| Physiographic regions of the U.S. Interior See:legend |
For purposes of description, the physical geography of the United States is split into several major physiographic divisions, three of which being the Laurentian Highlands, Interior Highlands and the Interior Plains (see subdivisions 1 and 11-15) lie in the interior of the U.S. Please refer to the Geography of the United States for the other areas.
| Table of contents |
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2 Region of the Great Lakes 3 The Prairie States 4 The Gulf Coastal Plain 5 The Great Plains |
An outlying upland of the Laurentian highlands of Canada projects into
the United States west and south of Lake Superior. This upland, part of the Canadian Shield along with the Adirondacks, is
a greatly deformed structure and is composed primarily of crystalline rocks
commonly associated with a rugged landscape. At some ancient period,
this had a strong relief, but today the upland as a whole is gently rolling
with the inter-streams surfaces being plateau-like in their evenness. Here
they have altitudes of 1,400 to 1,600 feet in their higher areas. In
this province, we find a part of those ancient mountains regions that
were initated by crustal deformation and then reduced by a long continued
erosion to a peneplain of modern relief. A peneplain with the occasional
moderately high monadnocks left behind during the peneplanation of
the rest of the surface. The erosion of the region must have been far
advanced in ancient times, even practically completed, because the even
peneplain surface is overlapped by fossiliferous marine strata from an
early geological data ( Cambrian ). This shows that the depression
of the region beneath an ancient sea took place after a long existence
as dry land.
The extent of the submergence and the area over which the
Palaeozoic strata were deposited are unknown. Because of the
renewed elevation without deformation, erosion in later periods has
stripped off an undetermined amount of the covering strata. The
valleys by which the uplands are here and there trenched to moderate
depth appear to be, in part at least, the work of streams that have
been superposed upon the peneplain through the now removed cover of
stratified rocks. Glaciation has strongly scoured away the
deeply-weathered soils that presumably existed here in preglacial time.
It left behand firm and rugged ledges in the low hills and swells of the
ground and spread an irregular drift cover over the lower parts, whereby the
drainage is generally disordered being deposited in lakes and
swamps and elsewhere rushing down rocky rapids.
The Palaeozoic strata, already mentioned as lapping on the southern
slope of the Superior Upland and around the western side of the
Adirondack Mountains are but parts of a great area of similar strata
hundreds of feet in thickness. These strata decline gently southward from
the great upland of the Laurentian highlands of eastern Canada.
The visible upland area of today was but a small part of the primeval
continent with the remainder of it still buried under a Palaeozoic cover.
The visible part was the last part of the primeval continent to sink under the
advancing Palaeozoic seas. When the upland and its overlap of stratified
deposits were elevated again, the overlapping strata must have had the
appearance of a coastal plain. Of course that was long ago, since then
the strata have eroded substantially and today possess neither the area nor
the smooth form of their initial extent. This district may be placed
considered an ancient coastal plains. As is always the case in the
broad denudation of the gently inclined strata of such plains, the
weaker layers are worn down in sub-parallel belts of lower land
between the upland and the belts of more resistant strata, which
rise in uplands.
Few better illustrations of this type of forms are to be found than
that presented in the district of the Great Lakes. The chief upland
belt or escarpment is formed by the firm Niagara limestone, which
takes its name from the gorge and falls cut through the upland by
the Niagara River. As in all such forms, the Niagara Escarpment has
a relatively strong slope or infacing escarpment on the side towards
the upland, and a long gentle slope on the other side. Its relief is
seldom more than 200 or 300 feet and is generally small. Its
continuity and its constrast with the associated lowlands on
the underlying and overlying weak strata suffice to make it an important
feature. The escarpment would lie straight east-west if the slant of
the strata were uniformly to the south. However, the strata are somewhat
warped and so the escarpment's course is strongly convex to the north in
the middle, gently convex to the south at either end.
The escarpment begins where its determining limestone begins, in west-central
New York. There, it separates the lowlands that containing Lake Ontario
from Lake Erie. It curves to the northwest through the Ontario province to
the island belt that divides the Georgian Bay from Lake Huron. From there,
it heads westward through the land-arm between Lake Superior and Lake Michigan
and sout-westward into the narrow points dividing Green Bay from Lake Michigan.
Finally, it fades away with the thinning out of the limestone and is hardly
traceable across the Mississippi River.
The arrangement of the Great Lakes is closely matches the course of the
lowlands worn on the two belts of weaker strata on either side of the
Niagara escarpment. Lake Ontario, Georgian Bay and Green Bay
occupy depressions in the lowland on the inner side of the escarpment.
Lake Erie, Lake Huron and Lake Michigan lie in depressions in the lowland
on the outer side. When the two lowlands are traced eastward, they become
confluent after the Niagara limestone has faded away in central New York
and the single lowland is continued under the name of Mohawk Valley. This is
an east-west longitudinal depression that has been eroded on a belt of
relatively weak strata between the resistant crystalline rocks of the
Adirondacks on the north and the northern escarpment of the Appalachian
plateau (Catskills-Helderbergs) on the south. Early in the U.S. history,
this provided a vital economic route between the Atlantic seaports
and the U.S. interior.
In Wisconsin, the inner lowland has an interesting feature. It is a knob of
resistant quartzites, known as Baraboo Ridge, rising from the buried upland
floor through the partly denuded cover of lower Palaeozoic strata. This knob
or ridge can be thought of as an ancient physiographic fossil as it is
an ancient monadnock having been preserved from destructive attacks of weather
by burial under sea-floor deposits. It has been recently re-exposed through
the erosion of its cover.
The occurrence of the lake basins in the lowland belts on either side of the
Niagara escarpment is an abnormal feature. Ordinary erosion does not explain
it. Glacial erosion has formed them through the glacial drift obstructing the
normal outlet valleys and to crustal warping in connection with or independent
of the glacial sheet.
Lake Superior is unlike the other lakes. The greater part of its basin
occupies a depression in the upland area, independent of the overlap
of Palaeozoic strata. The western half of the basin occupies a trough
of synclinal structure. The Great Lakes are peculiar in
receiving the drainage of but a small peripheral land area, enclosed
by an ill-defined water-parting from the rivers that run to Hudson Bay
or the Gulf of St Lawrence on the north and
to the Gulf of Mexico on the south.
The three lakes of the middle group stand at practically the same level:
The originally treeless prairies of the upper Mississippi basin began in
Indiana and extended westward and north-westward until they merged with
the drier region known as the Great Plains. An eastward extension of
the same region, originally tree-covered, extended to central Ohio.
Thus the prairies generally lie between the Ohio and Missouri rivers
on the south and the Great Lakes on the north.
The prairies are a contribution of the glacial period. They
consist for the most part of glacial drift, deposited unconformably on
an underlying rock surface of moderate or small relief. Here, the rocks
are an extension of the same stratified Palaeozoic formations
already described as occurring in the Appalachian region and around the
Great Lakes. They are usually fine-textured limestones and shales, lying
horizontal. The moderate or small relief that they were given by mature
preglacial erosion is now buried under the drift.
The greatest area of the prairies, from Indiana to North Dakota,
consists of till plains, that is, sheets of unstratified drift.
These plains are 30, 50 or even 100 ft. thick covering the underlying
rock surface for thousands of square miles except where postglacial
stream erosion has locally laid it bare. The plains have an extraordinarily
even surface. The till is presumably made in part of preglacial soils,
but it is more largely composed of rock waste mechanically transported
by the creeping ice sheets. Although the crystalline rocks from Canada
and some of the more resistant stratified rocks south of the Great Lakes occur
as boulders and stones, a great part of the till has been crushed and
ground to a clayey texture. The till plains, although sweeping in broad
swells of slowly changing altitude, often appear level to the eye with a
view stretching to the horizon. Here and there, faint depressions occur,
occupied by marshy sloughs, or floored with a rich black soil of postglacial
origin. It is thus by sub-glacial aggradation that the prairies have been
levelled up to a smooth surface, in contrast to the higher and
non-glaciated hilly country just to the south.
The great ice sheets formed terminal moraines around their border at
various end stages. However, the morainic belts are of small relief in
comparison to the great area of the ice. They rise gently from the till
plains to a height of 50, 100 or more feet. They may be one, two or three
miles wide and their hilly surface, dotted over with boulders, contains many
small lakes in basins or hollows, instead of streams in valleys. The morainic
belts are arranged in groups of concentric loops, convex southward,
because the ice sheets advanced in lobes along the lowlands of the
Great Lakes. Neighboring morainic loops join each other in re-entrants
(north-pointing cusps), where two adjacent glacial lobes came together
and formed their moraines in largest volume. The moraines are of too small
relief to be shown on any maps except of the largest scale. Small as they are,
they are the chief relief of the prairie states, and, in association with
the nearly imperceptible slopes of the till plains, they determine the
course of many streams and rivers, which as a whole are consequent
upon the surface form of the glacial deposits.
The complexity of the glacial period and its subdivision into several
glacial epochs, separated by interglacial epochs of considerable length
(certainly longer than the postglacial epoch) has a structural
consequence in the superposition of successive till sheets,
alternating with non-glacial deposits. It also has a physiographic consequence
in the very different amount of normal postglacial erosion suffered by the
different parts of the glacial deposits. The southernmost drift sheets, as
in southern Iowa and northern Missouri, have lost their initially
plain surface and are now maturely dissected into gracefully rolling forms.
Here, the valleys of even the small streams are well opened and graded, and
marshes and lakes are rare. These sheets are of early Pleistocene origin.
Nearer the Great Lakes, the till sheets are trenched only by the narrow
valleys of the large streams. Marshy sloughs still occupy the faint
depressions in the till plains and the associated moraines have
abundant small lakes in their undrained hollows. These drift sheets are of
late Pleistocene origin.
When the ice sheets extended to the land sloping southward to the Ohio River,
Mississippi River and Missouri River, the drift-laden streams flowed freely
away from the ice border. As the streams escaped from their subglacial
channels, they spread into broader channels and deposited some of their
load and thus aggraded their courses. Local sheets or aprons of gravel
and sand are spread more or less abundantly along the outer side of the
morainic belts. Long trains of gravel and sands clog the valleys that
lead southward from the glaciated to the non-glaciated area. Later, when
the ice retreated farther and the unloaded streams returned to their earlier
degrading habit, they more or less completely scoured out the valley deposits,
the remains of which are now seen in terraces on either side of the present
flood plains.
When the ice of the last glacial epoch had retreated so far that its front
border lay on a northward slope, belonging to the drainage area of the
Great Lakes, bodies of water accumulated in front of the ice margin,
forming glacio-marginal lakes. The lakes were small at first, and each
had its own outlet at the lowest depression of land to the south. As the ice
melted further back, neighboring lakes became confluent at the level of
the lowest outlet of the group. The outflowing streams grew in the same
proportion and eroded a broad channel across the height of land and far
down stream, while the lake waters built sand reefs or carved shore cliffs
along their margin, and laid down sheets of clay on their floors.
All of these features are easily recognized in the prairie region.
The present site of Chicago was determined by an Indian portage or
carry across the low divide between Lake Michigan and the headwaters
of the Illinois River. This divide lies on the floor of the former
outlet channel of the glacial Lake Michigan. Corresponding outlets
are known for Lake Erie, Lake Huron and Lake Superior. A very large sheet of
water, named Lake Agassiz, which once overspread a broad till plain in
northern Minnesota and North Dakota. The outlet of this glacial lake,
called river Warren, eroded a large channel in which the Minnesota River
evident today. The Red River of the North flows northward through a plain
formerly covered by Lake Agassiz.
Certain extraordinary features were produced when the retreat of the
ice sheet had progressed so far as to open an eastward outlet for the
marginal lakes. This outlet occurred along the depression between the
northward slope of the Appalachian plateau in west-central New York and
the southward slope of the melting ice sheet. When this eastward outlet
came to be lower than the south-westward outlet across the height of
land to the Ohio or Mississippi river, the discharge of the marginal
lakes was changed from the Mississippi system to the Hudson system.
Many well-defined channels, cutting across the north-sloping spurs
of the plateau in the neighborhood of Syracuse, New York, mark the temporary paths of the ice-bordered outlet river. Successive channels
are found at lower and lower levels on the plateau slope, indicating the
successive courses taken by the lake outlet as the ice melted farther
and farther back. On some of these channels, deep gorges were eroded
heading in temporary cataracts which exceeded Niagara in height but
not in breadth. The pools excavated by the plunging waters at the head
of the gorges are now occupied by little lakes. The most significant
stage in this series of changes occurred when the glacio-marginal
lake waters were lowered so that the long escarpment of Niagara
limestone was laid bare in western New York. The previously confluent
waters were then divided into two lakes. The higher one, Lake Erie,
supplying the outfiowing Niagara river, which poured its waters down
the escarpment of the escarpment to the lower, Lake Ontario. Lake Ontario's
outlet for a time ran down the Mohawk Valley to the Hudson. This gave
rise to the Niagra falls.
In certain districts, the subglacial till was not spread out in a smooth
plain, but accumulated in elliptical mounds, 100-200 feet. high and
0.5 - 1.0 mile long with axes parallel to the direction of the ice motion
as indicated by striae on the underlying rock floor. These hills are known
by the Irish name, drumlins, used for similar hills in north-western Ireland.
The most remarkable groups of drumlins occur in western New York, where
their number is estimated at over 6,000, and in southern Wisconsin, where
it is placed at 5,000. They completely dominate the topography of their
districts.
A curious deposit of an impalpably fine and unstratified silt, known by the
German name bess, lies on the older drift sheets near the larger river
courses of the upper Mississippi basin. It attains a thickness of 20 ft.
or more near the rivers and gradually fades away at a distance of ten
or more miles on either side. It contains land shells, and hence cannot
be attributed to marine or lacustrine submergence. The best explanation
is that, during certain phases of the glacial period, it was carried as
dust by the winds from the flood plains of aggrading rivers, and slowly
deposited on the neighboring grass-covered plains.
South-western Wisconsin and parts of the adjacent states of Illinois,
Iowa and Minnesota are known as the driftless area, because, although
bordered by drift sheets and moraines, it is free from glacial deposits.
It must therefore have been a sort of oasis, when the ice sheets from
the north advanced past it on the east and west and joined around its
southern border. The reason for this exemption from glaciation is the
converse of that for the southward convexity of the morainic loops.
For while they mark the paths of greatest glacial advance along lowland
troughs (lake basins), the driftless area is a district protected from
ice invasion by reason of the obstruction which the highlands of
northern Wisconsin and Michigan (part of the Superior upland) offered
to glacial advance.
The course of the upper Mississippi river is largely consequent upon
glacial deposits. Its sources are in the morainic lakes in northern
Minnesota. The drift deposits thereabouts are so heavy that the present
divides between the drainage basins of Hudson Bay, Lake Superior and the
Gulf of Mexico evidently stand in no very definite relation to the
preglacial divides. The course of the Mississippi through Minnesota
is largely guided by the form of the drift cover. Several rapids and
the Falls of Saint Anthony (determining the site of Minneapolis) are
signs of immaturity, resulting from superposition through the drift
on the under rock. Farther south, as far as the entrance of the Ohio,
the Mississippi follows a rock-walled valley 300 to 400 ft. deep,
with a flood-plain 2 to 4 m. wide. This valley seems to represent the
path of an enlarged early-glacial Mississippi, when much precipitation
that is today discharged to Hudson Bay and the Gulf of St Lawrence was
delivered to the Gtilf of Mexico, for the curves of the present river
are of distinctly smaller radii than the curves of the valley.
Lake Pepin (30 m. below St. Paul), a picturesque expansion of the
river across its flood-plain, is due to the aggradation of the
valley floor where the Chippewa River, coming from the northeast,
brought an overload of fluvio-glacial drift. Hence even the father of
waters, like so many other rivers in the Northern states, owes many of
its features more or less directly to glacial action.
The fertility of the prairies is a natural consequence of their origin.
During the mechanical transportation of the till no vegetation was present
to remove the minerals essential to plant growth, as is the case in the
soils of normally weathered and dissected peneplains. The soil is
similar to the Appalachian piedmont which though not exhausted by the
primeval forest cover, are by no means so rich as the till sheets of the
prairies. Moreover, whatever the rocky understructure, the till soil has
been averaged by a thorough mechanical mixture of rock grindings. Hence
the prairies are continuously fertile for scores of miles together.
The true prairies were once covered with a rich growth of natural grass and
annual flowering plants, but today are covered with farms.
The westward extension of the Atlantic coastal plain around the Gulf of
Mexico has certain features that were already described and several new ones.
As in the Atlantic coastal plain, it is only the lower, seaward part of
this region that deserves the name of plain, for there alone is the
surface unbroken by hills or valleys. The inner part, initially a plain
by, has been maturely dissected into an elaborate complex of hills and
valleys, usually of increasing altitude and relief as one passes inland.
The Gulf Plain features not found in the Atlantic coastal plain are:
A typical example of a belted coastal plain is found in Alabama and the
adjacent part of Mississippi. The plain is here about 1.50 m. wide.
The basal formation if chiefly a weak limestone, which has been stripped
from its original Alabama innermost extension and worn down to a flat
inner lowland of rich black soil, thus gaining the name of the black belt.
The lowland is enclosed by an upland or escarpment, known as Chunnenugga
Ridge, sustained by partly consolidated sandy strata. However, the upland
is not continuous, but a maturely dissected escarpment. It has a relatively
rapid descent toward the inner lowland, and a very gradual descent to the
coast prairies, which become very low, flat and marshy before dipping
under the Gulf waters, where they are generally fringed by off-shore reefs.
The coastal plain extends 500 miles inland on the axis of the Mississippi
embayment. Its inner border affords admirable examples of topographical
discordance where it sweeps northwestward square across the trend of the
piedmont belt, the ridges and valleys, and the plateau of the Appalachians.
All of which are terminated by dipping gently beneath the unconformable
cover of the coastal plain strata. In the same way the western side of the
embayment, trending south and southwest, passes along the lower southeastern
side of the dissected Ozark plateau of southern Missouri and northern
and central Arkansas. The southern Missouri and northern Arkansas Ozark plateau
resembles in many ways the Appalachian plateau. As the coastal plain
turns westward to Texas, it borders the Arbuckle hills in Oklahoma a small
analog of the Appalachian crystalline belt. In the embayment of the coastal
plain some low escarpment-like belts of hills with associated strips of
lowlands suggest the features of a belted coastal plain. The hilly belt or
dissected escarpment determined by the Grand Gulf formation in western
Mississippi is the most distinct. Important salt deposits occur in the
coastal plain strata near the coast. The most striking feature of the
embayment is the broad valley which the Mississippi has eroded across it.
The small proportion of total water volume supplied from the great Missouri
basin is due to the light precipitation in that region. The lower Mississippi
has no large tributaries from the lower east, but two important ones come
from the west. The Mississippi Arkansas drainage area being a little less
than the Ohio River and the basin of the Red River of Louisiana
being about half as large. The Mississippi River drains an area of about
one-third of the United States. The head of the coastal plain embayment
is near the junction of the Ohio and the Mississippi. It flows southward for
560 miles through the semi-consolidated strata of the plain. The river has
eroded a valley about 40-50 miles wide enclosed by bluffs one or two hundred
feet high in the northern part. These bluffs decrease towards the south, but
with local increase of height associated with a decrease in flood plain
breadth on the eastern side where the Grand Gulf escarpment is traversed.
This valley in the coastal plain, with the much narrower rock-walled
valley of the upper river in the prairie states, is the true valley of
the Mississippi River. However in popular usage, the Mississippi valley
is taken to include a large central part of the Mississippi drainage basin.
The valley floor is covered with a flood plain of fine silt, having a
southward slope of only half a foot to a mile. The length of the river
itself, from the Ohio mouth to the Gulf is about 1,060 miles due to its
windings. Its mean fall is about 3 inchs per mile. On account of the rapid
deposition of sediment near the main channel at times of overflow, the
flood plain, as is normally the case on mature valley floors, has a
lateral slope of as much as 5, 10, or even 12 ft. in the first mile
from the river, but this soon decreases to a less amount. Thus
just a short distance from the river, the flood plain is often swampy,
unless its surface is there aggraded by the tributary streams. For this
reason Louisiana, Arkansas and Mississippi rank immediately after Florida in
swamp area.
The great river receives an abundant load of silt from its tributaries,
and takes up and lays down silt from its own bed and banks with every change
of velocity. The swiftest current follows the outer side of every significant
curve in the channel. Thus the concave bank on the side of the fastest part of
the river is worn away. Any chance irregularity is exaggerated, and in time
a series of large serpentines or meanders is developed, the most-symmetrical
examples at present being those near Greenville, Mississippi. The growth of the
meanders tends to give the river continually increasing length. This
tendency is counteracted by the sudden occurrence of cut-offs from time
to time, so that a fairly constant length is maintained.
The floods of the Mississippi usually occur in spring or summer. Owing to
the great size of the drainage basin, it seldom happens that the three
upper tributaries are simultaneously flooded. It is a serious problem for
the lower river if two of the large tributaries flood at the same time.
In this case, the lower river will rise to 30, 40 or even 50 feet.
The fall of the river is significantly steepened and its velocity is
accelerated down stream from the point of highest rise. Conversely, the
fall and the velocity are both diminished up stream from the same point.
The load of silt carried down stream by the river finally, after many
halts on the way, reaches the waters of the Gulf. There, the decrease of
velocity aided by the salinity of the sea water, causes the formation of
a remarkable delta, leaving less aggraded areas as shallow lakes
(Lake Pontchartrain on the east, and Grand Lake on the west of the river).
The ordinary triangular form of deltas, due to the smoothing of the delta
front by sea action, is here wanting, because of the weakness of sea action
in comparison with the strength of the current in each of the four
distributaries or passes into which the river divides near its mouth.
After constriction from the Mississippi embayment to 250 miles in western
Louisiana, the coastal plain continues southwestward with this breadth
until it narrows to about 130 in. in southern Texas near the
crossing of the Colorado river (Texas Colorado River,
not the major Colorado River), but it again widens to 300 miles at the
national boundary as a joint effect of embayment up the valley of the
Rio Grande and of the seaward advance of this rivers rounded delta
front. These several changes take place in a distance of about 500 miles.
It includes a region of over 100,000 sq. miles less than half of the large
state of Texas. A belted arrangement of reliefs and soils, resulting from
differential erosion on strata of unlike composition and resistance,
characterizes almost the entire area of the coastal plain. Most of the
plain is treeless prairie, but the sandier belts are forested. Two of them
are known as cross timbers,
because their trend is transverse to the general course of the main
consequent rivers. An inland extension from the coastal plain in
north-central Texas leads to a large escarpment known as Grand Prairie
(not structurally included in the coastal plain), upheld at altitudes of
1,200 or 1300 ft. by a resistant Cretaceous limestone. This dips gently
seaward with its scalloped inland-facing escarpment overlooking a denuded
central prairie region of irregular structure and form. Its gentle coastward
slope (16 ft. to a mile) is dissected by many branching consequent streams.
In its southernpart as it approaches the Colorado river, the escarpment is
dissected into a belt of discontinuous hills. The western cross timbers
follow a sandy belt along the inner base of the ragged escarpment of
Grand Prairie. The eastern cross timbers follow another sandy belt in the
lowland between the eastern slope of Grand Prairie and the pale western
escarpment of the immediately eastward and lower Black Prairie escarpment.
This escarpment is supported at an altitude of 700 ft. or less by a
chalk formation, which gives an infacing slope some 200 ft. in height.
Its gently undulating or rolling seaward slope (2 or 3 ft. in a mile),
covered with marly strata and rich black soil, determines an important
cotton district. Then comes the East Texas timber belt, broad in the
northeast, narrowing to a point before reaching the Rio Grande,
a low and thoroughly dissected escarpment of sandy Eocene strata.
This is followed by the Coast Prairie, a very young plain, with a
seaward slope of less than 2 ft. in a mile, its smooth surface
interrupted only by the still more nearly level flood plains of
the shallow, consequent river valleys. Near the Colorado river,
the dissected escarpment of the Grand Prairie passes southward changing
to a more nearly horizontal structure into the dissected Edwards plateau.
The Edwards plateau is referred to later as part of the Great Plains.
The plateau terminates in a maturely dissected fault scarp approximately
300 or 400 feet in height as the northern boundary of the Rio Grande
embayment. From the Colorado to the Rio Grande, the Black Prairie,
the timber belt and the Coast Prairie merge in a vast plain, little
differentiated, overgrown with chaparral (shrub-like trees, often thorny),
widening eastward in the Rio Grande delta and extending southward into Meçico.
Although the Coast Prairie is a sea bottom of very modern uplift, it appears
already to have suffered a slight movement of depression. Its small rivers
all enter embayments. However, the larger rivers seem to have counteracted
the encroachment of the sea on the land by a sufficiently active delta
building with a resulting forward growth of the land into the sea.
The Mississippi has already been mentioned as rapidly building forward
its digitate delta. The Rio Granide, next in size, has built its delta
about 50 miles forward from the general coastline. Since this river is
much smaller than the Mississippi, its delta front is rounded by seashore
effects. In front of the Brazos and the Colorado, the largest of the Texan
rivers, the coast-line is very gently bowed forward as if by delta growth.
The sea touches the mainland in a nearly straight shore line. Nearly all
the rest of the coast is fringed by off-shore reefs, built up by waves from
the very shallow sea bottom. Due to the weak tides, the reefs continue in
long unbroken stretches between the few inlets.
A broad stretch of country underlaid by nearly horizontal strata extends
westward from the 97th meridian to the base of the Rocky Mountains,
a distance of from 300 to 500 miles. It extends northward from the Mexican
boundary far into Canada. This is the province of the Great Plains.
Although the altitude of plains increases gradually from 6oo or 1,200 ft.
on the east to 4,000-5,000 or 6,000 feet near the mountains, the local
relief is generally small. The sub-arid climate excludes tree growth and
opens far-reaching views. The plains are by no means a simple unit. They
are of diverse structure and of various stages of erosional development.
They are occasionally interrupted by buttes and escarpments. They are
frequently broken by valleys. Yet on the whole, a broadly extended surface
of moderate relief so often prevails that the name, Great Plains, for the
region as a whole is well deserved. The western boundary of the plains is
usually well defined by the abrupt ascent of the mountains. The eastern
boundary of the plains is more climatic than topographic. The line of
20 in of annual rainfall trends a little east of northward near the
97th meridian. If a boundary must be drawn where nature presents only
a gradual transition, this rainfall line may be taken to divide the drier
plains from the moister prairies. The plains may be described in northern,
intermediate, central and southern sections, in relation to certain peculiar
features.
The northern section of the Great Plains, north of latitude 44°, including
eastern Montana, north-eastern Wyoming and most of the Dakotas, is a
moderately dissected peneplain. This is one of the best examples of its kind.
The strata here are Cretaceous or early Tertiary, lying nearly horizontal.
The surface is shown to be a plain of degradation by a gradual ascent
here and there to the crest of a ragged escarpment, the escarpment-remnant
of a resistant stratum. There are also the occasional lava-capped mesas and
dike-ridges, surmounting the general level by 500 ft. or more and manifestly
demonstrating the widespread erosion of the surrounding plains. All these
reliefs are more plentiful towards the mountains in central Montana.
The peneplain is no longer in the cycle of erosion that witnessed its
production. It appears to have suffered a regional elevation, for the upper
Missouri River and its branches no longer flow on the surface of the
plain, but in well graded, maturely opened valleys, several hundred feet
below the general level. A significant exception to the rule of mature
valleys occurs, however, in the case of the Missouri, the largest river,
which is broken by several falls on hard sandstones about 50 miles east
of the mountains. This peculiar feature is explained as the result of
displacement of the river from a better graded preglacial valley by the
Pleistocene ice-sheet. Here, the ice sheet overspread the plains from
the moderately elevated Canadian highlands far on the north-east, instead
of from the much higher mountains near by on the west. The present altitude
of the plains near the mountain base is 4000 ft.
The northern plains are interrupted by several small mountain areas.
The Black Hills, chiefly in western South Dakota, are the largest group.
They rise like a large island from the sea, occupying an oval area of about
100 miles north-south by 50 miles east-west. At Harney Peak, they reach an
altitude of 7,216 feet and have an effective relief over the plains of 2000
or 3000 ft. This mountain mass is of flat-arched, dome-like structure,
now well dissected by radiating consequent streams. The weaker uppermost
strata have been eroded down to the level of the plains where their
upturned edges are evenly truncated. The next following harder strata
have been sufficiently eroded to disclose the core of underlying
crystalline rocks in about half of the domed area.
In the intermediate section of the plains, between latitudes 44° and 42°,
including southern South Dakota and northern Nebraska, the erosion of
certain large districts is peculiarly elaborate. Known as the Bad Lands,
it is a minutely dissected form with a relief of a few hundred feet. This is
due to several causes:
The southern section of the Great Plains, between latitudes 35.5° and
2~ ~. lies in eastern Texas and eastern New Mexico. Like the
central section, it is for the most part a dissected fluviatile plain.
However, the lower lands which surround it on all sides place it in so
strong relief that it stands up as a table-land, known from the time of
Mexican occupation as the Llano Estacado. It measures roughly 150 miles
east-west and 400 miles north-south. It is of very irregulal outline,
narrowing to the south. Its altitude is 5500 feet at the highest western
point, nearest the mountains whence its gravels were supplied. From there,
it slopes southeastward at a decreasing rate, first about 12 ft., then about
7 ft. per mile, to its eastern and southern borders, where it is 2000 ft.
in altitude. Like the High Plains farther north, it is extraordinarily smooth.
It is very dry, except for occasional shallow and temporary water sheets after
rains. The Llano is separated from the plains on the north by the mature
consequent valley of the Canadian river, and from the mountains on the west
by the broad and probably mature valley of the Pecos river. On the east,
it is strongly undercut by the retrogressive erosion of the headwaters of
the Red, Brazos and Colorado rivers of Texas and presents a ragged escarpment
approximately 500 to 800 ft. high, overlooking the central denuded area of
that state. There, between the Brazos and Colorado rivers, occurs a series
of isolated outliers capped by a limestone which underlies both the Llano
on the west and the Grand Prairies escarpment on the east. The southern and
narrow part of the table-land, called the Edwards Plateau, is more dissected
than the rest, and falls off to the south in a frayed-out fault scarp.
As already mentioned, this scrap overlooks the coastal plain of the Rio
Grande embayment. The central denuded area, east of the Llano, resembles
the east-central section of the plains in exposing older rocks. Between
these two similar areas, in the space limited by the Canadian and Red rivers,
rise the subdued forms of the Wichita MountaiIis in Oklahoma, the westernmost
member of the Ouachita system.The Superior Upland
Region of the Great Lakes
Lake Michigan and Lake Huron ar connected by the Mackinac Straits
with the Mackinac Bridge spanning the straits. Lake Huron and Lake Erie
are connected by the St Clair River and Detroit River with the
small Lake St. Clair between them. The land northeast of the rivers is
undergoing a slow elevation. The Niagara River connecting Lake Erie
and Lake Ontario, with a fall of 326 ft. (160 ft. at the cataract) in 30 miles,
is of very recent origin as an older river would have a mature valley. The
same is true for the St. Lawrence. The Prairie States
The Gulf Coastal Plain
A broad, low crustal arch extends southward at the junction of the Atlantic
and Gulf coastal plains. The emerged half of the arch, constitutes the
visible lowland peninsula of Florida. The submerged half extends westward
under the shallow Florida overlapping waters of the Gulf of Mexico. The
northern part of the peninsula is composed largely of a weak limestone.
Here, much of the lowland drainage is underground forming many sink-holes
(swallowholes). Many small lakes in the lowland appear to owe their basins
to the solution of the limestones. Valuable phosphate deposits occur in
certain districts. The southern part of the state includes the Everglades,
a large area of low, flat, marshy land, overgrown with tall reedy grass.
The eastern coast is fringed by long-stretching sand reefs, enclosing
lagoons so narrow and continuous that they are popularly called rivers.
At the southern end of the peninsula is a series of coral islands, known
as the Florida Keys. They appear to be due to the forward growth of
corals and other lime-secreting organisms towards the strong current
of the Gulf Stream from which they obtain their food.
The western coast has fewer, shorter off-shore reefs. Much of it is of
minutely irregular outline.The Great Plains
The central section of the Great Plains, between latitudes 42° and 36°,
occupying eastern Colorado and western Kansas, is, briefly stated,
for the most part a dissected fluviatile plain. That is, this section was
once smoothly covered with a gently sloping plain of gravel and sand that
had been spread far forward on a broad denuded area as a piedmont deposit
by the rivers which issued from the mountains. Since then, it has been more
or less dissected by the erosion of valleys. The central section of the
plains thus presents a marked contrast to the northern section. For while
the northern section owes its smoothness to the removal of local gravels
and sands from a formerly uneven surface by the action of degrading rivers
and their inflowing tributaries, the southern section owes its smoothness
to the deposition of imported gravels and sands upon a previously uneven
surface by the action of aggrading rivers and their outgoing distributaries.
The two sections are also unlike in that residual eminences still here and
there surmount the peneplain of the northern section, while the fluviatile
plain of the central section completely buried the pre-existent relief.
Exception to this statement must be made in the south-west, close to the
mountains in southern Colorado, where some lava-capped mesas
(Mesa de Maya, Raton Mesa) stand several thousand feet above the general
plain level, and thus testify to the widespread erosion of this region
before it was aggraded.