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Appletons' World Series
_THE REGIONS OF THE WORLD_
EDITED BY
H. J. MACKINDER, M.A.
_Reader in Geography in the
University of Oxford_
Each complete in One Volume, Large 8vo.
BRITAIN AND THE BRITISH SEAS
By the EDITOR
WESTERN EUROPE AND THE MEDITERRANEAN
By ELISEE RECLUS
CENTRAL EUROPE
By JOSEPH PARTSCH, Ph.D.
SCANDINAVIA AND THE ARCTIC REGION
By Sir CLEMENTS R. MARKHAM, K.C.B.,
F.R.S., President Royal Geog. Soc.
THE RUSSIAN EMPIRE
By PRINCE KROPOTKIN
THE NEARER EAST
By D. G. HOGARTH, M.A.
AFRICA
By J. SCOTT KELTIE, LL.D., Sec. R. G. S.
INDIA
By Colonel Sir THOMAS HOLDICH, K.C.I.E.,
C.B., R.E.
THE FARTHER EAST
By ARCHIBALD LITTLE
NORTH AMERICA
By ISRAEL C. RUSSELL, LL.D.
SOUTH AMERICA
By JOHN C. BRANNER, LL.D.
AUSTRALASIA AND ANTARCTICA
By H. O. FORBES, LL.D.
[Illustration: FIG. 1.--North America.
From a photograph of a relief map by Victor and Cosmos Mindeleff. Scales
of original: Horizontal, 120 miles to 1 inch; vertical, 40,000 feet to 1
inch: proportion, 1:16.]
NORTH AMERICA
BY
ISRAEL C. RUSSELL
PROFESSOR OF GEOLOGY IN THE UNIVERSITY OF MICHIGAN
_With Maps and Diagrams_
[Illustration]
NEW YORK D. APPLETON AND COMPANY 1904
COPYRIGHT, 1904
BY D. APPLETON AND COMPANY
PREFACE
The aim of this book is to give a condensed and, I trust, readable
account of the leading facts concerning the North American continent
which, from the point of view of the geographer, seem most interesting
and instructive. The area of the continent is so vast and the diversity
among its various parts so great, however, that the completeness of
treatment which characterizes the preceding volumes in the series to
which it belongs could not be attempted. To obviate in a measure this
confessed shortcoming, there has been appended to each chapter a list of
books which will enable the reader to continue the studies outlined in
it.
A complete review of the geography of a continent should, as it seems to
me, be divided into two parts: first, a discussion of the natural
conditions, or physical geography, and, second, man's dependence on and
use of the natural resources, or economic geography. Each of these two
leading phases of the subject was embraced in the preliminary outline of
the present volume, but owing to a desire to make each chapter as
complete as practicable, and also on account of limitations as to space,
the treatment of the economic phases of geography has been necessarily
brief. But little more can be claimed for the book as finished than that
it is an attempt to describe some of the more prominent and attractive
aspects of the natural conditions pertaining to North America.
While writing this book I have become more and more impressed with the
incompleteness and inadequacy of the printed records relating to the
geography of the continent of which it treats. Extensive tracts,
particularly in the far North, have not been traversed by observant men,
vast areas throughout the continent have not been surveyed and mapped,
and even in the somewhat thickly inhabited portions of the more
enlightened countries there are large districts in reference to the
geography of which there is but little critical information available.
Under these conditions it seemed best to select typical examples of
various geographical features from the better known portions of the
continent to represent the conditions throughout the less thoroughly
explored domain in which they are situated, and at the same time serve
to illustrate the highly creditable advances made by American
geographers in definitely formulating the principles of physiography.
The book may, in a measure, be considered as an attempt to present in
popular form a report of progress concerning the study of the
geographical development of North America at the beginning of the
twentieth century.
I. C. R.
CONTENTS
CHAPTER PAGE
I. THE MARGIN OF THE CONTINENT 1
The continental shelf--The submarine topography of the
Caribbean region--Movements of the ocean
waters--Islands--Topography of the coast--Estuaries and
harbours.
II. THE TOPOGRAPHY OF THE LAND 60
Coastal plains and plateaus--The Atlantic mountains--The
continental basin--The Pacific mountains--The Antillean
mountains.
III. CLIMATE 173
The elements of climate--Climatic provinces--Secondary
disturbances of the atmosphere--Evaporation.
IV. PLANT LIFE 215
The forests--Prairies, treeless plains, and
plateaus--The treeless mountain tops.
V. ANIMAL LIFE 258
General principles of geographical
distribution--Life-regions and life-zones--The
mammals--Some representative mammals--The birds.
VI. GEOLOGY 299
The growth of the continent--The rocks of which the
continent is composed--The concentration of mineral
substances.
VII. THE ABORIGINES 355
The Eskimos--The Indians.
VIII. POLITICAL GEOGRAPHY 408
Classification of boundaries--Political
control--Population in 1900.
INDEX 427
LIST OF ILLUSTRATIONS
COLOURED MAPS
PLATE PAGE
I. Orographical features 25
II. Mean annual rainfall and temperature 173
III. Climate and life provinces 185
IV. Leading geological features 306
V. Pleistocene glacial deposits 315
VI. Linguistic stocks of Indians North of Mexico 370
VII. Distribution of governments 410
VIII. Characteristic vegetation 418
OTHER ILLUSTRATIONS
FIG. PAGE
1. Relief map of North America _Frontispiece_
2. Profile of a continental shelf 2
3. Map of Gulf of Mexico and the Caribbean region,
showing topography of the sea-floor 17
4. Co-tidal lines 29
5. Profile of sea-cliff and cut-and-built terrace 33
6. Map of a portion of the Atlantic coast of the
United States 35
7. Map of Mobile Bay 36
8. Map of Cape Cod 37
9. Map of the coast of Texas 39
10. Map of a portion of the coast of Maine 45
11. Map of the coast of southeastern Alaska 47
12. Map of the delta of the Mississippi 53
13. Map giving the names of the larger physiographic
divisions of North America 61
14. Altitude map of North America 65
15. Map of the Appalachian Mountains 74
16. Section of anticlinal valleys and synclinal
mountains 78
17. Profile showing relation of ancient peneplains
in the Appalachians 81
18. Section through the Black Hills of Dakota 117
19. Sketch of the Grand Canyon of the Colorado River 134
20. Map of the Great Basin 137
21. Map of Crater Lake, Oregon 153
22. Mount Rainier, Washington 154
23. Map of Puget Sound 161
24. Map showing isobars for January and July 175
25. Ice-palace, Montreal 199
26. Map showing tracks of West Indian hurricanes 209
27. Map showing depth of evaporation
in the United States 212
28. Map showing distribution of forests 215
29. Douglas firs, Vancouver 241
30. View in redwood forest of California, from
photograph by U. S. Bureau of Forestry 242
31. Bison at Silver Heights, Winnipeg 275
32. Map showing range of bison 276
33. Maps showing the growth of the North American
continent 303
34. Map showing the distribution of coal-fields 336
35. Ideal section through an oil and gas
pool beneath an anticlinal 339
36. Lodge or tepee of Blackfoot
Indians, Manitoba 387
37. Panorama of Uxmal, Yucatan 390
38. Pointed arches in Central American ruins 393
39. Carved stonework, Uxmal, Yucatan 394
NORTH AMERICA
CHAPTER I
THE MARGIN OF THE CONTINENT--THE
CONTINENTAL SHELF
In beginning the study of the physical geography of North America, one
of the first facts to claim attention is that the true continental
border is in general many miles seaward from the present margin of the
land. The boundary of our field of study is defined with considerable
accuracy by a line drawn on the bottom of the sea adjacent to the
present coast-line of the continent so as to pass through all points
where the soundings show a depth of 100 fathoms of water. This
100-fathom contour in the topography of the sea-floor chances to
coincide in a general way with the outline of the submerged border of
the continent; landward from it the bottom rises with a gentle slope,
while seaward the descent is usually steep down to a depth of 2,000 or
more fathoms.
A gently sloping shelf-like border surrounds the deep central basin of
the Gulf of Mexico (Fig. 3). To the west and north of Yucatan and west
of Florida the shelf is from 140 to 160 miles broad, with a surface
slope towards the centre of the Gulf of less than 6 feet to a mile--a
slope so gentle that were the surface of the shelf exposed to view, no
eye could distinguish it from a perfect plain. The deepest sounding yet
obtained in the central part of the Gulf, approximately midway between
Yucatan and Florida, shows a depth of 2,119 fathoms. The remarkable fact
is that the slope from the 100-fathom line to the bottom of the central
basin of the Gulf is precipitous. In two places on the border of
the Yucatan bank a descent of about 8,500 feet occurs within a
horizontal distance of 15 or 20 miles.
On the east side of the southern extremity of Florida, and again on the
eastern shore of Yucatan, the continental shelf is only about 5 miles
broad; these are the nearest approaches of the present land to the
actual border of the continent to be found on the Atlantic coast. The
explanation of these exceptional conditions is that both Florida and
Yucatan are portions of the continental shelf which have been raised so
as to form low emerged plains.
From Cape Hatteras northward to the extremity of the Newfoundland Banks
the shelf increases gradually in breadth from about 15 miles in the
region of the Carolinas to over 100 miles off the coast of Maine. The
outer border of the shelf is an irregular curving line. Opposite the
coast of Massachusetts and Maine an extension of the Atlantic basin
reaches within 15 or 20 miles of the present margin of the land. The
manner in which the low plain fringing the eastern border of the United
States passes beneath the waters of the Atlantic and becomes a
continental shelf is illustrated by Fig. 2.
[Illustration: FIG. 2.--Ideal profile through a continental shelf.]
Southeast of Newfoundland the continental shelf has an irregular
surface, marked by shoals and depressions, and furnishes the most
valuable fishing-banks in the world. The 100-fathom curve is there over
500 miles from the coast. This is the broadest portion of the
continental shelf now known on the Atlantic border of the continent.
Northward of Newfoundland the Atlantic basin extends far into Davis
Strait and Baffin Bay, and then its border swings outward about
Greenland, but its true margin is there but imperfectly known.
To the north of the arctic coast of North America, as is suggested in
part by the soundings made by Nansen, the submerged margin of the
continent is probably broad and presents a steep escarpment to the
arctic basin, but the outline of the true continent, as in the case of
the present land extension in that direction, is unknown.
Soundings to the north of Cape Lisburne, on the northwest coast of
Alaska, show that the 100-fathom curve is there over 200 miles from
land. The exceptionally shallow sea covering this portion of the shelf
continues westward to the coast of Asia, and southward through Bering
Strait, so as to embrace the eastern portion of Bering Sea. The
continental mass of North America is thus directly connected with the
continental mass of Asia. A rise of the bottom of less than 200 feet in
Bering Strait would bring about a land connection between the Old and
the New World. This, as will appear later, is a most significant fact to
students of geography and geology.
On the Pacific coast of North America the continental shelf is
throughout much narrower than its average breadth on the Atlantic side
of the continent, and is also more deeply submerged on its seaward
border. The broad platform beneath the northern and eastern portions of
Bering Sea--from which rise the low islands, St. Lawrence, St. Matthew,
Nunivak, and the Pribilof group, now separated by water from 25 to 35
fathoms deep--extends to the south of the more easterly of the Aleutian
Islands, and is prolonged eastward along the south border of Alaska,
where the 100-fathom curve is from 10 to 20 miles from the coast-line,
and approaches still nearer the land in the neighbourhood of the islands
of southeastern Alaska and British Columbia. The shelf is narrow but
well defined along the coasts of Washington and Oregon. Adjacent to
California, Mexico, and Central America, its outer margin is barely 10
miles from land. Throughout the entire distance from the Aleutian
Islands to Panama the outer border of the shelf is in general well
defined, and its seaward escarpment descends abruptly to the floor of
the vast Pacific basin, where the sounding-line shows depths of
from 2,000 to 3,000 fathoms.
Could the waters of the sea be removed and North America viewed from a
distance, in the manner we are enabled to examine the surface features
of the moon through a powerful telescope, an observer would behold a
great plateau, having the present well-known triangular shape of the
continent, rising boldly between the Atlantic and the Pacific basins.
The surface of the plateau would be rough, in comparison with the
generally smooth contours of the adjacent troughs, but even the highest
mountains would be less in elevation above its general surface than the
crests of its bordering escarpments above the adjacent depressions. The
mountain-peaks when illuminated by the sun would appear as points of
light with long, tapering morning and evening shadows, and the east and
west plateau-borders would be strongly drawn bands of light or shadow,
according to the time of day, 6,000 or 8,000 miles in length. The
Bermuda, Hawaiian, and other islands now rising above the surface of the
deep sea would stand on its desiccated floor as isolated, gigantic
mountains--"Bermuda mountain" with an elevation of 15,000 feet, and the
Hawaiian group of peaks with a culminating point of light 30,000 feet
above the surrounding plain. The bordering slopes of the "North American
plateau" and its slightly bevelled margin forming the present
continental shelf would be lacking in details, and appear as a vast,
smooth, curving belt of light or shadow, in striking contrast to the
roughened surface now above water.
The North American continent is not exceptional in being partially
submerged at the present time. Similar conditions occur about the
margins of other continents which, as is well known, are fringed with
broad submarine terraces built in part of their own _debris_. In fact,
every large land mass on the earth under existing climatic conditions
and present distribution of life, if it remained moderately stable for a
sufficient length of time, would have a submarine shelf built about its
borders.
_Of what is the Continental Shelf Composed?_--The rocks forming the
present land surface of North America extend seaward from the existing
shores and constitute the basal portions of the continental shelf, thus
suggesting that the submerged platform is due, in part at least, to
shore erosion--the waves having eaten into the land so as to make a
terrace. That this is not the true explanation, however, may be shown in
several ways.
The superficial covering which gives the continental shelf its smooth
contours is composed largely of sediments such as rivers bring from the
land. This material is coarsest and in greatest abundance near shore and
decreases both in the size of the particles composing it and in
abundance towards the seaward borders of the shelf. The wash from the
land is mostly deposited within a few miles of the coast-line and, as
has been shown by dredging, is seldom carried, even under the most
favourable conditions, more than about 100 miles seaward. Supplementing
the fragmental material derived from the land, and increasing in
thickness towards the seaward margin of the continental
shelf--coincident with the increase in depth of the water--is a deposit
of light-coloured calcareous mud or ooze, formed of the hard parts of
animals and plants which live in the waters of the sea. The organisms
which supply this material are in the main microscopic and live
especially in the warmer seas in countless myriads. Their dead shells or
cases fall to the sea-floor in a constant shower, much as the snow falls
from the air, but continuously year after year and century after
century. This descent of the hard parts of organisms, both calcareous
and siliceous, from the waters of the sea has led to the accumulation of
a sheet of slimy sediment over almost the entire sea-bottom. How thick
this layer is we have no means of knowing, but it is probably many
hundreds of feet.
The organic _debris_ falling on the continental shelf descends through
only a few hundred feet of water and is but little affected by its
solvent action. The great number of organisms, such as the Foraminifera
which secrete calcareous tests or "shells" causes the slime on the
continental shelves to be calcareous and in the condition to form
limestone if cemented or subjected to sufficient pressure. In the deep
sea, where the hard parts of dead organisms fall through many thousands
of feet of water, their more soluble portions are removed and the bottom
is covered throughout vast areas with a pinkish clay composed of the
more insoluble residue of the calcareous shells and the cases of
silica-secreting animals and plants.
The continental shelves are, in general, within the influences of ocean
currents, and fine _debris_, as we seem justified in concluding, is
removed from their surfaces, carried beyond their margins, and deposited
on their seaward slopes. The shelves are thus built outward and are
largely constructional topographic forms. Their outer slopes, where best
defined, represent about the "angle of repose" in water of the fine
material of which they are composed. These slopes are in several regions
so precipitous that they probably would not retain their present forms,
but descend in landslides, should the restraining pressure of the
sea-water be removed.
In certain favoured regions, as about the southern extremity of Florida,
over an extensive area in the West Indies, and on both sides of Central
America, the conditions favour the growth of reef-building coral-polyps,
and portions of the continental shelf in that region are covered with an
irregular layer of living coral and dead coral rock. The importance of
this resistant superficial layer on the minor features of the relief of
the submarine banks, etc., needs to be considered in studying the
secondary topographic features of many portions of the floor of shallow
tropical seas.
In addition to the _debris_ from the land and the rain of the hard parts
of organisms from the water covering the continental shelf there is in
northern regions a third but less important source of material furnished
by floating ice. About the northern shores of America sea ice forms in
winter, some of which is frozen fast to boulders and stones in shallow
water, and when this ice-foot, as it is termed, is adjacent to steep
cliffs, rock _debris_ falls upon it. When the ice becomes broken
into cakes in the spring-time or during storms, it floats away, under
the influence of the winds and currents, and as it melts drops its
freight on the floor of the sea. This shore ice seldom travels far, and
is probably not an important factor in the building of continental
shelves. Of greater interest are the bergs derived from glaciers,
especially in Greenland, many of which contain hundreds of thousands and
even millions of cubic feet of ice and travel hundreds of miles before
melting. In some instances these bergs carry with them rock masses, mud,
etc., derived from the land over which their parent glaciers flowed, and
as they melt, distribute this material over the sea-floor. The greater
portion of this ice-carried freight derived from Greenland is dropped on
the continental shelf, and not infrequently reaches the latitude of
Halifax, and even journeys farther south. This berg-carried _debris_ is
mainly deposited on the continental shelf, for the reason that the cold
currents which bring the bergs southward follow the coast in a general
way, and are bordered on their seaward margins by warmer currents
flowing northward. To the north of Nova Scotia the additions of material
to the continental shelf through the agency of bergs is considerable in
the aggregate, and as the process has been in operation for thousands of
years, the banks or shoals in the sea off the Newfoundland coast are due
in part to this cause.
Ice-carried _debris_ forms an important source of material for the
building of the continental shelf from New England northward and
westward about the shores of North America, including Greenland, to
Bering Sea, and to a less extent on the south coast of Alaska, where
many comparatively small bergs are set afloat by glaciers which reach
tide-water. Supplementing the distribution of _debris_ over the
continental shelf by shore ice and bergs, is the similar work carried on
by the ice discharged into the sea by northern rivers, such as the St.
Lawrence, Mackenzie, and the Yukon.
During the glacial epoch great ice-sheets like those now discharging
bergs along the Greenland coast, but vastly larger, entered the
Atlantic all the way from New York to the Arctic Ocean, and along the
Pacific coast from the Aleutian Islands to the State of Washington.
During certain periods of this time of intense glaciation great
additions of ice-borne _debris_ must have been made to the continental
shelf. The banks to the east of Newfoundland and other similar shoals as
far south as Nantucket are probably due in large part to the _debris_
deposited by the glaciers which formerly entered the sea in that region.
It is of interest in this connection to note that the glaciers, even at
the time of their greatest expansion, could not have extended beyond the
seaward margin of the continental shelf, for the reason that on passing
that boundary and entering deep water they must have broken off and
given origin to bergs.
_Submerged River Channels._--One of the most interesting features in
connection with the continental shelf bordering North America is that
its generally plane surface is trenched in several places by canyon-like
depressions similar to the narrow steep-sided valleys which streams
sometimes cut in the surfaces of plateaus. This suggestion that the
surface of the continental shelf is crossed by stream-cut channels is
supported by the fact that several such depressions, leading seaward
from the present mouths of large rivers, have been discovered by the
sounding-line. The best known example occurs off the mouth of the Hudson
and has been traced from New York Bay about 120 miles seaward to the
edge of the continental shelf. It is deepest and best defined on the
outer portion of the submerged plateau, where for a distance of 23
miles, beginning 97 miles from Sandy Hook, it has an average width of 3
miles and a maximum depth of about 2,500 feet below the surface of the
bordering submarine plain, which has 20 fathoms of water over it. This
canyon opens out in the seaward face of the plateau and forms a deep
notch in the generally uniform crest-line of that escarpment. Farther
"up-stream," so to speak, the channel narrows to a mile and a quarter,
with some irregularities in depth, and near Sandy Hook it is not
apparent, owing to the amount of _debris_, largely sand, swept into
it by shore currents. This evidence, strengthened by the fact that the
true rock-cut valley of the Hudson as far as Troy is filled with clay
and sand to a considerable but unknown depth, is abundant proof that the
land was formerly higher than at present by at least 3,000 feet, and
that the now submerged continental shelf off Long Island was then a
plain above water, across which the ancient Hudson was extended. The
river flowed across this plain for a sufficient length of time to
excavate a canyon over 2,500 feet deep and 3 miles wide from crest to
crest of its walls in its seaward portion. This submerged channel has
the characteristics of a young, stream-cut valley and suggests that the
plain across which it flowed to the eastward of Long Island was a
submerged continental shelf previous to being upraised so as to be
trenched by the Hudson.
The evidence as to changes in the elevation of the Atlantic coast
furnished by the submerged valley of the Hudson does not stand alone.
Similar but less well-defined channels have been discovered by soundings
off the mouths of the Delaware and the Susquehanna, while the most
remarkable instance of all is furnished by the submerged valley of the
St. Lawrence, which has been traced through the Gulf of St. Lawrence and
out to the brink of the submerged continental escarpment some 200 miles
eastward of Nova Scotia. The tide now rises and falls in the St.
Lawrence to within a few miles of Montreal; that is, the "Greater St.
Lawrence" has lost about 1,000 miles of its length owing to a downward
movement of the land.
Evidence of the nature just considered is lacking, or, more correctly,
surveys and soundings which would perhaps reveal the presence of
submerged river channels have not been made about the shores of the more
northerly portion of the continent, but instructive results in this
connection are to be expected when that region is thoroughly studied.
On the Pacific coast several transverse channels in the continental
shelf, similar to the submerged valleys of the Hudson, have been
discovered by soundings, some of which are thought to be true
stream-cut valleys; others, however, start from the coast where there
are no rivers entering, and may be due to other and as yet unknown
causes.
An exceptional feature in the geography of the Pacific coast to the
south of the United States-Canadian boundary is furnished by the islands
off the southern portion of California. These islands, of which Santa
Rosa, Santa Cruz, Santa Catalina, and San Clemente are the most
important, rise from water that is 300 or more fathoms deep to a height
of from a few hundred to about 2,000 feet above the present sea-level.
These islands are the summit portions of mountains similar to those
which give a characteristic relief to southern California. One feature
concerning the islands referred to which is of interest in connection
with the study of the continental shelf is the presence on them of
numerous terraces. These occur on a series of level, step-like areas,
which sweep about the slopes of the islands, in a general way parallel
to the present shore-line, and are records of an upward movement of the
land. The highest well-defined terrace on San Clemente occurs at an
elevation of 1,320 feet, but there are less distinct beach-lines up to
1,500 feet above the present sea-level. The evidence of movements of the
land along the Pacific border of the continent might be multiplied, but
enough has been stated to show that the western border of North America,
like its eastern portion, is subject to fluctuation in reference to the
level of the sea. The line which marks the passage of the solid earth
beneath the waters of the sea is ever changing, owing to movements in
the earth's crust. It is ever changing, also, owing to the action of
waves and currents on the ocean's shores and other causes.
_Life on the Continental Shelf._--No attempt need be made here to give
an accurate description of the plants and animals which find a congenial
home on the continental shelf. One needs, however, to have in mind a
general idea of the wonderful abundance and variety of organic forms in
the shallow waters adjacent to the continent to fully appreciate the
changes in the relief of the ocean-floor, in which they play a
leading part, and also the vast economic importance of these marine
harvest-fields and pastures, as they may be termed.
The continental shelf fringing North America reaches from within 500
miles of the equator to probably a less distance of the north pole. Its
length following its broader curves is in the neighbourhood of 25,000
miles, and its average width is not far from 50 miles; its area is
therefore something like 1,000,000 square miles. On account of the vast
extent of this submarine plain and its great range in latitude, the
conditions influencing the lives of the plants and animals inhabiting
its surface or living in the waters covering it vary from place to place
between wide extremes. The waters resting on it have a mean annual
surface temperature of from 70 deg. to 80 deg. F. at the south, and about 32 deg. F.
at the north. In places great rivers and the turbid waters from glaciers
bring in sediments and form muddy deposits; at other localities the
currents, as in the path of the Gulf Stream off the Carolina coast,
sweep the bottom clear of all light _debris_; and again bare rocks of
limited extent are exposed. The depth of the water resting on the shelf
varies through all gradations down to 100 fathoms. At the south the
hours of light and darkness are approximately equal each day, but at the
north there are six months of sunlight and six months of darkness each
year. Still other variations, as of strength of currents, salinity,
etc., exert an influence in this realm and lead to great diversity in
its living organisms. Throughout its entire extent, however, the
continental shelf abounds in both plant and animal life.
The plant life of the sea, as has been shown in recent years by the use
of the dredge and net, is most abundant at the surface and is
practically absent at the bottom where the water is over 100 fathoms
deep. The animals of the sea, like those of the land, are dependent
primarily on plants for their food. By far the most abundant supply of
food plants in the sea is furnished by minute algae, which float free in
its water. Below a depth of about 100 fathoms algae are absent because of
lack of light, and all the deep-sea animals are believed to be
carnivorous. For these reasons the comparatively shallow waters
adjacent to the land and mainly covering the continental shelf are the
most favourably circumstanced of any portion of the sea for the support
of a teeming fauna.
On the continental shelf of North America, especially to the south of
Cape Cod on the Atlantic and south of the Aleutian Islands on the
Pacific coast, there is a warm temperature, light penetrates to the
bottom except in the unfavourable and fortunately restricted areas of
muddy water, and motion of the waters produced by currents and the
pulsations of waves is present. These several favourable conditions
permit of an exuberance of life such as is unknown to persons who
confine their attention to the study of land areas.
We may safely say, in the words of Alexander Agassiz, that the abundance
of life in the many favoured localities of the ocean far surpasses that
of the richest terrestrial faunal districts. The most thickly populated
tropical jungle does not compare in wealth of animal or vegetable life
with certain portions of the continental shelf on the western border of
the Gulf Stream. In this connection we may also cite Humboldt, who
before the marvellous revelations in reference to the life of the sea
made by recent dredging expeditions wrote: "Upon surfaces less varied
than we find on continents, the sea contains in its bosom an exuberance
of life of which no other portion of the globe could give us an idea."
The distribution of life in the sea is analogous to the distribution of
life on land, but in a reverse direction with reference to sea-level. A
traveller passing from the tropical plains of eastern Mexico and
ascending Orizaba, for example, crosses successive belts of vegetation,
each with its indigenous animals, but merging one with another so as to
make a gradation in the luxuriance of the flora and the abundance of
animal life from the wonderfully rich plains adjacent to the Gulf coast
to the snow-capped mountain top. In the sea, the tropical plains with
their tangled vegetation and plentiful animal life are represented by
the still more uniform plain forming the submerged continental
shelf with its strange forests of flowerless plants, the seaweeds. These
submarine jungles shelter hosts of animal species, many of which swarm
in countless myriads. This life embraces all grades of invertebrates,
such as the microscopic protozoa, sponges, radiate animals like the
coral-polyps, starfishes, sea-urchins, etc., and crustaceans in vast
variety, and, among vertebrates, includes fishes, reptiles, and mammals.
Even birds might be included in this category, since many of them are
more at home on the sea than on the land.
The struggle for food among this multitude is intense. As with many
animals on the land, adaptive coloration is here a means of escape from
enemies, and many of the animals assume the brilliant hues of the
surrounding vegetation. The water is less transparent than air, and in
the deep sea it is always night. Counteracting to some extent this
diminution or absence of sunlight, many marine animals are luminous and
shine with phosphorescent light of many different tints. This property
is shared also by the animals of the sunny, shallow sea as well as by
those always living in the cold midnight of the great deep and in the
polar oceans.
The luxuriant vegetation, both attached and floating, and varying from
giant kelp, scores of feet in length, to microscopic algae which an
amoeba might encompass, clothes the surface of the continental shelf
except in unfavourable localities or is carried here and there by the
currents moving over it, but has its lower limit at about the 100-fathom
line. This inferior limit of marine vegetation is probably more
definitely defined than the superior limit of land plants on snow-capped
mountains. All attached seaweeds are confined to the shallow seas, but
floating kelp, like the well-known Gulf weed or sargasso, which collects
in the eddies of the sea currents and forms more or less mythical
floating islands, is widely distributed, as are also many kinds of
minute algae which thrive in the upper 100 fathoms of the open ocean in
all latitudes. The primary source of food for the hungry millions of
marine animals, excepting the comparatively small quantity brought
by rivers or blown from the land, is supplied by the marine algae, and
mainly by the minute forms which float in the water.
So much space has just been given to the marvellous luxuriance of life
on the southern portion of the great shelf surrounding North America
that the reader may perhaps think the cold northern oceans are even more
lifeless and desolate than their adjacent shores. This, however, is not
the case.
Northward along the continental shelf, with decrease in the strength of
the sunlight, the plants and animals lose much of the brilliancy which
characterizes many of the denizens of southern waters, and in general
assume more subdued colours in harmony with the prevailing gray of their
surroundings. The great diversity of animal life to be found in shallow
tropical seas decreases as one traces the continental shelf northward,
but even in the Arctic Ocean adjacent to the land invertebrate life
literally swarms, although the number of species, genera, etc., is
comparatively limited. Seaweeds are not absent from the Arctic Ocean,
although its shores, owing to the destructive action of ice, usually
seem exceedingly barren, and the lower or smaller forms of algae float in
the waters in abundance. Food sufficient for an extensive fauna is thus
supplied, and where food is plentiful animals are present also, no
matter what the mean annual temperature may be.
The life of the Arctic Ocean has been but inadequately studied, but
enough is known concerning it to show that a promising field there
awaits the naturalist. On the continental shelf off Point Barrow, the
most northern portion of the arctic shore of Alaska (latitude 71 deg. 23'),
180 species of marine invertebrates have been collected. Of these, the
molluscs numbered 61 species; the crustaceans, 44 species; the worms, 20
species; and the echinoderms (sea-urchins, starfish, etc.), 17 species.
Nearly all of the Arctic Ocean adjacent to the coast of America is as
yet unexplored, and we have therefore no direct testimony as to its
flora and fauna. We may reasonably assume, however, that the life is
there practically the same as in the waters of similar depth to
the north of Eurasia. Nordenskioeld, in his narrative of the voyage of
the Vega, speaks of decapods, worms, mussels, crustacea, and asteroids
which crawled in myriads over the beds of clay and sand at the bottom of
the Kara Sea. A detailed account is given of one unusually successful
haul of the trawl when it brought up large asteroids, sponges, crinoids,
holothuria, a gigantic spider, masses of worms, crustacea, etc. This was
the most abundant yield of the trawl-net at any one time during the
voyage of the Vega on the north coast of Asia, and that, too, from the
sea off the northern extremity of the continent. The temperature of the
water at the surface was from zero to -1.4 deg. C. (32 deg. to 29.48 deg. F.), and
at the bottom from -1.4 deg. to 1.6 deg. C. (29.48 deg. to 34.88 deg. F.). In this
connection the same distinguished naturalist remarks: "It is singular
that a temperature under the freezing-point of pure water should be
advantageous for the development of an animal life so extremely rich as
that which is found here, and that this animal life should not suffer
any harm from the complete darkness which during the greater portion of
the year prevails at the bottom of the ice-covered sea."
To persons who have never visited the far north the statements that
travellers in those regions give in reference to the abundance of life
in the sea seems scarcely to be credited. The assertion, however, that
comparing equal areas in the most populous tropical sea and in portions
of the Arctic Ocean, the amount of life, or the tons of living animal
matter per square mile in the two regions, would be in favour of the
northern station is probably true. In addition to the direct evidence
indicated above as to the prolific invertebrate life of the cold waters
of the north, we have still more impressive testimony from the vast
numbers of birds and large-sized mammals which subsist on this
abundance, or feed on fishes, which in turn obtain their subsistence
from the invertebrate realm.
Every rocky island and headland at the north is a breeding-place for
sea-birds. They are to be numbered by millions, yet their daily food is
gathered from the surface of the sea. The seals live in large
numbers about all the arctic shore, and the walruses, each individual
weighing about a ton, occur in herds; while whales and narwhals inhabit
the same waters. The presence of such numbers of large mammals is proof
that the life on which they subsist is abundant.
THE SUBMARINE TOPOGRAPHY OF THE CARIBBEAN REGION
The studies which have given to the world so much information concerning
the continental shelf surrounding the main body of North America have
been continued, or, perhaps more properly, were initiated, in the West
Indian waters. For this important work we are indebted mainly to the
United States Coast and Geodetic Survey and the United States Fish
Commission. The work of officers of the United States Navy in charge of
Coast Survey and Fish Commission vessels, in making accurate
measurements of depths, temperatures, strength and direction of
currents, character of bottom, etc., has, in some instances, been
commemorated on maps of the sea-floor by such names as Brownson Deep,
Sigsbee Deep, Bartlett Deep, etc. The routine survey work referred to
has been supplemented and extended by the labours of Louis Agassiz, L.
F. Pourtales, Alexander Agassiz, and others in studying the life in the
sea, the origin and history of the material forming the sea-bottom, and
the nature of the shelves, banks, deeps, etc., which give diversity to
its topography.
The continental shelf bordering Florida on the east is separated from a
similar submarine embankment surrounding the Bahama Islands by a channel
56 miles wide and from 200 to 500 fathoms deep (Fig. 3).
This channel when followed northward becomes shallower and broader, and
opposite the Carolina coast is no longer discernible in the relief of
the broad continental shelf. The Gulf Stream flows northward through
this Florida channel, as it is termed, with a current of from 2 to 6
miles per hour. These conditions are such as to suggest that the channel
referred to has in part been excavated by the Gulf Stream.
[Illustration: FIG. 3.--Gulf of Mexico and the Caribbean region:
topography of sea-floor. The first contour line from the land indicates
a depth of 600 feet--the outer margin of the continental shelf. Contour
interval below the 600-foot curve, 3,000 feet. After R. T. Hill.]
The Great Bahama Bank, from which rise the low coral-built Andros
Islands and a large number of crags and rocks, measures about 360
geographical miles from southeast to northwest, and has a width of
approximately 200 geographical miles. Throughout its entire submerged
portion the water is less than 100, and over much of the area less than
10 fathoms deep. It is invaded and given an irregular shape, however, by
a "tongue of the ocean" which curves in from the northward, in which
soundings of from 700 to 1,000 fathoms have been obtained. To the north
of the Great Bahama Bank, and separated from it by water nearly 2,000
fathoms deep, is the Little Bahama Bank, measuring 50 by 150
geographical miles, from which rises the low islands known as Great
Bahama, Great Abaco, and a multitude of islets and crags, while beneath
the water, as is the case also on the greater submarine plateau to the
south, there are numerous shoals. Southeast from the Great Bahama Bank,
and in a general view to be classed with it, are several other shallow
areas in the sea, of similar character, and with numerous islands and
reefs rising from them. The southeastern terminus of this series of
plateaus, the surfaces of which have been built up practically to the
level of the surface of the sea, is the Navidad Bank, situated about 50
miles to the north of the eastern end of the island of Santo Domingo
(Haiti) and forms the west border of Brownson Deep. The length of the
series of banks to the north of the Greater Antilles is about 800, and
its average width 120 geographical miles.
The unevenness of the surface of the Bahama Banks (and the same is true
also of the southern portion of Florida, the Yucatan peninsula, and of
nearly all of the submarine plateaus or banks in West India waters) is
largely due to the coral reefs and the coral rock formed on them. While
the outer portion of the continental shelf, in most instances, is formed
of soft, unconsolidated calcareous mud or ooze, in the tropical seas,
where the depth, clearness of the water, etc., are favourable,
reef-building coral-polyps become attached and form massive corals. The
growth of these corals is irregular, and the surface of the plateaus
where they are attached becomes roughened. There is a delicate
adjustment between the growth of reef-building corals and strength of
current, freedom of exposure to the waves, etc., and they flourish in
certain localities, as on the windward border of islands, and die at
other localities. The growth of coral "heads" and reefs changes the
direction of currents, and the spaces of soft ooze and dead coral
between the localities most favourable for coral growth are liable to be
scoured out and the bottom lowered. When coral, together with the shells
of molluscs and other organic refuse of the teeming life of tropical
seas, reaches the surface of the water, fragments and even large masses
are broken off by the force of the waves, ground into calcareous sand
owing to the movements produced by the waves and currents, and much of
it heaped on the borders of the reefs so as to raise them above the
fair-weather level of the sea. Much of this material, when it becomes
dry, is moved by the winds and built into dunes, thus still further
increasing the height of the land. Many of the islands in the Bahamas
have thus been formed, but the process has been modified in the greater
part of that region by movements in the earth's crust which have
produced widely extended elevations and depressions. The larger islands
in the Bahama group are coral platforms which have been moderately
elevated, and bear on their surfaces extensive accumulations of
wind-deposited sand. The Yucatan peninsula is also, to a great extent,
an upraised coral platform. The surfaces of such exposed areas of easily
soluble calcareous rocks are roughened still more by the action of rain
and percolating waters, and if subsequently submerged to a moderate
depth would give origin to "banks" with uneven, and possibly
conspicuously roughened surfaces. Different stages in this varied
history are illustrated throughout the West India region.
About the Caribbean coast of Honduras and Nicaragua the continental
shelf is broad and is termed the Mosquito Bank, in reference to its
proximity to the widely known coast of that name. Off the northeast cape
of Honduras this submerged shelf has a breadth of about 125 geographical
miles, and is succeeded to the eastward by the much smaller,
isolated, submarine plateau known as the Rosalind Bank. Even on a small
map, like that forming Fig. 3, it is readily seen that in
general terms there is a series of banks and low islands extending from
the Mosquito Coast to Jamaica, Santo Domingo, Porto Rico, and the
Caribbees. The distance measured along the curved line connecting the
several areas of shallow water is about 1,700 geographical miles. Should
this region be upraised 600 feet, the new lands that would appear would
still, in several instances, be separated by deep water, thus showing
that although in a generalized view it is convenient to consider the
banks and shelves referred to as parts of a single great group, there
are several centres from which they have grown.
A third great group of banks and shoals occurs about the borders of
Cuba, especially along its southern margin. Associated with this
submerged plain of calcareous mud, roughened by countless coral crags,
is a narrow east-and-west ridge in the Caribbean Sea, known in part as
the Misteriosa Bank, which rises precipitously on its southeast border
from a depth of over 3,400 fathoms, and is indicated at the surface by
the Cayman Islands.
The Caribbean and Gulf of Mexico region has great depressions or "deeps"
as well as broad banks or shoals. The submarine topography is, in fact,
on a more Titanic scale than in any other known region. Brownson Deep,
some 50 miles north of Porto Rico, has a depth of 4,561 fathoms (27,366
feet), and the bordering slopes of the depression in certain places, and
for long distances, have an inclination of 35 degrees. Between 15 and 30
miles south of Porto Rico the bottom of the Caribbean Sea is 1,500
fathoms below its surface, and rapidly descends to over 2,400 fathoms.
Bartlett Deep, a long, narrow depression in the sea-floor, intervening
in its eastern portion between Cuba and Jamaica, has a depth of 3,428
fathoms (20,568 feet) measured from the surface of the sea. Sigsbee
Deep, in the central portion of the Gulf of Mexico, is a third basin of
similar nature, remarkable for the great extent of its nearly
level floor, which is from 2,035 to 2,071 fathoms below sea-level. (On
Fig. 3 only the general depths of these great depressions are
indicated.) Coupled with the profound depth of the sea in the West
Indian region are the rugged mountains of the Great Antilles and the
volcanic cones of the Caribbees. Some of the elevations of the land
referred to are, in feet, as follows: Porto Rico, 3,609; Jamaica, 7,360;
Cuba, 8,600; and Santo Domingo, 10,300. The extreme range in the relief
of the surface of the solid earth (lithosphere) between the bottom of
Brownson Deep and the summit of Santo Domingo is 37,666 feet--the
horizontal distance between the two is about 270 geographical miles. No
mountain on the earth has such an elevation above sea-level. The islands
of the West Indies are thus shown to be the summits of stupendous
mountains, the greater portions of which are submerged. The low-lying
islands, and even the banks which can be discovered only with the aid of
the sounding-line, are in reality the tops of wonderfully steep
mountains or plateaus some 20,000 feet in height.
It is a significant fact that the profiles of the partially or wholly
submerged mountains of the West Indies are, as a rule, steeper than the
slopes of the mountains on the land with which they may reasonably be
compared. This is due in part, perhaps, to the greater density of the
medium in which they stand, the sea-water affording a greater degree of
support than the air, but the main reason is that beneath a few hundred
feet of water there is no erosion except the exceedingly slow removal of
matter in solution. Could the waters of the sea be withdrawn so as to
reveal the Caribbean Mountains in all of their stupendous grandeur, the
vast, smooth, sweeping surfaces extending from the horizontal lines
drawn about the higher summits by the waves and by the deposition of
sediment and coral growths, down to their bases would be unmarked by
channels and ridges of the character that give details to the type of
mountains with which we are most familiar.
An instructive generalization concerning the relief of the West India
region, suggested by Alexander Agassiz and sustained by the later
studies of R. T. Hill, is that we there find topographic forms produced
by movements in the earth's crust which have not been modified by
erosion. The great elevations rising from the floors of the "deeps" are
upraised blocks of the earth's crust which have not been beaten by rain,
shattered by frost, or trenched by rills, creeks, or rivers. They
illustrate the character of the rough blocks of rock from which many of
the mountain forms of the land have been sculptured.
This sweeping view, which it seems safe to accept as a generalized
outline of the history of the topography of the region in question,
needs to be qualified, as there are known to have been extensive up and
down movements throughout large areas in that portion of the earth's
surface. The mountains on Jamaica are scored by horizontal lines marking
former sea-levels up to a height of 2,000 feet, and similar and still
higher records are plainly visible on several of the larger West India
islands. This evidence shows that the present land over a wide extent of
the Caribbean region was formerly deeply submerged. More than this, the
rocks forming the higher portions of the Greater Antilles are largely
composed of more or less consolidated ooze, such as is now found on the
sea-floor in deep water. This line of evidence shows that what in late
geological time was the sea-floor has been raised between 20,000 and
30,000 feet. It is thus known that both upward and downward movements of
great vertical and great horizontal extent have occurred in the
Caribbean region. Whatever minor changes the topography of the now
submerged sea-floor may have suffered owing to emergence, the general
relief, as suggested above, seems to have resulted from movements in the
earth's crust, and that these movements, in certain instances at least,
produced faults--that is, breaks or fissures--along which the rocks were
upraised on one side or depressed on the other, so as to form great
cliffs. The precipitous submarine slope forming the northwest border of
Bartlett Deep may reasonably be interpreted as a great fault scarp. A
portion of this escarpment rises above the sea and forms the remarkably
straight and exceedingly rugged south coast of Cuba in the region
of Santiago. In the main the remarkable submarine topography of the West
India region presents us with an example of what would have been the
leading features of several portions of the earth's surface which are
now land, as, for example, the Great Basin region of Utah, Nevada, etc.,
had deformation gone on without erosion.
MOVEMENTS OF THE OCEAN WATERS
To the student of the geography of a continent the climatic and other
influences of the great ocean currents, as well as the more tangible
results produced by the waves which break on the borders of the land,
demand extended and painstaking investigation. The most that we can hope
to do at present in this connection is to state briefly some of the more
important influences that the movements of the ocean waters have on the
climate of North America and on the topography of its shores.
_Currents._--The surface waters of both the north Atlantic and the north
Pacific, as is the case with all broad water bodies, have a drift and in
places flow in well-defined currents, mainly on account of the friction
of the wind on the surface of the sea, aided by variations in the
density of the water due to differences in temperature and salinity. In
each ocean there is a great swirl or eddy, for the reason that the
surface drift and the flow of the deeper currents carry the waters about
in a rudely circular path, parallel in a general way with the boundaries
of the respective basins. The direction of this motion, to one situated
in the central part of either basin, is from left to right, or with the
movements of the hands of a watch. In the southern portion of each basin
there is a westward-flowing equatorial current, which in each instance
is deflected northward on approaching the bordering land, and as it
continues is still more deflected owing to the influence of the earth's
rotation, and acquires a northeast trend; on reaching the eastern side
of the oceanic basins, the currents are again deflected, a portion of
the one in the Atlantic and all of the one in the Pacific being
turned southward so as to complete the circuit.
In the southern portion of the north Atlantic the surface drift is
westward at a rate of four or five miles a day. The waters, forced along
principally by the trade-winds, flow through the numerous passes between
the Lesser Antilles and enter the Caribbean Sea, and from thence are
carried through the Yucatan channel into the Gulf of Mexico. The waters
are piled up, as it were, in that great landlocked basin, at the same
time becoming warmer, and receive additions of fresh water from rain and
inflowing streams. Each of these causes tends to decrease the density of
the water, while evaporation has a counterbalancing influence. The
escape for the waters, both salt and fresh, which enter the Gulf, is by
evaporation and by flowing through the only notch in the rim of the Gulf
basin which is not in the path of the equatorial current, namely, the
strait separating Florida from Cuba and the Bahama Islands. These
outflowing waters form the justly celebrated Gulf Stream.
Between Florida and the shoal waters on the Bahama Banks the Gulf Stream
is about 50 miles wide, approximately 350 fathoms deep, and flows
northward at the rate of from four to five miles an hour. Its
temperature is about 80 deg. F. It is estimated that this great river in the
ocean carries 90,000,000,000 tons of water per hour past a given
cross-section. Its course is northward along the immediate border of the
continental shelf until it arrives opposite the Carolina coast, and
thence northeastward, thus giving it a constantly increasing distance
from the land. To the north of the Bahamas it receives as a tributary
the portion of the equatorial current, perhaps even greater in volume
than the true Gulf Stream, which is deflected northward by the West
India Islands and their associated banks. Continuing its course, it is
deflected still more towards the northeast owing to the influence of the
earth's rotation, at the same time expanding and losing velocity so as
to become a surface drift rather than a well-defined current. Under the
influence of the prevailing westerly winds of the north Atlantic, the
waters delivered by the Gulf Stream pass the vicinity of the
British Islands and in part enter the Greenland Sea.
[Illustration: PLATE I.--Orographical features.]
The transfer of the vast amount of warm water carried by the Gulf Stream
far to the north is counterbalanced in part by a southward-flowing cold
current which emerges from Davis Strait, and being joined by another
cold current from the eastward of Greenland, continues southward under
the name of the Labrador current, past Newfoundland and Nova Scotia to
the Massachusetts coast, and is thought to exert an influence on the
temperature of the sea even as far south as Cape Hatteras. While the
Gulf Stream in the northern portion of its course curves eastward and
departs from the American coast, the southward-flowing Labrador current
is turned westward and follows close along the border of the land, and
mainly over the continental shelf.
The chief effect of the cold current from the north in proximity to the
coast of the continent is to bring to the adjacent land a lower mean
annual temperature and especially colder and more stormy winters than it
would otherwise experience. This tendency is augmented by the icebergs
carried southward with the Labrador current. In a similar way, the
northward-flowing warm current gives Florida and the Carolinas a
subtropical climate, admits of the growth of reef-building corals about
the Bermuda Islands, and carries so much warmth to northwestern Europe
that its climate is milder and more humid than one would expect from its
geographical position.
The currents of the north Pacific are analogous to those of the north
Atlantic, but simpler, as there is nothing similar to the true Gulf
Stream, and as communication with the Arctic Ocean is practically
closed, there is no cold current flowing southward from that ocean; but
the conditions, so far as they influence the climate of North America,
are reversed. A warm current flowing northward off the coast of Japan,
and hence known as the Japan current, crosses the Pacific, and on
approaching the coast of Alaska and British Columbia is deflected
southward. The climate of the northwest coast is thus ameliorated, the
prevailing westerly winds are warm and humid, and the mean annual
precipitation from western Alaska to Oregon is in the neighbourhood of
100 inches. Under the influence of a mild equitable temperature and
abundant moisture, the land bordering the Pacific from southern Alaska
to northern California is clothed with the most magnificent forests that
the continent affords. The marked contrasts in climate, vegetation, and
the conditions that influence civilization between the two sides of the
North American continent, produced by the cold Labrador current on the
east and the warm Japan current on the west, is shown in a marked way by
the sweep of the lines of equal mean annual temperature (isotherms)
represented on the map forming Plate II, and again by the
distribution of forests, as will be described later. It is instructive
to note that the climate of Sitka, in north latitude 57 deg., is far more
temperate and equable than that of New York city, latitude 40 deg. 45',
although the cool summers on the northwest coast make the mean annual
temperature somewhat lower than on the coast of New York, or even of New
England.
The influence of the opposite conditions in reference to ocean currents
experienced by the eastern and western borders of the continent are even
more marked in the life of the adjacent waters than in the vegetation
and fauna of the land itself. The plant and invertebrate life of the
shoal waters of the Pacific coast, consisting largely of southern
species, is exceedingly rich and varied, even to the inlets of the
Alaska coast, where glaciers come down to the sea; while on the Atlantic
border, northern species occur on the New England coast, and even
farther south. The contrasts in temperature between the waters of the
Atlantic and Pacific which cover the submerged border of the continent
are well shown by the distribution of the cod, the most valuable of all
fishes to man, which, as is well known, belongs to the northern fauna
and ranges from the north Atlantic about the arctic coast of both the
Old and the New Hemispheres, to the north Pacific. On the east coast of
America this circumpolar fish, of which several species are known, is
found occasionally as far south as Cape Hatteras, but the most southern
"bank" on which it is extensively taken is off Cape Cod, in
latitude 42 deg.; on the west coast it travels perhaps as far south as the
mouth of the Columbia, but the most southern locality where it occurs in
commercial quantities is off the Shumagin Islands, in latitude 55 deg.. The
life of the continental shelf, as well as of the adjacent land, thus
bears testimony to the vast importance to North America of the great
ocean currents washing its shores.
_Tides._--The waters of the ocean are subject to wave-like undulations,
caused by the attraction of the moon and sun, termed the tides. Every
day, at the average interval of twelve hours and fifty-one minutes, the
"tide rises," and with equal regularity intermediate between these
periods it "falls." This rise and fall of the waters along the coast,
accompanied frequently by strong currents, are produced directly by the
arrival in the shoal water of a pulsation of the ocean, which becomes a
true onward-moving gravity wave as it nears the land. In the open sea
the amplitude of the tidal undulations is but two or three feet, and
their rate of travel in general 700 to 800 miles per hour. On reaching
shoal water, however, the onward movement is decreased by friction on
the bottom, the waves become higher, and when they meet an
outward-flowing bottom current, their bases are still more retarded and
the slope of their fronts increases until the water falls forward and
breaks into foam. On the Atlantic coast, each tidal wave reaches the
land broadside on, as it were (Fig. 4), and at the outer capes high
water occurs at practically the same time from Florida to New England,
but its farther landward progression is greatly modified by the shape of
the coast and the depth of water in the estuaries and other
indentations. When the wave as it rushes landward enters a broad water
body through a narrow entrance, as the Gulf of Mexico, for example, it
spreads, and as the impulse is transmitted to larger and larger volumes
of water, it decreases in height. (In a critical study the tides
originating in the Gulf itself should be considered.) At Galveston,
Texas, the mean range between high and low tide is less than one foot.
When, however, an estuary with a broad mouth receives a tidal wave from
the ocean, the impulse is more and more concentrated and the wave
rises higher. At the head of the Bay of Fundy the difference between
high and low water is from 50 to 60 feet. For the reason just stated,
the tidal wave is generally higher in the Atlantic coast estuaries than
on the ocean capes, and under favourable conditions may be transmitted
for long distances up the rivers emptying into such estuaries, and may
be felt where the mean elevation of the stream is several feet above the
mean level of the sea on the neighbouring open coast. Tidal waves pass
up the Hudson to Troy, a distance of 150 miles from the Narrows,
where the mean range is 2.3 feet. In St. John River, New Brunswick, the
tidal impulse is felt at Frederickton, 70 miles from the Bay of Fundy,
and at an elevation of 14 feet above its surface. In the St. Lawrence
estuary and river the tidal waves ascend 283 miles to Three Rivers, a
few miles below Montreal, where the mean elevation is about 11 feet and
the mean range of the tide 0.9 foot. In the Columbia the range of the
tide is about 6 inches at a distance of 140 miles from the ocean.
[Illustration: FIG. 4.--Co-tidal lines. A diagrammatic representation of
the advance of tidal waves in the Atlantic and Pacific. Figures refer to
noon and midnight. After R. S. Tarr.]
In the north Pacific (Fig. 4) the tidal waves come from the south and
expand much the same as the corresponding waves do in the north
Atlantic, but instead of striking the coast broadside on, sweep along
the shore from south to north.
There are two localities on the coast of North America, one at the head
of the Bay of Fundy and the other at the head of Cook's Inlet, Alaska,
where the tides present especially interesting features. In each of
these inlets the incoming tidal wave meets an outward-flowing current
which tends to hold it back. The incoming waters are thus piled up until
sufficient head is established to cause them to advance as a
steep-fronted wave termed a _bore_, which curls over and breaks in a
long line of foam as it rushes along. At the head of the Bay of Fundy
the bore travels at the rate of 6 or 7 miles an hour, and has a height
of from 4 to 6 feet. The great disturbance produced by the strong
current and breaking waves causes the mud of the bottom to be disturbed
and the waters to be charged with sediment. Much of this mud is
deposited during the interval of quiet water at high tide, and as the
outflow is not so impetuous as the inflow, broad mud-flats are formed.
At certain localities about the Bay of Fundy artificial dikes have been
made, which admit the mud-charged waters at high tide, and retain them
until much of their freight is deposited. In this manner, large areas of
rich lands have been secured.
The geographical influences of the tides and of the currents produced by
them are of interest in many ways. To navigators they are of special
importance. Even in this age of steam, the arrival and departure
of vessels from harbours is regulated so as to take advantage of the
incoming or outgoing tidal currents. Many harbours can be entered by
deep-draft vessels only at high water, for the reason in general that
sand-bars are frequently formed at the mouths of tidal estuaries. One of
the most marked illustrations of the influence of the rise and fall of
the tide on navigation occurs at St. John, New Brunswick, where the tide
flows in and out of St. John River so as to form a cascade each way,
dependent on the direction of the current. At low water the level of the
river is from 11 to 15 feet above the Bay of Fundy, and at high water
the level of the bay is from 8 to 12 feet above that of the river when
not affected by the tide. There are four periods of from ten to fifteen
minutes each during each twenty-four hours when vessels can pass in and
out of the river's mouth.
In the Arctic Ocean the tides are small. At Point Barrow, the most
northern locality on the coast of Alaska, the difference between high
and low water is but 6 or 7 inches. The tide comes from the southward
and westward, and there is a prevailing current setting to the eastward.
At Herschel Island, near where the east boundary of Alaska reaches the
Arctic Ocean, the mean range of the tide is but 1.8 foot. At Cape
Sheridan, the northeast point of Grinnell Land, north latitude 82 deg. 25',
there is a range of 2.6 feet during two periods each month when the
tides are highest, and but 1.2 foot at the lowest or neap tide periods.
In addition to the weakness of the tides and tidal currents along the
arctic coast, there is an absence or great diminution of the influence
of wave and currents, owing to the prevalence of ice on the sea. Shore
erosion is there at a minimum in spite of the abrasion produced by the
ice-packs when forced landward by the wind.
_Islands._--In the classification of islands used by A. R. Wallace in
his Island Life two primary divisions are recognised, namely,
continental and oceanic islands.
Continental islands are land masses which have been separated from
continents and are rarely far removed from their borders, and,
besides, are composed of rocks similar to those of the neighbouring
mainland and inhabited by terrestrial animals which are related to the
fauna of the larger land area. Ancient and modern continental islands
have also been recognised, their age being indicated by the degree of
similarity between their faunas and the fauna of the continent with
which they were formerly connected. Those of ancient origin are commonly
surrounded by deep water, while those which are more modern usually rise
from continental shelves, the channels intervening between them and the
mainland being less than 100 fathoms deep. Oceanic islands rise from
deep water, are either volcanic or so far as their emerged portions are
concerned composed of coral rock, and are without warm-blooded
terrestrial animals.
About the borders of North America there are islands belonging to each
of these classes. The numerous examples rising from the continental
shelf all about the margin of the land, but in the Atlantic most
numerous from New York northward, and in the Pacific from the Strait of
Fuca northward, are plainly recent continental islands. The larger of
the West Indies and the group of small islands off the California coast
are also continental islands, but show by the character of their faunas
and the depth of the water about them that they have been long separated
from the main mass of the continent. Typical examples of oceanic islands
are furnished by Bermuda, in the Atlantic, and Guadalupe, in the
Pacific. In this same class, but less remote from the mainland, and in
their faunas and floras showing a nearer relationship to South than to
North America, belong the Caribbees.
TOPOGRAPHY OF THE COAST
The generalized coast-line of North America measures about 35,000 miles
in extent, and presents a great variety of scenery. The range in
diversity embraces all classes of coast topography from the low, sandy
mangrove-fringed borders of Florida and the Gulf of Mexico, to the
magnificent sea-cliffs of Labrador and British Columbia and the
marvellous ice-walls of Greenland and Alaska where tide-water glaciers
enter the ocean.
Like nearly all the features of the earth's surface, this narrow
intricate belt where the sea and land meet is constantly undergoing
changes. The principal processes which lead to alterations in the
coast-line may be considered as forming three groups: First, the wearing
away of the land through the action of waves and currents and the
deposition of the _debris_ thus produced so as to make additions to the
borders of the continent; second, the upward and downward movements of
the land; and third, the changes produced by glaciers, ice-flows, and
icebergs.
With these more active agencies by which the coast-line is being
modified may be included chemical solution and deposition, the influence
of plants and animals, the weathering of the margin of the land, etc.;
but a critical review of all these processes is impracticable in the
present treatise.
_Changes in the Coast-Line due to Waves and Currents._--The waves of the
sea beat on the land with never-ceasing activity, but exert the greatest
force during storms. The blow which a great surge strikes when it breaks
at the base of a cliff, amounting in many instances to 3 or more tons to
the square foot, tends to disrupt the rocks both directly by its impact
and by the compression of air and water in their interstices. The
greatest work of the breaking waves is performed, however, with the aid
of the stones which accumulate on the beaches. These are hurled against
the land by the force of the landward-rushing waters and break and
abrade the rocks with which they come in contact. The friction produced
by the impact of waves charged with sand, pebbles, and boulders against
the land leads to its removal along a horizontal belt with a narrow
vertical range. The waves of the sea, in fact, act like a horizontal
saw, the edge of which slowly advances landward. As a result of this
process of under-cutting, highly characteristic and frequently most
picturesque forms are given to rocky coasts. Whenever the sea is
bordered by hard rocks standing well above the surface, but not rising
too precipitously from deep water, we find cliffs facing seaward.
At the base of each of these sea-cliffs there is a shelf or terrace
which records, in part at least, the advance that the sea has made
inland.
A cross profile of a wave-cut seashore (Fig. 5) shows two prominent
features, namely, a sea-cliff with a horizontal base, and a terrace
sloping seaward from the foot of the cliff. Of these, the cliff is by
far the more prominent as it stands up boldly to view, while the terrace
is in large part and perhaps wholly submerged. These two leading
characteristics in the topography of wave-cut shores are shown in the
following diagram:
[Illustration: FIG. 5.--Ideal profile of a sea-cliff and current-built
terrace.]
The water carried landward by each wave as it rushes up the sloping
surface of a terrace again finds its way seaward, either wholly or in
part, as an "undertow." Much of the rock _debris_ ground fine by the
ceaseless beating of the surf is separated from the coarser material,
thus leaving the latter free to be moved by succeeding waves, and is
carried seaward by the bottom current or undertow. During storms
especially there is usually to be seen a belt of discoloured water
seaward from the white breakers which margin the land. The finer
_debris_ carried away from the shore by the undertow is sooner or later
deposited, and much of it is laid down on the terrace bordering the land
and serves to build out its seaward margin. A normal sea-terrace is thus
in part the result of the cutting away of the land, and in part of the
deposition of the material removed. The sea not only cuts away the land,
however, but at many localities makes important additions to it.
Where the water is shallow the larger waves break at a distance perhaps
of several miles from the coast-line, and build up long narrow bars,
usually of sand, which form barriers, more or less parallel with the
shore, and shelter it from further encroachments of the sea. Again, when
the wind from the sea blows obliquely to the coast, currents are
established in the water which sweep along the loose material on the
beach and on the submerged portion of the terrace of which the beach is
a visible part, and cause it to travel in the general direction of the
prevailing on-shore winds. This action also leads to the building of
bars more or less parallel with the coast and at the extremities of
capes, particularly where the shore currents enter deeper water and give
origin to spits of various shapes, which are frequently curved towards
the land and at their extremities become hooks and loops.
There are thus two important processes, one destructive and the other
constructive, by which the sea is continually modifying the border of
the land.
When once the underlying principles on which depend the characteristics
of coastal topography are suggested, any observant person can apply them
for himself and thus be able to read the history as well as admire the
beauties of seacoast scenery. It is not necessary, therefore, to attempt
to present a detailed account of the coasts of North America from a
purely geographical point of view; there are certain results of the
processes just referred to, however, which are of wide-reaching
economic, and especially of commercial interest.
[Illustration: FIG. 6.--A portion of the Atlantic coast of the United
States.]
From Central America northward to Cape Cod the rocks bordering the sea
are soft or easily soluble, and the adjacent land of low relief.
Throughout this section the work of the sea is mostly constructive, and
the margin of the land is sheltered by sand-bars from the attack of
waves and currents. Where the waves of the open ocean do reach the land,
as on the coast of New Jersey, the sea-cliffs are low and the topography
of a mild type. Very generally, as along the coast of Mexico and Texas,
and from Florida to Long Island, there are long narrow bars adjacent to
the shore, with lagoons intervening between them and the mainland.
These features are well illustrated on the accompanying map (Fig. 6) of
a portion of the Atlantic coast where long narrow bars, sometimes
forming skeleton capes, are a characteristic feature. On the middle
Atlantic coast of the United States the prevailing winds blow southward
and there is a general southward flow of the shore currents, which carry
with them the sand on the beaches and bars. An interesting fact in this
connection, pointed out by N. S. Shaler, is that although the sands are
continually being moved they are not worn out. After the sand-grains
have been reduced to a certain size they retain films of water which
separate them one from another, and act as cushions which prevent the
grains from coming in contact, thus greatly retarding further
comminution. But for the protection thus afforded the sand-bars would be
removed and the border of the land exposed to the attack of the waves
and cut away; whereas under existing conditions lagoons are formed,
which in many instances are utilized as harbours or are filled by
wind-blown sand, the sediment brought by streams, plant growths, etc.,
and valuable additions are made to the continent.
[Illustration: FIG. 7.--Mobile Bay.]
The sand-bars just referred to frequently cross the mouths of rivers,
and in such instances a struggle ensues between the currents moving
along the shore and the outflowing river-waters aided by the currents
produced by the tides. This conflict leads to the formation of sand
banks and bars, generally submerged, across the entrances of bays and
inlets and to the building of sand-spits from the seaward capes. A
typical instance is furnished at the entrance of Mobile Bay (Fig. 7),
where a spit from each side has been built by shore currents so as
to greatly contract the tideway between. Similar features are presented
by Sandy Hook and Coney Island, each of which has been built of sand
deposited by shore currents at the seaward entrance of the lower New
York Bay. Another illustration of this same general character is
furnished by the curved extremity of Cape Cod (Fig. 8), which is a
sand-spit of large size with a hooked extremity. Spits of this nature
are common on our coasts, and in many instances themselves form
harbours, as at Coney Island and near the extremity of Cape Cod. Many
other similar examples of the importance of lagoons, sand-bars, spits,
etc., to shipping, which occur, especially along the Atlantic coast of
the United States, may be studied to advantage on the admirable charts
of the United States Coast and Geodetic Survey.
[Illustration: FIG. 8.--Cape Cod, Massachusetts.]
The sand-bars, spits, and other similar structures along the Atlantic
coast are also of strategic importance, for the reason that they afford
advantageous sites for fortifications, as is illustrated by the strong
forts at Sandy Hook which guard the entrance of New York Bay. These
sea-built foundations are also utilized in a large number of localities
for lighthouses. The waterways shut off from the sea by off-shore bars
in some instances permit of the passage of vessels from one harbour to
another. In this connection it is of interest to note that an important
system of canals is under consideration for making a continuous waterway
for deep-draft vessels, some 700 miles long, which will connect the
estuaries and lagoons from New York to the Carolinas.
While the islands of sand referred to present many conditions favourable
to commerce, fisheries, and other industries, their apparent durability
is deceptive, and in some instances faith in their permanence has led to
disastrous results. They owe their existence to the action of waves and
currents, and unless blown sand is heaped upon them are raised but a few
feet above mean sea-level, and are liable to inundation if a high tide
is accompanied by an on-shore gale. A sad illustration of this plain
conclusion is furnished by the disaster that overwhelmed the city of
Galveston on the night of September 8, 1900, during which some 3,000
people perished and $20,000,000 to $30,000,000 worth of property was
destroyed. This great loss was in large part due to the fact that the
city was inundated by the advance over its site of the storm-driven
waters of the Gulf of Mexico. The island on which Galveston stands (Fig.
9) was built by the waters of the Gulf, and during the hurricane
referred to they again claimed their own.
[Illustration: FIG. 9.--Coast of Texas.]
Northward of Cape Cod, the rocks adjacent to the ocean are mostly hard
and resistant, consisting largely of schist, gneiss, granite, trap,
etc., which when undercut by the waves stand as bold cliffs and
headlands. This portion of the continental border abounds in picturesque
scenery and is abundantly supplied with fine harbours and well-sheltered
havens in which boats may take refuge. Typical portions of this
rugged coast are furnished by the magnificent sea-cliffs of Mount Desert
and Grand Manan islands, the bold shores of Newfoundland and Labrador,
and the precipitous border of Greenland. The scenery throughout nearly
all of this vast extent of wave and storm beaten rocks is in striking
contrast to the mild and generally monotonous sand-built shores to the
south of Cape Cod. Between the angular headlands and rugged capes at the
north, with their white girdles of surf, there are frequently curved
beaches and numerous spits and bars of yellow sand which connect the
salients of the shore or extend from them so as to furnish safe
anchorages.
On the arctic coast of North America the action of the waves and
currents on the land is greatly retarded by ice, and the tides are
small, but to what extent these conditions unfavourable to the work of
the sea are counterbalanced by the abrasion performed by ice-floes is
unknown. The northern border of Alaska, as well as the shore of Bering
Sea, is mostly low and the rocks soft, although certain of the sea-capes
are bold and are evidently composed of resistant material.
The Aleutian Islands present a peculiar exception to the general coast
topography of the rest of the continent. Although this region has not
been studied in detail, it seems to furnish an example of a rugged
mountain range that has been partially submerged at a comparatively
recent date. The rocks in many places descend precipitously into deep
water, leaving no room for the formation of beaches, and hence the
waves, to a great extent, are without tools with which to cut away the
land. At the heads of the many bays and inlets, however, one finds
beautiful sand-beaches with gracefully curving lines, in striking
contrast to the dark, rugged cliffs bordering their seaward extensions.
The southern and southeastern borders of Alaska are exceedingly bold,
and present some of the most sublime coast scenery to be found in the
world, but to the geographer the greatest interest of this portion of
the continental border, as is true also of the entire Pacific coast of
North America, centres in its relation to up and down movements of the
land.
_Changes in the Coast-Line due to Oscillation of the Land._--Land areas
are exposed to the erosive action of wind, rain, streams, etc., and are
sculptured by these agencies into valleys, canyons, peaks, ridges, and
other familiar topographic forms. The various processes by which land
areas are modified lead in general to a roughening of the surface. As an
extreme illustration, a high plateau becomes dissected by streams so as
to form an intricate system of rugged mountain ridges and peaks, with
deep, steep-sided valleys between. The degree of this roughening depends
principally on the elevation of the land, together with contrasts in the
resistance of the rocks due mainly to variation in hardness, climatic
conditions, etc., but in general one may say the higher the land
is raised above the sea the more rugged will be its topography as the
process of wearing away progresses. It is to be remembered in this
connection, however, that land areas pass through a somewhat definite
series of changes, from topographic youth to topographic old age, each
stage being accompanied by changes in the relief. It is during
topographic maturity that the greatest roughness of the surface of a
land area is produced.
Land areas are continually wasting away, owing especially to the attacks
of the streams, and the material removed is deposited in the sea. The
_debris_ brought from the continents by streams is laid down in shallow
water--about the shores of North America almost entirely on the surface
of the continental shelf--and in this region of deposition the hollows
are filled and a generally smooth surface given to the sea-floor.
The topography of the land, for the reason stated above, is nearly
everywhere uneven; while the topography of the sea-floor is
characterized by uniformity. We can easily predict, therefore, the
general character of the changes in a coast which would result from
either a subsidence of the land, thus allowing the sea to encroach upon
it, or of an elevation, which would expose a portion of the sea-bottom,
thereby increasing the area of the land. A subsidence of the land
adjacent to the sea permits an extension of the waters landward; the sea
will enter the valleys so as to form estuaries, bays, straits, etc.,
while the high land between the partially water-filled depressions will
rise above the water-level and appear as peninsulas, capes, and islands.
A bold, deeply sculptured coast when depressed will give origin to an
intricate, and what may be termed a ragged shore-line; while a lower
region crossed by large river-valleys would be changed to a system of
broad estuaries.
An upward movement in the earth's crust along the ocean's shore would
expose a portion of the sea-floor and add a strip of generally level
country to the previous land area. The boundary between the old and new
topography in such an instance would be the upraised coast-line with
its sea-cliffs, wave-cut caves, terraces, beaches, and other
characteristic features of coast topography.
There are thus two strongly contrasted types of coast scenery, produced
by oscillations of the earth's crust where ocean and continents meet. In
each class there is a wide range in details, which vary in harmony with
the amount the land rises or falls in reference to sea-level.
When one has these general laws in mind a map of the coast-line of North
America acquires great significance.
From about the latitude of New York southward to Central America many
comparatively small oscillations of the land have occurred in recent
geological time, and what was formerly a portion of the continental
shelf is now exposed and forms a coastal plain. This plain, in general
from 50 to 100 miles broad, slopes gently seaward, and its continuation
under the sea forms the present continental shelf (Fig. 2). Evidently a
slight up or down movement or a gentle tilting of this partially
submerged plain in an east and west direction would cause a marked
advance or recession of the sea. Each time the sea advanced the country
submerged would be smoothed over by the action of the waves and currents
and a sheet of sediment laid down upon it; and each time the sea receded
the emerged land would be trenched by the rivers flowing across it. The
records show that many such changes have occurred.
The Gulf border of Mexico and Texas, composed of soft marine sediments,
forms a gently sloping plain bordered on the west by a roughened upland,
and illustrates the general feature of a recently emerged coastal plain
(Fig. 9). The same is true also of the entire coast from Texas to New
York, but it happens that a recent movement through this region was of
such a nature as to allow the sea to encroach on the land, and the
previously excavated stream valleys are now, in part, occupied by the
sea. This feature is most marked from the Carolinas to New York (Fig.
6), where there are several great estuaries and drowned river-valleys
which extend far into the land. The best examples are Albemarle Sound
and Chesapeake and Delaware Bays. The James River channel is submerged
as far as Richmond, the Potomac to Washington, the Susquehanna to
Harrisburg, the Delaware to Trenton, and the Hudson to Troy. These are
typical illustrations of what geographers term drowned river-valleys.
They are evidence that the land formerly stood higher than now, was
trenched by the rivers that flowed across it, and was then depressed or
tilted so as to allow the sea to encroach upon it. The importance of
these events in the settlement of North America by Europeans and on the
subsequent development of commerce, manufactures, the location of
cities, etc., needs only to be suggested to permit the reader to fill in
the details for himself.
On the Gulf coast and about Florida the later movements of the land have
been less than in the region from Albemarle Sound to New York, and
estuaries are there absent, with the somewhat marked exception of Mobile
Bay. Certain secondary conditions need to be introduced here, but space
will not permit of more than a brief presentation of them. Not only have
the recent movements of the land been less about the shores of the Gulf
of Mexico than in the middle Atlantic region of the United States, but
the rivers at the south are in general smaller and less swift than those
farther north, and hence are less able to excavate broad valleys. The
Southern rivers, such as the Alabama, Mississippi, Rio Grande, etc., are
silt-laden and tend to fill their estuaries, while the weaker streams
are unable to resist the encroachments of sand-bars and spits built by
shore currents, and their mouths have been practically closed. The coast
of Texas gives evidence of slight modern subsidence, but the small
estuaries formed have, for the most part, been separated from the Gulf
by sand-bars.
Northward of the middle Atlantic region the recent oscillations of the
land continued to increase and reached a maximum about the shores of the
Arctic Ocean; on the Pacific coast also there is similar evidence of an
increase in the recent earth movements from the south northward.
In an outline sketch of the present coastal topography of the continent
we can generalize, and say that the whole continent during the late
Tertiary, glacial, and recent times has swayed up and down about a
hinge-line situated in the region of the Gulf of Mexico, and the
movements, although not uniform, have increased in amount from the south
northward. Let us glance at the evidence on which this broad statement,
involving the up and down surging of a vast continent, is based.
The Hudson, as stated above, is a drowned river as far as Troy, a
distance from the present land margin of 160 miles. In the next great
river to the northward, the St. Lawrence, the tide rises and falls
nearly up to Montreal, a distance of about 800 miles from the general
shore-line. Still farther north are Hudson Strait and Hudson Bay, which,
although but imperfectly explored, seem to be an example not only of the
drowning of a river-valley, but of the largest part of a river-basin.
The geography of the arctic archipelago fringing the north shore of the
continent also suggests that a strongly stream-cut plateau has there
been deeply submerged.
In addition to the drowned river-valleys and ragged coasts which record
a subsidence of the land, there are raised terraces and beaches which
begin at the south near New York and increase in elevation above the
present sea-level, when followed northward, all the way to the arctic
region, and have in the far north an altitude of about 1,200 feet. These
old beaches and terraces show that the land was formerly depressed and
has since risen; but, as shown above, has not regained the elevation it
had previous to the glacial epoch.
The marked differences in the geography of the coast from New York
northward to the Arctic Ocean, and from the same locality southward to
Central America, are due primarily to the fact that the oscillations of
the land have been such that at the north the continental shelf is
entirely submerged and the sea has encroached on a rough land; while at
the south the recent oscillations have been less and a broad margin of
the continental shelf is exposed and forms the coastal plain.
At the north, we find innumerable islands, bold, rocky shores with many
capes and headlands, separated by deep inlets, sounds, straits,
bays, etc., or, in brief, a ragged coast such as finds typical
illustration on the shores of Maine (Fig. 10), while at the south (Fig.
6) the shores are low, sandy, remarkably uniform in trend, and without
islands, excepting such as are built by the waves and currents. The West
India Islands will, no doubt, be recalled by the reader, but their
history is again different. Intermediate between the land that has
experienced great oscillation at the north and the region of less
energetic movements at the south is the series of large estuaries
mentioned above, in the narrower portions of the coastal plain.
[Illustration: FIG. 10.--A portion of the coast of Maine.]
The northern and western coasts of Alaska are mostly low, and correspond
in a general way with the coastal plan of the Carolina region. The last
well-marked movement of the land in that region has been in the
direction of an elevation, and we find low shores, with but few
harbours, similar in many ways to the coast of Texas.
It is probably true, as already stated, that the Aleutian Islands,
although in part the result of recent volcanic activity, owe their
peculiar and exceptional characteristics to the partial subsidence of a
deeply sculptured mountain range. On the south coast of Alaska, in the
region of Mount St. Elias and Mount Fairweather, a recent and
extensive elevation has occurred, which, however, did not bring the
bottom of the adjacent portion of the ocean above the sea-level. This
apparent anomaly seems to be due to an uprising of the rocks along the
north side of a break, or belt of branching fractures, which closely
approximates to the coast-line and has determined the position of the
continental border in that region. The facts, so far as known, appear to
show that we have here what geologists term a fault, the north or
landward side of which has been raised at least 5,000 feet in very
modern times, but, so far as we can judge, without disturbing the
seaward border of the break. The coast between Mount Fairweather and
Mount St. Elias is by far the boldest, and from a scenic point of view
the most impressive, portion of the entire shore-line of North America.
The mountains are young and among the highest on the continent. They
rise precipitously from the margin of the sea, and are sheathed in snow
and ice from base to summit throughout the year.
The margin of the continent southward from Mount Fairweather to the
Columbia River, a distance in a straight line of about 1,200 miles,
furnishes some of the best illustrations of the changes in coastal
geography due to subsidence that our continent affords (Fig. 11). This
wonderfully irregular coast is fringed with a belt of mountainous
islands from 50 to 100 or more miles broad. The inlets between the bold
capes and the straits separating the numerous islands are deep. The
rugged, forest-clothed slopes with precipitous, and in many instances
nearly vertical walls, descend into water that is frequently from 50 to
over 200 fathoms deep. In brief, a deeply dissected mountain range more
than 1,000 miles in length has there been depressed at least 2,000 feet
below its former altitude, thus allowing the sea to flood its deep,
picturesque valleys.
Puget Sound, with its numerous and frequently narrow arms (Fig. 23), is
the southward extension of the partially inundated country considered
above. To the west of this magnificent sound rise the Olympic Mountains,
which barely escape being an island at the present stage of the
swaying of the land. On the west, as on the east border of the
continent, there are drowned river-valleys, such as the Stikine, Frazer,
Columbia, and Sacramento. It is not to be understood, however, that the
entire Pacific coast region has been raised or depressed as a unit.
There have been differential movements in some of its parts, but these
are not as yet well known. In southern California, for example, raised
beaches and a narrow coastal plain about Los Angeles give evidence of a
modern rise of the land.
[Illustration: FIG. 11.--Coast of southeastern Alaska.]
In reference to the broad generalization that the continental mass of
North America has undergone up and down movements, greatest at the north
and decreasing southward, as if moving on a hinge-line running east and
west in the region of the Gulf of Mexico, it is of interest to note that
the ragged coasts of Maine, Nova Scotia, Newfoundland and
Labrador, due to the partial submergence of a rugged land, lie in the
same latitudes as the equally ragged coast of Washington, British
Columbia, and Alaska. This is more than a coincidence. The rocks on the
two coasts are similar, being for the most part resistant crystalline
schists, gneisses, granites, etc., and in each instance stood high above
the sea for a long period during which they were deeply trenched by
streams and by great glaciers, and then at about the same time, as
nearly as can be judged, each region was depressed so as to allow the
sea to encroach upon it.
While a deeply sculptured land when partially submerged gives origin to
a ragged coast, a region of similar elevation, but not cut by streams or
other agencies so as to have deep valleys, when subsidence occurs
produces a bold, harbourless shore without islands. The striking
contrast between the deeply indented border of the continent, with its
broad fringe of islands, from Mount Fairweather southward to Mount
Olympus, and the remarkably uniform although bold coast-line from Mount
Olympus southward to Mexico, and indeed nearly to Cape Horn, has much
significance in this connection.
The mountains bordering the Pacific coast of the United States are among
the younger on the continent. These coast ranges, largely on account of
their youth, have not been deeply sculptured, but rise boldly from the
ocean's shore throughout nearly the entire distance from the Strait of
Fuca to the end of the peninsula of Lower California. The mountains of
Central America, although but little known, are of comparatively recent
date, but differ from the coast ranges in being more largely built of
young volcanic rocks. Both the coast ranges and the mountains of Central
America are much less deeply sculptured than the mountains bordering the
Pacific to the north of Puget Sound, and a subsidence along this shore
would produce but moderate changes in the coast-line. In this great
extent of coast, measuring nearly 5,000 miles, there are but few
harbours; in the portion belonging to the United States the generally
bold coast-line is broken but in two places, one where the
Columbia reaches the sea, and the other where the Sacramento finds an
outlet through the portions of its drowned valley known as the Golden
Gate.
The Bay of San Francisco owes its origin to a subsidence of the land
which has admitted the sea into the valley of the Sacramento, but this
valley, which, uniting with the one at the south drained by the San
Joaquin, forms the Great Valley of California, is not due to stream
erosion, as in the case of the drowned valley of the Hudson or of the
St. Lawrence, but to the upraising of the mountains bordering it. During
a former time of greater subsidence than at present the Bay of San
Francisco was larger than now, and has been contracted both by the
deposition of sediment and by a partial re-elevation of the land. The
exceptional character of the Bay of San Francisco and its marked
excellence as a harbour give to the city on its shore promises of
marvellous development.
The Gulf of California is due, in a general view, to what may be
considered as a departure of the Coast mountains away from the general
trend of the continental border. We have but little detailed information
concerning this region, however, and the studies of modern geographers
have likewise been meagre throughout all the coast-line farther south.
The Pacific coast of Mexico is geographically similar to that of
California, but instead of a single great harbour there are four of
moderate size and excellence, the histories of which have not been
studied. Farther south, along the Central American coast, the shores are
bold, but several indentations, due in part at least to volcanic
agencies, furnish shelter for vessels and offer encouragement to
commerce.
The bold and not deeply sculptured mountains along the nearly unbroken
coast from the Strait of Fuca to Panama, rise close to the true border
of the continent. The continental shelf of this portion of the shore of
the Pacific is narrow. An elevation of 100 fathoms would add scarcely
more than 10 miles to the extent of the land. This narrowness of the
continental shelf seems to be due to the recency of the uplifting
of the Coast mountains, and the lack of time for the _debris_ from the
land and the organic refuse of the sea to shoal the water. The Pacific
basin is deep close to the land bordering it, thus restricting the
seaward extension of the continental shelf.
_Changes in the Coast-Line due to Ice._--It is now well known that
glacial ice many hundreds of feet thick formerly covered the northern
half of North America and flowed outward across the present position of
the coast-line throughout all of the northern border of the continent
from Staten Island in the east and Puget Sound in the west, with the
exception of the Arctic and Bering Sea coasts of Alaska.
The effects of this outward-flowing ice on the topography of the
continental border crossed by it were in general in two directions.
Where the land was rough or moderately so previous to the coming of the
ice-sheets the inequalities of surface were increased; but where the
land was smooth or but gently undulating its elevations were planed away
by the glaciers and made still more smooth. The reason for these
differences is that when the land from which the ice flowed was rugged
or had previously been deeply trenched by streams, the valleys gave
direction to the ice currents and the margins of the continental
ice-sheets became divided into separate ice-streams, as is the case in
Greenland at the present day. This localization of the ice currents
served to deepen and broaden the pre-existing valleys, and especially on
the bold coast of Alaska and British Columbia increased in a marked way
the inequalities of the surface and favoured the production of a ragged
coast-line when the ice melted and was replaced in part by the sea.
When, however, the topography of the land was not sufficiently accented
to cause the ice flowing over it to gather into well-defined currents
the general surface was worn down, thus favouring the production of an
even coast-line after the melting of the ice-sheets.
Where the coast-lands were high and rugged, the deepening and broadening
of the valleys led to the origin of deep, narrow, canal-like waterways
termed fiords (_fjords_), when the ice withdrew from the partially
submerged land or when subsequent depression carried the glaciated
troughs below sea-level. On the Atlantic coast from Maine to Labrador,
and thence northward to the Arctic Ocean, there are numerous examples of
fiords, as is also the case on the Pacific coast from Mount St. Elias to
Puget Sound. At the present time the localized ice-streams from the
great central ice-sheet of Greenland are continuing this process of
fiord excavation. The same is true also, but on a much smaller scale, of
the tide-water glaciers of southern Alaska.
The shores of the northern portion of the continent from New England to
the Aleutian Islands are now being modified by the grinding of
ice-floes, which are driven against the land by the wind. This process,
however, although locally important, need claim but little attention in
a general view of the geography of the continent.
_Changes in the Coast-Line due to the Deposits made by Streams._--The
visible loads of silt and sand in suspension carried to the ocean by
streams, as well as the material the streams roll and push along their
bottoms, is delivered to the waters of the ocean and deposited in
various ways. Much of this material, notably the coarser portion, is
dropped near land and the finer portion floated far out from the coast
before settling to the bottom. Two classes of deposits made in this way
may be recognised, namely, those laid down by the streams themselves as
they drop their loads on entering still water, or delta deposits; and
those spread over the sea-floor by waves and currents after receiving
the _debris_ brought from the land. Which of these two modes of
deposition will prevail depends on whether the waters of the ocean at
the localities where the streams deliver their loads are essentially
still or are affected by strong currents. In the former instance all but
the finer of the _debris_ derived from the land is quickly dropped and
deltas are formed; and in the second instance the currents bear the
material away and deposit it either in the shallow water adjacent to the
neighbouring shore, forming shoals, bars, embankments, spits, etc., or
spread it in a sheet over the sea-floor. The most notable changes
in the coast-line resulting from this general process occur where
silt-laden streams enter still water and form deltas.
On the coast of North America many of the streams which enter estuaries
deliver their loads to waters which are agitated, especially by tidal
currents, and ill-defined shoals, sand-banks, etc., are produced. In
three conspicuous instances, however, large rivers are engaged in
building deltas, and thus producing well-marked changes in the
coast-line.
At the north, the Mackenzie enters the nearly tideless Arctic Ocean,
where floating ice almost completely counteracts the tendency of the
wind to produce currents, and a great delta is being extended seaward.
The river divides on its delta into many _distributaries_ and enters the
sea by several mouths. The sea near the mouths of the river is reported
to be shallow, and obstructed by many sand-banks and islands. No survey
of the Mackenzie delta has as yet been made, and but little definite
information concerning it is available.
The Yukon on entering the shallow eastern portion of Bering Sea, where
the influence of the tides is small and floating ice is present
throughout about nine months each year, is also engaged in building a
great delta which projects into the sea and gives the coast-line a bold
outward curve. The Yukon begins to divide into separate channels,
several of which enter the sea as independent distributaries at a
distance of about 150 miles from the outer border of its delta. The
distance between the outer finger-like division of the stream is about
90 miles. The Yukon is a graded stream--i. e., is able to carry material
in suspension, but not to deepen or fill its channel--in the lower
portion of its course, and is making an important addition to the land
owing to the dropping of its burden of silt as soon as the still water
into which it flows is reached. The stream is thus being extended, and
in order to enable it to continue its task of transportation and the
delivery of its load to the sea, the extended portion of its channel is
built up so as to give a slope down which the waters can
flow--that is, the beds of the distributaries are raised, and they also
shift their positions from time to time and make additions to the entire
surface of the delta. This extension of the stream and deposition of
silt by its distributaries have added about 1,000 square miles to the
land. Although the delta of the Yukon presents an admirable example of
the change in a coast-line produced by the sediment dropped by a great
river, the partial surveys of it that have been made are not as yet
available for study.
Fully as characteristic of the modification of coast-lines made by a
stream as any in the world is the well-known example of the delta of the
Mississippi. This classical instance illustrates not only the manner in
which coast-lines are modified, but the behaviour of a large silt-laden
stream which has reduced its valley to a low gradient, and throughout
hundreds of miles of its lower course is spreading out a wide flood
plain. The extension seaward of this flood plain forms the broad delta
at the river's mouth.
[Illustration: FIG. 12.--Delta of the Mississippi. After United States
Coast and Geodetic Survey.]
During high-water stages the Mississippi widely over-spreads its banks
and during such inundations of its valley drops much of the silt it
previously held in suspension. The material deposited is laid down most
abundantly on the immediate border of its low-water channel. Each side
of the channel is thus raised so as to form what is termed a natural
levee. During this process also the bed of the stream is raised by the
deposition of sediment upon it, thus tending to cause the stream to flow
on a raised ridge and producing an unstable condition which from time to
time enables the river to break across its confining levees and divide
into two or more separate channels. In the lower portion of the river
some of the new channels thus formed reach the sea and furnish
independent outlets for its waters. The first of these distributaries
now departs from the main channel at a distance of 200 miles from the
Gulf of Mexico, and farther seaward several other divisions occur (Fig.
12). The area of the delta is about 1,230 square miles. Each
distributary is engaged in building a pair of embankments, or natural
levees (although this process in recent years has been modified by the
construction of artificial embankments for the sake of improving
navigation), and each subdivision of the river is also building a delta.
Each of the finger-like extensions of the delta, shown on the
accompanying map, is due to the prolongation of a pair of embankments
into the Gulf by each distributary and the growth of a secondary delta
at its mouth. The river is thus building a highly compound delta,
composed of the secondary deltas formed at the mouth of each of its
distributaries. A conspicuous modification of the otherwise generally
evenly curved border of the Gulf of Mexico is thus produced, a result
that could only be reached in a water body but little disturbed by wind
or tidal currents.
ESTUARIES AND HARBOURS
The features of a coast of greatest importance to civilization are its
harbours. A coast without harbours is like a Chinese wall, and tends to
isolate a people inclosed by it. An indented coast with numerous havens
for the shelter of vessels fosters the interests of navigation,
including sea fisheries, invites commerce from other lands, and
stimulates its inhabitants to explore and travel. A diversity of
industries is thus favoured and the people adjacent to an indented coast
with good harbours tend to become more progressive and more cosmopolitan
than if intercourse with other communities is confined to overland
routes.
The Atlantic border of North America is abundantly supplied with fine
harbours, which not only favour communication with distant countries,
but are within easy reach of agricultural and forest lands and important
coal and other mineral deposits adjacent to the coast or in the
interior, and are near extensive and valuable fishing grounds. The best
of these harbours are at the mouths of rivers which have been depressed
so as to form estuaries with wide entrances. These sea-gates, however,
are frequently contracted, owing to the presence of sand-bars and spits
deposited by shore currents.
The great St. Lawrence estuary reaches to Montreal, and beyond lie the
Great Lakes, the rich lands of Ontario and New York, and the now
highly productive States of the Middle West. Two geographical features
in this basin detract from the conditions otherwise highly favourable to
commercial development, namely: the rapids in the St. Lawrence between
Montreal and Lake Ontario and the fall in the Niagara, and the winter
climate of Canada, which causes the rivers and estuaries to be ice-bound
for a considerable part of each year. To obviate the first of these
unfavourable conditions far-reaching plans for a deep waterway between
the Great Lakes and the Atlantic are now being matured. The splendid
harbours from Nova Scotia southward are never seriously obstructed by
ice, and south of Virginia ice is practically unknown.
The estuaries at the mouth of the Hudson, Delaware, Susquehanna,
Potomac, James, and the Alabama, together with the distributaries of the
Mississippi (which is not a partially drowned river, but one that is
building up and extending its channel), are the natural outlets of
portions of the continent of great fruitfulness. When other, and
especially climatic, conditions are considered, it will be seen that to
the geographer the Atlantic sea-border from the Gulf of St. Lawrence to
the Gulf of Mexico seems destined to be the next great commercial centre
in the succession from Greece to Britain. An important adjunct to the
present highly favourable geographical conditions pointing to a great
future for civilization on the Atlantic coast is the construction of a
ship-canal across the isthmus uniting North and South America. This step
must soon be taken.
A glance at a map of North America must impress one with the belief that
the Pacific coast with its great extent of harbourless water-front is
far less favourable to the growth or ideas, institutions, and industries
than the deeply indented Atlantic shore-line. From the Isthmus of Panama
to the State of Washington there is, as we have seen, but one harbour of
the first class, the estuary of the Sacramento, and one of the second or
third class, the estuary of the Columbia. From Puget Sound northward
harbours are numberless. There are two important geographical reasons,
however, why the general absence of good harbours to the south of
Puget Sound is not so serious as it perhaps might seem. First, the
mountain ranges run north and south parallel with the coast, and the
natural lines of interior travel lead to the outlets through the Coast
Ranges traversed by the Sacramento and the Columbia. The second and more
general reason is that, owing to the warm currents in the Pacific, the
portion of the west coast most favourable for a high degree of
civilization is situated farther north than the similar belt on the
Atlantic border.
* * * * *
By way of a summary of this chapter, the reader is asked to bear in mind
the fact that the land forming North America, as is the case with all
continents, is not at rest, but is subject to movements which cause
elevations and depressions of various portions of its area with
reference to sea-level. These movements have been in progress since the
birth of the continent, and still continue. An upward movement of the
earth's crust where the land and ocean meet causes a portion of the
sea-floor to emerge and an addition usually of the nature of a coastal
plain to be made to the border of a continent; while a reverse movement
enables the sea to advance on the land and to flood the low-grade
valleys opening to the ocean.
In a generalized view of the recent history of the coast-line of North
America the dominant fact is that to the north of the latitude of the
north shore of the Gulf of Mexico the resultant of the later movements
of the continent is downward; the amount of the depression thus caused
increases in a general way with increase in latitude on both the
Atlantic and Pacific coasts. This downward movement has permitted the
sea to encroach on the land and to flood many pre-existing valleys. On
the Atlantic coast it produced such estuaries as Chesapeake and Delaware
Bays, the tide-water portions of such rivers as the Hudson and the St.
Lawrence, and farther north, where the submergence was greater,
permitted the sea to invade the continental basin and form Hudson Bay.
To this same wide-reaching cause is due also the bold ragged coast-line
of the Atlantic from New England northward. On the Pacific border
the downward movement is recorded by the tide-water portions of the
Sacramento, Columbia, etc., and the deep picturesque fiords of the
Canadian and Alaskan coasts. The most decided influence of these changes
in the geography of the continent's margin on the affairs of men
resulted from the production of numerous fine harbours and the extension
of estuaries far inland, thus favouring commerce and fisheries in a high
degree.
As a result of the oscillations just referred to, the sea has gained
important characteristics as well as the land. The broad submerged shelf
fringing the continent furnishes conditions highly favourable to both
plant and animal life, and affords some of the most valuable
fishing-banks of the world.
The Caribbean region is a marked exception to the broader changes that
have affected the coast-line throughout the central and northern
portions of the continent, and stands by itself as a conspicuous
illustration of more localized earth movements which produced a
remarkable submarine topography.
LITERATURE
In continuation of the studies outlined in this chapter the following
books, most of which contain more special references, will be found of
assistance:
AGASSIZ, A. _Three Cruises of the Blake._ 2 vols. Houghton,
Mifflin & Co., Boston, 1888.
GILBERT, G. K. _The Topographic Features of Lake Shores._ In
United States Geological Survey, Fifth Annual Report, 1883-'84,
pp. 69-123.
GULLIVER, F. P. _Shoreline Topography._ In Proceedings of the
American Academy of Arts and Sciences, vol. xxxiv, 1899, pp.
151-258. Contains a valuable bibliography.
HILL, R. T. _The Geological History of the Isthmus of Panama and
Portions of Costa Rica._ Bulletin of the Museum of Comparative
Zoology at Harvard College, vol. xxviii, 1898, pp. 151-285.
HILL, R. T. _Cuba and Porto Rico with the Other Islands of the
West Indies._ The Century Co., New York, 1899.
SHALER, N. S. _Seacoast Swamps of the Eastern United States._ In
United States Geological Survey, Sixth Annual Report, 1884-'85,
pp. 353-398.
SHALER, N. S. _The Geological History of Harbors._ In United
States Geological Survey, Thirteenth Annual Report, Part II,
1891-'92, pp. 93-209.
SHALER, N. S. _Sea and Land._ Scribner, New York, 1894. _United
States Coast and Geodetic Survey._ Annual Reports, Coast Pilot,
Tide Tables, etc.
CHARTS AND MAPS
To the student of the geography of the coast and submerged border
of North America, the extensive series of charts published by
the _United States Coast and Geodetic Survey_ supply a vast
amount of accurate information. A catalogue of these charts,
with prices, etc., is issued by the Survey. Many of the charts
issued by the _United States Hydrographic Office_, and a large
number of the topographic maps published by the _United States
Geological Survey_, are of value in this same connection. An
account of these charts and maps, with instructions as to
methods of obtaining them, etc., is given in _Government Maps
for Use in Schools_, by Messrs. Davis, King, and Collie,
published by H. Holt & Co., New York, 1894.