_
(a comprehensive term applied to all kinds of rock emitted from
volcanoes in a molten state), and _explosive_ eruptions, in which the
rock is blown out in fragments great and small by the expansive force
of steam.
Eruptions of the Effusive Type
=The Hawaiian volcanoes.= The Hawaiian Islands are all volcanic
in origin, and have a linear arrangement characteristic of many
volcanic groups in all parts of the world. They are strung along a
northwest-southeast line, their volcanoes standing in two parallel
rows as if reared along two adjacent lines of fracture or folding. In
the northwestern islands the volcanoes have long been extinct and are
worn low by erosion. In the southeastern island. Hawaii, three
volcanoes are still active and in process of building. Of these Mauna
Loa, the monarch of volcanoes, with a girth of two hundred miles and a
height of nearly fourteen thousand feet above sea level, is a lava
dome the slope of whose sides does not average more than five degrees.
On the summit is an elliptical basin ten miles in circumference and
several hundred feet deep. Concentric cracks surround the rim, and
from time to time the basin is enlarged as great slices are detached
from the vertical walls and engulfed.
Such a volcanic basin, formed by the insinking of the top of the cone,
is called a _caldera_.
[Illustration: Fig. 211. Mauna Loa]
[Illustration: Fig. 212. Caldera of Mauna Loa]
On the flanks of Mauna Loa, four thousand feet above sea level, lies the
caldera of Kilauea, an independent volcano whose dome has been joined to
the larger mountain by the gradual growth of the two. In each caldera
the floor, which to the eye is a plain of black lava, is the congealed
surface of a column of molten rock. At times of an eruption lakes of
boiling lava appear which may be compared to air holes in a frozen
river. Great waves surge up, lifting tons of the fiery liquid a score of
feet in air, to fall back with a mighty plunge and roar, and
occasionally the lava rises several hundred feet in fountains of
dazzling brightness. The lava lakes may flood the floor of the basin,
but in historic times have never been known to fill it and overflow the
rim. Instead, the heavy column of lava breaks way through the sides of
the mountain and discharges in streams which flow down the mountain
slopes for a distance sometimes of as much as thirty-five miles. With
the drawing off of the lava the column in the duct of the volcano
lowers, and the floor of the caldera wholly or in part subsides. A black
and steaming abyss marks the place of the lava lakes (Fig. 213). After a
time the lava rises in the duct, the floor is floated higher, and the
boiling lakes reappear.
[Illustration: Fig. 213. Portion of Caldera of Kilauea after
Collapse following an Eruption]
The eruptions of the Hawaiian volcanoes are thus of the effusive type.
The column of lava rises, breaks through the side of the mountain, and
discharges in lava streams. There are no explosions, and usually no
earthquakes, or very slight ones, accompany the eruptions. The lava in
the calderas boils because of escaping steam, but the vapor emitted is
comparatively little, and seldom hangs above the summits in heavy
clouds. We see here in its simplest form the most impressive and
important fact in all volcanic action, molten rock has been driven
upward to the surface from some deep-lying source.
=Lava flows.= As lava issues from the side of a volcano or overflows
from the summit, it flows away in a glowing stream resembling molten
iron drawn white-hot from an iron furnace. The surface of the stream
soon cools and blackens, and the hard crust of nonconducting rock may
grow thick and firm enough to form a tunnel, within which the fluid
lava may flow far before it loses its heat to any marked degree. Such
tunnels may at last be left as caves by the draining away of the lava,
and are sometimes several miles in length.
[Illustration: Fig. 214. Pahoehoe Lava, Hawaii]
=Pahoehoe and aa.= When the crust of highly fluid lava remains unbroken
after its first freezing, it presents a smooth, hummocky, and ropy
surface known by the Hawaiian term _pahoehoe_ (Fig. 214). On the other
hand, the crust of a viscid flow may be broken and splintered as it is
dragged along by the slowly moving mass beneath. The stream then appears
as a field of stones clanking and grinding on, with here and there from
some chink a dull red glow or a wisp of steam. It sets to a surface
called _aa_, of broken, sharp-edged blocks, which is often both
difficult and dangerous to traverse (Fig. 215).
[Illustration: Fig. 215. Lava Flow of the _Aa_ Type, Cinder
Cones in the Distance, Arizona]
=Fissure eruptions.= Some of the largest and most important outflows
of lava have not been connected with volcanic cones, but have been
discharged from fissures, flooding the country far and wide with
molten rock. Sheet after sheet of molten rock has been successively
outpoured, and there have been built up, layer upon layer, plateaus of
lava thousands of feet in thickness and many thousands of square miles
in area.
=Iceland.= This island plateau has been rent from time to time by
fissures from which floods of lava have outpoured. In some instances
the lava discharges along the whole length of the fissure, but more
often only at certain points upon it. The Laki fissure, twenty miles
long, was in eruption in 1783 for seven months. The inundation of
fluid rock which poured from it is the largest of historic record,
reaching a distance of forty-seven miles and covering two hundred and
twenty square miles to an average depth of a hundred feet. At the
present time the fissure is traced by a line of several hundred
insignificant mounds of fragmental materials which mark where the lava
issued (Fig. 216).
The distance to which the fissure eruptions of Iceland flow on slopes
extremely gentle is noteworthy. One such stream is ninety miles in
length, and another seventy miles long has a slope of little more than
one half a degree.
Where lava is emitted at one point and flows to a less distance there
is gradually built up a dome of the shape of an inverted saucer with
an immense base but comparatively low. Many _lava domes_ have been
discovered in Iceland, although from their exceedingly gentle slopes,
often but two or three degrees, they long escaped the notice of
explorers.
The entire plateau of Iceland, a region as large as Ohio, is composed
of volcanic products,--for the most part of successive sheets of lava
whose total thickness falls little short of two miles. The lava sheets
exposed to view were outpoured in open air and not beneath the sea;
for peat bogs and old forest grounds are interbedded with them, and
the fossil plants of these vegetable deposits prove that the plateau
has long been building and is very ancient. On the steep sea cliffs of
the island, where its structure is exhibited, the sheets of lava are
seen to be cut with many _dikes_,--fissures which have been filled by
molten rock,--and there is little doubt that it was through these
fissures that the lava outwelled in successive flows which spread far
and wide over the country and gradually reared the enormous pile of
the plateau.
Eruptions of the Explosive Type
In the majority of volcanoes the lava which rises in the pipe is at
least in part blown into fragments with violent explosions and shot
into the air together with vast quantities of water vapor and various
gases. The finer particles into--which the lava is exploded are called
_volcanic dust_ or _volcanic ashes_, and are often carried long
distances by the wind before they settle to the earth. The coarser
fragments fall about the vent and there accumulate in a steep,
conical, volcanic mountain. As successive explosions keep open the
throat of the pipe, there remains on the summit a cup-shaped
depression called the _crater_.
=Stromboli.= To study the nature of these explosions we may visit
Stromboli, a low volcano built chiefly of fragmental materials, which
rises from the sea off the north coast of Sicily and is in constant
though moderate action.
Over the summit hangs a cloud of vapor which strikingly resembles the
column of smoke puffed from the smokestack of a locomotive, in that it
consists of globular masses, each the product of a distinct explosion.
At night the cloud of vapor is lighted with a red glow at intervals of
a few minutes, like the glow on the trail of smoke behind the
locomotive when from time to time the fire box is opened. Because of
this intermittent light flashing thousands of feet above the sea,
Stromboli has been given the name of the Lighthouse of the
Mediterranean.
Looking down into the crater of the volcano, one sees a viscid lava
slowly seething. The agitation gradually increases. A great bubble
forms. It bursts with an explosion which causes the walls of the
crater to quiver with a miniature earthquake, and an outrush of steam
carries the fragments of the bubble aloft for a thousand feet to fall
into the crater or on the mountain side about it. With the explosion
the cooled and darkened crust of the lava is removed, and the light of
the incandescent liquid beneath is reflected from the cloud of vapor
which overhangs the cone.
At Stromboli we learn the lesson that the explosive force in volcanoes
is that of steam. The lava in the pipe is permeated with it much as is
a thick boiling porridge. The steam in boiling porridge is unable to
escape freely and gathers into bubbles which in breaking spurt out
drops of the pasty substance; in the same way the explosion of great
bubbles of steam in the viscid lava shoots clots and fragments of it
into the air.
=Krakatoa.= The most violent eruption of history, that of Krakatoa, a
small volcanic island in the strait between Sumatra and Java, occurred
in the last week of August, 1883. Continuous explosions shot a column
of steam and ashes. seventeen miles in air. A black cloud, beneath
which was midnight darkness and from which fell a rain of ashes and
stones, overspread the surrounding region to a distance of one hundred
and fifty miles. Launched on the currents of the upper air, the dust
was swiftly carried westward to long distances. Three days after the
eruption it fell on the deck of a ship sixteen hundred miles away, and
in thirteen days the finest impalpable powder from the volcano had
floated round the globe. For many months the dust hung over Europe and
America as a faint lofty haze illuminated at sunrise and sunset with
brilliant crimson. In countries nearer the eruption, as in India and
Africa, the haze for some time was so thick that it colored sun and
moon with blue, green, and copper-red tints and encircled them with
coronas.
At a distance of even a thousand miles the detonations of the eruption
sounded like the booming of heavy guns a few miles away. In one
direction they were audible for a distance as great as that from San
Francisco to Cleveland. The entire atmosphere was thrown into
undulations under which all barometers rose and fell as the air waves
thrice encircled the earth. The shock of the explosions raised sea
waves which swept round the adjacent shores at a height of more than
fifty feet, and which were perceptible halfway around the globe.
At the close of the eruption it was found that half the mountain had
been blown away, and that where the central part of the island had
been the sea was a thousand feet deep.
=Martinique and St. Vincent.= In 1902 two dormant volcanoes of the
West Indies, Mt. Pelee in Martinique and Soufrière in St. Vincent,
broke into eruption simultaneously. No lava was emitted, but there
were blown into the air great quantities of ashes, which mantled the
adjacent parts of the islands with a pall as of gray snow. In early
stages of the eruption lakes which occupied old craters were
discharged and swept down the ash-covered mountain valleys in torrents
of boiling mud.
On several occasions there was shot from the crater of each volcano a
thick and heavy cloud of incandescent ashes and steam, which rushed
down the mountain side like an avalanche, red with glowing stones and
scintillating with lightning flashes. Forests and buildings in its
path were leveled as by a tornado, wood was charred and set on fire by
the incandescent fragments, all vegetation was destroyed, and to
breathe the steam and hot, suffocating dust of the cloud was death to
every living creature. On the morning of the 8th of May, 1902, the
first of these peculiar avalanches from Mt. Pelee fell on the city of
St. Pierre and instantly destroyed the lives of its thirty thousand
inhabitants.
[Illustration: Fig. 219. An Eruption of Vesuvius, 1872]
The eruptions of many volcanoes partake of both the effusive and the
explosive types: the molten rock in the pipe is in part blown into the
air with explosions of steam, and in part is discharged in streams of
lava over the lip of the crater and from fissures in the sides of the
cone. Such are the eruptions of Vesuvius, one of which is illustrated
in Figure 219.
=Submarine eruptions.= The many volcanic islands of the ocean and the
coral islands resting on submerged volcanic peaks prove that eruptions
have often taken place upon the ocean floor and have there built up
enormous piles of volcanic fragments and lava. The Hawaiian volcanoes
rise from a depth of eighteen thousand feet of water and lift their
heads to about thirty thousand feet above the ocean bed. Christmas
Island (see p. 194), built wholly beneath the ocean, is a coral-capped
volcanic peak, whose total height, as measured from the bottom of the
sea, is more than fifteen thousand feet. Deep-sea soundings have
revealed the presence of numerous peaks which fail to reach sea level
and which no doubt are submarine volcanoes. A number of volcanoes on
the land were submarine in their early stages, as, for example, the
vast pile of Etna, the celebrated Sicilian volcano, which rests on
stratified volcanic fragments containing marine shells now uplifted
from the sea.
Submarine outflows of lava and deposits of volcanic fragments become
covered with sediments during the long intervals between eruptions.
Such volcanic deposits are said to be _contemporaneous_, because they
are formed during the same period as the strata among which they are
imbedded. Contemporaneous lava sheets may be expected to bake the
surface of the stratum on which they rest, while the sediments
deposited upon them are unaltered by their heat. They are among the
most permanent records of volcanic action, far outlasting the greatest
volcanic mountains built in open air.
From upraised submarine volcanoes, such as Christmas Island, it is
learned that lava flows which are poured out upon the bottom of the
sea do not differ materially either in composition or texture from
those of the land.
Volcanic Products
Vast amounts of steam are, as we have seen, emitted from volcanoes,
and comparatively small quantities of other vapors, such as various
acid and sulphurous gases. The rocks erupted from volcanoes differ
widely in chemical composition and in texture.
[Illustration: Fig. 220. Cellular Lava]
=Acidic and basic lavas.= Two classes of volcanic rocks may be
distinguished,--those containing a large proportion of silica (silicic
acid, SiO_{2}) and therefore called _acidic_, and those containing less
silica and a larger proportion of the bases (lime, magnesia, soda,
etc.) and therefore called _basic_. The acidic lavas, of which
_rhyolite_ and _thrachyte_ are examples, are comparatively light in
color and weight, and are difficult to melt. The basic lavas, of which
_basalt_ is a type, are dark and heavy and melt at a lower
temperature.
=Scoria and pumice.= The texture of volcanic rocks depends in part on
the degree to which they were distended by the steam which permeated
them when in a molten state. They harden into compact rock where the
steam cannot expand. Where the steam is released from pressure, as on
the surface of a lava stream, it forms bubbles (steam blebs) of
various sizes, which give the hardened rock a cellular structure
(Fig. 220), In this way are formed the rough slags and clinkers called
_scoria_, which are found on the surface of flows and which are also
thrown out as clots of lava in explosive eruptions.
On the surface of the seething lava in the throat of the volcano there
gathers a rock foam, which, when hurled into the air, is cooled and
falls as _pumice_,--a spongy gray rock so light that it floats on
water.
[Illustration: Fig. 221. Amygdules in Lava]
=Amygdules.= The steam blebs of lava flows are often drawn out from a
spherical to an elliptical form resembling that of an almond, and
after the rock has cooled these cavities are gradually filled with
minerals deposited from solution by underground water. From their
shape such casts are called amygdules (Greek, _amygdalon_, an almond).
Amygdules are commonly composed of silica. Lavas contain both silica
and the alkalies, potash and soda, and after dissolving the alkalies,
percolating water is able to take silica also into solution. Most
_agates_ are banded amygdules in which the silica has been laid in
varicolored, concentric layers (Fig. 222).
[Illustration: Fig. 222. Polished Section of an Agate]
[Illustration: Fig. 223. Microsection showing the Beginnings of
Crystal Growth in Glassy Lava]
=Glassy and stony lavas.= Volcanic rocks differ in texture according
also to the rate at which they have solidified. When rapidly cooled,
as on the surface of a lava flow, molten rock chills to a glass,
because the minerals of which it is composed have not had time to
separate themselves from the fused mixture and form crystals. Under
slow cooling, as in the interior of the flow, it becomes a stony mass
composed of crystals set in a glassy paste. In thin slices of volcanic
glass one may see under the microscope the beginnings of crystal
growth in filaments and needles and feathery forms, which are the
rudiments of the crystals of various minerals.
Spherulites, which also mark the first changes of glassy lavas toward
a stony condition, are little balls within the rock, varying from
microscopic size to several inches in diameter, and made up of
radiating fibers.
Perlitic structure, common among glassy lavas, consists of microscopic
curving and interlacing cracks, due to contraction.
[Illustration: Fig. 224. Perlitic Structure and Spherulites,
_a_, _a_]
[Illustration: Fig. 225. Flow Lines in Lava]
=Flow lines= are exhibited by volcanic rocks both to the naked eye and
under the microscope. Steam blebs, together with crystals and their
embryonic forms, are left arranged in lines and streaks by the
currents of the flowing lava as it stiffened into rock.
[Illustration: Fig. 226. Porphyritic Structure]
=Porphyritic structure.= Rocks whose ground mass has scattered through
it large conspicuous crystals (Fig. 226) are said to be _porphyritic_,
and it is especially among volcanic rocks that this structure occurs.
The ground mass of porphyries either may be glassy or may consist in
part of a felt of minute crystals; in either case it represents the
consolidation of the rock after its outpouring upon the surface. On
the other hand, the large crystals of porphyry have slowly formed deep
below the ground at an earlier date.
=Columnar structure.= Just as wet starch contracts on drying to
prismatic forms, so lava often contracts on cooling to a mass of
close-set, prismatic, and commonly six-sided columns, which stand at
right angles to the cooling surface. The upper portion of a flow, on
rapid cooling from the surface exposed to the air, may contract to a
confused mass of small and irregular prisms; while the remainder forms
large and beautifully regular columns, which have grown upward by slow
cooling from beneath (Fig. 227).
Fragmental Materials
Rocks weighing many tons are often thrown from a volcano at the
beginning of an outburst by the breaking up of the solidified floor of
the crater; and during the progress of an eruption large blocks may be
torn from the throat of the volcano by the outrush of steam. But the
most important fragmental materials are those derived from the lava
itself. As lava rises in the pipe, the steam which permeates it is
released from pressure and explodes, hurling the lava into the air in
fragments of all sizes,--large pieces of scoria, _lapilli_ (fragments
the size of a pea or walnut), volcanic "sand" and volcanic "ashes."
The latter resemble in appearance the ashes of wood or coal, but they
are not in any sense, like them, a residue after combustion.
[Illustration: Fig. 227. Columnar Structure in Basaltic Lava,
Scotland]
Volcanic ashes are produced in several ways: lava rising in the
volcanic duct is exploded into fine dust by the steam which permeates
it; glassy lava, hurled into the air and cooled suddenly, is brought
into a state of high strain and tension, and, like Prince Rupert's
drops, flies to pieces at the least provocation. The clash of rising
and falling projectiles also produces some dust, a fair sample of
which may be made by grating together two pieces of pumice.
Beds of volcanic ash occur widely among recent deposits in the western
United States. In Nebraska ash beds are found in twenty counties, and
are often as white as powdered pumice. The beds grow thicker and
coarser toward the southwestern part of the state, where their
thickness sometimes reaches fifty feet. In what direction would you
look for the now extinct volcano whose explosive eruptions are thus
recorded?
=Tuff.= This is a convenient term designating any rock composed of
volcanic fragments. Coarse tuffs of angular fragments are called
_volcanic breccia_, and when the fragments have been rounded and sorted
by water the rock is termed a _volcanic conglomerate_. Even when
deposited in the open air, as on the slopes of a volcano, tuffs may be
rudely bedded and their fragments more or less rounded, and unless
marine shells or the remains of land plants and animals are found as
fossils in them, there is often considerable difficulty in telling
whether they were laid in water or in air. In either case they soon
become consolidated. Chemical deposits from percolating waters fill
the interstices, and the bed of loose fragments is cemented to hard
rock.
The materials of which tuffs are composed are easily recognized as
volcanic in their origin. The fragments are more or less cellular,
according to the degree to which they were distended with steam when
in a molten state, and even in the finest dust one may see the glass
or the crystals of lava from which it was derived. Tuffs often contain
_volcanic bombs_,--balls of lava which took shape while whirling in
the air, and solidified before falling to the ground.
[Illustration: Fig. 228. Volcanic Bombs, Cinder Cone, California]
[Illustration: Fig. 229. A Volcanic Cone, Arizona]
=Ancient volcanic rocks.= It is in these materials and structures
which we have described that volcanoes leave some of their most
enduring records. Even the volcanic rocks of the earliest geological
ages, uplifted after long burial beneath the sea and exposed to view
by deep erosion, are recognized and their history read despite the
many changes which they may have undergone. A sheet of ancient lava
may be distinguished by its composition from the sediments among which
it is imbedded. The direction of its flow lines may be noted. The
cellular and slaggy surface where the pasty lava was distended by
escaping steam is recognized by the amygdules which now fill the
ancient steam blebs. In a pile of successive sheets of lava each flow
may be distinguished and its thickness measured; for the surface of
each sheet is glassy and scoriaceous, while beneath its upper portions
the lava of each flow is more dense and stony. The length of time
which elapsed before a sheet was buried beneath the materials of
succeeding eruptions may be told by the amount of weathering which it
had undergone, the depth of ancient soil--now baked to solid
rock--upon it, and the erosion which it had suffered in the interval.
If the flow occurred from some submarine volcano, we may recognize the
fact by the sea-laid sediments which cover it, filling the cracks and
crevices of its upper surface and containing pieces of lava washed
from it in their basal layers.
Long-buried glassy lavas devitrify, or pass to a stony condition,
under the unceasing action of underground waters; but their flow lines
and perlitic and spherulitic structures remain to tell of their
original state.
Ancient tuffs are known by the fragmental character of their volcanic
material, even though they have been altered to firm rock. Some
remains of land animals and plants may be found imbedded to tell that
the beds were laid in open air; while the remains of marine organisms
would prove as surely that the tuffs were deposited in the sea.
In these ways ancient volcanoes have been recognized near Boston, in
southeastern Pennsylvania, about Lake Superior, and in other regions
of the United States.
The Life History of a Volcano
The invasion of a region by volcanic forces is attended by movements
of the crust heralded by earthquakes. A fissure or a pipe is opened
and the building of the cone or the spreading of wide lava sheets is
begun.
=Volcanic cones.= The shape of a volcanic cone depends chiefly on the
materials erupted. Cones made of fragments may have sides as steep as
the angle of repose, which in the case of coarse scoria is sometimes
as high as thirty or forty degrees. About the base of the mountain the
finer materials erupted are spread in more gentle slopes, and are also
washed forward by rains and streams. The normal profile is thus a
symmetric cone with a flaring base.
[Illustration: Fig. 230. Sarcoui, a Trachyte Dome, France]
Cones built of lava vary in form according to the liquidity of the
lava. Domes of gentle slope, as those of Hawaii, for example, are
formed of basalt, which flows to long distances before it congeals.
When superheated and emitted from many vents, this easily melted lava
builds great plateaus, such as that of Iceland. On the other hand,
lavas less fusible, or poured out at a lower temperature, stiffen when
they have flowed but a short distance, and accumulate in a steep cone.
Trachyte has been extruded in a state so viscid that it has formed
steep-sided domes like that of Sarcoui (Fig. 230).
Most volcanoes are built, like Vesuvius, both of lava flows and of
tuffs, and sections show that the structure of the cone consists of
outward-dipping, alternating layers of lava, scoria, and ashes.
[Illustration: Fig. 231. Section of Vesuvius
_V_, Vesuvius; _S_, Somma, a mountainous rampart half encircling
Vesuvius, and like it built of outward-dipping sheets of tuff and
lava; _a_, crystalline rocks; _b_, marine strata; _c_, tuffs
containing seashells. Which is the older mountain, Vesuvius or
Somma? Of what is Somma a remnant? Draw a diagram showing its
original outline. Suggest what processes may have brought it to its
present form. What record do you find of the earliest volcanic
activity? What do you infer as to the beginnings of the volcano?]
From time to time the cone is rent by the violence of explosions and
by the weight of the column of lava in the pipe. The fissures are
filled with lava and some discharge on the sides of the mountain,
building parasitic cones, while all form dikes, which strengthen the
pile with ribs of hard rock and make it more difficult to rend.
Great catastrophes are recorded in the shape of some volcanoes which
consist of a circular rim perhaps miles in diameter, inclosing a vast
crater or a caldera within which small cones may rise. We may infer
that at some time the top of the mountain has been blown off, or has
collapsed and been engulfed because some reservoir beneath had been
emptied by long-continued eruptions (Fig. 230).
The cone-building stage may be said to continue until eruptions of
lava and fragmental materials cease altogether. Sooner or later the
volcanic forces shift or die away, and no further eruptions add to the
pile or replace its losses by erosion during periods of repose. Gases
however are still emitted, and, as sulphur vapors are conspicuous
among them, such vents are called _solfataras_. Mount Hood, in Oregon,
is an example of a volcano sunk to this stage. From a steaming rift on
its side there rise sulphurous fumes which, half a mile down the wind,
will tarnish a silver coin.
[Illustration: Fig. 232. Crater Lake, Oregon
How wide and deep is the basin which holds the lake? The
mountain walls which enclose it are made of outward-dipping
sheets of lava. Draw a diagram restoring the volcano of which
they are the remnant. No volcanic fragments of the same nature
as the materials of which the volcano is built are found about
the region. What theory of the destruction of the cone does
this fact favor? _W´_, Wizard Island, is a cinder cone. When was
it built?]
=Geysers and hot springs.= The hot springs of volcanic regions are
among the last vestiges of volcanic heat. Periodically eruptive
boiling springs are termed geysers. In each of the geyser regions of
the earth--the Yellowstone National Park, Iceland, and New
Zealand--the ground water of the locality is supposed to be heated by
ancient lavas that, because of the poor conductivity of the rock,
still remain hot beneath the surface.
[Illustration: Fig. 233. Old Faithful Geyser in Eruption,
Yellowstone National Park]
=Old Faithful=, one of the many geysers of the Yellowstone National
Park, plays a fountain of boiling water a hundred feet in air; while
clouds of vapor from the escaping steam ascend to several times that
height. The eruptions take place at intervals of from seventy to
ninety minutes. In repose the geyser is a quiet pool, occupying a
craterlike depression in a conical mound some twelve feet high. The
conduit of the spring is too irregular to be sounded. The mound is
composed of porous silica deposited by the waters of the geyser.
Geysers erupt at intervals instead of continuously boiling, because
their long, narrow, and often tortuous conduits do not permit a free
circulation of the water. After an eruption the tube is refilled and
the water again gradually becomes heated. Deep in the tube where it is
in contact with hot lavas the water sooner or later reaches the
boiling point, and bursting into steam shoots the water above it high
in air.
[Illustration: Fig. 234. Terrace and Cones of Siliceous Sinter
deposited by Geysers, Yellowstone National Park]
=Carbonated springs.= After all the other signs of life have gone, the
ancient volcano may emit carbon dioxide as its dying breath. The
springs of the region may long be charged with carbon dioxide, or
carbonated, and where they rise through limestone may be expected to
deposit large quantities of travertine. We should remember, however,
that many carbonated springs, and many hot springs, are wholly
independent of volcanoes.
[Illustration: Fig. 235. Mount Shasta, California]
[Illustration: Fig. 236. Mount Hood, Oregon]
=The destruction of the cone.= As soon as the volcanic cone ceases to
grow by eruptions the agents of erosion begin to wear it down, and the
length of time that has elapsed since the period of active growth may
be roughly measured by the degree to which the cone has been
dissected. We infer that Mount Shasta, whose conical shape is still
preserved despite the gullies one thousand feet deep which trench its
sides (Fig. 235), is younger than Mount Hood, which erosive agencies
have carved to a pyramidal form (Fig. 236). The pile of materials
accumulated about a volcanic vent, no matter how vast in bulk, is at
last swept entirely away. The cone of the volcano, active or extinct,
is not old as the earth counts time; volcanoes are short-lived
geological phenomena.
[Illustration: Fig. 237. Crandall Volcano]
=Crandall volcano.= This name is given to a dissected ancient volcano
in the Yellowstone National Park, which once, it is estimated, reared
its head thousands of feet above the surrounding country and greatly
exceeded in bulk either Mount Shasta or Mount Etna. Not a line of the
original mountain remains; all has been swept away by erosion except
some four thousand feet of the base of the pile. This basal wreck now
appears as a rugged region about thirty miles in diameter, trenched by
deep valleys and cut into sharp peaks and precipitous ridges. In the
center of the area is found the nucleus (_N_, Fig. 237),--a mass of
coarsely crystalline rock that congealed deep in the old volcanic
pipe. From it there radiate in all directions, like the spokes of a
wheel, long dikes whose rock grows rapidly finer of grain as it leaves
the vicinity of the once heated core. The remainder of the base of the
ancient mountain is made of rudely bedded tuffs and volcanic breccia,
with occasional flows of lava, some of the fragments of the breccia
measuring as much as twenty feet in diameter. On the sides of canyons
the breccia is carved by rain erosion to fantastic pinnacles. At
different levels in the midst of these beds of tuff and lava are many
old forest grounds. The stumps and trunks of the trees, now turned to
stone, still in many cases stand upright where once they grew on the
slopes of the mountain as it was building (Fig. 238). The great size
and age of some of these trees indicate, the lapse of time between the
eruption whose lavas or tuffs weathered to the soil on which they grew
and the subsequent eruption which buried them beneath showers of
stones and ashes.
Near the edge of the area lies Death Gulch, in which carbon dioxide is
given off in such quantities that in quiet weather it accumulates in a
heavy layer along the ground and suffocates the animals which may
enter it.
[Illustration: Fig. 238. Fossil Tree Trunks, Yellowstone National Park]
CHAPTER XII
UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN
It is because long-continued erosion lays bare the innermost anatomy
of an extinct volcano, and even sweeps away the entire pile with much
of the underlying strata, thus leaving the very roots of the volcano
open to view, that we are able to study underground volcanic
structures. With these we include, for convenience, intrusions of
molten rock which have been driven upward into the crust, but which
may not have succeeded in breaking way to the surface and establishing
a volcano. All these structures are built of rock forced when in a
fluid or pasty state into some cavity which it has found or made, and
we may classify them therefore, according to the shape of the molds in
which the molten rock has congealed, as (1) dikes, (2) volcanic necks,
(3) intrusive sheets, and (4) intrusive masses.
=Dikes.= The sheet of once molten rock with which a fissure has been
filled is known as a dike. Dikes are formed when volcanic cones are
rent by explosions or by the weight of the lava column in the duct,
and on the dissection of the pile they appear as radiating vertical
ribs cutting across the layers of lava and tuff of which the cone is
built. In regions undergoing deformation rocks lying deep below the
ground are often broken and the fissures are filled with molten rock
from beneath, which finds no outlet to the surface. Such dikes are
common in areas of the most ancient rocks, which have been brought to
light by long erosion.
In exceptional cases dikes may reach the length of fifty or one
hundred miles. They vary in width from a fraction of a foot to even as
much as three hundred feet.
[Illustration: Fig. 239. Dikes, Spanish Peaks, Colorado]
Dikes are commonly more fine of grain on the sides than in the center,
and may have a glassy and crackled surface where they meet the
inclosing rock. Can you account for this on any principle which you
have learned?
[Illustration: Fig. 240. A Dissected Volcanic Cone
_N_, volcanic neck; _l_, _l_, lava-topped table mountains;
_t_, _t_, beds of tuff; _d_, _d_, dikes; dotted lines indicate
the initial profile]
=Volcanic necks.= The pipe of a volcano rises from far below the base
of the cone,--from the deep reservoir from which its eruptions are
supplied. When the volcano has become extinct this great tube remains
filled with hardened lava. It forms a cylindrical core of solid rock,
except for some distance below the ancient crater, where it may
contain a mass of fragments which had fallen back into the chimney
after being hurled into the air.
[Illustration: Fig. 241. Mount Johnson, a Volcanic Neck near
Montreal]
As the mountain is worn down, this central column known as the
_volcanic neck_ is left standing as a conical hill (Fig. 240). Even
when every other trace of the volcano has been swept away, erosion
will not have passed below this great stalk on which the volcano was
borne as a fiery flower whose site it remains to mark. In volcanic
regions of deep denudation volcanic necks rise solitary and abrupt
from the surrounding country as dome-shaped hills. They are marked
features in the landscape in parts of Scotland and in the St. Lawrence
valley about Montreal (Fig. 241).
[Illustration: Fig. 242. The Palisades of the Hudson, New Jersey]
=Intrusive sheets.= Sheets of igneous rocks are sometimes found
interleaved with sedimentary strata, especially in regions where the
rocks have been deformed and have suffered from volcanic action. In
some instances such a sheet is seen to be _contemporaneous_ (p. 248).
In other instances the sheet must be _intrusive_. The overlying
stratum, as well as that beneath, has been affected by the heat of the
once molten rock. We infer that the igneous rock when in a molten
state was forced between the strata, much as a card may be pushed
between the leaves of a closed book. The liquid wedged its way between
the layers, lifting those above to make room for itself. The source of
the intrusive sheet may often be traced to some dike (known therefore
as the _feeding dike_), or to some mass of igneous rock.
Intrusive sheets may extend a score and more of miles, and, like the
longest surface flows, the most extensive sheets consist of the more
fusible and fluid lavas,--those of the basic class of which basalt is
an example. Intrusive sheets are usually harder than the strata in
which they lie and are therefore often left in relief after long
denudation of the region (Fig. 315).
[Illustration: Fig. 243. Diagram of the Palisades of the Hudson
_i_, intrusive sheet; _s_, sandstone; _d_, feeding dike;
_HR_, Hudson River]
On the west bank of the Hudson there extends from New York Bay north
for thirty miles a bold cliff several hundred feet high,--the
_Palisades of the Hudson_. It is the outcropping edge of a sheet of
ancient igneous rock, which rests on stratified sandstones and is
overlain by strata of the same series. Sandstones and lava sheet
together dip gently to the west and the latter disappears from view
two miles back from the river.
It is an interesting question whether the Palisades sheet is
_contemporaneous_ or _intrusive_. Was it outpoured on the sandstones
beneath it when they formed the floor of the sea, and covered
forthwith by the sediments of the strata above, or was it intruded
among these beds at a later date?
[Illustration: Fig. 244. Section of Electric Peak. E. and Gray
Peak, G, Yellowstone National Park
Intrusive sheets and masses of igneous rock are drawn in black]
The latter is the case: for the overlying stratum is intensely baked
along the zone of contact. At the west edge of the sheet is found the
dike in which the lava rose to force its way far and wide between the
strata.
_Electric Peak_, one of the prominent mountains of the Yellowstone
National Park, is carved out of a mass of strata into which many
sheets of molten rock have been intruded. The western summit consists
of such a sheet several hundred feet thick. Studying the section of
Figure 244, what inference do you draw as to the source of these
intrusive sheets?
[Illustration: Fig. 245. Stone Mountain, Georgia, a Granite Boss]
Intrusive Masses
=Bosses.= This name is generally applied to huge irregular masses of
coarsely crystalline igneous rock lying in the midst of other
formations. Bosses vary greatly in size and may reach scores of miles
in extent. Seldom are there any evidences found that bosses ever had
connection with the surface. On the other hand, it is often proved
that they have been driven, or have melted their way, upward into the
formations in which they lie; for they give off dikes and intrusive
sheets, and have profoundly altered the rocks about them by their
heat.
[Illustration: Fig. 246. Map of Granite Bosses near Baltimore
(areas horizontally Lined)
The texture of the rock of bosses proves that consolidation proceeded
slowly and at great depths, and it is only because of vast denudation
that they are now exposed to view. Bosses are commonly harder than the
rocks about them, and stand up, therefore, as rounded hills and
mountainous ridges long after the surrounding country has worn to a
low plain (Fig. 245).
Figure 246 exhibits a few small bosses of granite near Baltimore as
examples of numerous areas of igneous rock within the Piedmont Belt
which represent bodies of molten rock which solidified deep below the
surface.
The _Spanish Peaks_ of southeastern Colorado were formed by the
upthrust of immense masses of igneous rock, bulging and breaking the
overlying strata. On one side of the mountains the throw of the fault
is nearly a mile, and fragments of deep-lying beds were dragged upward
by the rising masses. The adjacent rocks were altered by heat to a
distance of several thousand feet. No evidence appears that the molten
rock ever reached the surface, and if volcanic eruptions ever took
place either in lava flows or fragmental materials, all traces of them
have been effaced. The rock of the intrusive masses is coarsely
crystalline, and no doubt solidified slowly under the pressure of vast
thicknesses of overlying rock, now mostly removed by erosion.
A magnificent system of dikes radiates from the Peaks to a distance of
fifteen miles, some now being left by long erosion as walls a hundred
feet in height (Fig. 239). Intrusive sheets fed by the dikes penetrate
the surrounding strata, and their edges are cut by canyons as much as
twenty-five miles from the mountain. In these strata are valuable beds
of lignite, an imperfect coal, which the heat of dikes and sheets has
changed to coke.
[Illustration: Fig. 247. Section of a Laccolith]
=Laccoliths.= The laccolith (Greek laccos, cistern; lithos, stone) is
a variety of intrusive masses in which molten rock has spread between
the strata, and, lifting the strata above it to a dome-shaped form,
has collected beneath them in a lens-shaped body with a flat base.
The _Henry Mountains_, a small group of detached peaks in southern
Utah, rise from a plateau of horizontal rocks. Some of the peaks are
carved wholly in separate domelike uplifts of the strata of the
plateau. In others, as Mount Hillers, the largest of the group, there
is exposed on the summit a core of igneous rock from which the
sedimentary rocks of the flanks dip steeply outward in all directions.
In still others erosion has stripped off the covering strata and has
laid bare the core to its base; and its shape is here seen to be that
of a plano-convex lens or a baker's bun, its flat base resting on the
undisturbed bedded rocks beneath. The structure of Mount Hillers is
shown in Figure 248. The nucleus of igneous rock is four miles in
diameter and more than a mile in depth.
[Illustration: Fig. 248. Section of Mount Hillers]
=Regional intrusions.= These vast bodies of igneous rock, which may
reach hundreds of miles in diameter, differ little from bosses except
in their immense bulk. Like bosses, regional intrusions give off dikes
and sheets and greatly change the rocks about them by their heat. They
are now exposed to view only because of the profound denudation which
has removed the upheaved dome of rocks beneath which they slowly
cooled. Such intrusions are accompanied--whether as cause or as
effect is still hardly known--by deformations, and their masses of
igneous rock are thus found as the core of many great mountain ranges.
The granitic masses of which the Bitter Root Mountains and the Sierra
Nevadas have been largely carved are each more than three hundred
miles in length. Immense regional intrusions, the cores of once lofty
mountain ranges, are found upon the Laurentian peneplain.
=Physiographic effects of intrusive masses.= We have already seen
examples of the topographic effects of intrusive masses in Mount
Hillers, the Spanish Peaks, and in the great mountain ranges mentioned
in the paragraph on regional intrusions, although in the latter
instances these effects are entangled with the effects of other
processes. Masses of igneous rock cannot be intruded within the crust
without an accompanying deformation on a scale corresponding to the
bulk of the intruded mass. The overlying strata are arched into hills
or mountains, or, if the molten material is of great extent, the
strata may conceivably be floated upward to the height of a plateau.
We may suppose that the transference of molten matter from one region
to another may be among the causes of slow subsidences and elevations.
Intrusions give rise to fissures, dikes, and intrusive sheets, and
these dislocations cannot fail to produce earthquakes. Where intrusive
masses open communication with the surface, volcanoes are established
or fissure eruptions occur such as those of Iceland.
The Intrusive Rocks
The igneous rocks are divided into two general classes,--the
_volcanic_ or _eruptive_ rocks, which have been outpoured in open air
or on the floor of the sea, and the _intrusive_ rocks, which have been
intruded within the rocks of the crust and have solidified below the
surface. The two classes are alike in chemical composition and may be
divided into acidic and basic groups. In texture the intrusive rocks
differ from the volcanic rocks because of the different conditions
under which they have solidified. They cooled far more slowly beneath
the cover of the rocks into which they were pressed than is permitted
to lava flows in open air. Their constituent minerals had ample
opportunity to sort themselves and crystallize from the fluid mixture,
and none of that mixture was left to congeal as a glassy paste.
They consolidated also under pressure. They are never scoriaceous, for
the steam with which they were charged was not allowed to expand and
distend them with steam blebs. In the rocks of the larger intrusive
masses one may see with a powerful microscope exceedingly minute
cavities, to be counted by many millions to the cubic inch, in which
the gaseous water which the mass contained was held imprisoned under
the immense pressure of the overlying rocks.
Naturally these characteristics are best developed in the intrusives
which cooled most slowly, i.e. in the deepest-seated and largest
masses; while in those which cooled more rapidly, as in dikes and
sheets, we find gradations approaching the texture of surface flows.
=Varieties of the intrusive rocks.= We will now describe a few of the
varieties of rocks of deep-seated intrusions. All are even grained,
consisting of a mass of crystalline grains formed during one
continuous stage of solidification, and no porphyritic crystals appear
as in lavas.
_Granite_, as we have learned already, is composed of three
minerals,--quartz, feldspar, and mica. According to the color of the
feldspar the rock may be red, or pink, or gray. Hornblende--a black or
dark green mineral, an iron-magnesian silicate, about as hard as
feldspar--is sometimes found as a fourth constituent, and the rock is
then known as _hornblendic granite_. Granite is an acidic rock
corresponding to rhyolite in chemical composition. We may believe that
the same molten mass which supplies this acidic lava in surface flows
solidifies as granite deep below ground in the volcanic reservoir.
_Syenite_, composed of feldspar and mica, has consolidated from a less
siliceous mixture than has granite.
_Diorite_, still less siliceous, is composed of hornblende and
feldspar,--the latter mineral being of different variety from the
feldspar of granite and syenite.
_Gabbro_, a typical basic rock, corresponds to basalt in chemical
composition. It is a dark, heavy, coarsely crystalline aggregate of
feldspar and _augite_ (a dark mineral allied to hornblende). It often
contains _magnetite_ (the magnetic black oxide of iron) and _olivine_
(a greenish magnesian silicate).
In the northern states all these types, and many others also of the
vast number of varieties of intrusive rocks, can be found among the
rocks of the drift brought from the areas of igneous rock in Canada
and the states of our northern border.
[Illustration: Fig. 249. Ground Plan of Dikes in Granite.
(Scale 80 feet to the inch)
What is the relative age of the dikes _aa_, _bb_, and _cc_?]
[Illustration: Fig. 250. _A_ and _B_. Mountains of coarsely
Crystalline Igneous _i_, surrounded by Sedimentary Strata _s_
and _s´_
Copy each diagram and complete it, so as to show whether the
mass of igneous rock is a volcanic neck, a boss, or a laccolith]
=Summary.= The records of geology prove that since the earliest of
their annals tremendous forces have been active in the earth. In all
the past, under pressures inconceivably great, molten rock has been
driven upward into the rocks of the crust. It has squeezed into
fissures forming dikes; it has burrowed among the strata as intrusive
sheets; it has melted the rocks away or lifted the overlying strata,
filling the chambers which it has made with intrusive masses. During
all geological ages molten rock has found way to the surface, and
volcanoes have darkened the sky with clouds of ashes and poured
streams of glowing lava down their sides. The older strata,--the
strata which have been most deeply buried,--and especially those which
have suffered most from folding and from fracture, show the largest
amount of igneous intrusions. The molten rock which has been driven
from the earth's interior to within the crust or to the surface during
geologic time must be reckoned in millions of cubic miles.
[Illustration: Fig. 251.
1, limestone; 2, tuff; 3, 5, 7, shale with marine shells; 4, 6,
lava, dotted portions scoriaceous. Give the history recorded in
this section]
[Illustration: Fig. 252.
_a_, sedimentary strata with intrusive sheets; _b_, sedimentary
strata; _c_, lava flow; _d_, dike. Give the succession of
events recorded in this section]
[Illustration: Fig. 253.
Which of the lava sheets of this section are contemporaneous
anti which intrusive,--_A_, whose upper surface is overlain
with a conglomerate of rolled lava pebbles; _B_, the cracks and
seams of whose upper surface are filled with the material of
the overlying sandstone; _C_, which breaks across the strata in
which it is imbedded; _D_, which includes fragments of both the
underlying and overlying strata and penetrates their crevices
and seams?]
[Illustration: Fig. 254. Mato Tepee, Wyoming
This magnificent tower of igneous rock three hundred feet in
height has been called by some a volcanic neck. Is the
direction of the columns that which would obtain in the
cylindrical pipe of a volcano? The tower is probably the
remnant of a small laccolith, an outlying member of a group of
laccoliths situated not far distant]
The Interior Condition of the Earth and Causes of Vulcanism and
Deformation
The problems of volcanoes and of deformation are so closely connected
with that of the earth's interior that we may consider them together.
Few of these problems are solved, and we may only state some known
facts and the probable conclusions which may be drawn as inferences
from them.
=The interior of the earth is hot.= Volcanoes prove that in many parts
of the earth there exist within reach of the surface regions of such
intense heat that the rock is in a molten condition. Deep wells and
mines show everywhere an increase in temperature below the surface
shell affected by the heat of summer and the cold of winter,--a shell
in temperate latitudes sixty or seventy feet thick. Thus in a boring
more than a mile deep at Schladebach, Germany, the earth grows warmer
at the rate of 1° F. for every sixty-seven feet as we descend. Taking
the average rate of increase at one degree for every sixty feet of
descent, and assuming that this rate, observed at the moderate
distances open to observation, continues to at least thirty-five
miles, the temperature at that depth must be more than three thousand
degrees,--a temperature at which all ordinary rocks would melt at the
earth's surface. The rate of increase in temperature probably lessens
as we go downward, and it may not be appreciable below a few hundred
miles. But there is no reason to doubt that _the interior of the earth
is intensely hot_. Below a depth of one or two score miles we may
imagine the rocks everywhere glowing with heat.
Although the heat of the interior is great enough to melt all rocks at
atmospheric pressure, it does not follow that the interior is fluid.
Pressure raises the fusing point of rocks, and the weight of the crust
may keep the interior in what may be called a solid state, although so
hot as to be a liquid or a gas were the pressure to be removed.
=The interior of the earth is dense and heavy.= The earth behaves as a
globe more rigid than glass under the strains to which it is subjected
by the attractions of the sun and moon and other heavenly bodies. The
jar of world-shaking earthquakes passes through the earth's interior
with nearly twice the velocity with which it would traverse solid steel,
and since the speed of elastic waves depends on the density and
elasticity of the medium, it follows that the globe is as a whole more
dense and rigid than steel. _The interior of the earth is extremely
dense and rigid._
The common rocks of the crust are about two and a half times heavier
than water, while the earth as a whole weighs five and six-tenths
times as much as a globe of water of the same size. _The interior is
therefore much more heavy than the crust._ This may be caused in part
by compression of the interior under the enormous weight of the crust,
and in part also by an assortment of material, the heavier substances,
such as the heavy metals, having gravitated towards the center.
Between the crust, which is solid because it is cool, and the
interior, which is hot enough to melt were it not for the pressure
which keeps it dense and rigid, there may be an intermediate zone in
which heat and pressure are so evenly balanced that here rock
liquefies whenever and wherever the pressure upon it may be relieved
by movements of the crust. It is perhaps from such a subcrustal layer
that the lava of volcanoes is supplied.
=The causes of volcanic action.= It is now generally believed that the
_heat_ of volcanoes is that of the earth's interior. Other causes,
such as friction and crushing in the making of mountains and the
chemical reactions between oxidizing agents of the crust and the
unoxidized interior, have been suggested, but to most geologists they
seem inadequate.
There is much difference of opinion as to the _force_ which causes
molten rock to rise to the surface in the ducts of volcanoes. Steam is
so evidently concerned in explosive eruptions that many believe that
lava is driven upward by the expansive force of the steam with which
it is charged, much as a viscid liquid rises and boils over in a test
tube or kettle.
But in quiet eruptions, and still more in the irruption of intrusive
sheets and masses, there is little if any evidence that steam is the
driving force. It is therefore believed by many geologists that it is
_pressure due to crustal movements and internal stresses_ which
squeezes molten rock from below into fissures and ducts in the crust.
It is held by some that where considerable water is supplied to the
rising column of lava, as from the ground water of the surrounding
region, and where the lava is viscid so that steam does not readily
escape, the eruption is of the explosive type; when these conditions
do not obtain, the lava outwells quietly, as in the Hawaiian
volcanoes. It is held by others not only that volcanoes are due to the
outflow of the earth's deep-seated heat, but also that the steam and
other emitted gases are for the most part native to the earth's
interior and never have had place in the circulation of atmospheric
and ground waters.
=Volcanic action and deformation.= Volcanoes do not occur on wide
plains or among ancient mountains. On the other hand, where movements
of the earth's crust are in progress in the uplift of high plateaus,
and still more in mountain making, molten rock may reach the surface,
or may be driven upward toward it forming great intrusive masses. Thus
extensive lava flows accompanied the upheaval of the block mountains
of western North America and the uplift of the Colorado plateau. A
line of recent volcanoes may be traced along the system of rift
valleys which extends from the Jordan and Dead Sea through eastern
Africa to Lake Nyassa. The volcanoes of the Andes show how conspicuous
volcanic action may be in young rising ranges. Folded mountains often
show a core of igneous rock, which by long erosion has come to form
the axis and the highest peaks of the range, as if the molten rock had
been squeezed up under the rising upfolds. As we decipher the records
of the rocks in historical geology we shall see more fully how, in all
the past, volcanic action has characterized the periods of great
crustal movements, and how it has been absent when and where the
earth's crust has remained comparatively at rest.
=The causes of deformation.= As the earth's interior, or nucleus, is
highly heated it must be constantly though slowly losing its heat by
conduction through the crust and into space; and since the nucleus is
cooling it must also be contracting. The nucleus has contracted also
because of the extrusion of molten matter, the loss of constituent
gases given off in volcanic eruptions, and (still more important) the
compression and consolidation of its material under gravity. As the
nucleus contracts, it tends to draw away from the cooled and solid
crust, and the latter settles, adapting itself to the shrinking
nucleus much as the skin of a withering apple wrinkles down upon the
shrunken fruit. The unsupported weight of the spherical crust develops
enormous tangential pressures, similar to the stresses of an arch or
dome, and when these lateral thrusts accumulate beyond the power of
resistance the solid rock is warped and folded and broken.
Since the planet attained its present mass it has thus been lessening
in volume. Notwithstanding local and relative upheavals the earth's
surface on the whole has drawn nearer and nearer to the center. The
portions of the lithosphere which have been carried down the farthest
have received the waters of the oceans, while those portions which
have been carried down the least have emerged as continents.
Although it serves our convenience to refer the movements of the crust
to the sea level as datum plane, it is understood that this level is
by no means fixed. Changes in the ocean basins increase or reduce
their capacity and thus lower or raise the level of the sea. But since
these basins are connected, the effect of any change upon the water
level is so distributed that it is far less noticeable than a
corresponding change would be upon the land.
CHAPTER XIII
METAMORPHISM AND MINERAL VEINS
Under the action of internal agencies rocks of all kinds may be
rendered harder, more firmly cemented, and more crystalline. These
processes are known as _metamorphism_, and the rocks affected, whether
originally sedimentary or igneous, are called _metamorphic rocks_. We
may contrast with metamorphism the action of external agencies in
weathering, which render rocks less coherent by dissolving their
soluble parts and breaking down their crystalline grains.
=Contact metamorphism.= Rocks beneath a lava flow or in contact with
igneous intrusions are found to be metamorphosed to various degrees by
the heat of the cooling mass. The adjacent strata may be changed only
in color, hardness, and texture. Thus, next to a dike, bituminous coal
may be baked to coke or anthracite, and chalk and limestone to
crystalline marble. Sandstone may be converted into quartzite, and
shale into _argillite_, a compact, massive clay rock. New minerals may
also be developed. In sedimentary rocks there may be produced crystals
of mica and of _garnet_ (a mineral as hard as quartz, commonly
occurring in red, twelve-sided crystals). Where the changes are most
profound, rocks may be wholly made over in structure and mineral
composition.
In contact metamorphism, thin sheets of molten rock produce less
effect than thicker ones. The strongest heat effects are naturally
caused by bosses and regional intrusions, and the zone of change about
them may be several miles in width. In these changes heated waters and
vapors from the masses of igneous rocks undoubtedly play a very
important part.
Which will be more strongly altered, the rocks about a closed dike in
which lava began to cool as soon as it filled the fissure, or the
rocks about a dike which opened on the surface and through which the
molten rock flowed for some time?
Taking into consideration the part played by heated waters, which will
produce the most far-reaching metamorphism, dikes which cut across the
bedding planes or intrusive sheets which are thrust between the
strata?
=Regional metamorphism.= Metamorphic rocks occur widespread in many
regions, often hundreds of square miles in area, where such extensive
changes cannot be accounted for by igneous intrusions. Such are the
dissected cores of lofty mountains, as the Alps, and the worn-down
bases of ancient ranges, as in New England, large areas in the
Piedmont Belt, and the Laurentian peneplain.
In these regions the rocks have yielded to immense pressure. They have
been folded, crumpled, and mashed, and even their minute grains, as
one may see with a microscope, have often been puckered, broken, and
crushed to powder. It is to these mechanical movements and strains
which the rocks have suffered in every part that we may attribute
their metamorphism, and the degree to which they have been changed is
in direct proportion to the degree to which they have been deformed
and mashed.
Other factors, however, have played important parts. Rock crushing
develops heat, and allows a freer circulation of heated waters and
vapors. Thus chemical reactions are greatly quickened; minerals are
dissolved and redeposited in new positions, or their chemical
constituents may recombine in new minerals, entirely changing the
nature of the rock, as when, for example, feldspar recrystallizes as
quartz and mica.
Early stages of metamorphism are seen in _slate_. Pressure has
hardened the marine muds, the arkose (p. 186), or the volcanic ash
from which slates are derived, and has caused them to cleave by the
rearrangement of their particles.
Under somewhat greater pressure, slate becomes _phyllite_, a clay
slate whose cleavage surfaces are lustrous with flat-lying mica
flakes. The same pressure which has caused the rock to cleave has set
free some of its mineral constituents along the cleavage planes to
crystallize there as mica.
[Illustration: Fig. 255. A Foliated Rock]
=Foliation.= Under still stronger pressure the whole structure of the
rock is altered. The minerals of which it is composed, and the new
minerals which develop by heat and pressure, arrange themselves along
planes of cleavage or of shear in rudely parallel leaves, or _folia_.
Of this structure, called _foliation_, we may distinguish two
types,--a coarser feldspathic type, and a fine type in which other
minerals than feldspar predominate.
_Gneiss_ is the general name under which are comprised coarsely
foliated rocks banded with irregular layers of feldspar and other
minerals. The gneisses appear to be due in many cases to the crushing
and shearing of deep-seated igneous rocks, such as granite and gabbro.
_The crystalline schists_, representing the finer types of foliation,
consist of thin, parallel, crystalline leaves, which are often
remarkably crumpled. These folia can be distinguished from the laminae
of sedimentary rocks by their lenticular form and lack of continuity,
and especially by the fact that they consist of platy, crystalline
grains, and not of particles rounded by wear.
_Mica schist_, the most common of schists, and in fact of all
metamorphic rocks, is composed of mica and quartz in alternating wavy
folia. All gradations between it and phyllite may be traced, and in
many cases we may prove it due to the metamorphism of slates and
shales. It is widespread in New England and along the eastern side of
the Appalachians. _Talc schist_ consists of quartz and _talc_, a
light-colored magnesian mineral of greasy feel, and so soft that it
can be scratched with the thumb nail.
_Hornblende schist_, resulting in many cases from the foliation of
basic igneous rocks, is made of folia of hornblende alternating with
bands of quartz and feldspar. Hornblende schist is common over large
areas in the Lake Superior region.
_Quartz schist_ is produced from quartzite by the development of fine
folia of mica along planes of shear. All gradations may be found
between it and unfoliated quartzite on the one hand and mica schist on
the other.
Under the resistless pressure of crustal movements almost any rocks,
sandstones, shales, lavas of all kinds, granites, diorites, and
gabbros may be metamorphosed into schists by crushing and shearing.
Limestones, however, are metamorphosed by pressure into _marble_, the
grains of carbonate of lime recrystallizing freely to interlocking
crystals of calcite.
These few examples must suffice of the great class of metamorphic
rocks. As we have seen, they owe their origin to the alteration of
both of the other classes of rocks--the sedimentary and the
igneous--by heat and pressure, assisted usually by the presence of
water. The fact of change is seen in their hardness arid cementation,
their more or less complete recrystallization, and their foliation;
but the change is often so complete that no trace of their original
structure and mineral composition remains to tell whether the rocks
from which they were derived were sedimentary or igneous, or to what
variety of either of these classes they belonged.
[Illustration: Fig. 256. Contorted Gneiss, the Ottawa River,
Canada]
[Illustration: Fig. 257. Quartz Veins in Slate]
In many cases, however, the early history of a metamorphic rock can be
deciphered. Fossils not wholly obliterated may prove it originally
water-laid. Schists may contain rolled-out pebbles, showing their
derivation from a conglomerate. Dikes of igneous rocks may be followed
into a region where they have been foliated by pressure. The most
thoroughly metamorphosed rocks may sometimes be traced out into
unaltered sedimentary or igneous rocks, or among them may be found
patches of little change where their history maybe read.
Metamorphism is most common among rocks of the earlier geological
ages, and most rare among rocks of recent formation. No doubt it is
now in progress where deep-buried sediments are invaded by heat either
from intrusive igneous masses or from the earth's interior, or are
suffering slow deformation under the thrust of mountain-making forces.
Suggest how rocks now in process of metamorphism may sometimes be
exposed to view. Why do metamorphic rocks appear on the surface
to-day?
Mineral Veins
In regions of folded and broken rocks fissures are frequently found to
be filled with sheets of crystalline minerals deposited from solution
by underground water, and fissures thus filled are known as _mineral
veins_. Much of the importance of mineral veins is due to the fact
that they are often metalliferous, carrying valuable native metals and
metallic ores disseminated in fine particles, in strings, and
sometimes in large masses in the midst of the valueless nonmetallic
minerals which make up what is known as the _vein stone_.
The most common vein stones are _quartz_ and _calcite_. _fluorite_
(calcium fluoride), a mineral harder than calcite and crystallizing in
cubes of various colors, and _barite_ (barium sulphate), a heavy white
mineral, are abundant in many veins.
[Illustration: Fig. 258. Placer Deposits in California
_g_, gold-bearing gravels in present river beds; _g´_, ancient
gold-bearing river gravels; _a_, _a_, lava flows capping table
mountains; _s_, slate. Draw a diagram showing by dotted lines
conditions before the lava flows occurred. What changes have
since taken place?]
The gold-bearing quartz veins of California traverse the metamorphic
slates of the Sierra Nevada Mountains. Below the zone of solution (p.
45) these veins consist of a vein stone of quartz mingled with pyrite
(p. 13), the latter containing threads and grains of native gold. But
to the depth of about fifty feet from the surface the pyrite of the
vein has been dissolved, leaving a rusty, cellular quartz with grains
of the insoluble gold scattered through it.
The _placer deposits_ of California and other regions are gold-bearing
deposits of gravel and sand in river beds. The heavy gold is apt to be
found mostly near or upon the solid rock, and its grains, like those
of the sand, are always rounded. How the gold came in the placers we
may leave the pupil to suggest.
Copper is found in a number of ores, and also in the native metal.
Below the zone of surface changes the ore of a copper vein is often a
double sulphide of iron and copper called _chalcopyrite_, a mineral
softer than pyrite--it can easily be scratched with a knife--and
deeper yellow in color. For several score of feet below the ground the
vein may consist of rusty quartz from which the metallic ores have
been dissolved; but at the base of the zone of solution we may find
exceedingly rich deposits of copper ores,--copper sulphides, red and
black copper oxides, and green and blue copper carbonates, which have
clearly been brought down in solution from the leached upper portion
of the vein.
=Origin of mineral veins.= Both vein stones and ores have been
deposited slowly from solution in water, much as crystals of salt are
deposited on the sides of a jar of saturated brine. In our study of
underground water we learned that it is everywhere circulating through
the permeable rocks of the crust, descending to profound depths under
the action of gravity and again driven to the surface by hydrostatic
pressure. Now fissures, wherever they occur, form the trunk channels
of the underground circulation. Water descends from the surface along
these rifts; it moves laterally from either side to the fissure plane,
just as ground water seeps through the surrounding rocks from every
direction to a well; and it ascends through these natural water ways
as in an artesian well, whenever they intersect an aquifer in which
water is under hydrostatic pressure.
The waters which deposit vein stones and ores are commonly hot, and in
many cases they have derived their heat from intrusions of igneous
rock still uncooled within the crust. The solvent power of the water
is thus greatly increased, and it takes up into solution various
substances from the igneous and sedimentary rocks which it traverses.
For various reasons these substances stances are deposited in the vein
as ores and vein stones. On rising through the fissure the water cools
and loses pressure, and its capacity to hold minerals in solution is
therefore lessened. Besides, as different currents meet in the
fissure, some ascending, some descending, and some coming in from the
sides, the chemical reaction of these various weak solutions upon one
another and upon the walls of the vein precipitates the minerals of
vein stuffs and ores.
As an illustration of the method of vein deposits we may cite the case
of a wooden box pipe used in the Comstock mines, Nevada, to carry the
hot water of the mine from one level to another, which in ten years
was lined with calcium carbonate more than half an inch thick.
The Steamboat Springs, Nevada, furnish examples of mineral veins in
process of formation. The steaming water rises through fissures in
volcanic rocks and is now depositing in the rifts a vein stone of
quartz, with metallic ores of iron, mercury, lead, and other metals.
=Reconcentration.= Near the base of the zone of solution veins are
often stored with exceptionally large and valuable ore deposits. This
local enrichment of the vein is due to the reconcentration of its
metalliferous ores. As the surface of the land is slowly lowered by
weathering and running water, the zone of solution is lowered at an
equal rate and encroaches constantly on the zone of cementation. The
minerals of veins are therefore constantly being dissolved along their
upper portions and carried down the fissures by ground water to lower
levels, where they are redeposited.
Many of the richest ore deposits are thus due to successive
concentrations: the ores were leached originally from the rocks to a
large extent by laterally seeping waters; they were concentrated in
the ore deposits of the vein chiefly by ascending currents; they have
been reconcentrated by descending waters in the way just mentioned.
=The original source of the metals.= It is to the igneous rocks that
we may look for the original source of the metals of veins. Lavas
contain minute percentages of various metallic compounds, and no doubt
this was the case also with the igneous rocks which formed the
original earth crust. By the erosion of the igneous rocks the metals
have been distributed among sedimentary strata, and even the sea has
taken into solution an appreciable amount of gold and other metals,
but in this widely diffused condition they are wholly useless to man.
The concentration which has made them available is due to the
interaction of many agencies. Earth movements fracturing deeply the
rocks of the crust, the intrusion of heated masses, the circulation of
underground waters, have all coöperated in the concentration of the
metals of mineral veins.
While fissure veins are the most important of mineral veins, the
latter term is applied also to any water way which has been filled by
similar deposits from solution. Thus in soluble rocks, such as
limestones, joints enlarged by percolating water are sometimes filled
with metalliferous deposits, as, for example, the lead and zinc
deposits of the upper Mississippi valley. Even a porous aquifer may be
made the seat of mineral deposits, as in the case of some
copper-bearing and silver-bearing sandstones of New Mexico.
* * * * *
[Illustration: Fig. 260. Geological Map of the United states
and Part of Canada]
* * * * *
PART III
HISTORICAL GEOLOGY