Geology of the Appalachians
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| Missing image US_east_coast_physiographic_regions_map.jpg image:US_east_coast_physiographic_regions_map.jpg |
| Physiographic regions of the U.S. East Coast Courtesy of USGS See:legend |
The Geology of the Appalachians describes a complex system of two belts:
- one on the southeast - consisting largely crystalline rocks.
- one on the northwest - consisting of folded strata.
These belts create the Applachian Highlands, which include not just the Appalachian Mountains, but also the surrounding plateaus and valleys.
The southeast portion of the highlands is largely composed of ancient and greatly deformed crystalline rocks (e.g. igneous and metamorphic), while the northwest portion is a heavy series of folded Palaeozoic strata. It is convenient to associate a third belt, farther northwest, consisting of the same Palaeozoic strata but being essentially horizontal. This belt constitutes the Appalachian plateau. At least in part, the crystalline belt represents the ancient highlands from whose ruins the sedimentary sandstones, shales and limestones of the stratified series were formed.
The deformation of the Appalachians occurred in two major periods of compressive deformation:
- one near the start of the Palaeozoic period and
- the other towards its close.
Both occasions were undoubtedly of long duration, though the effects of the second period extended its effects farther northwest than the first. These periods were followed by a time of minor tilting and faulting in the early Mesozoic, next by a moderate upwarping in the Tertiary, and finally by a moderate uplift in the post-Tertiary time. The later small movements are of importance because they helped form some of the Appalachians' more interesting topography. Each of the disturbances altered the attitude of the mass with respect to the general base level of the ocean surface, thus each movement introduced a new cycle of erosion, a cycle which was interrupted only by a later movement.
Thus the Appalachian topographical features of today may be ascribed to three cycles of erosion:
- a nearly complete Mesozoic cycle, in which most of the previously folded and faulted mountain masses were reduced in Cretaceous time to a peneplain or lowland of small relief. In the northeast though, the most resistant rocks give rise to monadnocks, standing isolated or in groups above their surroundings.
- an incomplete Tertiary cycle, initiated by the moderate Tertiary upwarping of the Mesozoic peneplain, and of sufficient length to develop mature valleys in the more resistant rocks of the crystalline belt or in the horizontal strata of the plateau, and to develop late mature or old valleys in the weaker rocks of the stratified belt, where the harder strata were left standing up in ridges
- And finally, a brief post-Tertiary cycle, initiated by an uplift of moderate amount and in progress long enough only to erode narrow and relatively immature valleys. Glacial action complicated the work of the latest cycle in the northern part of the system.
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.
In a northeastern section , 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 Allegheny Mountains.
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.
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Middle Section of the Appalachian
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:
- The pattern of the folded strata on the low lying Cretaceous peneplain must have resembled the pattern of the curved grain of wood on a planed board. When the peneplain was uplifted the weaker strata were worn down almost to a lowland of a second generation, while the resistant sandstones, of which there are three chief members, retained a great part of their new-gained altitude in the form of long narrow even-crested ridges with an occasional sharp bending in peculiar zigzags. These strata are well deserving of the name of Endless Mountains given them by the Indians. The post Tertiary uplift resulting in the present altitude of approximately 1000 to 1500 ft (300 to 450 m). in Pennsylvania and of 2500 to 3500 ft. in Virginia, has not significantly altered the forms thus produced. Rather, it has only incited the rivers to entrench themselves 100 feet (30 m) or more beneath the lowlands of tertiary erosion. As a rule, the watercourses today are longitudinal, following the strike of the weaker strata in paths that they appear to have gained by spontaneous adjustment during the long Mesozoic cycle. However, now and again they cross from one longitudinal valley to another by a transverse course, and there they have cut down sharp notches or water gaps in the hard strata that elsewhere stand up in the long even-crested ridges.
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| Middle Appalachians Courtesy of USGS |
The transition from the strongly folded structure of the Allegheny 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 explanation. 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
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| 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 the Hoosac Range along the boundary of Massachusetts and New York.
- in the linear series of the Green Mountain summits (Mt Mansfield. 4364 ft., Killington Peak, 4241 ft.) and their (west) piedmont ridges farther north in Vermont.
- in the ridges of northern Maine.
These are all representative of the various Appalachian structures, as are:
- certain open valleys such as the Berkshire (limestone) Valley in western Massachusetts and the correspondin Rutland (limestone and marble) Valley in western Vermont.
- the long Connecticut Valley from northern New Hampshire across Massachusetts to the sea at the southern border of Connecticut. The populous southern third of this valley is broadly eroded along a belt of red Triassic sandstones with trap ridges.
However in general, the dissection of the New England upland is as irregular as is the distribution of the surmounting monadnocks. One such is Mount Monadnock in southwestern New Hampshire, a fine example of an isolated residual mass rising from an upland some 1500 ft. in altitude and reaching a summit height of 3186 ft. A still larger example is seen in Mt. Katahdin (5200 ft.) in north-central Maine. Mt. Katahdin is the largest of several similar isolated mountains that are scattered over the interior uplands seemingly randomly. The White Mountains of northern New Hampshire may be thought of as a complex group of monadnocks, all of subdued forms. All that is except for a few cliffs at the head of cirque-like valleys. Mt Washington is the highest of these reaching 6293 ft. Thirteen other summits reach over 5000 ft. The absence of range-like continuity is emphasized here by the occurrence of several low passes or notches leading directly through the group. The best-known of these is Crawford's Notch (1900 ft.).
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:
- the Connecticut
- the Merrimack
- the Kennebec
- the Penobscot and
- the St John (in the province of New Brunswick.)
The drainage of New England is unlike that of the middle and southwestern Appalachians in the occurrence of numerous lakes and falls. These irregular features are few south of the limits of Pleistocene glaciation. Further south, the rivers had time in the latest cycle of erosion, to establish a continuous flow, and as a rule, to wear down their courses to a smoothly graded slope. In New England in preglacial times, the rivers were undoubtably also able to establish drainage. Partly due to the irregular scouring of the rock floor, but even more because of the very irregular deposition of unstratified and stratified drift1 in the valleys, the drainage is now in great disorder.
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 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.
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.
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.
Appalachian Great Valley
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.
Coastal Plain
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 Lakes 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.
Appalachian plateau
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 further 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.
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.
- 1drift - rock material transported and left deposited.
- 2Fluviatile - Latin meaning river.