THE TOPOGRAPHY OF THE LAND
Taking the better known portions of North America as a basis on which to
classify the leading geographical features of the continent, it is
convenient, and in the main sufficiently accurate, to recognise five
primary physiographic provinces. These are, in their general order, from
east to west:
1. Coastal plains and plateaus, of which the country between the
Atlantic Ocean and the Appalachian Mountains furnishes the most typical
examples.
2. A series of mountain ranges embracing all of the more elevated
country on the east side of the continent from Georgia northward to the
arctic archipelago, and in this book termed the _Atlantic Mountains_.
3. The great system of plains and plateaus extending from the Gulf of
Mexico northward to the Arctic Ocean and bordered on the east by the
Atlantic Mountains and on the west by a still greater series of
mountains, which may be designated with sufficient accuracy as the
_Continental Basin_.
4. A group of mountain chains and mountain ranges on the west side of
the continent, including the Rocky Mountains, Sierra Nevada, Cascades,
etc., and sometimes termed the Cordilleras. Under the scheme of
classification here used, this highly complex belt of rugged country
extending from south-central Mexico northward to the Arctic Ocean is
termed the _Pacific Cordillera_, or, in less technical language, the
_Pacific Mountains_.[1]
[1] The propriety of using the names here employed for the larger
physiographic provinces of North America has been discussed by several
writers in the _Bulletin of the Geographical Society of Philadelphia_,
vol. ii, 1899, pp. 55-69.
[Illustration: FIG. 13.--The larger physiographic divisions of North
America.]
Each of the four physiographic provinces briefly described above is in a
conspicuous manner elongated in a north and south direction. The
mountains, valleys, and plateaus, as well as the controlling lines of
structure in the rocks below the surface, throughout the main body of
the continent coincide in direction more or less nearly with the
parallels of longitude. At the south, however, and crossing the trend of
each of the provinces named above, is the:
Fifth, or Caribbean province, which includes the West Indies, the
southern part of Mexico, and all of Central America. In this
province are the _Antillean Mountains_, now mostly submerged, the
principal axes of which trend east and west.
With this brief outline of the larger physical divisions of North
America in mind, let us endeavour to become acquainted with the leading
characteristics of each of the provinces as they exist to-day, and at
the same time learn something of their long and varied histories.
COASTAL PLAINS AND PLATEAUS
_The Coastal Plains._--From New York to Key West and thence about the
borders of the Gulf of Mexico to the neighbourhood of Vera Cruz, the
border of the present land area of the continent is formed by a low
plain, from 30 to 50 miles broad in New Jersey, but increasing in width
southward to Georgia and Florida, where its somewhat indefinite inland
margin is more than 100 miles from the sea, and reaching its greatest
development in the delta of the Mississippi. Extending southward about
the west coast of the Gulf, it forms the low, featureless eastern border
of Texas, about 50 miles broad, and passes into Mexico, but gradually
narrows as the Pacific Mountains approach the coast, and ends in the
vicinity of Vera Cruz.
The Atlantic and Gulf coastal plain everywhere slopes gently seaward,
and on its landward margin has an elevation in general of from 200 to
300 feet. The character of the material of which the coastal plain is
composed, the fossils contained in it, as well as its geographical
features, show that it is a continuation of the continental shelf, and
was formed at a time when the border of the continent was more deeply
submerged than at present. Minor oscillations of the earth's crust have
time and again allowed the sea to extend inland, only to be forced to
recede when the land again rose. Each invasion of the sea left a sheet
of soft sediment over the portion of the land that was submerged. These
oscillations are still in progress, as is indicated by the fact that
along the New Jersey coast a downward movement at the rate of about 2
feet per century is taking place. A similar depression of the land
is also thought to be in progress along the south Atlantic coast and in
the delta of the Mississippi. The Atlantic coastal plain has its most
characteristic development in South Carolina, and is roughly divisible
according to its topography and soil into several belts parallel with
the shore-line. At the same time it is transversely divided into strips
by the several rivers which flow across it and by the many branches of
these rivers originating on the plain itself.
The junction of the portion of the gently sloping border of the
continent now above sea-level, with the submerged portion, is
characterized by the presence of a belt of swamps, in part marine
marshes where the salt water ebbs and flows, and in part fresh-water
morasses in which the drainage is obstructed largely by decaying
vegetation. Inland from the coastal swamps the surface becomes higher,
is for the most part well drained, and when not too sandy furnishes rich
agricultural lands. The Atlantic plain as a whole thus has three
principal divisions: a submerged portion, a marsh portion, and a
subaerial portion. During past ages the position of each of these belts
migrated, owing to movements in the earth's crust, but their succession
in reference to each other has been the same since the Tertiary period.
One of the most typical portions of the fringe of swamps now bordering
the land is situated in eastern Virginia and North Carolina, and is
known as the Dismal Swamp. In the central portion of this marshy region,
embracing some 700 square miles, lies Lake Drummond, an example of a
large number of small fresh-water lakes which are retained by rims
composed of plant growths and decaying vegetable matter. The mound of
vegetable _debris_ in the summit of which Lake Drummond is situated is
from 20 to 30 miles broad and rises some 12 feet above tide-level. The
lake is nearly circular, from 2 to 21/2 miles in diameter, and from 6
to 10 feet deep. The water is amber-coloured on account of the vegetable
matter in solution, but is clear and without sediment in suspension, and
is considered as remarkably wholesome. The lake was without definite
outlet previous to the cutting of drainage-canals, and is entirely
encircled by a dense forest, which has encroached on its border in such
a manner as to render its boundaries indefinite. The wall of rank
vegetation surrounding the open waters of the lake marks the beginning
of the encircling swamp. Standing in the lake and supported by their
widely expanded roots are several aged cypress-trees.
Along the coast of the Carolinas and Georgia sand-bars thrown up by the
sea have formed many lagoons (Fig. 6), which are being filled by the
wash of detritus from the land, by sand blown from their confining
ridges, and by vegetation and the hard parts of molluscs, crustaceans,
etc., living in their waters. In part, these areas have been converted
into swamps, and are gradually being transformed into dry land. Farther
southward, about the shores of Florida, and thence along the Gulf
border, the low, indefinite margin of the coastal plain is fringed in
many places by dense thickets of mangrove-trees, which extend their
aerial roots into the salt water, and by retaining sediment and dead
vegetation as well as by furnishing conditions favourable for animal
life, lead to a gradual extension of the land.
The west border of the coastal plain from New York southward to central
Georgia is at the junction of the soft, unconsolidated sands and clays
of the emerged portion of the continental shelf, with hard and usually
crystalline rocks of great geological age forming, an upland known as
the Piedmont plateau, which extends westward to the base of the
Appalachian Mountains. The sharply defined boundary between the plain
and the plateau is termed the _fall line_, for the reason that it is
marked by the lowest falls and rapids in the streams flowing eastward
from the Appalachian Mountains. Throughout the courses of these streams
to the west of the fall line they are shallow and swift and broken by
many picturesque rapids, while to the east of the fall line they broaden
in the soft sediments of the coastal plain, and are deep, placid streams
which widen into estuaries. The influence of the tides is felt in these
drowned rivers to the fall line. The most important fact in this
connection is that the lower courses of the larger rivers, such as
the Delaware, Susquehanna, Potomac, James, etc., are navigable for
ocean-going vessels, while their upper courses to the west of the fall
line are difficult to traverse even in canoes.
[Illustration: FIG. 14.--Relief map of North America. After United
States Geological Survey and Canadian Geological Survey.]
The fall line is thus the head of navigation in a number of rivers, and
for this reason it has determined the sites of several important cities.
Its course is marked by Trenton, Philadelphia, Baltimore, Washington,
Richmond, Weldon, Raleigh, Augusta, and Macon. Farther south, about the
landward margin of the portion of the coastal plain bordering the Gulf
of Mexico, the fall line is less distinct, largely for the reason that
the rocks bordering it on the north and west are less resistant than
those forming the plateau at the east base of the Appalachians.
Exceptions to the fact that the coastal plain is composed mostly of soft
sediments occur in southern Florida and in Yucatan, where coral rock has
been upraised. Southeastward from Yucatan a coastal plain is wanting and
rocky bluffs separated by stream-cut valleys come boldly down to the
surf line. Partially drowned valleys on each side of Central America
bear record of a recent but moderate downward movement of the land.
From New York northward along the border of the continent the coastal
plain is mostly lacking, or if recognisable, is greatly modified by
glacial deposits, and the Piedmont plateau, as it is known farther
south, swings eastward and becomes a coastal plateau with a more or less
roughened surface, which extends northward to Labrador and the Arctic
Ocean.
The geologically recent oscillations of the continent, as stated in the
preceding chapter, have been greatest in high latitudes, where the last
movement, as there are reasons for believing, was upward and is still
continuing. This rise, although it has not fully counteracted the
changes produced by a preceding downward movement, has caused the
shore-line to recede and a great area on the arctic border of the
continent which was previously submerged has thus become exposed. The
coastal plain on the west side of Hudson Bay, as described by T. B.
Tyrrell, is about 50 miles wide in the vicinity of Fort Churchill,
latitude 55 deg., and broadens rapidly northward of that locality. In
latitude 64 deg. the boundary between these new lands and the older plains
of the interior is about 300 miles from the present shore; thence
northwestward it has not been traced, but may be expected to cross the
Mackenzie some 250 miles from its mouth and pass westward into Alaska.
This arctic coastal plain is known in part as the Barren Grounds, but in
general may be designated as a _tundra_, as over extensive areas it is
similar to the still greater tundras of Siberia. This tundra forms the
extreme northern and northwestern border of the continent in arctic
Canada and northern and northwestern Alaska, and although but
imperfectly explored, has a length of probably 2,000 miles and a width
of from 50 to 60, and in places of over 100 miles. On the west coast of
Hudson Bay the tundra region slopes gradually from 500 to 600 feet above
the sea down to the present coast, and is traversed by sand and gravel
terraces and beaches or ridges which mark the former positions of the
sea margin. The lower ridges referred to are thickly strewn with shells
of molluscs belonging to species still living in the adjacent ocean
waters, thus indicating the recency of the emergence of the land. This
arctic coastal plain has the same general geographical features as the
coast plain on the southern Atlantic and Gulf border of the continent;
but, owing mainly to different climatic conditions, differs from its
southern representative in nearly every detail.
The tundra may be briefly defined as a vast frozen morass. The dense
mat-like vegetation consists principally of mosses and lichens (but not
noticeably of _Sphagnum_ or peat-moss, as is sometimes stated), and
during the short and not infrequently hot summers is beautified by a
multitude of low flowering herbaceous plants. Trees are absent, except
along the inland border, where the tundra merges with the subarctic
forest. To the north, or seaward from the isolated groves of stunted
spruce-trees marking the "continental timber-line," the only
representative of arboreal vegetation is usually the slim osier-like
arctic willow which grows in sheltered localities and attains a height
of 3 to 5 feet. Near the streams there are in some localities broad
areas covered with dark-green meadow-like growths of rushes
(_Equiseta_). The luxuriant flowering plants spring into existence
as if by magic as soon as the winter's snow melts, and under the warmth
and light of the nightless arctic summer grow with wonderful rapidity.
In winter the tundra is snow-covered, but the snow is less deep than in
more humid regions, and the cold is intense. The bog becomes deeply
frozen, and is not completely thawed during the succeeding summer. Even
in midsummer, when the surface is a luxuriant garden of flowers and
fresh gray-green moss, ice exists a foot or two beneath the luxuriant
carpet and extends to a great but unknown depth. Excavations made in
Alaska have shown that the perennial, dirt-stained ice beneath the
tundra is at least 25 feet thick, but this is by no means its maximum
depth. On the shore of Eschscholtz Bay and along the Kowak River
sections of the tundra exposed in cliffs indicate a thickness of 150 to
300 feet of ice, covered by a thin layer of black peaty soil. The
similar region in Siberia, as shown by borings, is known to be
permanently frozen to a depth of 380 feet deep. The subsoil ice is
sheltered by the vegetation and the peaty soil resting on it, from the
heat of the short summers, and the part softened by the summer's sun is
refrozen during the long intensely cold winters. It is probable that
under the present climatic conditions a sheet of perennial ice would be
formed beneath the tundra, but the suggestion that the ice now present
is in part an inheritance from a former period of greater cold is not
without support. The vegetation of the tundra grows each year at the
surface, while the partially decayed material below is frozen and
preserved. This increase in depth of the vegetable matter is much the
same as the growth of peat in temperate latitudes, except that the
partially decayed material is preserved in cold storage. It was in the
tundra of Siberia that the completely preserved bodies of the mammoth
and the woolly rhinoceros have been discovered. Similar finds are to be
looked for in the tundra of North America, where the bones of these
animals have already been found.
On the Pacific border of the continent the shores are mostly bold, and
coastal plains comparable with those on its eastern and northern
margins are absent. In southern California, however, in the vicinity of
Los Angeles, a modern and apparently local elevation of the land has
produced a highly fertile plain, now, owing to the magic touch of
irrigation, beautified by gardens and orchards.
_The Piedmont and Coastal Plateaus._--Adjacent to the western margin of
the Atlantic coastal plain, and extending from Alabama northward to New
England, there is a plateau region about 150 miles broad in its central
part, but narrowing towards its extremities so as to be from 40 to 60
miles broad in Maryland and New Jersey, and of about the same width at
the south, in Georgia. The slope of the plateau surface is seaward from
an elevation of about 1,000 feet along its western margin to 250 or 300
feet at the fall line where it joins the coastal plain.
From its position at the foot of the Appalachian Mountains this
moderately elevated plain is termed the Piedmont plateau. The same
plateau extends northeastward, however, where it is known as the New
England plateau, and is without any definite boundary to separate it
from the similar region in the maritime provinces of Canada. While local
divisions of this great extent of moderately elevated plateau country
are recognised, yet in a general view of the continent it is evident
that the Piedmont plateau, the New England plateau, and the similar
region, mostly of crystalline and igneous rocks, extending from Maine to
Hudson Strait and beyond, in reality forms a single great geographical
unit in which the geological structure and geographical features are
much the same. The general history of this great Atlantic plateau, as it
may, perhaps, be termed, shows that it consists mainly of metamorphic
rocks, such as mica schist, gneiss, slates, etc., together with granite
and other igneous rocks, and, to a minor extent, of sandstones, shales,
and limestones, mostly of Jura-Trias and Carboniferous age. These rocks
were upraised probably in part into lofty mountains, and then worn down
by erosion nearly to sea-level, thus forming what is termed a
_peneplain_, or a plain of subaerial denudation. It is not intended by
this statement to imply that all of the Atlantic plateau was ever
a single great peneplain, but the same general history seems to apply to
the entire region. The upheaval of the plains produced by erosion gave
the streams greater energy, and they have begun the task of again
reducing the land to sea-level, but have not as yet broadened their
valleys so as to greatly modify the general plateau character of the
region they traverse. The softer or more easily soluble rocks have been
eroded away, leaving broad valleys, as in the several instances where
sandstones and shales of what is known as the Newark system (Jura-Trias)
occur in detached areas from South Carolina to Nova Scotia. Then, too,
from northern New Jersey northward to Labrador and beyond, great
glaciers have crossed the plateau or developed upon its broad north
portion and have ground down its surface or left widely extended hills
and ridges of morainal material upon it.
Where the process, just referred to, of planing down a tract of country
nearly to sea-level is incomplete and remnants of former uplands still
remain as isolated hills or groups of hills, such inheritances from the
pre-peneplain stage may still exist when the region is elevated into a
plateau and give diversity to its surface. An example of such a residual
hill is furnished by Mount Monadnock, in southern New Hampshire, and, as
proposed by W. M. Davis, the name of this old landmark is adopted as a
technical term by which to designate all similar remnants of old uplands
left standing on a peneplain. On the Atlantic plateau there are many
_monadnocks_. They range in size from well-characterized hills to
mountain-like forms, and may be isolated or occur in groups. When a
monadnock stands alone its history may be easily read, but groups of
such eminences, especially when of large size, become ranges of hills or
even mountains, and may preserve so much of their former characteristics
that they outrank the adjacent peneplain and become the dominant
geographic feature of the region to which they give diversity. Such a
passage from monadnocks to mountains seems to be furnished by the
numerous isolated hills on the Atlantic plateau and the mountains of New
England and of eastern Canada.
The most characteristic portions of what has just been termed
provisionally the Atlantic plateau are the Piedmont plateau, which
skirts the east base of the Appalachian Mountains from New York to
central Alabama and the Labrador plateau. The eastern border of the
Piedmont plateau is determined by the fall line described above, where
the hard crystalline rocks of the Piedmont region meet the softer rocks
of the Atlantic coastal plain. The rivers flowing eastward from the
Appalachian, such as the Delaware, Susquehanna, Potomac, and the James,
cross the Piedmont plateau in well-defined but narrow channels, usually
from 100 to 200 feet deep, leaving the interstream spaces with generally
level surfaces, although etched as it were by the lateral tributaries of
the master streams. These rivers are shallow and rapid in their courses
across the plateau, or in somewhat technical geographical language are
not as yet graded, but on crossing the fall line become sluggish
tide-water streams which widen into estuaries, as already described.
Owing to the warm humid climate of this region, the rocks in the
interstream spaces are usually deeply decayed and furnish clay soils
which have characteristic red and yellow colours. Much of the cotton and
tobacco of the South Atlantic States is grown on these residual soils
which were left as the more soluble portions of the rocks were removed
in solution.
Labrador, although in great part unexplored, is known to present the
characteristic features of an irregular plateau, with a general
elevation of 1,500 to 2,000 feet above the sea. The surface is
undulating and has hills and hollows, the latter frequently holding
lakes and swamps, but the inequalities seldom exceed 500 feet in
vertical range. Although the western boundary of the Labrador plateau is
indefinite, its area may be taken at about 500,000 square miles. In its
western part, and apparently rising from the plateau as a group of
residual hills left by erosion, are the so-called Laurentian or
Laurentide Mountains. The eastern border of the plateau forms the bold
and excessively rugged Atlantic coast-line of Labrador, characterized by
steep cliff with a fringe of small rocky islands. The adjacent sea
is deep and the continental shelf narrow. On the south the plateau is
bordered by a series of terraces which lead down to the St. Lawrence
River and on the west it merges indefinitely with the plains of the
continental basin.
The rocks of Labrador are largely metamorphic, but include ancient
igneous intrusions, and are hard and resistant. The present surface is
the result of deep erosion which has removed a great but unknown
thickness of material and left exposed what was once the deeply buried
basal portion of a mountainous region. This is a part of the oldest
known land of the continent, and, so far as can be learned, has never
been covered by the sea since a very ancient geological period. In
addition to the long eras of erosion, during which the _debris_ removed
was deposited in part farther south, and contributed to the formation of
the stratified rocks of the Appalachian region and interior continental
basin, there was a comparatively recent extension of great glaciers over
the plateau which removed the previously disintegrated and decayed rocks
and left the present bare, rounded, and generally subdued hills with
intervening basins. The soils are thin, for the reason that under the
present climatic conditions rock decay is retarded, and are confined
principally to the depression where peaty material has accumulated.
Owing to the lack of soil on the uplands, the excess of water in the
hollows, and to the severity of the climate, the forest is not
continuous, the trees are small, and the vegetation generally of a
subarctic character. This vast region is without agricultural
possibilities, and thus far has been of value to man almost solely on
account of its fur-bearing animals and the fisheries of its coast.
The northern border of the Atlantic plateau cannot at present be
accurately defined. Seemingly it should include the Arctic archipelago,
which provisionally may be considered as a deeply dissected plateau
region, at present less elevated than formerly, thus allowing the sea to
enter the valleys and to transform old uplands into islands. The bold
and highly instructive explorations conducted in recent years by
Robert Bell, for the Canadian Geological Survey, have shown that in the
region adjacent to Hudson Strait the plateau features characteristic of
the greater part of the Labrador peninsula are absent and mountains
occur which rank as the highest on the eastern border of the continent.
On the Pacific coast plateaus corresponding closely with those adjacent
to or bordering the Atlantic are wanting. What geographers recognise as
deeply dissected plateaus, so extremely rough that they pass for
mountain ranges, do occur on the western border of the continent,
however, and will be described later.
THE ATLANTIC MOUNTAINS
This title will no doubt appear novel to many persons, and is, perhaps,
open to adverse criticism, but it serves to unite in one group all of
the mountains in the eastern half of North America. A cordillera, as
usually defined, consists of two or more mountain chains associated
geographically, but not necessarily of the same age. On the Atlantic
border of the continent we have an example of such a family of
mountains. The Atlantic mountains, although comprising ranges, systems,
etc., of widely different ages, are all geologically old, and have
resulted from upheavals along two generally parallel and slightly
overlapping northeast and southwest belts adjacent to the Atlantic
Ocean. The growth of this group of mountains is believed to have been
from the north southward, and several periods of upheaval have been
recognised.
The two main divisions or chains referred to are separated by the valley
of the St. Lawrence. The mountains at the north are known as the
Laurentides or Laurentian Highlands, and those at the south comprise the
mountains of New Brunswick and Maine, the White Mountains of New
Hampshire, the Green Mountains of Vermont, the Adirondack Mountains of
northeastern New York, and the Appalachians. The most convenient method
of reviewing the characteristics and histories of these several uplifts
is to begin with the Appalachians, which are at the same time the
most important and best known, and consider them in their order from
south to north.
[Illustration: FIG. 15.--Appalachian Mountains.]
_The Appalachian Mountains._--This beautiful and frequently exceedingly
picturesque series of long, narrow ridges separated one from another by
trough-like valleys, constitutes a mountain system some 900 miles long
and 50 to 130 or more miles wide (Fig. 15). The truly mountainous
portion in its widest part, in western North Carolina and eastern
Tennessee, is about 70 miles across, but a portion of the adjacent
plateau on the west partakes of the same structural features and is a
part of the Appalachian uplift. The system is considered as extending
from the Hudson southward to central Alabama and central Georgia. At the
north its terminus is indefinite, as it merges with the highlands to the
east of the Hudson and with the Berkshire Hills of Massachusetts, which
in turn are not strictly separable from the Green Mountains of Vermont.
At the south, the system ends somewhat abruptly where the crystalline
rocks comprising its southern terminus pass beneath the soft sediments
of the coastal plain. The eastern border of the system is well defined
by its junction with the Piedmont plateau, but on the west it merges
through a series of lessening folds with the plateaus and plains of the
eastern border of the interior continental basin. The Alleghany plateau,
which skirts the western border of what is usually recognised as the
Appalachian Mountains, but which is really its moderately disturbed
border, extends from the Hudson to Alabama, and in its various portions
is known by distinct names. Its northern extension overlooking the
Hudson forms the Catskill Mountains; farther south it becomes locally
the Alleghany plateau, and still farther south the Cumberland plateau.
Separating the bold eastern escarpment of this series of plateaus from
the generally higher mountains to the eastward lies the great
Appalachian Valley, which under various names extends from the Hudson to
central Alabama. This important and highly fruitful valley is underlaid
to a great extent by thick bedded limestones and soft shales, and owes
its existence to erosion and largely to the removal of limestone
in solution.
The Appalachians are nowhere lofty, and only approach the
characteristics of great mountains in their southern portion. The
culminating summit is Mount Mitchell, in western North Carolina, which
has an elevation of 6,711 feet. Roan Mountain, 27 miles to the northward
of Mount Mitchell, rises 6,287 feet above the sea. In the neighbouring
Unaka and Great Smoky Mountains, to the southwest and west of Mount
Mitchell, there are many boldly rounded domes ranging in height from
5,000 to over 6,000 feet Northward of the highly picturesque southern
Appalachians, the system decreases in height and is really a deeply
dissected plateau, as will be shown later, in which the long,
even-crested ridges have a general elevation of 4,000 feet in Virginia
and about 2,000 feet in Pennsylvania and New Jersey. At its northern
extension in New York it decreases still more in height, and is
surpassed in elevation by the plateau on the west, there represented by
the Catskill Mountains, the highest portion of which is 3,660 feet above
the sea.
The characteristic structural feature of the Appalachians is the
presence of a great series of up and down folds, or anticlinals and
synclinals as geologists term them, which run in a nearly parallel
northeast and southwest direction, but in Pennsylvania especially
exhibit many broad curves in their general course. These folds are
similar to the waves that may be produced in a heavy rug or carpet by
pressing against one of its margins. The rocks have been thrown into a
series of great wrinkles which are not continuous throughout the length
of the system, but as one dies out another takes its place. The folds
overlap at the ends or are arranged _en echelon_. The longer axes of the
folds are seldom horizontal, but have usually a gentle pitch; for this
reason one end of a fold frequently passes beneath the surface, while
the other end is exposed to erosion. Another characteristic is that the
anticlinals, as a rule, are steep on their western margins, and slope
more gently on their eastern flanks, or are unsymmetrical. The
overturning of the folds where most pronounced has led to the
breaking of the rocks on the west side of an upward wrinkle where the
descending limb of an anticlinal is sharply bent in order to pass into
the ascending limb of the adjacent synclinal. These breaks or faults in
certain instances form thrust planes along which one portion of a series
of beds has been carried westward, sometimes for several miles, over
another portion of the same series. This highly characteristic system of
unsymmetrical folds, passing at times, and especially in Tennessee and
Alabama, into great thrust planes, is accounted for on the general
theory that there has been lateral pressure or a tangential thrust,
which has forced the strata into a series of elongated arches, in much
the same manner as in the case of a rug, as above suggested, one margin
of which has been forced by lateral pressure towards its central part.
The rocks composing the greater portion of the Appalachians are
stratified marine sediment such as sandstone, shale, limestone, etc.,
which were laid down one on another until a great depth was attained,
corresponding, as we may fancy, to a pile of rugs, the original
thickness in Pennsylvania being about 40,000 feet. Lateral pressure
resulting, as it is believed, from the cooling and consequent
contraction of the earth's highly heated interior, and the movement of
the cool and rigid crust in order to keep in contact with the shrinking
mass beneath, has led to the folding and occasional breaking of the
rocks, which at the same time were elevated above the sea. A crushing
together or folding of the rocks similar to that which has taken place
along the central part of the Atlantic border of North America, as is
well known, has occurred also in many other regions, and the
Appalachians may be taken as the type of a class of mountains, sometimes
termed corrugated mountains, which includes the Alps and Pyrenees, the
Coast Range of California, etc. For convenience we may speak of such
mountains as being of the Appalachian type.
Had the folding in the Appalachian region gone on without erosion, the
surface would to-day be a series of great, elongated arches or
upward folds, rising in many instances 5,000 or more feet above the
intervening valleys, and where breaks or faults occur their upraised
borders would stand as mighty cliffs, in some localities a mile or more
high. The central part of the region with this strange topography had
there been no erosion would, perhaps, be fully as prominent as the
Himalayan Mountains are at present. No sooner, however, were the
Appalachian Mountains upraised above the sea than the destructive
agencies of the atmosphere began their attacks upon them. The rocks were
shattered by changes of temperature, and at times at least crumbled by
the freezing of absorbed water and also underwent chemical changes which
softened and disintegrated them. The rains beat upon them, and streams
flowing to the sea cut channels and carried away the material forming
the land. These processes of disintegration and erosion have been in
progress since islands and continents first appeared on the earth, and
every mountain range now giving diversity to the surface of the land
represents the net result of elevation over denudation. The Appalachians
are not an exception, but a typical illustration of this general law.
The great folds of which they are composed have been truncated by
erosion and the surfaces thus produced, etched, as it were, by the
action of the air, rain, and by streams, so as to leave the edges of the
more resistant layers in relief.
One conspicuous result in this general process of erosion is due to the
fact that the folded strata consist in many instances of alternating
hard, or insoluble and soft, or readily soluble layers. Where resistant
layers underlaid by soft, or readily soluble strata formed the summits
of arches they have in many instances been broken in the process of
folding or cut through by streams flowing down their flanks and the weak
beds beneath exposed. After this stage was reached the erosion of the
upward folds went on more rapidly than the removal of rock from the
compressed downward folds, so that what is structurally a ridge became a
valley: while the bordering troughs or synclinals floored with hard
layers were left in relief as ridges or table-lands. The anticlinal
ridges have thus been transformed into topographic valleys and the
original synclinal troughs left in relief as plateaus and ridges.
[Illustration: FIG. 16.--Section of anticlinal valleys and synclinal
mountains.]
This reversion of what would have been ridges and troughs had there been
no erosion, is illustrated by the following cross-section through
Lookout Mountain in Alabama, which is an example of what is known as a
synclinal mountain. Many such synclinal mountains or plateaus, separated
by narrower anticlinal valleys, occur throughout the Appalachians.
The characteristics in the present topography of the Appalachians just
considered are but a minor portion of the great changes that have
resulted from erosion. The history of the system has not been the same
in this connection throughout, but retains evidences of successive
upward movements with long periods of erosion intervening which have
produced certain striking differences in its northern and southern
portions. These differences are so well marked that it is convenient to
divide the system into two portions, termed the northern Appalachians
and southern Appalachians. The most conspicuous difference between the
two is shown by the direction of flow of the larger rivers. At the
north, the principal rivers--the Delaware, Susquehanna, Potomac, and
James--rise well to the west of the mountains and flow southeast athwart
the numerous folds, and after crossing the Piedmont plateau and coastal
plain discharge into the Atlantic. At the south, however, the rivers,
particularly New River and the Tennessee, rise on the eastern border of
the Appalachians and flow westward, cutting through the Alleghany
plateau, and are tributary to the Mississippi and the Gulf of Mexico.
The somewhat arbitrary dividing line between these two provinces
follows the divide to the north of New River, or in a general way, as
has been stated by C. W. Hayes, is marked by a line drawn from the most
easterly point of Kentucky southeastward to Cape Fear, on the Atlantic
coast.
The fact that several large rivers rising to the northwest of the
northern Appalachians flow directly across or through the numerous
ridges composing the system in deep, narrow valleys, and the similar
behaviour of the streams rising on the eastern border of the southern
Appalachians, but flowing westward, are among the most interesting
features of the entire region. Why is it that the mountains have not
formed a divide or water-parting so as to force all of the streams
having their sources on its west side to take what would seem the easier
course, and to flow to the Gulf of Mexico, and cause the waters falling
on its eastern slopes to flow to the Atlantic? The answer to this
apparently puzzling question has been furnished by Davis, Willis, Hayes,
Campbell, and others, who have shown that the mountains were not raised
all at once, but experienced upward movements at widely separated
intervals, with intervening periods of rest during which the elevations
previously produced were more or less completely planed away by erosion.
During one of these intervals the north Appalachians more especially
were worn down to approximately sea-level and a gently sloping plain
produced across which the larger rivers flowed to the Atlantic. This
peneplain was later upraised into a plateau and its downward inclination
towards the east increased. The streams were thus given greater energy
and began again to deepen their channels. They held their right of way
acquired on the featureless erosion plain and cut deep trenches through
the edges of the hard layers which crossed their courses. At the same
time lateral branches were developed which followed the outcrops of the
less resistant beds and eroded them away so as to leave the hard beds in
bold relief. As the edges of the more resistant beds became more and
more prominent the eastward-flowing streams cut deeper and deeper into
them. The even summits of the ridges, one of the most striking
features in the beautiful scenery of the Appalachians, still mark the
position of the elevated erosion plain.
In the southern Appalachian the old erosion plain formed nearly at
sea-level was tilted gently westward, and the streams flowing over its
surface given initial courses in that direction, which were maintained
as they deepened their channels, and on account of increased energy
originating from the upraising of the region drained by them, developed
lateral branches, as is the case of the more northern streams just
referred to, and the process of carving away the land to sea-level was
again renewed.
Portions of the original upland or mountain mass left unconsumed during
the long period of planation, which reduced most of the region nearly to
sea-level, still remain in eastern Tennessee, western North Carolina,
and northern Georgia, and form the highest and most picturesque portion
of the Appalachians.
After the upraised peneplain from which the long, even-crested ridges of
the Appalachians were produced by the excavation of the bordering
valleys had been deeply dissected and the valleys broadened, another
upward movement took place and the streams again deepened their valleys.
This is the stage in which we now find the mountains. The crests of the
ridges, characteristically displayed in eastern Pennsylvania, are
portions of the first peneplain of which a definite record is preserved,
while the broad valleys with sharply cut channels in their bottoms
represent the much less complete second stage of planation.
The two ancient peneplains referred to above, the histories of which are
recorded in the topography, have received definite names in order that
they may be readily designated. The older and higher one is termed the
Schooley peneplain[2] on account of the preservation of a typical
portion at Schooley Mountain in New Jersey, while the lower one,
represented by the broad valley through which flows the Shenandoah
River, Virginia, is known as the Shenandoah peneplain. A generalized
profile in a northwest and southeast direction through a portion of the
Appalachians is shown in the following diagram, which will serve to make
more definite the description just given. The highest summits in the
diagram represent portions of the Schooley peneplain; if the depressions
could be refilled the surface of the great plateau formed by the
elevation of this plain would be restored. The bottoms of the broad
depressions represent the Shenandoah peneplain, which is sharply
trenched by the modern river channels.
[2] Also known as the "Kittatinny peneplain," but the name used above
has priority.
[Illustration: FIG. 17.--Generalized east-and-west profile showing
relation of peneplains.]
The Appalachians thus furnish not only a typical example of a mountain
system produced by the folding and upheaval of the rocks of the earth's
crust, accompanied in many localities by breaks or faults and
overthrusts, but also preserve the records of two well-characterized
peneplains. The long and varied history of the range has been in part
interpreted by geologists from the character of the rocks, the fossils
they contain, and the structure that has been impressed upon them; but
some of the most instructive chapters are recorded in the topography,
and their study has led to a highly creditable advance in methods of
geographical research.
The Appalachian Mountains when first seen by Europeans were clothed
throughout with a varied and beautiful forest consisting largely of
hardwood trees. Nowhere do they invade the region of perpetual snow, and
glaciers are absent. These statements are true also for all of the
mountains on the eastern side of the continent to the south of Hudson
Strait.
The Appalachians abound in beautiful scenery, but, except about a few of
the very highest domes and ridges, have little of the stern
ruggedness which is typical of truly great mountains. Their countless
valleys are now mostly cleared of their primitive forests and under
cultivation. To a large extent also even the steep hillsides are tilled.
The larger trees which formerly grew on the mountains have nearly all
been felled, and where the land is not suitable for cultivation their
place is taken by a dense second growth. Under the mild, humid climate
that prevails, more especially from the vicinity of the Susquehanna
River southward, the rocks are deeply disintegrated and decayed, and
even steep mountainsides are mantled with soil and rock _debris_. It is
the excess of disintegration and decay over erosion which gives to the
mountains their usually flowing outlines and pleasingly picturesque
rather than rugged scenery. The valleys still retain much of the
material washed from the uplands, and are deeply floored with rich soil.
The characteristic colours of this decayed rock-waste are many shades of
red and yellow, which harmonize in a most artistic manner with the
prevailing green of the plant-covered uplands and abandoned fields.
These red and yellow soils, particularly about the bases of the higher
summits of the southern Appalachians, afford abundant crops of cotton,
corn (maize), and tobacco.
_The Mountains of New England, New York, New Brunswick, etc._--The
picturesque Berkshire Hills, in the western portion of Massachusetts,
have rounded and flowing outlines and a generally subdued relief. The
more prominent of these greatly eroded remnants of what was once a
mountain range rise but 2,000 to 3,500 feet above the sea. No
satisfactory boundary between these hills of gneiss, schist, and allied
metamorphic rocks, and the others of the same general character in the
neighbouring portions of New York and New Jersey, has been determined.
So far as the relief is concerned, and so far also as the complex
geological history has been deciphered, there seems no good reason for
separating the Berkshire Hills from the Appalachian Mountains. It is
convenient, however, to consider the Appalachians as terminating at the
Hudson. The Berkshire Hills when traced northward merge with a
region of similar topography which unites them with the Green Mountains
of Vermont, the highest summit of which, Mount Mansfield, attains an
elevation of 4,364 feet above the sea. To the east of the Green
Mountains are situated the still higher and more rugged White Mountains
of New Hampshire, which culminate in Mount Washington. This widely known
and greatly admired peak has an elevation of 6,293 feet, and, next to
Mount Mitchell in the southern Appalachians, is the highest mountain on
the eastern side of the continent to the south of the newly discovered
group of peaks near Hudson Strait. Associated with Mount Washington are
at least 15 peaks, each of which is over 5,000 feet high, and a still
larger number of lesser summits which exceed 4,000 feet in elevation
above the sea. The remarkable natural beauties of the Green and White
Mountains, the ease with which they can be reached by means of
railroads, and the numerous summer hotels, and hospitable farmhouses
interspersed among them, make this, the most mountainous portion of New
England, a favourite region for summer rest and recreation. The Green
and White Mountains are nearly parallel north and south ranges, from 30
to 60 miles apart, and separated by a tract of lower but hilly country
with a generally southern slope, where many streams unite to form the
southward-flowing Connecticut River.
The Adirondack Mountains, in northeastern New York, are situated some 25
miles to the west of the Green Mountains, and separated from them by
another tract of hilly country similar to the one dividing the mountains
of Vermont from those of New Hampshire. In this space lies the irregular
sheet of water over 100 miles long known as Lake Champlain. This
beautiful lake discharges northward through Richelieu River to the St.
Lawrence. In the same tract of hills, but to the southward of Lake
Champlain and tributary to it, lies the smaller but still more charming
Lake George.
The Adirondacks are rudely circular in ground plan, and measure from 60
to 70 miles from east to west, and about 100 miles from north to south.
The entire area, known to the early settlers of New York State as
the North Woods, is rugged and most pleasingly diversified. Its leading
charms are the large number of dark, densely forested summits, the many
beautiful lakes and clear, sparkling streams. The highest of the
numerous steep-sided peaks is Mount Marcy, 5,344 feet, and second in
rank is the equally beautiful eminence known as Whiteface, which rises
4,872 feet above the sea and about 3,000 feet above the adjacent
valleys. Over 20 neighbouring forest-covered summits have elevations in
excess of 4,000 feet.
The rugged region in northeastern New York and the adjacent portion of
New England is in general without well-marked boundaries. On the north
it extends into Canada, and is margined by the great valley through
which flows the St. Lawrence. In the province of Ottawa, to the south of
the St. Lawrence, there is a group of bold hills similar in many ways to
the Green Mountains, known as the Notre Dame Mountains, which decreases
in height when traced northward and merge with a roughened plateau which
extends far to the northeast and embraces the Gaspe Peninsula and the
table-land and hills of New Brunswick. Much of the country adjacent to
the St. Lawrence on the south is rolling and hilly and contains large
tracts of rich agricultural land which is highly favourable for dairying
and sheep-raising. Mount Sutton, the highest elevation in the Notre Dame
Mountains, is 4,000 feet high, and several other forest-covered
mountain-like hills range in elevation from 1,000 to 3,000 feet. In the
irregular valleys of this region there are a large number of lakes,
situated in general from 700 to 1,000 feet above the sea. The Gaspe
Peninsula to the north of New Brunswick, bordered on the north by the
valley of the St. Lawrence, and on the east by the Gulf of St. Lawrence,
has a rough relief and dense forests and is still a wilderness. The
general elevation of the uplands in this little known region is about
1,500 feet. The surface is in reality a broad plateau in which numerous
valleys have been excavated and from which rises a range of hills termed
the Shikshock Mountains, some 65 miles long and 4 or 5 miles wide,
with peaks ranging from 3,000 to 4,000 feet in height.
Much of Maine and New Brunswick is similar to the region just referred
to, and, in a generalized geographical view, may be considered as a part
of the great coastal plateau of the northeastern portion of the
continent, roughened by erosion so as to appear to one travelling
through its valleys as an endless succession of rugged hills. The
highest of the numerous prominences in Maine is Mount Katahdin, 5,200
feet, and in New Brunswick the culminating summit is Bald Mountain,
2,470 feet.
The rugged region embracing the Adirondacks, together with the more
elevated portions of New England and of the adjacent provinces of
Canada, has many geographical features that are similar to those of the
southern Appalachians, but at the same time this, the central portion,
differs in a marked way from the southern extension of the Atlantic
mountains. The higher mountains in each of these picturesque regions are
at least in a general way to be considered as the unconsumed remnants of
ancient uplands, the greater part of which have been eroded away. The
most marked contrast in the scenery of these two regions of similar
elevation is due to the presence of a great number of lakes at the
north, many of them of large size, and the total absence of such
beautifying elements in the landscapes at the south. The streams at the
north are frequently impetuous and broken by many cataracts and rapids,
thus furnishing abundant water-power; while at the south the streams
flow through more evenly graded channels and are without cascades except
near their sources in the mountains. These contrasts are such as are to
be found the world over between regions of young and old topography. The
differences in the degree of development reached by the streams of the
New England region as contrasted with those of the southern
Appalachians, finds an explanation in the fact that New England, Canada,
etc., was formerly covered with glacial ice, and on the retreat of the
glaciers the surface of the land was left with an essentially new
relief, while the southern Appalachians were well to the south of
the great ice invasion, and the streams of that region have reached a
mature development, except near the sources of their head-water
branches, which, like the topmost twigs of a tree, are always young.
The central, like the southern portion of the Atlantic mountains, is
forest-clothed. All but a few of the highest summits in the Adirondack
and White Mountains are concealed beneath a dense and varied growth of
trees and shrubs. The summits, which are nearly bare of vegetation, like
the upper 800 or 1,000 feet of Mount Washington, owe this condition to
lack of soil rather than to elevation. Nowhere in the Atlantic mountains
to the south of the but little known peaks near Hudson Strait, is the
elevation sufficient to reach above what would be the timber-line under
favourable soil conditions. The trees of the White and neighbouring
mountains are principally various species of conifers, such as the pine,
spruce, hemlock, larch, etc., which grow thickly on all but the most
precipitous slopes. Before man disfigured the beauties of the land the
lower hills, the river-valleys, and the borders of the numerous lakes
and tarns were clothed with a more varied flora than the uplands. In
these valley forests the dark foliage of evergreens is in summer mingled
with the lighter green of maples, beeches, birches, oaks, locusts, and
other broad-leaf trees. The forests are thus highly diversified and
partake of the characteristics of both the northern and southern floras.
It is in these northern woods that the glorious autumnal colouring for
which North America is justly famous is to be seen in its greatest
splendour. October is here truly the golden month of the year. At that
season the bold hills, with their sombre robes of coniferous trees, rise
like dark rugged islands above an undulating sea from which the most
gorgeous sunset colours seem to be reflected. The brilliant colouring of
the ripe foliage beautifies the land as with a cloth of gold. It is at
this season also, during the tranquil days of what is known as Indian
Summer, that a purple haze is thrown like a veil over the harlequin
landscape, as if to subdue its glories and bring them within the range
of man's appreciation.
Only a few of the higher summits in the New England region approach the
scenic conditions usually associated with truly lofty mountains. In
fact, the general lack of rugged escarpments as well as of great
elevation leads the geographer to rank even the highest of these rounded
summits as hills of large size rather than attempt to burden them with
the dignity that the term mountain carries with it. They are beautiful
hills, separated one from another by lovely valleys, which draw the
beholder to them and fill his memory with tender longings and vague
dreamy fancies such as the sterner grandeurs of great mountains fail to
awaken.
_The Laurentian Highlands._--A vast area in the eastern portion of
Canada, to the north of the valley of the St. Lawrence, including
Labrador, is underlaid by very ancient crystalline rocks of the same
general character as those forming the Adirondacks. This same geological
system, the Archean, has a wide development in the continental basin to
the north of Lake Superior and about Hudson Bay. To the north of Quebec,
in the region drained by the Saguenay and Ottawa Rivers, the land has a
general elevation of 1,500 to 1,600 feet, and is known as the Laurentian
Highlands, although sometimes dignified by the name Laurentian
Mountains, or, more briefly, as the Laurentides. In reality, this broad,
indefinitely defined region from a geographical point of view is a
roughened plateau and not a mountain range or group of ranges. When the
structure and metamorphosed condition of the rocks are considered,
however, it is found that they have the characteristics pertaining to
the central and more deeply seated portions of true mountains. The rocks
are mainly crystalline schist, gneiss, granite, etc., together with
igneous intrusions, all of which have been intensely folded, crumpled,
and broken. The general interpretation of the existing conditions is
that deep erosion has occurred and, in fact, a mountain range or a
mountain chain worn down to a generally plane surface. The thickness of
the rocks thus removed, or the depth of erosion, is unknown, and owing
mainly to the complexity of the geological structure of the terranes
remaining, will perhaps never be ascertained, but can be safely
estimated as not only hundreds, but several thousands of feet. Erosion
has laid bare portions of the earth's crust which were once deeply
buried, and reveals the character of the "basement complex," as it has
been termed, which forms the foundation of the continent. Owing to the
great age of the rocks and the depth to which they were once depressed
in the earth's crust, they have experienced great changes. They are not
only intensely folded and crushed, but in large part have been caused to
flow under great pressure, and have thus acquired a schistose structure.
Fissures have been filled with molten rock injected from below so as to
form dikes, and possibly still greater or regional intrusions have
occurred. Over large areas the amount of once molten and intruded rock
exceeds the surface exposure of what are usually, but with some
hesitation, classed as metamorphosed sediments.
Long exposure to the air in a region of mild relief is usually
accompanied by the formation of a deep soil. The soil over the
Laurentian Highlands, however, is generally thin, and large areas of
bare rock are exposed. The explanation of this apparent anomaly is that
glaciers during a geologically recent period were formed on this region
and flowed away from it, carrying most of the previously formed rock
_debris_ with them. The time since the melting of the glaciers has been
too short for a new soil to form, except in the valleys and depressions
among the bare glaciated hills, which hold a peaty accumulation
resulting from the partial decay of vegetation. The scarcity of soil is
also due in part to the climatic conditions now prevailing, which are
unfavourable to rapid rock decay.
To the north of the Laurentian Highlands and in the vicinity of Hudson
Strait, the land becomes higher, and as recently reported by Robert
Bell, of the Canadian Geological Survey, forms true mountains with
elevations in the neighbourhood of 8,000 feet. What revelations are to
come from the inhospitable and in large part ice-covered lands still
farther north can only be told as exploration and surveys are extended
in that direction.
This brief review of some of the leading characteristics of the
mountains and hills adjacent to the Atlantic coast will, I think, serve
to show that they bear a family relationship; like the members of a
family, they are of various ages, although all of them are past their
prime, and may with propriety be termed the _Atlantic Cordillera_.
THE CONTINENTAL BASIN
An inspection of the map forming Fig. 14, on which the larger
geographical features of North America are indicated, will assist the
reader in appreciating the general relations and extent of the plains
and plateaus which collectively form the Continental basin.
This medial region of the continent is bordered on the east for some
2,000 miles by the Atlantic mountains, and on the west throughout its
entire extent by the Pacific mountains. It is open to the sea at both
the north and the south, and extends in one continuous series of plains
and plateaus from the Gulf of Mexico to the Arctic Ocean. The southern
portion of this interior basin or trough has already been briefly
described in discussing the characteristics of the Gulf plains. The
northern portion has also been considered in describing the tundra
region adjacent to the Arctic Ocean.
The leading geographical features of the North American continental
basin are its generally low elevation, the mildness of its topographic
details, and, with two exceptions, the absence within its borders of
elevations having a mountainous structure. In general the rocks beneath
the surface are horizontally stratified marine sediments. The stream-cut
valleys are shallow and usually broad, except in the bordering plateaus
and foot-hills on the east and west sides, where the streams frequently
flow several hundred feet below the surface of the broad, flat-topped
interstream spaces. The drainage of the continental basin serves as a
convenient basis for subdividing it into three separate portions. These
are the Gulf slope, which discharges its surplus waters into the Gulf of
Mexico and is drained principally by the Mississippi; the St.
Lawrence slope, occupied in part by the Great Lakes and drained by the
St. Lawrence River; and the arctic slope, down which the Mackenzie,
Nelson, and other rivers flow to the Arctic Ocean or to Hudson Bay. At
no place are the Pacific mountains broken by cross-drainage, so as to
allow the continental basin to send a tribute to the Pacific Ocean.
The vast extent of the Continental basin, embracing, as it does, some
three-fourths of the entire area of North America, makes it necessary,
even in a general review of the large geographical features of the
continent, to recognise smaller subdivisions than the three great
drainage slopes referred to above. For this purpose we select the more
or less well-defined plains and plateaus into which the region is
naturally subdivided. The portion of the Continental basin embraced
within the boundaries of the United States has been shown by J. W.
Powell to consist of the following physiographic regions, namely, the
Gulf plains; the Prairie plains; the Lake plains, including the region
draining to the Great Lakes; and the Great plateaus or Great plains, as
they are more generally termed, adjacent to the eastern border of the
Pacific mountains. Several of these divisions need to be extended and
still others recognised in order to include the entire region under
review. The portion of the Continental basin to the north of the United
States-Canadian boundary has been only partially explored, and the
subdivisions of it suggested below are to be considered as provisional.
The Lake plains include in Canada the country to the north of the Great
Lakes, which drains to them, but excepting the flat lands bordering
Lakes Erie and Ontario and once covered by their waters, the region
referred to is rather a roughened plateau than a plain. From a
geological point of view the hilly country composed of crystalline rocks
to the north of Lakes Superior and Huron and included within their
hydrographic basins partakes more of the character of the Laurentian
Highlands than it does of the features of the portion of the Lake plains
situated in the United States.
The Prairie plains also extend far to the north of the international
boundary, and on their northern border merge with the forest-covered
plains in central Manitoba and the northern portion of Saskatchewan,
which are drained by northward-flowing rivers. These plains in the far
north differ from the Prairie plains in the fact that they are forested
and acquire greater diversity from the presence of innumerable lakes,
several of which are of large size. For convenience we may designate
this vast and but little known northern region as the Subarctic Forest
plains. Still farther north, where the forest dies away, lie the Barren
Grounds, which merge on their northern border with the frozen morasses
or tundra of the arctic coastal plain.
To acquire just conceptions of the topographic and other characteristics
of the several regions of mild relief which make up the Continental
basin is a difficult task, as each one is of great extent and possesses
many peculiarities of its own, and besides, in two separate regions,
each embracing many hundreds of square miles, movements in the earth's
crust have occurred of such a nature as to elevate the rocks and give
them the general structure commonly found in mountain ranges. Reference
is here made to the Ozark uplift in the southwestern portion of the
Prairie plains and the Black Hills of Dakota which rise from the Great
plateaus.
_The Ozark Uplift._--There is an area embracing about 75,000 square
miles in southern Missouri, northern Arkansas, and the eastern border of
the Indian Territory, in which the rocks have been upraised above the
surrounding Gulf and Prairie plains. The uplift, if we imagine it
uneroded, would have the general form of an inverted canoe; that is, it
would form an elongated ridge, broad and dome-like in the central
portion and dying away on all sides into the great surrounding region of
undisturbed and essentially horizontal rocks. The major axis of the
uplift, although exhibiting a double curvature, has a general northeast
and southwest trend. It is about 500 miles long, and in the widest part
is approximately 200 miles broad. What the height of the dome
would be had the rocks composing it not yielded to the destructive
influences of the air or been removed by streams cannot be readily
estimated, since the movements of the earth's crust which upraised it
occurred at several widely separated intervals with intervening periods
of decay and erosion, and downward movements have also been experienced
which submerged the region and permitted the deposit of sheets of
sediment over it. If the results of the upbuilding agencies had not in a
large measure been counteracted in these several ways, the dome to-day
would have a height of several thousand feet. In the present condition
the deeply eroded dome presents the net result of elevation over
subsidence and erosion. The dome-like form is lost, and in its place is
a complex series of ridges and valleys. The higher summits now
remaining, situated principally in the Iron Mountain district in
northeastern Missouri, rise from 1,400 to 1,800 feet above the
neighbouring plains, and from 1,800 to 2,100 feet above the sea.
The greater intensity with which the rocks in the southwestern portion
of the Ozark uplift have been folded than in the more northern portion
and the varying degrees to which the beds have yielded to denudation
have resulted in giving to its various parts different types of
topography. This diversity has led to the recognition of several
distinct divisions, such as the Shawnee Hills, at the extreme
northeastern end of the uplift, where the rocks have been folded and the
ridges cut across by the Mississippi; the St. Francois Mountains, in
southeastern Missouri, composed of a large number of isolated hills and
rising from 500 to 800 feet above the adjacent valleys; the Ozark
plateau, in southwestern Missouri and northwestern Arkansas, the central
part of which has a general elevation of 1,500 feet above the sea, and
to one travelling over it seems a boundless and featureless plain
underlaid by apparently horizontal but in reality gently westward
dipping sheets of stratified rocks; the Boston Mountains, in central and
western Arkansas, consisting of rugged irregular ridges and truncated
summits with a general crest-line elevation of 1,000 feet above
the sea; and the Ouachita (pronounced _Wichitaw_) Mountains, formed of
numerous rudely parallel upward folds of hard rock, which rise from 500
to 1,000 feet above the adjacent valleys and form a belt of unusually
picturesque, forest-crowned hills, extending from Little Rock, Arkansas,
westward into Indian Territory.
The study of the island-like Ozark region in the broad, ocean-like
expanse of the prairies is far from being complete. Although
topographically distinct and appearing as one of the minor units in the
geology of the continent, geologists are inclined to the view that the
Ozark uplift as above described should be considered as consisting of
two independent but contiguous areas of upheaval, namely, the Ozark
Hills, situated mainly in Missouri, and the Ouachita Hills, lying mainly
in Arkansas and the Indian Territory. This Ozark-Ouachita region--by
whatever name finally designated--is one with a long and varied, nay,
even a poetic history. In writing of the Archean rocks of the Iron
Mountain region, Missouri, Arthur Winslow states that they "are truly
ancient elevations, older than any others in the State, older than the
mountains of Arkansas, older than the Appalachians, older than the Rocky
Mountains; if venerable be an attribute of great age, they certainly
merit that appellation. For not only are all other rocks of Missouri
youthful as compared with these, but there is a genetic relationship,
and the former are in a sense descendants of the latter. For when the
limestones and other sedimentary rocks were yet unformed these
crystalline rocks must have existed as parts of a continental mass, and
from the degradation of this continent resulted the materials of the
later formed sedimentary rocks. The present granite and porphyry hills
are but protruding parts of the remnant of this ancient continent which
stood as islands above the ocean waters while the beds of limestone and
sandstone were being formed about them, which rose with these beds when
they were lifted from the waters, which now, rugged and weather-beaten,
yet tempered by age and varied experience, rear themselves above the
surrounding younger rocks and bid fair still to live when the
latter have yielded to the forces of degradation."
Besides its pleasing scenery, varied and abundant mineral resources, and
health-giving springs, this oasis of hills amid the unvaried monotony of
the grass-covered plains in the southern portion of the continental
basin derives an additional attraction from its forest growths in which
southern pines are mingled with oaks, hickories, walnuts, and other
broad-leaved trees. The soil is generally productive, and great fields
of corn and cotton may be seen side by side.
_The Gulf Plains._--The Gulf plains include the western portion of
Florida, and extend westward and southward about the borders of the Gulf
of Mexico in a continuous belt from 50 to 60 to perhaps 100 miles wide,
to where the Pacific mountains approach the coast in east-central
Mexico. This low, gently seaward-sloping region, underlaid by soft
horizontal strata, possesses a generally rich soil well adapted for the
cultivation of cotton, corn, sugar-cane, and rice. In the low, hot
country of eastern Mexico nearly all tropical fruits can be successfully
raised. The most characteristic as well as the broadest portion of this
productive belt is in the States of Mississippi and Louisiana, and
extends northward with a gradually decreasing width to the mouth of the
Ohio. This is the lower Mississippi basin, which owes its existence
mainly to the deposits of silt laid down by the river after which it is
named. Much of the land is really the delta of the "Father of Waters,"
over which that river spreads out in vast inundations each year.
The Gulf plains skirt the southern end of the Appalachian Mountains, and
to the west of the Mississippi are bordered in part by the Ozark uplift.
There are certain reasons for believing that these two regions of
elevation, characterized by a similar geological structure, are portions
of a single greatly disturbed belt, but are now separated by a broad
area which has been depressed and deeply covered with comparatively
recent sediments. But that this general view of the origin of the larger
features in the relief of the Gulf States can be accepted with
entire confidence is questionable. True it is, however, that the delta
region of the Mississippi has undergone many up and down movements, and
that several successive sheets of sediment have been laid down upon it,
but that the folds and crumplings characteristic of the southern
Appalachians and of the Ozark uplift extend across the intervening space
beneath the covering of horizontal rocks has not been demonstrated.
The Gulf plains throughout are less than 500 feet above the sea, and
much of the Gulf margin and the similar tract which extends northward to
the mouth of the Ohio has an elevation of less than 100 feet. The fringe
of lowland bordering the Gulf and extending up the course of the
Mississippi is generally swampy and contains numerous small water bodies
which owe their existence to the cutting off of the beds of the river so
as to form what are termed ox-bow lakes.
Previous to the settlement of the Gulf plains by Europeans and the
clearing of much of the land for plantations it was clothed with such a
dense growth of trees and vines as to be almost impenetrable. The
southern pine there reaches its greatest perfection and is the basis of
a great lumber industry, and oaks of several species, the wide-spreading
white-trunked sycamore, the still more stately tulip-tree with its
cup-like blossoms of yellow, the fragrant magnolia, the seemingly always
aged cypress, the gum-tree, and many other species of arboreal
vegetation also find most congenial conditions for their growth. The
dwarf palmetto, which forms such a characteristic growth in Florida,
extends northward in the Mississippi basin to the southern border of the
Ozark uplift. Much of the luxuriant moss and lichen draped forest of the
Gulf plains with all its primitive network of shrubs and vines still
remains.
_The Prairie Plains._--A prairie in the current use of the term is a
generally level region, either a plain or a plateau, without forests but
clothed with a carpet of luxuriant grasses and flowering annuals. A
rolling prairie is an undulating or hilly, grass-covered region. The
Great plains of the west-central portion of the Continental basin
meet these requirements, and are typical prairies. On their eastern and
northern border the Prairie plains merge with the adjacent forested
plains, and on the west from Mexico northward to the subarctic forest
pass by still less tangible gradations into the more elevated and drier
Great plateaus or high plains, where bunch-grass, with bare intervals
between the scattered tufts, takes the place of the continuous sod of
the true prairies. The reasons for the change from forest to prairie and
beyond to the land of the bunch-grass as one travels from east to west
across the interior basin, lie in differences in the humidity of the
climate.
The Prairie plains have their beginning at the south in Mexico a short
distance from the Rio Grande, and are prolonged northward through
central Texas, meeting to the north of Red River the forest-covered
Ouachita Hills. But to the west of the Ozark uplift the Prairie plains
extend northward in a belt about 100 miles wide which expands in Kansas,
northern Missouri, eastern Nebraska, Iowa, Illinois, and western Ohio to
fully 800 miles. In this highly fertile region, now the most productive
agricultural area of comparable size in North America, if not in the
world, one may travel in a straight line for nearly 1,000 miles through
a land without high hills but pleasingly diversified by undulations of
the generally level surface and by winding stream-formed valleys
bordered by swelling bluffs, without losing sight of towns, villages, or
comfortable farmhouses. In spring this entire region is bright green
with pastures and sprouting grain-fields, and in autumn yellow with the
harvest. Miles on miles of rustling corn-fields form the most
characteristic feature of the summer landscapes.
The Prairie plains contract to the north of Illinois and Iowa to a width
of about 200 miles, being encroached upon by the forests of the Great
Lakes region, but are prolonged northward through Minnesota and the
Dakotas far into Canada. The length of these natural meadows from south
to north is nearly 2,000 miles; their entire area is not far from
500,000 square miles. On the north they merge with the vast region
of similar relief which is darkened by the pines and spruces of the
subarctic forest.
The northern portion of the original prairie region has been given a new
and in some respects a more pleasing aspect by the sowing of millions of
acres with wheat. This is the most favourable large area for wheat
culture in North America, and one of the three great wheat-growing
regions in the world. The most productive portion of these northern
wheat-lands lies in the valley of the Red River of the North, situated
in part in Minnesota and the eastern portion of the Dakotas, but
including also the plains of Manitoba. Could we view the broad extent of
the Prairie plains as do the birds in their southward migrations, we
would see them golden with the sheen of ripening wheat at the north,
green and russet in the central portion with corn, and white with cotton
to the south. Everywhere from south to north and east to west the vast
expanse is dotted with the curling wreaths arising from household fires,
and at hundreds of localities blotted by the smoke of towns, factories,
smelting-works, and coal-mines.
Throughout the entire extent of the Prairie plains the underlying rocks
are essentially horizontal, and consist largely of limestone. An ancient
sea-bottom has been broadly upraised with but slight disturbances of the
strata to a general elevation of about 800 feet in Minnesota and the
Dakotas. From this low continental divide the land slopes gently both to
the north and south. The local variations of surface are due mainly to
the unequal weathering of the rocks and the excavation of stream-formed
valleys. To the north of the mouth of the Ohio, however, the prairie, in
common with the adjacent regions, was formerly occupied by glacial ice,
which on melting left widely spread deposits of clay, stones, gravel,
etc., which gave the region a new surface, and in certain instances
turned the streams from their former courses. Much of the rolling
prairie inherits its billowy surface from the glaciers. In the midst of
the young topography of glacial and more recent date there is an area of
about 10,000 square miles in southwestern Wisconsin and adjacent
portions of Minnesota and Iowa which is surrounded by the deposits of
the ancient ice-sheets (glacial drifts), but not covered by them. This
driftless area, as it is termed, has an old topography in striking
contrast to the relief of the region about it, in which broad
river-valleys bordered by the pinnacled and castellated rocks exposed in
the bordering slopes of the adjacent uplands are among the most
conspicuous features.
The soil of this driftless region is a ferruginous clay, resulting from
the prolonged weathering of the rocks, principally limestone, on which
it rests, while the surfaces formerly covered by glacial ice are mantled
with soil of a mixed character containing many fragments and large
boulders of compact rock. In the prairies to the south of the glacial
boundary the soils are mainly of a sedentary origin, and have resulted
from the disintegration and decay of the rocks on which they rest, but
usually rendered black by the humus resulting from the partial decay of
numberless generating grasses and other lowly plants. This black soil is
wonderfully productive and furnishes the basis of the greater part of
the wealth and industries of the region it covers. The minor exceptions
to the general fertility occur where the rocks immediately underlying
the surface, as in the zinc and lead region of southwestern Missouri,
are highly charged with flint-like material, which remains when the
limestone once containing it is dissolved and carried away. The
horizontal sheets of rock beneath the broad central portion of the
Prairie plains belong to the Carboniferous system and contain highly
valuable seams of bituminous coal. The area of these coal-producing
lands is estimated at 125,000 square miles. In this same region also
there are extensive tracts in which natural gas and petroleum are
obtained in remarkable abundance. In southern Wisconsin and the adjacent
portions of Illinois valuable deposits of lead occur under conditions
similar to those associated with the lead and zinc mines about the
northern border of the Ozark uplift.
Owing to the demand for transportation facilities and the mild
relief of the land, the entire extent of the Prairie plains is covered
with a double-lined network of steel. The ganglia in this pulsating
nerve system of intercommunication are Chicago (here included, as it
belongs to the prairie as well as to the Great Lakes region), St. Louis,
Kansas City, Omaha, Minneapolis, St. Paul, etc., cities with from
100,000 to over 1,500,000 inhabitants, and hundreds of lesser centres of
trade, manufacture, and education.
_The Lake Plains._--The region draining to the Great Lakes--or the
Laurentian lakes, as they may, perhaps, be more properly designated,
since they form the source of the river of that name--presents many
striking contrasts to the more monotonous treeless prairies skirting it
on the south and west.
The name "Lake plains," suggested by J. W. Powell for the portion of the
region here referred to within the borders of the United States, when
extended to the entire area draining to the Laurentian lakes, is in part
a misnomer, since much of its surface is rough and irregular. In a
certain sense, however, the term _plain_ is applicable, since it
includes a plain of water over 95,000 square miles in area. The combined
areas of the lakes are greater than that of the region draining to them.
The land bordering the Laurentian lakes is underlaid to a large extent
by horizontal or but slightly disturbed sedimentary rocks, but includes
on the north a portion of the contorted, crystalline terranes already
referred to as forming the Laurentian Highlands, and in general is
characterized by the mildness of its relief. The elevations of the
surfaces of the several Laurentian lakes above the sea are, in feet, as
follows: Superior, 602; Michigan and Huron, 582; Erie, 373; and Ontario,
247. The land forming the margins of these water bodies rises in general
less than 300 feet above their surfaces. In portions of northern
Michigan and in the region of crystalline rocks to the north of Lakes
Superior and Huron, however, the relief is more pronounced and there are
many bold rounded hills with basins between them.
The principal part of the nearly plane land surface about the Laurentian
lakes is in immediate proximity to their borders, and records the
former extent of their waters. These plains, composed of clay deposited
from the lakes when more widely expanded than at present, form a fringe
from 5 to 50 or more miles broad all about the present lake margins.
Across this gently sloping surface the streams from the uplands,
increasing in length as the lakes were lowered, have excavated narrow,
steep-sided channels. These modern plains furnish typical illustrations
of young topography.
In its primitive condition nearly the entire Laurentian lakes region was
densely covered with trees. Previous to the destruction which followed
the advance of the lumbermen its northern portion contained some of the
finest and most valuable white-pine forests on the continent. To the
south of the Laurentian Lakes, and in a general way adjacent to the
Prairie plains, there were park-like areas in the forest, known as
oak-openings, where picturesque bur-oak grew in open groves amid
luxuriant natural meadows. These sunlit gardens, yellow and purple with
golden-rods and asters in autumn, owed their existence to soil
conditions determined long previously by the streams issuing from the
margin of the retreating ice-sheet, which formed level areas of sand and
gravel. The loose open texture of these deposits renders them less
retentive of moisture than the neighbouring morainal hills, and during
the long hot summers all but the most deeply rooted of the trees that
spring up upon them perished.
The soil throughout the Great Lake region is nearly all of glacial
origin and presents many local variations, dependent principally on the
fact that the streams flowing from the ice assorted the _debris_
delivered to them. The surface material, technically speaking, is of
both glacial and fluvio-glacial origin. The former consists principally
of stony clay or _till_, and the latter of gravel. About the immediate
border of the existing lakes lacustral clays form the surface. The
leading characteristics of the glacial and fluvio-glacial soils are
their varied composition and endurance under cultivation. The glaciers
that ploughed the land preparatory to the present harvest gathered
together a great variety of rock _debris_, much of it broken and
unweathered and not leached of its more soluble constituents.
The most typical portion of the Lake plains, including the southern part
of the province of Ontario and the southern shores of the Laurentian
lakes from Minnesota to New York, is highly favourable for agricultural
pursuits, and produces in abundance a great variety of crops as well as
richly flavoured fruits, luscious berries, and healthful vegetables. The
beneficial influence of the neighbouring water bodies on the climate,
tempering the heat of the summers and moderating the severity of the
winters, is shown especially in the distribution of the fruit belts of
Michigan, Ohio, and New York, which are in regions where the prevailing
winds blowing over them come from the lakes.
_The Subarctic Forest Plains._--The Prairie plains merge at the north
with a great tract of forest-covered lowlands, which extend from the
Laurentian hills on the east to near the base of the Rocky Mountains on
the west. The change as one travels northward from the grassy prairies
to the country of equally mild relief, but clothed with trees adapted to
a rigorous climate, is gradual. Along the irregular and in part
indefinite junction of these two vast plains, the alignment of the
forest is broken in many places, and its margin fringed by a picket-line
of groves and of isolated trees, which has advanced southward and
invaded the grass-lands. Between these outposts the prairie with its
wealth of summer bloom reaches well into the realm of perennial shadow.
The southward extensions of the forest are mainly in the valleys and
adjacent to the streams, while the drier steppes between are open
grass-lands. No conspicuous change in the topography of the land or of
the rocks or the soil coincides with the change from grass to forest.
The differences in vegetation must therefore be sought in climatic
conditions, and mainly in the influence of atmospheric changes on the
water contained in the soil.
Throughout practically the whole of the region occupied by the subarctic
forest, between Hudson Bay and the foot-hills of the Rocky Mountains,
the land is low and the valleys monotonous. Many lakes are
present, several of them of large size, and the rivers are remarkable
for their lengths, low gradients, and large volumes.
The subdued topography of the region here considered, and the presence
of vast numbers of lakes and swamps, is due in general to the influence
of the ice-sheets which formerly covered it. In a minor way the presence
of the innumerable small lakes and swamps is owing to the obstructions
formed by growing vegetation, the damming of streams by driftwood, the
work of beavers, and possibly the influence of subsoil ice.
To the north of the Subarctic Forest plain, as already described, occur
the desolate tracts known in Canada as the Barren Grounds, which form a
part or merge into the tundras bordering the Arctic Ocean.
_The Great Plateaus._--The boundary between the prairie plains of the
central portion of the interior Continental basin and the Great plateaus
(Great plains) bordering them on the west is usually indefinite. The
prairies pass into the more elevated and drier plateaus by insensible
gradations. The plateaus rise gradually from east to west, and along
their western margin, adjacent to the east base of the Pacific
mountains, attain a general elevation of from 5,000 to 6,000 feet. Over
vast areas these monotonous plateaus, with their even sky-lines, are
higher above the sea than the crests of the Appalachians, and along
their western margin in many localities even surpass in elevation the
most prominent peaks in the eastern portion of the United States.
Accompanying this increase in elevation from east to west there is a
decrease in precipitation, and in consequence a marked change in the
vegetation. The plateaus, like the prairies, are treeless in their most
characteristic portions, but the larger rivers winding across them are
margined in many instances by giant cottonwoods.
The mental picture that a traveller over the broad plateaus retains in
after-years is of a vast treeless level tract of country, boundless as
the ocean, which is bright green and decked with lowly flowers in the
early spring, but becomes yellowish brown as the heat and dryness
of summer increase and the grasses lose their freshness. Various
portions of the plateaus, however, have their own individuality and
present characteristics which make them conspicuously different from
other portions of the same great series of steppes. At the south, in the
region of the Rio Grande and of the Pecos and Canadian Rivers, the
plateau is dissected by stream-cut valleys 1,000 feet or more deep, and
from one to two score miles across, which divide it into a number of
individual table-lands. The plateau margins for many miles on each side
of the larger river-valleys have been carved by a complex system of
secondary and usually ephemeral streams into a great variety of rock
forms with deep trenches between. These conspicuously sculptured areas
constitute what are commonly termed Bad Lands. In certain regions also
the surfaces of the plateaus, more especially in Nebraska and South
Dakota, are broadly undulating or reveal a seemingly endless succession
of ridges and hills separated by shallow depressions, due to the
presence of large tracts of drifting sand. In spite of these several
variations, however, the leading characteristics of by far the larger
portion of the plateau country are the generally level grass-covered
surfaces extending away in all directions far beyond the reach of
vision. On the rolling prairie one can frequently see the undulating
surface about him for a distance of 15 or 20 miles, but the curvature of
the earth usually draws still narrower limits to the region within the
view of the plainsman. In riding over the plains the scene changes but
little from day to day and from week to week. Monotony is the one word
that best describes the lives of those whose lot is cast on these broad
featureless surfaces. In journeying westward across the plateau over any
one of the transcontinental railways a moment of excitement occurs when
the even line of the western horizon is broken by the summit of a
cloud-like mountain-peak. "Land ho!" is no more thrilling to voyagers on
the ocean than the shouts which first made known the presence of a
mountain-peak to the bands of immigrants who slowly voyaged across this
sea of grass with their picturesque "prairie schooners" previous
to the building of the railroads which now bind together the East and
the West.
The Great plateaus begin indefinitely to the south of the Rio Grande,
broaden in the United States to a general width of about 400 miles, and
extend far northward into Canada. Their northern limit has not as yet
been determined, but is to be looked for near the head waters of the
Mackenzie. The length of the plateau country is in the neighbourhood of
2,000 or 2,500 miles, and its average width about 300 miles. An estimate
of the area with a generally plane surface and an elevation of from
1,000 to 6,000 feet above the sea places it at about 700,000 square
miles.
The eastern portion of the Great plateaus includes western Texas,
Oklahoma, the central and western portions of Kansas and Nebraska, the
western half of South Dakota, western North Dakota, western Assiniboia,
and thence extend northward so as to include portions of Saskatchewan,
Alberta, and Athabasca. On the west the plateau region includes the
eastern portions of New Mexico and Colorado, extends far into Wyoming,
and embraces central and eastern Montana, and thence reaches northward
to Mackenzie.
This region of essentially level plateaus, extending as it does from the
hot lands of eastern Mexico nearly to the arctic circle, presents great
diversity of climate and also well-marked variations in the secondary
features of its relief. Of necessity it needs to be subdivided for more
detailed study. The rivers flowing eastward from the Rocky Mountains
have excavated valleys in the plateau region, and may be used as a basis
for its subdivision. This has been done by J. W. Powell for the portion
within the borders of the United States, and the terms Pecos plateau,
Arkansas plateau, Platte plateau, and Missouri plateau have been
proposed; this category may be extended especially to the northward, so
as to include the less well-known Saskatchewan, Athabasca, Peace, and
Laird plateaus. Each of these divisions is in reality a group of
plateaus, for the reason that the broad areas between the
eastward-flowing rivers are trenched by lateral stream channels
tributary to the main waterways, and thus subdivided into smaller units.
This subdivision of the plateau region by stream channels leaving
flat-topped areas between them makes one instructive geographical
process prominent--that is, the great table-land has been dissected. The
depths of the channels cut across it depend mainly on the elevation of
the land and the distance the streams have to travel to reach the sea;
but modifying conditions are furnished by the degree of resistance the
rocks offer to erosion, the amount of precipitation, etc. If the
elevation is great, the stream can cut deeply, and leave bold secondary
plateaus between them; if the distance to the sea is short, other
conditions being the same, the streams can cut more deeply than when
their courses are long; if the rocks are resistant, they are left in
bold escarpment bordering the valleys and the margins of the secondary
plateaus are well defined, but if they are soft and crumble easily,
their _debris_ is washed and blown into the rivers, and a general
lowering of the surface without the formation of deep trenches is the
result. These and still other conditions have influenced the manner in
which the Great plateaus have been dissected, and are of necessity to be
considered in a critical discussion of the history of the land as
recorded in its relief.
The main reason for the dissection of the region under consideration is
to be found in the fact that it is bordered on the west by high
mountains where precipitation is abundant, and the streams, supplied
largely by the melting of the snow in summer, flow across a
comparatively rainless country. The stream channels in general have been
deepened at a more rapid rate than the areas between them have been
lowered by erosion. Valleys running east and west have thus been
excavated, leaving the intervening spaces as uplands, which, however, in
certain instances have been minutely dissected by the streams
originating on them and supplied by local winter precipitation. Added to
these general conditions are differences in rock texture, which have led
to great variations in the details due to erosions, particularly on the
valley borders.
One other condition which has modified the history of the plateau
region throughout, but most decidedly at the north, is the climatic
change which culminated in the Glacial epoch. During the time referred
to the northern portion of the Great plateaus situated in Canada and the
adjacent part of the United States was invaded by glacial ice which
spread an irregular sheet of detritus over the country it occupied.
Decided changes occurred also in the central and southern portion owing
to increased precipitation, the flooding of the rivers leading from the
melting ice-front, and to movements in the earth's crust of as yet
undetermined extent and amplitude. It is apparent to the geographer that
much of the history of the climatic changes of glacial and post-glacial
times is recorded in the relief of the interior Continental basin to the
south of the limit reached by the ice and in the terraces and alluvial
deposits of the valleys, but as yet for the most part this interesting
story remains unread.
The most deeply dissected portion of the Great plateaus occurs in
western Texas, eastern New Mexico, and Oklahoma. In that region the
rivers having their sources in the Rocky Mountains and flowing to the
Gulf of Mexico have excavated deep and wide valleys, leaving broad
intervening areas in bold relief.
The Pecos River drains a large part of the mountainous region in eastern
New Mexico, and flows through a valley of its own making, which is some
30 or more miles broad and its bottom about 1,200 feet below the general
surface of the plateau lying to the eastward. The Canadian River has
excavated a similar valley, which is some 40 miles broad throughout much
of its course, and is bordered by bold rocky escarpments from 1,000 to
1,200 feet high, in which the edges of the horizontal strata underlying
the adjacent plateaus are exposed. This region of large and strongly
defined topographic features illustrates in a remarkable manner the
nature of the work performed by streams which rise amid high mountains
and flow across a dry plateau standing well above sea-level.
_El Llano Estacado._--A typical portion of the great plateau region left
by deep dissection is furnished by the table-land named by early
Mexican explorers "El Llano Estacado," or the Staked Plains, in
reference to the fact that owing to the monotony of the surface and the
scarcity of water the routes of travel were at first marked by stakes.
This region, celebrated in the traditions of the Southwest frontier, is
described by Captain Marcy, who crossed its eastern portion in 1849, as
being "much elevated above the surrounding country, very smooth and
level, and spreading out in every direction as far as the eye can
penetrate, without a tree, shrub, or any other herbage to intercept the
vision. The traveller in passing over it sees nothing but one dreary and
monotonous plain of barren solitude. It is an ocean of desert prairie,
where the voice of man is seldom heard, and where no living being
permanently resides. The almost total absence of water causes all
animals to shun it; even the Indians do not venture to cross it, except
at two or three points, where they find a few small pools of water." As
will be shown below, the barrenness and desolation of this arid tract is
not so great as it seemed to those who first invaded its primeval
solitude.
El Llano Estacado, or the _Llano_, as it is frequently termed, is about
500 miles across from north to south, and 280 miles wide from east to
west. It is bordered on nearly all portions of its periphery by
descending escarpments which lead down to the adjacent valleys. Its
surface, although appearing horizontal, in reality slopes eastward at
the rate of about 20 feet per mile, and on its highest, northwest
border, has an elevation of 5,500 feet above the sea. This great
table-land has a smooth floor, and, as reported by recent explorers, is
clothed with an abundance of bunch-grass, which formerly furnished
sustenance to herds of antelope and deer. It was in this general region
also that some of the immense herds of buffalo which once inhabited the
broad plateaus found a winter range.
The Llano, together with its southward extension, having the same
characteristics and known as the Edwards plateau, is bordered on the
west by the deep and broad valley of the southward-flowing Pecos River,
and on the north by the equally deep and broad valley carved in the
plateau country by the eastward-flowing Canadian River. The
eastward slope of the surfaces of the two plateaus is continued
throughout the region bordering them on the east all the way to the
shore of the Gulf of Mexico. The streams originating on the eastern
border of the eastward sloping plateaus and flowing to the Gulf of
Mexico, represented at the present time by the Colorado (of Texas), the
Brazos, Trinity, and Red Rivers, extended their trunks by head-water
corrasion and developed numerous branches so long as the rainfall was
sufficient to maintain a surface drainage. But as the streams were
lengthened they cut farther and farther westward and into a region that
became drier and drier, until finally they reached a land in which all
of the scanty rain that fell was absorbed by the thirsty soil. The
drainage from this higher and drier region is subterranean, and reaches
the head waters of the streams to the eastward to a considerable extent
as springs. The streams which lowered the country to the eastward of the
Llano developed many branches, some of which were extended westward into
the drier plateau country in such a manner as to give the eastern margin
of the remaining upland a scalloped and irregular border.
In travelling westward up the courses of the rivers of eastern Texas,
one passes from a low region of old topography to one where the head
branches of the streams flow in canyons, and the relief has the
ruggedness of youth; on gaining the western border of the belt of
country having surface streams one ascends to the smooth surface of the
high plateau, which is young as regards stream development, although in
years older than the country with a deeply eroded surface to the
eastward. The Llano and Edwards plateau present us with examples of
perpetuated topographic youthfulness.
The Llano, although dreaded by early explorers and shunned so far as
possible even by experienced plainsmen, on account of a lack of water,
has in recent years become more favourably known. It is crossed at
present by two railroads. Water has been found beneath the surface in
numerous localities, and the desert-like region now bids fair to
become a favourable cattle-raising country. It is not to be expected,
however, that all the glowing predictions which have been published
concerning this and neighbouring table-lands will be more than partially
fulfilled through the use of the subsurface waters.
_The Arkansas Plateau._--To the north of the Canadian River the region
termed above the Great plateaus is less deeply dissected than in the
portion of which the Llano is typical, and the streams from the
mountains flow through shallow valleys with less rugged and less
picturesque borders than those of the deep wide valleys of western Texas
and eastern New Mexico. The broad plateau surfaces adjacent to the
valleys of the Arkansas and Platte Rivers probably come nearer to the
popular idea of the essential characteristics of the "Great plains" than
any other of the larger divisions of the region under review. The most
conspicuous geographic features of the Arkansas plateau have been
described by W. D. Johnson as consisting of an assemblage of low and
broad table-lands separated by shallow erosion valleys. The plateaus are
immense unsculptured remnants in light relief of an older and originally
perfect plain. The few long and feeble streams, wide apart and flowing
eastward from the distant mountains in parallel courses and without
tributaries, have blocked out by dissection the larger features of the
broad landscape which in future ages will be slowly etched into a
multitude of details. The scenery of these featureless plains is
ordinarily depressing when once the novelty of being adrift on a sea of
grass has passed away. There is nothing that can be termed scenery
except that which once a year for a brief period the sky affords when
clouds of extraordinary grandeur darken the air. Throughout nearly the
entire annual course there is no material for landscape effect except
the straight line of the horizon with a featureless breadth of sun-faded
brown below it and above a merely broader space of faded blue. There is
nowhere a curved line, and though as a scientific fact there is vast
expanse of flat plain, there is little to suggest it when the sky is
empty of clouds. In June the clouds come with a gradual maturing at some
point along the even sky-line, and increase rapidly until the
heavens are filled with magnificent vapour banks; but the display is
simply spectacular, and passes away in a few hours as quickly as it
came, with only local showers to refresh the land.
The one industry that can thrive on the Arkansas plateau, which was
formerly at certain seasons blackened by herds of bison, is
stock-raising. Wells from which water is pumped by windmills furnish
sufficient water for herds of cattle and horses, but not for irrigation.
_Bad Lands._--In the northern portion of the Great plateaus within the
United States the surface rocks over great areas are soft or but
irregularly hardened sediments of ancient lakes and streams and have
been sculptured by rain, wind, and ephemeral rills into a most
marvellous array of monumental and castellated forms. Localities where
this minute dissection of the soft horizontal strata is especially well
marked over hundreds of square miles occur in South Dakota and Montana,
and especially on the borders of the valleys carved by the Loup Fork,
Niobrara, White, Yellowstone, and Missouri Rivers. In this region the
rainfall is light, the mean annual precipitation being in the
neighbourhood of 15 inches, and occurs mostly during the winter months.
In the summer season the lands far out on the plateaus are dry and hot,
and all but the larger streams disappear. The rocks, consisting mostly
of unconsolidated clays and soft sandstones, with occasional hard layers
and irregular concretions, have been cut into innumerable channels,
leaving steep-sided remnants of the former plain between. The maze of
trench-like valleys, the similarity of the sculptured land-forms one to
another, and the absence of water, make these desert regions excessively
difficult to traverse. The Canadian-French who explored the
north-central portion of the Great plateaus in early days of American
settlement termed these tracts of country, so difficult to cross,
_Mauvaises Terres_, a name now seldom used, but replaced by the English
name Bad Lands. Although _bad_ to the hunter and the plainsman, these
desert regions are of fascinating interest to men of scientific
training. The intense heat, the choking alkaline dust, the absence of
water, and the danger of being lost and of perishing of thirst in
these wild silent regions, have not checked the ardour of explorers. Not
only do the Bad Lands present a most attractive field to the student of
erosion and of the origin of earth forms, but their deathlike solitudes
have been made to yield the most wonderful procession of strange extinct
animals yet unearthed by geologists. They are vast cemeteries in which
are interred the skeletons of many genera and hundreds of species of
animals which lived in the ancient lakes or wandered through the almost
tropical forest that in distant ages clothed the adjacent country. The
great lesson to be learned by the geographer in these uninviting regions
as they seem to most people relates to the way in which the rocks have
been eroded. The prevailing softness of the beds with occasional hard
layers, the scarcity of vegetation, the occasional heavy rains, and the
considerable height of the country above the master streams combine to
favour rapid and deep sculpturing. The precipitous slopes of the small
mesas and castle-like rock forms destitute of all vegetation excepting
succulent cacti and scattered clumps of bunch-grass, reveal a multitude
of sunken lines and raised edges, produced by the ephemeral streams, and
a less complex series of horizontal ledges due to the prominent edges of
hard layers. The steep slopes are worn into alcoves and irregular
recesses by the transient rills, and smoothed or etched by the
wind-driven sands. The result is an assemblage of architectural forms
such as only the most fantastic dream pictures or the strange tricks of
the mirage on northern ice-fields can simulate. Nor are the wonderfully
intricate topographical forms the sole attraction. The rocks are
variously coloured, and present endless combinations of yellow, red,
green, purple, etc., in many tints and shades, rendered seemingly
brilliant by contrast with the gray of shales and the blackness of
occasional coal-seams. Owing to the burning of coal-beds, the rocks are
sometimes altered over broad areas and given unusually striking colours,
among which various shades of red predominate. Standing on some
commanding crag in the Bad Lands in the early morning or when the purple
shadows of evening fill the gorges and ravines, the most unimaginative
traveller sees in the silence about him the ruins of a vast city, with
cathedrals, temples, and palaces of varied colours and weird designs
such as no mortal hand ever fashioned. It is at such times that the
picturesque and gorgeous, although desolate, landscape kindles the fancy
and suggests day dreams which distract one's attention from the more
prosaic study of these earth ruins.
The best developed portions of the strange region here referred to occur
on the borders of the uplands overlooking the larger valleys, excavated
by the rivers flowing eastward from the mountains, and are simply larger
examples of erosion, such as may be seen in many bluffs and valley-sides
in nearly every country, but rendered conspicuous by their size, extent,
endless variety, and unusual colours.
_Sand-Hills._--The sands winnowed by the winds from the bare plains and
steep bluffs are in certain places on the Great plateaus gathered into
dunes which cover great areas with a succession of low dome-shaped
hills. On the borders of the Niobrara River there is a detached area of
about 20,000 square miles, which has been covered in this manner with
loose sands. This region, as described by F. V. Hayden, presents a
succession of round-topped hills, some of them scooped out by the
swirling winds so as to resemble volcanic craters. These sand-hills were
formerly a favourite resort of the bison, which fed upon the scanty but
very nutritious grasses in the little valleys and intervals among the
mounds and ridges. There is, for the most part, an abundant supply of
water in the lakelets scattered through the region, and fed by the
seepage from the porous sands, which drink in all the water that falls
upon them and allow it to percolate slowly into the adjacent
depressions. Some of the lakelets and ponds are highly alkaline, while
others are fresh; the former can be easily distinguished from the latter
by the absence of vegetation about their borders. The hills, although
seemingly utterly desolate, on a nearer view sometimes reveal
considerable vegetation, including yuccas or "Spanish needles," which
shelter the sands from the winds.
Many other regions on the western border of the Great plateaus, in the
valleys of the Rocky Mountains and on the desert plains of Utah, Nevada,
and California, are buried beneath drifting sands, which have the
characteristic features of a dune-covered seacoast. The sources of the
sands in these interior plains are usually to be found in the
disintegration of the rocks under the action of the dry air with its
many and sudden changes of temperature, but occasionally they come from
desiccated lake beds not yet clothed with vegetation.
_Central Portion of the Great Plateaus._--The State of Nebraska, about
400 miles in length from east to west, and extending across the Great
plateaus to within some 60 miles of the mountains bordering them on the
west, furnishes a typical example of the west-central portion of the
interior Continental basin. As described by Hayden, this State may be
divided into two portions, one agricultural and the other pastoral. The
eastern part, included in the Prairie plains, contains some of the most
beautiful, gently rolling, and fertile agricultural lands in America.
But the western part is a treeless, almost waterless plain; yet thick,
low, sweet, nutritious grasses cover the entire surface, and render it
well adapted for the raising of large numbers of horses, cattle, and
sheep. Over western Nebraska not more than 15 or 20 inches of moisture
fall annually; the snows of winter are very light and soon pass away,
the winds rapidly gathering them into the valleys and gorges, leaving
vast areas entirely bare. The grasses, instead of decaying, as in all
temperate countries with a humid climate, slowly wither, retaining all
their nutritious qualities, and thus continue until April or May, when
the fresh shoots spring up, so that all kinds of stock thrive throughout
the winter on the open plains without artificial shelter. In this
account, however, the author cited fails to note that the winters are
frequently marked by exceedingly severe storms termed blizzards, during
which gales blow while the temperature is far below freezing, and that
at such times cattle have been known to perish by thousands.
In late summer and autumn the streams in this portion of the plateau
region for the most part become dry, although water may usually be
discovered at long intervals in pools in their beds. In ascending the
valleys the water appears and disappears as if by magic. Here one finds
a swift-running stream several yards broad, and then for a considerable
distance nothing is to be seen but a dry and dusty creek bed, resembling
a sunken roadway. Even the broad Platte has so far forgotten itself for
several seasons as to cease to be a running stream. It is not uncommon
for a river originating in the mountains on the west to be considerably
larger towards its source than near its mouth. Many of the important
streams that flow from the Black Hills towards the Missouri are lost on
their way through the plains. The Yellowstone and the Missouri, the two
most important rivers crossing the Great plateaus in the northern
portion of the United States, retain their existence throughout the
year, although becoming greatly shrunken in autumn, and send eastward a
never-ceasing tribute to the Mississippi.
_Northern Extension of the Great Plateaus._--The Great plateaus cross
the United States-Canadian boundary and extend northwestward through the
western portion of Assiniboia and Saskatchewan, and embrace nearly the
whole of Alberta and western Athabasca. At the international boundary
the plateau region is about 470 miles broad, and extends from longitude
103 deg. 30' westward to the foot-hills of the Rocky Mountains, and in this
region embraces what is frequently termed the third prairie steppe,
known in part as the Missouri Coteau. The east border of this high
plateau throughout much of its extent is well marked by an escarpment
which descends some 300 or 400 feet to the second prairie steppe, which
together with the first or most easterly of the series, embracing the
Red River Valley, is usually considered as belonging to the Prairie
plains. The third steppe in the series, or the one extending from the
Red River to the Rocky Mountains, has an elevation along its eastern
border of about 2,000 feet, rises gradually to the westward, and attains
a general elevation of over 4,000 feet on its western border. All of the
region of the Great plateaus north of the international boundary,
with the exception of about 20,000 square miles tributary to the
Missouri, is drained by rivers flowing eastward to Hudson Bay or
northward to the Arctic Ocean. It is thus a portion of the northern or
arctic slope of the Continental basin. The eastern border of the plateau
country trends northwestward, and finally reaches the Rocky Mountains in
the vicinity of the head waters of the Mackenzie, but as this region is
but imperfectly explored, our knowledge of the boundaries of the natural
division of the continent we have been endeavouring to trace there
becomes indefinite. The Great plateaus in Canada merge into the Prairie
plains bordering them on the east, and in large part the position of the
dividing line between the two is arbitrary.
The portion of the Great plateaus in Canada, like the similar region to
the south, is covered with bunch-grass, which dries as it stands and
forms highly nutritious self-cured hay. Formerly this region was the
winter feeding-ground for vast herds of bison. The winters, although
cold, are not characterized by a heavy snowfall, and even in midwinter
the warm dry _chinook_ winds, as they are termed, similar to the _foehn_
winds of Switzerland, frequently cause the snow to disappear and leave
the brown plateau surfaces free for grazing. Now that the bison has
disappeared, this immense region is favourable in many ways for
stock-raising, but, unlike the lower prairies to the east with their
rich black soil and long hot summers, is not suitable for agriculture.
The main difficulties in the way of successful farming lie in the
dryness of the summers, and the scarcity of water available for
irrigation. The rivers flow in valleys several hundred feet below the
general plateau surfaces, and hence cannot be made available for
irrigating the uplands without too great an expense. In the bottoms of
the valleys, however, adjacent to the stream, limited areas are now
under cultivation, and it is to be expected that the wheat-fields of the
prairie region will be gradually extended into the valley to the
westward, and perhaps even to the eastern margin of the plateaus. A
greater extension of the wheat-belt to the north and west than is now
thought practicable has been predicted, but what the ultimate
limit will be cannot be told.
_The Black Hills of Dakota._--As stated in the brief account already
given of the Prairie plains, their monotony of surface and of geological
structure is broken by a single area of disturbance termed the Ozark
uplift. Similarly, the vast generally level expanse of the Great
plateaus is broken by a single rudely circular region of elevation, the
Black Hills of Dakota, which has been sculptured by atmospheric agencies
and given a diversified topography, in striking contrast with the even
monotony of the country surrounding it.
This protuberance on the surface of the Great plateaus is situated in
the southwest portion of South Dakota, and embraces also a part of
Wyoming, and about 140 miles east of the nearest range--the Big Horn
Mountains--of the Rocky Mountain chain. It rises from the surrounding
plateau to a height on an average of about 2,000 to 2,500 feet; the
highest summit, Harney Peak, is 3,000 feet above the plain, and 7,216
feet above the sea. The uplift is elliptical in ground plan, with a
northwest and southeast axis measuring about 120 miles, and a transverse
diameter of 40 to 50 miles. Its area is in the neighbourhood of 6,000
square miles.