Bible Encyclopedias
United States

Physical G eography

Coast.

The Atlantic coast of the United States is, with minor exceptions, low; the Pacific coast is, with as few exceptions, hilly or mountainous. The Atlantic coast owes its oblique N.E.S.W. trend to crustal deformations which in very early geological time gave a beginning to what later came to be the Appalachian mountain system; but this system had Its climax of deformation so long ago (probably in Permian time) that it has since then been very generally reduced to moderate or low relief, and owes its present altitude either to renewed elevations along the earlier lines or to the survival of the most resistant rocks as residual mountains. The oblique trend of the coast would be even more pronounced but for a comparatively modern crustal movement, causing a depression in the northeast, with a resulting encroachment of the sea upon the land, and an elevation. in the south-west, with a resulting advance of the land upon the sea. The Pacific coast has been defined chiefly by relatively recent crustal deformations, and hence still preserves a greater relief than that of the Atlantic. The minor features of each coast will be mentioned in connection with the lani districts of which the coast-line is only the border.

General Topography and Drainage.

The low Atlantic coast and the hilly or mountainous Pacific coast foreshadow the leading features in the distribution of mountains within the United States. The Appalachian system, originally forest-covered, on the eastern side of the continent, is relatively low and narrow; it is bordered on the south-east and south by an important coastal plain. The Cordilleran system on the western side of the continent is lofty, broad and complicated, with heavy forests near the north-west coast, but elsewhere with trees only on the higher ranges below the Alpine region, and with treeless or desert intermont valleys, plateaus and basins, very arid in the south-west. Between the two mountain systems extends a great central area of plains, stretching from the Gulf of Mexico northward, far beyond the national boundary, to the Arctic Ocean. The~ rivers that drain the Atlantic slope of the Appalachians are comparatively short; those that drain the Pacific slope include only two, the Columbia and the Colorado, which rise far inland, near the easternmost members of the Cordilleran system, and flow through plateaus and intermont basins to the ocean. The central plains are divided by a hardly perceptible height of land into a Canadian and a United States portion; from the latter the great Mississippi system discharges southward to the Gulf of Mexico. The upper Mississippi and some of the Ohio basin is the prairie region, with trees originally only along the watercourses; the uplands towards the Appalachians were included in the great eastern forested area; the western part of the plains has so dry a climate that its herbage is scanty, and in the south it is barren. The lacustrine system of the St Lawrence flows eastward from a relatively narrow drainage area.

Relation of General Topography to Settlement.

The aboriginal occupants of the greater part of North America were comparatively few in number, and except in Mexico were not advanced beyond the savage state, The geological processes that placed a much narrower ocean between North America and western Europe than between North America and eastern Asia secured to the New World the good fortune of being colonized by the leading peoples of the occidental Old World, instead of by the less developed races of the Orient. The transoceanic invasion progressed slowly through the 17th and ~8th centuries, delayed by the head winds of a rough ocean which was crossed only in slow sailing vessels, and by the rough backwoods of the Appalachians, which retarded the penetration of wagon roads and canals into the interior. The invasion was wonderfully accelerated through the I9th century, when the vast area of the treeless prairies beyond the Appalachians was offered to the settler, and when steam transportation on sea and land replaced sailing vessels and wagons. The frontier was then swiftly carried across the eastern half of the central plains, but found a second delay in its advance occasioned by the dry climate of the western plains. It was chiefly the mineral wealth of the Cordilleran region, first developed on the far Pacific slope, and later in many parts of the inner mountain ranges, that urged pioneers across the dry plains into the apparently inhospitable mountain region; there the adventurous new-corners rapidly worked out one mining district after another, exhausting and abandoning the smaller camps to early decay and rushing in feverish excitement to new-found river fields, but establishing important centres of varied industries in the more important mining districts. It was not until the settlers learned to adapt themselves to the methods of wide-range cattle raising and of farming by irrigation that the greater value of the far western interior was recognized as a permanent home for an agricultural population.

The purchase of Louisiana a great area west of the Mississippi river from the French in 1803 has sometimes been said to be the cause of the westward expansion of the United States, but the Louisiana purchase has been better interpreted as the occasion for the expansion rather than its cause; for, as Lewis Evans of Philadelphia long ago recognized (1749), whoever gained possession of the Ohio Valleythe chiet eastern part of the central plainswould inevitably become the masters of the continent.

Physiographic Subdivisions

The area of the United States may be roughly divided into the Appalachian belt, the Cordilleras and the central plains, as already indicated. These large divisions need physiographic subdivision, which will now be made, following the guide of structure, process and stage; that is, each subdivision or province will be defined as part of the earths crust in which some similarity of geological structure prevails, and upon which some process or processes of surface sculpture have worked long enough to reach a certain stage in the cycle of physiographic development.

The Appalachians.

The physiographic description of the Appalachian mountain system offers an especially good opportunity for the application of the genetic method based on structure, process and stage. This mountain system consists essentially of two belts: one on the south-east, chiefly of ancient and greatly deformed crystalline rocks, the other on the north-west, a heavy series of folded Palaeozoic strata; and with these it will be convenient to associate a third belt, farther north-west, consisting of the same Palaeozoic strata lying essentially horizontal and constituting the Appalachian plateau. The crystalline belt represents, at least in part, the ancient highlands from whose ruins the sandstones, shales and limestones of the stratified series were formed, partly as ~narine, partly as fluviatile deposits. The deformation of the Appalachianswas accomplished in two chief periods of compressive deformation, one in early Palaeozoic, the other about the close of Palaeozoic time, and both undoubtedly of long duration; the second one extended its effects farther northwest than the first. These were followed by a period of minor tilting and faulting in early Mesozoic, by a moderate upwarping in Tertiary, and by a moderate uplift in post-Tertiary time. The later small movements are of importance because they are related to the existing topography with which we are here concerned. Each of the disturbances altered the attitude of the mass with respect to the general base-level of the ocean surface; each movement therefore introduced a new cycle of erosion, which was interrupted by a later movement and the beginning of a later cycle.

Thus interpreted, the Appalachian forms of to-day 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, surmounted, however, in the north-east and in the south-west by monadnocks of the most resistant rocks, standing singly or in groups; 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 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. In view of all this it is possible to refer nearly every element of Appalachian form to its appropriate cycle and stage of development. The more resistant rocks, even though dissected by Tertiary erosion, retain in their summit tiplands an indication of the widespread peneplain of Cretaceous tinie, now standing at the altitude given to it by the Tertiary upwarping and post-Tertiary uplift; and 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. Evi-Jently, therefore, the Appalachians as we now see them are not the still surviving remnants of the mountains of late Palaeozoic deformation; they owe their present height chiefly to the Tertiary upwaroing and uoliftinr. and their form to the normal urocesses of sculpture which, having become nearly quiescent at the close,of the Mesozoic cycle, became active again in Tertiary and later times.

The belts of structure and the cycles of erosion thus briefly described are recognizable with more or less continuity from the Gulf of St Lawrence i 500 m. south-westward to Alabama, where the deformed mountain structures pass out of sight under nearly horizontal strata of the Gulf coastal plain. But the dimensions of the several belts and the strength of the relief developed by their later erosion varies greatly along the system. In a north-eastern 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; and here the plateau member is wanting, being replaced, as it were, by the Adirondacks, an outlier of the Laurentian highlands of Canada which immediately succeeds the deformed stratified belt west of Lake Champlain. In a middle section of the system, from the Hudson river in southern New York to the James river in southern Virginia, the crystalline belt is narrowed, as if by the depression of its south-eastern part beneath the Atlantic Ocean or beneath the strata of the Atlantic coastal plain which now represents the ocean; but the stratified belt is here 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; and the plateau assumes full strength southward from the monochinal Mohawk valley which separates it from the Adirondacks. The linear ridges of this mIddle section are often called the Alleghany Mountains. In a south-western 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.

The middle section of the Appalachians, rather arbitrarily limited by the Hudson and the James rivers, may be described first because it contains the best representation of the three longitudinal belts of which the mountain system as a whole is The Middle composed. The mountain-making compression of the ,4pialaheavy series of Palaeozoic strata has here produced a chians. marvellous series of rock folds with gently undulating axes, trending north-east and south-west through a belt 70 or 80 m. wide; no less wonderful is the form that has been produced by the processes of sculpture. The peculiar configuration of thr~ ridges may be apprehended as follows: The pattern of the folded ~trata 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 a1~- three chief members, retained a great part of their new-gained altitude in the form of long, narrow, even-crested ridges, well deserving of the name of Endless Motintains given them by the Indians, but here and there bending sharply in peculiar zigzags which give this Alleghany section of the mountains an unusual individuality. The postTertiary uplift, giving the present altitude of 1000 or 1500 ft. in Pennsylvania, and of 2500 or 3500 ft. in Virginia, has not significantly altered the forms thus produced; it has only incited the rivers t0 intrench themselves 100 or more feet beneath the lowlands of tertiary erosion. The watercourses to-day are, as a rule, longitudinal, following the strike of the weaker strata in paths that they appear to have gained by spontaneous adjustment during the long Mesozoic cycle; but 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.

The transition from the strongly folded structure of the Alleghany ridges and valleys to the nearly horizontal structure of the Appala; chian plateau is promptly made; and with the change of structure comes an appropriate change of form. 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 plateau is made in a later section.

The crystalline belt of the middle Appalachians, 60 or 80 m- wide, is to-day of moderate height because the Tertiary upwarping was there of moderate amotint. The height is greatest along the inner or north-western border of the belt, and here a sub-mountainous topography has been produced by normal dissection, chiefly in the Tertiary cycle; the valleys being 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 b farms and villages, but many of the valleys are wooded glens. Wit continued decrease of altitude south-eastward, the crystalline belt dips under the coastal plain, near a line marked by the Delaware river from Trenton to Philadelphia in Pennsylvania, and thence south-south-westward through Maryland and Virginia past the cities of Baltimore, Washington and Richmond.

The Pennsylvania portionof the crystalline belt is narrow, as has been said, because of encroachment upon it by the inward overlap of the coastal olain: it ~s low because of small Tertiary unlift: but.

still more, it is discontinuous, because of the inclusion of certain 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 north-eastern and south-western sections by the arrangeDrain age. ment of 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 most perfectly exemplify this habit are the Delaware, Susquehanna and Potomac; the Hudson, the north-eastern 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 south-western 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 over-simple, Gordian explanation. The south-east course of the middle-section rivers is the result of many changes from the initial drainage; the Mesozoic and Tertiary upwarprngs 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 they frequently pass through sharp-cut notches in the na1row ridges of the stratified beltthe 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, of which 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 occur also 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 01 the lower Susquehanna; and in the falls of the Schuylkill, a branch which joins the Delaware at Philadelphia, where the water-power has long been used in extensive factories. Hence rivers in the Appalachians are not navigable; it is only farther down-stream, where the rivers have been converted into estuaries and bayssuch as Chesapeake and Delaware baysby a slight depression of the coastal plain belt, that they serve the purposes of navigation. But the Hudson is strikingly exceptional in this respect; it possesses a deep and navigable tide-water channel all through its gorge in the highlands, a feature which has usually been explained as the result of depression of the land, but may also be explained by glacial erosion without change of land-level; a feature which, in connection with the Mohawk Valley, has been absolutely determinative of the metropolitan rank reached by New York City at the Hudson mouth.

The community of characteristics that is suggested by the association of six north-eastern states under the name New England The North- is in large measure warranted by the inclusion of easternA all these states within the broadened crystalline belt palachians of the north-eastern Appalachians, which is here 150 m. wide. The uplands which prevail through the centre of this area at altitudes of about iooo ft. rise to 1500 o~ 2000 ft. in the north-west, before descent is made to the lowlands of the stratified belt (St Lawrence-Champlain-Hudson valleys, described later on as part of the Great Appalachian valley), 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 south-east or south to the coast, their altitude and their relief over the valleys gradually decrease; and thus 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; there are large tracts of unbroken forest in northern Maine, hardly 150 m. 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 (N.E.S.W.) 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. They may be seen 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; and in the ridges of northern Maine: these are all in synipathy with Appalachian structure: so also are certain open valleys, as the Berkshire (limestone) Valley in western Massachusetts and the correspondin Rutland (limestone and marble) Valley in western Vermont; an more particularly the long Connecticut Valley from northern New Hampshire across Massachusetts to the sea at the southern border of Connecticut, the populous southern third of which is broadly &roded along a belt of red Triassic sandstones with trap ridges.

But in general the dissection of the New England upland is as irregular as is the distribution of the surmounting monadnocks. The type of this class of forms is Mt Monadnock in south-western 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, the greatest of several similar isolated mountains that-are scattered over the interior uplands without apparent system. The White Mountains of northern New Hampshire may be treated as a complex group of rnonadnocks, all of subdued forms, except for a few cliffs at the head of cirque-like valleys, with Mt Washington, the highest of, the dome-like or low pyramidal summits, reaching 6293 ft., and thirteen other summits over 5000 ft. The absence of range-like continuity is here emphasized by the occurrence of several low passes or notches leading directly through the group; the best-known being Crawfords Notch (1900 ft.).

In consequence of the general south-eastward slope of the highlands and uplands of New England, the divide between the Atlantic rivers and those which flow northward an~j westward D ~t into the lowland of the stratified belt in Canada and r New York is generally close to the boundary of these two physiographic districts. The chief rivers all flow south or south-east:

theyare the Connecticut, Merrimack, Kennebec, Penobscot and St John, the last being shared with the province of New Brunswick.

The drainage of New England is unlike that of the middle and south-western Appalachians in the occurrence of numerous lakes and falls. These irregular features are wanting south of the limits of Pleistocene glaciation; there the rivers have had time, in the latest cycle of erosion into which they have entered, to establish themselves in a continuous flow, and as a rule to wear down their courses to a smoothly graded slope. In New England also a wellestablished drainage undoubtedly prevailed in preglacial times; but partly in consequence of the irregular scouring of the rock floor, and even more because of the very irregular deposition of unstratified and stratified drift 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; the largest is Moosehead Lake, about 35 m. long and of a very irregular shore line.

The features of a coast can be appreciated only when it is perceived that they result from the descent of the land surface beneath the sea and from the work of the sea ,upon the shore line thus determined; and it is for this reason that through- Coast, out this article the coastal features are described in connection with the districts of which they are the border. 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; but the great part of the shore line is rocky, and there the change from initial pattern due to submergence is as yet small. Hence the coast as a whole is irregular, with numerous embayments, peninsulas and islands; and in Maine this irregularity reaches a disadvantageous climax.

As in the north-east, so in the south-west, the crystalline belt widens and gains in height; but while New England is an indivisible unit, the southern crystalline belt must be subdivided The Southinto a higher mountain belt on the north-west, 60 m. western A wide where broadest, and a lower piedmont belt on the p,,Jachjan~ south-east, 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, which is rather a narrow upland belt than a ridge proper where the Potomac cuts across it; while 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 risesto 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, becatise the widespread forest cover is replaced over their heads by a grassy cap.

The height and massiveness of the mountains decrease to the south-west, 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; the very type of vanishing forms, Certain districts within the mountains, apparently consisting of less resistant crystalline rocks, have been reduced to basin-like peneplains in the same 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 m. across; but in consequence of later elevation, its general surface, now standing at an altitude of 2500 ft is mattirely 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; Asheville on its southern border is a noted health resort.

The rivers of the mountain belt, normally dividing and subdividing in apparently fnsequent 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; they thus contrast with glaciated mountains such as the Alps and the Canadian Rockies, where the laterals habitually 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 north-westward 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; it is indeed one of the best examples of that class of forms. 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. The uplands are here and there 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 class of forms. The chief rivers flow south-eastward 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; rapids are rare and lakes are unknown.

The bofindary between the mountains and the piedmont belt is called the Blue Ridge all along its length; and although the nan:e 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; and as such it is a form of unusual occurrence. 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 south-eastward 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.

There is one feature of the Appalachians that has greater continuity than any other; this is the Great Valley. Itis determined The Great structurally by a belt of topographically weak limestones VaJie and shales (or slates) next inland from the crystalline ~ uplands; hence, whatever the direction of the rivers which drain the belt, it has been worn down by Tertiary erosion to a continuous lowland from the Gulf of St Lawrence to central Alabama. Through all this distance of 1500 in. the lowland is nowhere interrupted by a transverse ridge, although longittidinal ridges of moderate heiyht occasionally diversify its surface. In the middle section, as already stated, 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 north-east 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 south-west section the valley broadens from the North Carolina highlands on the south-east almost to the Cumberland plateau on the north-west, for here also the ridge-making formations weaken, although they do not entirely disappear.

A strikin,g contrast between New England and the rest of the Appalachians is found in the descent of the New England uplands Th At! ~ to an immediate frontage on the sea; while to the south of New York harbour the remainder of the Appala Plain chians are set back from the sea by the interposition of a coastal plain, one of the most characteristic examples of this class of forms anywhere to be found. As in all such cases, the plain consists of marine (with some estuarine and flu viatile) 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 to-day. 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 harbour to its greatest breadth of 110 or 120 m. 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 upon 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 extended seaward from the older land anti by their inntimerable branches, which are often of insequent arrangement; while the seaward border, latest uplifted, is prevailingly low and smooth, with a hardly perceptible seaward slope of but a few feet in a mile; and the shallow sea deepens very gradually for many nules 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 or pocossins 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 ri~e 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. This is a ferm of relief frequently occurring elsewhere, as in the Niagara cuesta of the Great Lake district of the northern United States and in the Cotswold and Chiltern hills of England, typical examples of the cuesta class. The Delaware river, unlike its southern analogues, which pursue a relatively direct course to the sea, turns south-westward along the inner lowland for some 50 m.,

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; and that, as in the New Jersey section the plain was here stripped from a significant breadth of inland overlap and worn down so as to form an inner lowland enclosed by a longitudinal upland or cuesta; and that when 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 thy plain and the inner lowland, leaving only the higher parts of the cuesta as islands. Thus Long Island (fronting Connecticut, but belonging to New York state), Block Island (part of the small state of Rhode Island), Marthas Vineyard and Nantucket (parts of Massachusetts) may be best explained. Heavy terminal moraines and outwashed fluviatile plains have been laid on the cuesta 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 shore line thus originated, reducing the size of the more exposed islands farther east, and even consuming some islands which are now represented by the Nantucket shoals.

The same Paiaeozoic formations that are folded in the belt of the Alleghany ridges lie nearly horizontal in the plateau district next north-west. The exposed strata are in large part resistant sandstones. While they have suffered active e dissection by streams during the later cycles of erosion, ~ the hilltops have retained so considerable an altitude ~

that the district is known as a plateau; it might be better described as a dissected plateau, inasmuch as its uplands are not contiQuous but are nearly everywhere interrupted by ramifying insequent valleys. The unity and continuity of the district, expressed in the name Appalachian plateau, is seldom recognized in local usage. Its iiorth-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. The border of this part of the plateau descends eastward by a single strong escarpment to the Hudson valley, from which the mountains present a fine appearance, and northward by two escarpments (the second being called the Helderberg Mountains) to the Mohawk Valley, north of which rise the Adirondacks; but to the south west the dissected highland continues into Pennsylvania and Virginia, where it is commonly known as the Alleghany plateau. A curious feature appears in northern Pennsylvania: here the lateral pressure of the Palaeozoic mountain-making forces extended its effects through a belt about fifty miles wider than the folded belt of the Hudson Valley, thus compressing into great rock waves a part of the heavy stratified series which in New York lies horizontal and forms the Catskills; hence one sees, in passing south-west from the horizontal to the folded strata, a beautiful illustration of the manner in which land sculpture is controlled by land structure. Altitudes of 1200 ft. prevail in Pennsylvania and increase in Virginia; then the altitude falls to about 1000 ft. in Kentucky and Tennessee, where the name Cumberland plateau is used for the highest portion, and to still less in northern Alabama, where the plateau, like the mountain belt, disappears under the Gulf coastal plain. Through all this distance of 1000 m. the border of the plateau on the south-east is an abrupt escarpment, eroded where the folded structure of the mountain belt reveals a series of weaker strata; but in the north-west the plateau suffers only a gradual decrease of height and of relief, until the prairie plains are reached in central Ohio and southern Indiana and Illinois, about 150 m. inland from the escarpment. Two qualifications must, however, be added. In certain parts of the plateau there are narrow anticlinal uplifts, an outlying effect of mountain-making compression; here a ridge rises if the exposed strata are resistant, as in Chestnut ridge of western Pennsylvania; but here a valley is excavated if the exposed strata are weak, as in Sequatchie Valley, a long narrow trough which cuts off a strip of the plateau from its greater body in Tennessee. Again, in Kentucky and Tennessee, there is a double alternation of sandstone and limestone in the plateau-making strata; and as the skyline of the plateau bevels across these formations, there are west-facing escarpments, made ragged by mature dissection, as one passes from the topographically strong sandstone to the topographically weak limestone.

In the north-east (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 south-west, the plateau is drained to the Ohio rrver 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 Superior Oldland.An outlying upland of the Laurentian highlands of Canada projects into the United States west and south of Lake Superior. Although composed chiefly of crystalline rocks, which are commonly associated with a rugged landscape, and although possessing a greatly deformed structure, which must at some ancient period have been associated with strong relief, the upland as a whole is gently rolling, and the inter-stream surfaces are prevailing plateau-like in their evenness, with altitudes of 1400 to 1600 ft. in their higher areas. In this province, therefore, we find a part of one of those ancient mountain regions, initiated by crustal deformation, but reduced by long continued erosion to a peneplain of modern relief, with occasional surmounting monadnocks of moderate height not completely consumed during the peneplanation of the rest of the surface. The erosion of the region must have been far advanced, perhaps practically completed, in very ancient times, for the even surface of the peneplain is overlapped by fossiliferous marine strata of early geological date (Cambrian); and this shows that a 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; for in consequence of 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 perieplain through the now removed cover of stratified rocks. Glaciation has strongly scoured away the deeply-weathered soils that presumably existed here in preglacial time, revealing firm and rugged ledges in the low hills and swells of the ground, and spreading an irregular drift cover over the lower parts, whereby the drainage is often much disordered; here being detained in lakes and swamps (muskegs) and there rushing down rocky rapids. The region is therefore generally unattractive to the farmer, but it is inviting to the lumberman and the miner.

The Adirondack Mountains .T his rugged district of northern New York may be treated as an outlier in the United States of the Laurentian highlands of Canada, from which it is separated by the St Lawrence Valley. It is of greater altitude (Mt Marcy 5344 ft.) and of much greater relief than the Superior Oldland; its heights decrease gradually to the north, west and south, where it is unconformably overlapped by Palaeozoic strata like those of Minnesota and Wisconsin; it is of more broken structure and form on. the east, where the disturbances of the Appalachian system have developed ridges and valleys of linear trends, which are wanting or but faintly seen elsewhere. (See ADIRONDACKS.)

Region of tile Great LakesThe Palaeozoic strata, already mentioned as lapping on the southern slope of the Superior Olclland and around the western side of the Adirondacks- are but parts of a great area of similar strata, hundreds of feet in thickness, which dec]ine gently southward from the great oldland of the Laurentian highlands of eastern Canada. The strata are the deposits of an ancient sea, which in the earlier stage of geological investigation was thought to be part of the primeval ocean, while the Laurentian highlands were taken to be the first land that rose from the primeval waters. Inasmuch, however, as the floor on which the overlapping strata rest is, like the rest of the Laurentian and Superior Oldland, a worn-down mountain region, and as the lowest member of the sedimentary series usually contains pebbles of the oldiand rocks, the better interpretation of the relation between the two is that the visible oldiand area of to-day is but a small part of the primeval continent, the remainder of which is still buried under the Palaeozoic cover; and that the visible oldiand, far from being the first part of the continent to rise from the primeval ocean, was the last part of the primeval continent to sink under the advancing Palaeozoic seas. When the oldland and its overlap of stratified deposits were elevated again, the overlapping strata must have had the appearance of a coastal plain; but that was long ago; the strata have since then been much eroded, and to-day possess neither the area nor the smooth form of their initial extent. I-fence this district may be placed in the class of 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 oldiand and the belts of more resistant strata, which rise in uplands.

Few better illustrations of this class of forms are to be found than that presented in the district of the Great Lakes. The chief upland belt or cuesta 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 cuesta has a relatively strong slope or infacing escarpment on the side towards the oldland, and a long gentle slope on the other side. Its relief is seldom more than 200 or 300 ft., and is commonly of small measure, but its continuity and its contrast with the associated lowlands worn on the underlying and overlying weak strata suffice to sake it a feature of importance. The cuesta would be straight from east and west if the slant of the strata were uniformly to the south; but the strata are somewhat warped, and hence the course of the cuesta is strongly convex to the north in the middle, gently convex to the south at either end. The cuesta begins where its determining limcstone begins, in west-central New York; there it separates the lowlands that contain the basins of lakes Ontario and Erie; thence it curves to the north-west through the province of Ontario to the belt of islands that divide1 Georgian Bay from Lake Huron; then westward throtigh the land-arm between lakes Superior and Michigan, and south-westward into the narrow points that divide Green Bay from Lake Michigan, and at last westward to fade away again with the thinning out of the limestone; it is hardly traceable across the Mississippi river. The arrangement of the Great Lakes is thus seen to he closely synipathetic with the course of the lowlands worn on the two belts of weaker strata on either side of the Niagara cuesta; Ontario, Georgian Bay and Green Bay occupy depressions in the lowland on the inner side of the cuesta; Erie, Huron and 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, 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; forming a pathway of great historic and economic importance between the Atlantic seaports and the interior.

In Wisconsin the inner lowland presents an interesting feature in a knob of resistant quartzites, known as Baraboo Ridge, rising from the buried oldland floor through the partly denuded cover of lower Palaeozoic strata. This knob or ridge may be appropriately regarded as an ancient physiographic fossil, inasmuch as, being a monadnock of very remote origin, it has long been preserved from the destructive attack of the weather by burial under sea-floor deposits, and recently laid bare, like ordinary organic fossils of much smaller size, by the removal of part of its cover by normal erosion.

The occurrence of the lake basins in the lowland belts on either side of the Niagara cuesta is an abnormal feature, not to be explained by ordinary erosion, which can produce only valleys. The basins have been variously ascribed to glacial erosion, to obstruction of normal outlet valleys by barriers of glacial drift, and to crustal warping in connection with or independent of the presence of the glacial sheet. No satisfactory solution of this problem has been reached; but the association of the Great Lakes and other large lakes farther north in Canada with the great North American area of strong and repeated glaciation is highly suggestive.

Lake Superior is unlike the other lakes; the greater part of its basin occupies a depression. in the oldland area, independent of the overlap of Palaeozoic strata. The western half of the basin occupies a trough of synclinal structure; but the making of this syndine is so ancient that it cannot be directly connected with the occurrence of the lake to-day. A more reasonable explanation ascribes the lake basin to a geologically modern depression. within the Superior oldland area; but there is at present no direct evidence in favor of this hypothesis. The Great Lakes are peculiar in receiving the drainage of but a sma]l 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.

Large canals and locks on both sides of the Sault (pronounced Soo) Ste Marie in the outlet of Lake Superior are actively used except during three or four winter months. The three lakes of the middle group stand at practically the same level: Michigan and Huron are connected by the Strait of Mackinac (pronounced Mackinaw); Huron and Erie by the St Clair and Detroit rivers, with the small Lake St Clair between them. The navigable depth of these two short rivers is believed to be the result of a slow elevation of the land in the north-east, still in progress, whereby the, waters have risen on their former shores near Detroit. Niagara river, connecting lakes Erie and Ontario, with a fall of 326 ft. (160 ft. at the cataract) in 30 m, is manifestly a watercourse of very modern origin; for a large river would now have a thoroughly matured valley had it long followed its present course; the same is true of the St Lawrence, which in its several rapids and in its subdivision into many channels at the Thousand Islands, presents every sign of youth. Canals on the Canadian side of these unnavigable stretches admit vessels of a considerable size to lakes Ontario and Erie.

The Prairie States.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 described Leyond as the Great Plains. An eastward extension of the same region, originally tree-covered, extended to central Ohio. Thus the prairies may be described as lying in a general way between the Ohio and Missouri rivers on the south and the Great Lakes on the north. Under the older-fashioned methods of treating physical geography, the prairies were empirically described as level prairies, rolling prairies, and so on. The great advance in the interpretation of land forms now makes it possible to introduce as thoroughly explanatory a description of these fertile plains as of forms earlier familiar, such as sand dunes, deltas and sea cliffs. The prairies are, in brief, 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. The rocks here concerned are the 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, but is known by numerous borings for oil, gas and water.

The greatest area of the prairies, from Indiana to North Dakota, consists of till plains, that is, sheets of unstratified drift, 30, 50 or even 100 ft. thick, which cover the underlying rock surface for thousands of square miles (except where postglacial stream erosion has locally laid it bare), and present an extraordinarily even surface. The till is presumably made in part of preglacial soils, but it is more largely composed of rock waste mechanically comminuted by the crccpiiig 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, are often level to the eye, and the view across them stretches to the horizon, unless interrupted by groves of trees along the watercourses, or by belts of low morainic hills. Here and there faint depressions occur, occupied by marshy sloughs, or floored with a rich black soil of pestglacial origin. It is thus by sub-glacial aggradation that the prairies have been leyelled up to a smooth surface, in contrast to the higher and non-glaciated hilly country next south.

The great ice sheets formed terminal moraines around their border at various halting stages; but 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 discovery of this significant looped arrangement of the morainic belts is the greatest advance in interpretation of glacial phenomena since the first suggestion of a glacial period; it is also the strongest proof that the ice here concerned was a continuous sheet of creeping land ice, and not a discontinuous series of floating icebergs, as had been supposed. The moraines are of too small relief to be shown on any maps but those of the largest scale; yet small as they are, t

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