| Physiographic regions of the U.S. East Coast Courtesy of USGS See:legend |
For purposes of description, the physical geography of the United States is split into several major physiographic divisions, two of which being the Atlantic Plain and the Appalachian Highlands (see subdivisions 2-10) lie on or near the east coast. Please refer to the Geography of the United States for the other areas.
The Appalachain Highlands includes not just the Appalachian Mountains, but also the surrounding plateaus and valleys. The physiographic description of the system consists essentially of two belts:
The deformation of the Appalachians occurred in two major periods of compressive deformation:
Thus the Appalachian topographical features of today may be ascribed to
three cycles of erosion:
The more resistant rocks, even though dissected by Tertiary erosion, retain in their summit tiplands an indication of the widespread peneplain of Cretaceous time, now standing at the altitude given to it by the Tertiary upwarping and post-Tertiary uplift. The most resistant rocks surmount the Cretaceous peneplain as unconsumed monadnocks of the Mesozoic cycle. On the other hand, the weaker rocks are more or less completely reduced to lowlands by Tertiary erosion, and are now trenched by the narrow and shallow valleys of the short post-Tertiary cycle. Evidently, the Appalachians as we now see them are not the still surviving remnants of the mountains of late Palaeozoic deformation. Instead, they owe their present height chiefly to the Tertiary upwarping and uplifting and their form to the normal processes of sculpture which, having become nearly quiescent at the close, of the Mesozoic cycle, became active again in Tertiary and later times.
The aforementioned belts and the cycles of erosion can be seen nearly uniformly throughout the region stretching from the Gulf of St. Lawrence to Alabama some 500 miles southwestward. There, the deformed mountain structures pass out of sight under the nearly horizontal strata of the Gulf coastal plain. The dimensions of the several belts and the strength of the relief developed by their later erosion varies greatly along the whole system.
Northeastern Appalachians - New England
In a northeastern section (subdiv 9), practically all of
New England is occupied by the older crystalline belt.
The corresponding northern part of the stratified belt in the St Lawrence
and Champlain - Hudson valleys on the inland side of New England is
comparatively free from the ridge-making rocks which abound farther south.
Here, the plateau is nearly absent being replaced by the Adirondacks.
The Adirondacks are an outlier of the Laurentian Highlands (subdiv 1) of Canada
which immediately succeeds the deformed stratified belt west of
Lake Champlain.
From the Hudson river in southern New York to the James River
in southern Virginia, the crystalline belt is narrowed, but the stratified belt is broadly developed in a remarkable series of ridges and valleys determined by the action of erosion on the many alternations of strong and weak folded strata. The plateau assumes full strength southward from the Mohawk Valley which separates it from the Adirondacks. The linear ridges of this middle section are often called the Alleghany Mountains.
The middle section of the Appalachians, rather arbitrarily limited by the
Hudson and the James rivers contains the best representation of the three
longitudinal belts that comprise the whole mountain system.
Here, the mountain making compression of the Palaeozoic strata has
produced a marvellous series of rock folds with gently undulating axes,
trending northeast and southwest through a belt 70 or 80 miles wide.
This peculiar configuration of three ridges may be understood as follows:
Middle Appalachians
Southern Appalachians
In a southwestern section, the crystalline belt again assumes importance
in breadth and height, and the plateau member maintains the strength that
it had in the middle section, but the intermediate stratified belt again
has fewer ridges, because of the infrequence here of ridge, making strata
as compared to their frequency in the middle section.Middle Section of the Appalachian
| Middle Appalachians Courtesy of USGS |
The transition from the strongly folded structure of the Alleghany ridges and valleys to the nearly horizontal structure of the Appalachian plateau is promptly made. With this change in structure comes a change in the form. As the horizontal strata of the plateau present equal ease or difficulty of erosion in any direction, the streams and the submature valleys of the plateau therefore ramify in every direction, thus presenting a pattern that has been called insequent, because it follows no apparent control. - Further mention of the Appalachian plateau is made in a later section.
The crystalline belt of the middle Appalachians, approximately 60 to 80 miles wide, is today of moderate height because the Tertiary upwarping there was more moderate. The height is greatest along the inner or northwestern border of the belt. Here a sub-mountainous topography has been produced by normal dissection, chiefly in the Tertiary cycle. Its valleys are narrow because the rocks are resistant. The relief is strong enough to make occupation difficult. The slopes are forested. The uplands are cleared and well occupied farms and villages, but many of the valleys are wooded glens. As the altitude decreases southeastward, the crystalline belt dips under the coastal plain, near a line marked by the Delaware river from Trenton to Philadelphia, and from there south-southwestward through Maryland and Virginia past the cities of Baltimore, Washington and Richmond.
The Pennsylvania portion of the crystalline belt is narrow, because of encroachment upon it by the inward overlap of the coastal plain. It is low not only because of the small Tertiary uplift, but more importantly its even discontinous because of the inclusion of belts of weak non-crystalline rock. Here the rolling uplands are worn down to lowland belts, the longest of which reaches from the southern corner of New York, across New Jersey, Pennsylvania and Maryland, into central Virginia.
The middle section of the Appalachians is further distinguished from the northeastern and southwestern sections by its drainage. Its chief rivers rise in the plateau belt and flow across the ridges and valleys of the stratified belt and through the uplands of the crystalline belt to the sea. The rivers which best exemplify this are the Delaware, Susquehanna and Potomac. The Hudson, the northeastern boundary of the middle section, is peculiar in having headwaters in the Adirondacks as well as in the Catskills (northern part of the plateau). The James, forming the southwestern boundary of the section, rises in the inner valleys of the stratified belt, instead of in the plateau. The generally transverse course of these rivers has given rise to the suggestion that they are of antecedent origin. But there are many objections to this oversimplified, Gordian explaination. The southeast course of the middle section rivers is the result of many changes from the initial drainage. The Mesozoic and Tertiary upwarpings were probably very influential in determining the present general courses.
For the most part the rivers follow open valleys along belts of weak strata but will frequently pass through sharp cut notches in the narrow ridges of the stratified belt. The Delaware Water Gap is one of the deepest of these notches, and in the harder rocks of the crystalline belt they have eroded steep-walled gorges. The finest is that of the Hudson, because of the greater height and breadth of the crystalline highlands there than at points where the other rivers cross it. The rivers are shallow and more or less broken by rapids in the notches. Rapids also occur near the outer border of the crystalline belt, as if the rivers there had been lately incited to downward erosion by an uplift of the region, and had not yet had time to regrade their courses. This is well shown in the falls of the Potomac a few miles above Washington, in the rapids of the lower Susquehanna and in the falls of the Schuylkill. The Hudson possesses a deep and naturally navigable tide-water channel all through its gorge in the highlands, a feature which, in connection with the Mohawk Valley, has been absolutely determinative of the metropolitan rank reached by New York City at the mouth of the Hudson.
Northeastern Section of the Appalachians
| Northeast Appalachians Courtesy of USGS |
The community of characteristics that is suggested by the association of six northeastern states under the name New England is in large measure warranted. All of these states lie within the broadened crystalline belt of the northeastern Appalachians which is approximately 150 miles wide. The uplands which prevail through the center of this area at altitudes of about 1,000 ft. rise to 1,500 or 2,000 ft. in the northwest. Thereafter, these uplands descend to the lowlands of the stratified belt, the St Lawrence-Champlain-Hudson valleys, and at the same time the rising uplands are diversified with monadnocks of increasing number and height and by mature valleys cut to greater and greater depths. Thus, the interior of New England is moderately mountainous. When the central uplands are followed southeast or south to the coast, their altitude and their relief over the valleys gradually decrease. Finally near the coast, the surface gradually passes under the sea. The lower coastal parts, from their accessibility and their smaller relief, are more densely populated. The higher and more rugged interior is still largely forested and thinly settled with there are large tracts of unbroken forest in northern Maine, hardly 150 miles from the coast. In spite of these contrasts, no physiographic line can be drawn between the higher and more rugged interior and the lower coastal border, one merges into the other. New England is a unit, though a diversified unit.
The Appalachian trends northeast-southwest that are so prominent in the stratified belt of the middle Appalachians and are fairly well marked in the crystalline belt of New Jersey and Pennsylvania, are prevailingly absent in New England. However, they may be seen in a few areas on the western border:
In consequence of the general southeastward slope of the highlands and uplands of New England, the divide between the Atlantic rivers and those which flow northward and westward into the lowland of the stratified belt in Canada and New York is generally close to the boundary of these two physiographic districts. The chief rivers all flow south or south-east. These are:
Many lakes of moderate size and irregular outline have been formed where drift deposits formed barriers across former river courses. The lake outlets are more or less displaced from former river paths. Smaller lakes were formed by the deposition of washed drift around the longest-lasting ice remnants. When the ice finally melted away, the hollows that it left came to be occupied by ponds and lakes. In Maine, lakes of both classes are numerous with the largest being Moosehead Lake, a lake about 35 miles long and with a very irregular shore line.
Understanding coastal features requires knowing how the descent of
the land surface beneath the sea and from the work of the sea. Thus,
the coastal features are always described in connection with their
bordering districts. The maturely dissected and recently glaciated
uplands of New England are now somewhat depressed with respect to
sea level, so that the sea enters the valleys, forming bays and
estuaries, while the interfiuve uplands and hills stand forth in
headlands and islands. Narragansett Bay, with the associated headlands
and islands on the south coast, is one of the best examples.
Where drift deposits border the sea, the shore line has been cut
back or built forward in beaches of submature expression, often
enclosing extensive tidal marshes. However, the bulk of the shore line
is is rocky and so the change from the initial pattern due
to submergence is as yet small. Hence the coast as
a whole is irregular, with numerous embayments, peninsulas and islands.
In Maine, this irregularity reaches a climax.
The height and massiveness of the mountains decrease to the southwest,
where the piedmont belt sweeps westward around them in western Georgia
and eastern Alabama. Some of the residual mountains hereabouts are
reduced to a mere skeleton or framework by the retrogressive
penetration of widening valleys between wasting spurs. Certain
districts within the mountains, apparently consisting of less
resistant crystalline rocks, have been reduced to basin-like
peneplains in the same amount of time that served only to grade the slopes and
subdue the summits of the neighboring mountains of more resistant
rocks. The best example of this kind is the Asheville peneplain in
North Carolina, measuring about 40 by 20 miles across. In consequence
of later elevation, its general surface, now standing at an altitude
of 2500 ft is maturely dissected by the French Broad river and its
many branches in valleys 300 ft. deep. The basin floor is no longer a
plain, but a hilly district in the midst of the mountains.
The rivers of the mountain belt, normally dividing and subdividing in
apparently subsequent fashion between the hills and spurs, generally
follow open valleys. There are few waterfalls. The streams being as
a rule fairly well graded, though their current is rapid and their
channels are set with coarse waste. The valley floors always join at
accordant levels, as is the habit among normally subdued mountains.
Thus, they contrast with glaciated mountains such as the Alps and the
Canadian Rockies. In those, the laterals open as hanging valleys
in the side slope of the main valleys. It is a peculiar feature of the
drainage in North Carolina that the headwaters lie to the east of the
highest mountains and that the chief rivers flow northwestward through
the mountains to the broad valley lowland of the stratified belt and then
through the plateau, as the members of the Mississippi system. It is
probable that these rivers follow in a general way courses of much more
ancient origin than those of the Atlantic rivers in the middle Appalachians.
The piedmont belt may be described as a maturely dissected peneplain over
much of its extent. Indeed, It is one of the best examples of kind.
Its uplands are of fairly accordant altitude, which gradually
decreases from 500 to 1000 ft. near the mountain belt to half that
height along the coastal plain border. Here and there, the uplands are
surmounted by residual monadnocks in the form of low domes and knobs.
These increase in height and number towards the mountain belt and
decrease towards the coastal plain. Stone Mountain, near Atlanta,
Georgia, a dome of granite surmounting the schists of the uplands,
is a striking example of this. The chief rivers flow
southeastward in rather irregular courses through valleys from 200 to
500 ft. deep. The small branches ramify indefinitely in typical insequent
arrangement. The streams are nearly everywhere well graded with rapids
being rare and lakes unknown.
The boundary between the mountains and the piedmont belt is called the
Blue Ridge all along its length. Although the name Blue Ridge
is fairly appropriate in northern Virginia, it is not deserved in the
Carolinas, where the ridge is only an escarpment descending abruptly
1000 or 1500 ft. from the valleys of the mountain belt to the rolling
uplands of the piedmont belt. This form is unusual. It is
not defined by rock structure, but appears to result from the retrogressive
erosion of the shorter Atlantic rivers, whereby the highlands, drained by
much longer rivers, are undercut. The piedmont belt merges southeastward
into the coastal plain, the altitudes of the piedmont uplands and of the
coastal plain hills being about the same along their line of junction.
Many of the rivers, elsewhere well graded, have rapids as they pass
from the harder rocks of the piedmont to the semi-consolidated strata
of the coastal plain.
Extending from the Hudson River valley to Alabama, the Great Valley is
one feature of the Appalachians that has greater continuity than any
other. It is determined structurally by a belt of topographically weak
limestones and shales (or slates) just inland from the crystalline
uplands. Hence regardless of the direction of the rivers draining the
belt, the valley has been worn down by Tertiary erosion to a
continuous lowland from the Gulf of St Lawrence to central Alabama.
The lowland is uninterrupted by any transverse ridge throughout its
distance of 1,500 miles, though longittidinal ridges of moderate
height occasionally diversify its surface. In the middle section, the
Great Valley is somewhat open on the east, by reason of the small
height and broad interruptions of the narrow crystalline belt. On the
west, it is limited by the complex series of Alleghany ridges and
valleys. In the northeast section, the valley is strongly enclosed on
the east by the New England uplands, and on the west by the
Adirondacks and Catskills (see below). In the southwest section, the
valley broadens from the North Carolina highlands on the southeast
almost to the Cumberland plateau on the northwest due to the weaken,
though still present, ridge-making formations.
A striking contrast between New England and the rest of the Appalachians
is found in the descent of the New England uplands to an immediate frontage
on the sea. South of New York harbor, the remainder of the
Appalachians are set back from the sea by the interposition of a coastal
plain. The plain consists of marine (with some estuarine and
fluviatile2) stratified deposits, more or less indurated,
which were laid down when the land stood lower and the sea had its shore line
farther inland than today. An uplift, increasing to the south, revealed part
of the shallow sea bottom in the widening coastal plain, from its narrow
beginning at New York harbor to its greatest breadth of 110 or 120 miles
in Georgia. There, it turns westward and is continued in the Gulf coastal
plain, described farther on. The coastal plain, however, is the result,
not of a single recent uplift, but of movements dating back to Tertiary
time and continued with many oscillations to the present. Nor is its
surface smooth and unbroken, for erosion began on the inner part of the
plain long before the outer border was revealed.
Indeed, the original interior border of the plain has been well stripped
from its inland overlap. The higher standing inner part of the plain is now
maturely dissected with a relief of 200 to 500 ft. by rivers extending
seaward from the older land anti by their inntimerable branches, which are
often of insequent arrangement. The seaward border being the latest uplifted,
is prevailingly low and smooth with a hardly perceptible seaward slope of
a few feet per mile. The shallow sea deepens very gradually for many miles
off shore.
South Carolina and Georgia furnish the broadest and most typical section of
this important physiographic province. Here, the more sandy and hilly
interior parts are largely occupied by pine forests which furnish much
hard or yellow pine lumber, tar and turpentine. Farther seaward, where the
relief is less and the soils are richer, the surface is cleared and cotton
is an important crop.
A section of the coastal plain, from North Carolina to southern New Jersey,
resembles the plain farther south in general form and quality of soils, but
besides being narrower, it is further characterized by several embayments or
arms of the sea, caused by a slight depression of the land after mature
valleys had been eroded in the plain. The coastal lowland between the sea
arms is so flat that, although distinctly above sea level, vegetation hinders
drainage and extensive swamps occur. Dismal Swamp, on the border of
North Carolina and Virginia, is the largest example.
The small triangular section of the coastal plain in New Jersey north of
Delaware Bay deserves separate treatment because of the development there
of a pectiliar topographic feature, which throws light on the occurrence
of the islands off the New England coast, described in the next paragraph.
The feature referred to results from the occurrence here of a weak basal
formation of clay overlaid by more resistant sandy strata. The clay belt
has been stripped for a score or more of miles from its original inland
overlap, and worn down in a longitudinal inner lowland, while the sandy
belt retains a significant altitude of 200 or 300 ft. overlooking the
inner lowland in a well-defined slope dissected by many inland-flowing
streams, and descending from its broad crest very gently seaward, thus
giving rise to what has been called a belted coastal plain in which
the relief is arranged longitudinally and the upland member, with its
very unsymmetrical slopes, has sometimes been called a
cuesta also known as an escarpment.
This is a form of relief frequently occurring elsewhere, as in the
Niagara escarpment of the Great Lake district of the northern
United States and in the Cotswold and Chiltern hills of England,
typical examples of the escarpment class. The Delaware river,
unlike its southern analogs, which pursue a relatively direct
course to the sea, turns southwestward along the inner lowland for some
50 miles.
There is good reason for believing that at least along the southern border of
New England a narrow coastal plain was for a time added to the continental
border. As in the New Jersey section, this plain was stripped
from a significant breadth of inland overlap and worn down so as to form an
inner lowland enclosed by a longitudinal upland or escarpment. After
this stage was reached, a submergence of the kind which has produced the
many embayments of the New England coast drowned the outer part of the
plain and the inner lowland. This left only the higher parts of the escarpment
as islands. Long Island, Block Island (part of Rhode Island),
Martha's Vineyard and Nantucket (parts of Massachusetts)
were probably similarly formed. Heavy terminal moraines and outwashed
fluviatile plains have been laid on the escarpment remnants,
increasing their height as much as 100 ft. and burying their seaward
slope with gravel and sand. Moreover, the sea has worked on the original
shore line reducing the size of the more exposed islands farther east,
and even consuming some islands which are now represented by the
Nantucket shoals.
The Catskill Mountains, Adirondacks, Alleghany plateau and the
Cumberland plateau are part of the Appalachian plateau. The same
Paiaeozoic formations that are folded in the belt of the Alleghany
ridges lie nearly horizontal in the plateau district just to its
northwest. The exposed strata are in large part resistant sandstones.
While they have suffered active dissection by streams during the later
cycles of erosion, the hilltops have retained a considerable altitude.
In fact, that district is known as a plateau. It might be better
described as a dissected plateau since its uplands are not contiguous,
rather nearly everywhere the uplands are interrupted by ramifying
insequent valleys. In fact, local usage rarely refers to this as the
Appalachian plateau since it is not perceived to be one
plateau. Its north-eastern part in eastern New York is known as the
Catskill Mountains. Here, it reaches truly mountainous heights in
great dome-like masses of full-bodied form, with two summits rising a
little over 4000 ft. At the eastern border of this part of the plateau
is a single strong escarpment descending to the Hudson valley. There
are two escarpments on the northern borders towards the Mohawk
Valley. Above this is the Adirondacks. The plateau extends southwest
into Pennsylvania and Virginia where it is called the Alleghany
plateau. In northern Pennsylvania, the lateral pressure of the
Palaeozoic moutain-making forces extended its effects through a belt
about fifty miles wider than in the folded belt of the Hudson
Valley. This caused the compressing the heavy stratified series into
great rock waves while in New York, this same stratified series,
forming the Catskills, lies horizontal. Altitudes of 1,200 feet
prevail in Pennsylvania and increase in Virginia. In the Kentucky and
Tennessee portion, where the highest section is called the
Cumberland plateau, the altitude falls to about 1,000 feet. The
altitude drops furher in Alabama where the plateau, like the mountain
belt, disappears under the Gulf coastal plain.
Throughout this distance of 1,000 miles, the southwestern border of the
plateau is an abrupt escarpment. It is eroded at places where the folded
structure of the mountain belt reveals a series of weaker strata. In the
northwest, the plateau loses height and relief more gradually until
it reaches the praire plains in central Ohio, southern Indiana
and Illinois, about 150 miles inland from the escarpment.
Two qualifications must be added. In certain parts of the plateau,
there are narrow anticlinal uplifts which are an outlying effect of
the mountain-making compression. A ridge will rise if the exposed strata
are resistant to erosion such as the Chestnut ridge of western
Pennsylvania or a valley will be excavated if the exposed strata are
more easily eroded such as the Sequatchie Valley which is a long
narrow trough cutting fof a strip of the plateau from the larger body
found in Tennessee. In Kentucky and Tennessee, there is a double
alternation of sandstone and limestone in the strata. As the
plateau's skyline bevels across these formations, there are
west-facing escarpments made ragged by mature dissection as one
passess from the topographically strong sandstone to the
topographically weak limestone.
In the northeast (New York and Pennsylvania), the higher parts of the plateau
are drained by the Delaware and Susquehanna rivers directly to the Atlantic.
Farther west and southwest, the plateau is drained to the Ohio river and its
branches. The submature or mature dissection of the plateau by its branching
insequent streams results in giving it an excess of sloping surface, usually
too steep for farming, and hence left for tree growth.
Southwest Section of Appalachians
As in the northeast, so in the southwest, the crystalline belt widens and
gains in height. In New England this belt is an indivisible unit,
but the southern crystalline belt must be subdivided into a higher
mountain belt on the northwest and a lower piedmont belt on the
Appalachian southeast, 100 m. wide, from southern Virginia to South Carolina.
This subdivision is already necessary in Maryland, where the mountain belt
is represented by the Blue Ridge. At the place where the
Potomac cuts across it, it is more of a narrow upland belt than a
ridge proper. The piedmont belt, relieved by occasional monadnocks,
stretches from the eastern base of the Blue Ridge to the coastal
plain, into which it merges. Farther south, the mountain belt widens and
attains its greatest development, a true highland district, in North Carolina,
where it includes several strong mountain groups. Here Mt Mitchell rises to
6711 ft., the highest of the Appalachians, and about thirty other summits
exceed 6000 ft., while the valleys are usually at altitudes of about 2000 ft.
Although the relief is strong, the mountain forms are rounded rather than
rugged. Few of the summits deserve or receive the name of peaks. Some are
called domes, from their broadly rounded tons. Others are known as balds,
because the widespread forest cover is replaced over their heads by a grassy
cap.Appalachian Great Valley
Coastal Plain
Appalachian Plateau
The Adirondack Mountains
This rugged district of northern New York may be treated as an outlier
in the United States of the Laurentian highlands of Canada that is
separated by the St Lawrence Valley. It is part of the Canadian Shield along with the Superior Upland that extends into the U.S. It is of greater altitude
(Mt Marcy 5344 ft.) and of much greater relief than the Superior
Upland. Its heights decrease gradually to the north, west and south,
where it is unconformably overlapped by Palaeozoic strata like those
of Minnesota and Wisconsin. On the east, its structure and
form is more broken where the disturbances of the Appalachian
system have developed ridges and valleys of linear trends. Elsewhere,
these ridges and valleys are smaller or barely seen.
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