1911 Encyclopædia Britannica/United States, The/Physical Geography
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 north-east, 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 connexion with the land 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 18th 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 19th 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-comers 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 Valley—the chief eastern part of the central plains—would 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 earth's 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 marine, partly as fluviatile deposits. The deformation of the Appalachians was 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 north-west than the first. These were followed by a period of minor tilting and faulting in early Mesozoic, by a moderate up warping 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 uplands 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; 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. Evidently, 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 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 belts of structure and the cycles of erosion thus briefly described are recognizable with more or less continuity from the Gulf of St Lawrence 1500 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 monoclonal 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 The Middle Appalachians. it contains the best representation of the three longitudinal belts of which the mountain system as a whole is composed. The mountain-making compression of the heavy series of Palaeozoic strata has here produced a 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 the ridges may be apprehended 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, well deserving of the name of Endless Mountains 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 post-Tertiary 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 to 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 Appalachian 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 amount. 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 by farms and villages, but many of the valleys are wooded glens. With 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 portion of the crystalline belt is narrow, as has been said, because of encroachment upon it by the inward overlap of the coastal plain; it is low because of small Tertiary uplift; 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 Drainage. arrangement 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 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 they 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, 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 of 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, w ere the rivers have been converted into estuaries and bays—such as Chesapeake and Delaware bays—by 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 connexion 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-eastern Appalachians. is in large measure warranted by the inclusion of all these states within the broadened crystalline belt 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 1000 ft. rise to 1500 or 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 sympathy with Appalachian structure; so also are certain open valleys, as the Berkshire (limestone) Valley in western Massachusetts and the corresponding Rutland (limestone and marble) Valley in western Vermont; and 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 eroded 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 monadnocks, 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 Crawford's Notch (1900 ft.).
In consequence of the general south-eastward slope of the highlands and uplands of New England, the divide between the Atlantic Drainage. 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; they are 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 well established 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 Coast. sea and from the work of the sea, upon the shore line thus determined; and it is for this reason that throughout this article the coastal features are described in connexion 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 interfluve 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 The South-western Appalachians. unit, the southern crystalline belt must be subdivided into a higher mountain belt on the north-west, 60 m. wide where broadest, and a lower piedmont belt on the 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 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 tops, 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 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
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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 neighbouring 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 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; Asheville on its southern border is a noted health resort.
The rivers of the mountain belt, normally dividing and subdividing in apparently in sequent 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 boundary between the mountains and the piedmont belt is called the Blue Ridge all along its length; and although the name 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. It is determined
structurally by a belt of topographically weak limestones
and shales (or slates) next inland from the crystalline
Valley. 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 m. the lowland is nowhere interrupted by a transverse ridge, although longitudinal ridges of moderate height 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 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; while to the south
of New York harbour the remainder of the Appalachians
Coastal Plain. 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 fluviatile) 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 and by their innumerable 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 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 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 peculiar 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.” This is a form 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 the 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), Martha’s 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 Palaeozoic 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
Plateau. 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 continuous 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 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. 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 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 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 peneplain through the now removed cover of stratified rocks. Glaciation has strongly scoured away the deeply-weathered soils that presumably existed here in preglacial time, 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.—This 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 the Great Lakes.—The Palaeozoic strata, already mentioned as lapping on the southern slope of the Superior Oldland and around the western side of the Adirondacks are but parts of a great area of similar strata, hundreds of feet in thickness, which decline 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 oldland rocks, the better interpretation of the relation between the two is that the visible oldland 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 oldland, 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. Hence 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 oldland 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 out through the upland by the Niagara river. As in all such forms, the Niagara cuesta as 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 make 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 limestone 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 divides Georgian Bay from Lake Huron; then westward through 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 be closely sympathetic 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 quartzite's, 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 connexion 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 syncline 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 favour of this hypothesis. The Great Lakes are peculiar in receiving the drainage of but a small peripheral land area, enclosed by an ill-defined water-parting from the rivers that run to Hudson Bay or the Gulf of St Lawrence on the north and to the Gulf of Mexico on the south.
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 beyond 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 post glacial 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 creeping ice sheets; although the crystalline rocks from Canada and some of the more resistant stratified rocks south of the Great Lakes occur as boulders and stones, a great part of the till has been crushed and ground to a clayey texture. The till plains, although sweeping in broad swells of slowly changing altitude, 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 post-glacial origin. It is thus by sub-glacial aggradation that the prairies have been levelled up to a smooth surface, in contrast to the higher and non-glaciated hilly country 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; neighbouring 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, they are the chief relief of the prairie states, and, in association with the nearly imperceptible slopes of the till plains, they determine the course of many streams and rivers, which as a whole are consequent upon the surface form of the glacial deposits.
The complexity of the glacial period and its subdivision into several glacial epochs, separated by interglacial epochs of considerable length (certainly longer than the post glacial epoch) has a structural consequence in the superposition of successive till sheets, alternating with non-glacial deposits, and also a physiographic consequence in the very different amount of normal post-glacial erosion suffered by the different parts of the glacial deposits. The southernmost drift sheets, as in southern Iowa and northern Missouri, have lost their initially plain surface and are now maturely dissected into gracefully rolling forms; here the valleys of even the small streams are well opened and graded, and marshes and lakes are wanting: hence these sheets are of early Pleistocene origin. Nearer the Great Lakes the till sheets are trenched only by the narrow valleys of the large streams; marshy sloughs still occupy the faint depressions in the till plains, and the associated moraines have abundant small lakes in their untrained hollows: hence these drift sheets are of late Pleistocene origin.
When the ice sheets fronted on land sloping southward to the Ohio, Mississippi and Missouri rivers, the drift-laden streams flowed freely away from the ice border; and as the streams, escaping from their sub glacial channels, spread in broader channels, they ordinarily could not carry forward all their load; hence they acted not as destructive but as constructive agents, and aggraded their courses. Thus local sheets or “aprons” of gravel and sand are spread more or less abundantly along the outer side of the morainic belts; and long trains of gravel and sands clog the valleys that lead southward from the glaciated to the non-glaciated area. Later when the ice retreated farther and the unloaded streams returned to their earlier degrading habit, they more or less completely scoured out the valley deposits, the remains of which are now seen in terraces on either side of the present flood plains.
When the ice of the last glacial epoch had retreated so far that its front lay on a northward slope, belonging to the drainage area of the Great Lakes, bodies of water accumulated in front of the ice margin, forming glacio-marginal lakes. The lakes were small at first, and each had its own outlet at the lowest depression in the height of land to the south; but as the ice melted back, neighbouring lakes became confluent at the level of the lowest outlet of the group; the outflowing streams grew in the same proportion and eroded a broad channel across the height of land and far down stream, while the lake waters built sand reefs or carved shore cliffs along their margin, and laid down sheets of clay on their floors. All of these features are easily recognized in the prairie region. The present site of Chicago was determined by an Indian portage or “carry” across the low divide between Lake Michigan and the headwaters of the Illinois river; and this divide lies on the floor of the former outlet channel of the glacial Lake Michigan. Corresponding outlets are known for the glacial lakes Erie, Huron and Superior, and for a very large sheet of water, named Lake Agassiz, which once overspread a broad till plain in northern Minnesota and North Dakota. The outlet of this glacial lake, called river Warren, eroded a large channel in which the Minnesota river, of to-day is an evident “misfit.”
Certain extraordinary features were produced when the retreat of the ice sheet had progressed so far as to open an eastward outlet for the marginal lakes along the depression between the northward slope of the Appalachian plateau in west-central New York and the southward slope of the melting ice sheet; for when this eastward outlet came to be lower than the south-westward outlet across the height of land to the Ohio or Mississippi river, the discharge of the marginal lakes was changed from the Mississippi system to the Hudson system. Many well-defined channels, cutting across the north-sloping spurs of the plateau in the neighbourhood of Syracuse, N.Y., mark the temporary paths of the ice-bordered outlet river. Successive channels are found at lower and lower levels on the plateau slope, thus indicating the successive courses taken by the lake outlet as the ice melted farther and farther back. On some of these channels deep gorges were eroded heading in temporary cataracts which exceeded Niagara in height but not in breadth; the pools excavated by the plunging waters at the head of the gorges are now occupied by little lakes. The most significant stage in this series of changes occurred when the glacio-marginal lake waters were lowered so that the long cuesta of Niagara limestone was laid bare in western New York; the previously confluent waters were then divided into two lakes; the higher one, Erie, supplying the outflowing Niagara river, which poured its waters down the escarpment of the cuesta to the lower lake, Ontario, whose outlet for a time ran down the Mohawk Valley to the Hudson: thus Niagara falls began. (See Niagara.)
Many additional features associated with the glacial period might be described, but space can be given to four only. In certain districts the subglacial till was not spread out in a smooth plain, but accumulated in elliptical mounds, 100 or 200 ft. high, half a mile or a mile long, with axes parallel to the direction of the ice motion as indicated by striae on the underlying rock floor; these hills are known by the Irish name, drumlins, used for similar hills in north-western Ireland. The most remarkable groups of drumlins occur in western New York, where their number is estimated at over 6000, and in southern Wisconsin, where it is placed at 5000. They completely dominate the topography of their districts.
A curious deposit of an impalpably fine and unstratified silt, known by the German name loess, lies on the older drift sheets near the larger river courses of the upper Mississippi basin. It attains a thickness of 20 ft. or more near the rivers and gradually fades away at a distance of ten or more miles on either side. It is of inexhaustible fertility, being in this as well as in other respects closely like the loess in China and other parts of Asia, as well as in Germany. It contains land shells, and hence cannot be attributed to marine or lacustrine submergence. The best explanation suggested for loess is that, during certain phases of the glacial period, it was carried as dust by the winds from the flood plains of aggrading rivers, and slowly deposited on the neighbouring grass-covered plains.
South-western Wisconsin and parts of the adjacent states of Illinois. Iowa and Minnesota are known as the “driftless area,” because, although bordered by drift sheets and moraines, it is free from glacial deposits. It must therefore have been a sort of oasis, when the ice sheets from the north advanced past it on the east and west and joined around its southern border. The reason for this exemption from glaciation is the converse of that for the southward convexity of the morainic loops; for while they mark the paths of greatest glacial advance along lowland troughs (lake basins), the driftless area is a district protected from ice invasion by reason of the obstruction which the highlands of northern Wisconsin and Michigan (part of the Superior oldland) offered to glacial advance.
The course of the upper Mississippi river is largely consequent upon glacial deposits. Its sources are in the morainic lakes in northern Minnesota; Lake Itasca being only one of many glacial lakes which supply the headwater branches of the great river. The drift deposits thereabouts are so heavy that the present divides between the drainage basins of Hudson Bay, Lake Superior and the Gulf of Mexico evidently stand in no very definite relation to the preglacial divides. The course of the Mississippi through Minnesota is largely guided by the form of the drift cover. Several rapids and the Falls of St Anthony (determining the site of Minneapolis) are signs of immaturity, resulting from superposition through the drift on the under rock. Farther south, as far as the entrance of the Ohio, the Mississippi follows a rock-walled valley 300 to 400 ft. deep, with a flood-plain 2 to 4 m. wide; this valley seems to represent the path of an enlarged early-glacial Mississippi, when much precipitation that is to-day discharged to Hudson Bay and the Gulf of St Lawrence was delivered to the Gulf of Mexico, for the curves of the present river are of distinctly smaller radius than the curves of the valley. Lake Pepin (30 m. below St Paul), a picturesque expansion of the river across its flood-plain, is due to the aggravation of the valley floor where the Chippewa river, coming from the north-east, brought an overload of fluvio-glacial drift. Hence even the “father of waters,” like so many other rivers in the Northern states, owes many of its features more or less directly to glacial action.
The fertility of the prairies is a natural consequence of their origin. During the mechanical comminution of the till no vegetation was present to remove the minerals essential to plant growth, as is the case in the soils of normally weathered and dissected peneplains, such as the Appalachian piedmont, where the soils, though not exhausted by the primeval forest cover, are by no means so rich as the till sheets of the prairies. Moreover, whatever the rocky under structure, the till soil has been averaged by a thorough mechanical mixture of rock grindings; hence the prairies are continuously fertile for scores of miles together.
The true prairies, when first explored, were covered with a rich growth of natural grass and annual flowering plants. To-day they are covered with farms. The cause of the treelessness has been much discussed. It does not seem to lie in peculiarities of temperature or of precipitation; for trees thrive where they are properly planted on the prairies; every town and farm to-day has its avenues and groves of trees; but it should be noted that west of the Mississippi river increasing aridity becomes an important factor, and is the chief cause of the treelessness of the Great Plains (see below). The treelessness of the prairies cannot be due to insufficient time for tree invasion since glacial evacuation; for forests cover the rocky uplands of Canada, which were occupied by ice for ages after the prairies were laid bare. A more probable cause is found in the fineness of the prairie soil, which is inimical to the growth of young trees in competition with the grasses and annual plants. Prairie fires, both of natural and artificial origin, are also a contributive cause; for young trees are exterminated by fires, but annual plants soon reappear.
The Gulf Coastal Plain.—The westward extension of the Atlantic coastal plain around the Gulf of Mexico carries with it a repetition of certain features already described, and the addition of several new ones. As in the Atlantic coastal plain, it is only the lower, seaward part of this region that deserves the name of plain, for there alone is the surface unbroken by hills or valleys; the inner part, initially a plain by reason of its essentially horizontal (gently seaward-sloping) structure, has been converted by mature dissection into an elaborate complex of hills and valleys, usually of increasing altitude and relief as one passes inland.
The special features of the Gulf Plain are the peninsular extension of the plain in Florida, the belted arrangement of relief and soils in Alabama and in Texas, and the Mississippi embayment or inland extension of the plain half-way up the course of the Mississippi river, with the Mississippi flood plain there included.
A broad, low crustal arch extends southward at the junction of the Atlantic and Gulf coastal plains; the emerged half of the arch Florida. constitutes the visible lowland peninsula of Florida; the submerged half extends westward under the shallow overlapping waters of the Gulf of Mexico. The northern part of the peninsula is composed largely of a weak limestone; here much of the lowland drainage is underground, forming many sink-holes (swallow-holes). Many small lakes in the lowland appear to owe their basins to the solution of the limestones. Valuable phosphate deposits occur in certain districts. The southern part of the state includes the “Everglades” (q.v.), a large area of low, flat, marshy land, overgrown with tall reedy grass, a veritable wilderness; thus giving Florida an unenvied first rank among the states in marsh area. The eastern coast is fringed by long-stretching sand reefs, enclosing lagoons so narrow and continuous that they are popularly called “rivers.” At the southern end of the peninsula is a series of coral islands, known as “keys”; they appear to be due to the forward growth of corals and other lime-secreting organisms towards the strong current of the Gulf Stream, by which their food is supplied: the part of the peninsula composed of coral reefs is less than has been formerly supposed. The western coast has fewer and shorter off-shore reefs; much of it is of minutely irregular outline, which seems to be determined less by the work of the sea than by the forward growth of mangrove swamps in the shallow salt water.
A typical example of a belted coastal plain is found in Alabama and the adjacent part of Mississippi. The plain is here about Alabama. 150 m. wide. The basal formation is chiefly a weak limestone, which has been stripped from its original innermost extension and worn down to a flat inner lowland of rich black soil, thus gaining the name of the “black belt.” The lowland is enclosed by an upland or cuesta, known as Chunnenugga Ridge, sustained by partly consolidated sandy strata; the upland, however, is not continuous, and hence should be described as a “maturely dissected cuesta,” It has a relatively rapid descent toward the inner lowland, and a very gradual descent to the coast prairies, which become very low, flat and marshy before dipping under the Gulf waters, where they are generally fringed by off-shore reefs.
The coastal plain extends 500 m. inland on the axis of the Mississippi embayment. Its inner border affords admirable examples of topographical discordance where it sweeps north-westward square across the trend of the piedmont belt, the ridges and valleys, and the plateau of the Appalachians, which are all terminated by dipping The Mississippi Embayment. gently beneath the unconformable cover of the coastal plain strata. In the same way the western side of the embayment, trending south and south-west, passes along the lower south-eastern side of the dissected Ozark plateau of southern Missouri and northern Arkansas, which in many ways resembles the Appalachian plateau, and along the eastern end of the Massern ranges of the Ouachita mountain system in central Arkansas, which in geological history and topographical form present many analogies with the ridges and valleys of the Appalachians; and as the coastal plain turns westward to Texas it borders the Arbuckle hills in Oklahoma, a small analogue of the crystalline Appalachian belt. In the embayment of the coastal plain some low cuesta-like belts of hills with associated strips of lowlands suggest the features of a belted coastal plain; the hilly belt or dissected cuesta determined by the Grand Gulf formation in western Mississippi is the most distinct. Important salt deposits occur in the coastal plain strata near the coast. The most striking feature of the embayment is the broad valley which the Mississippi has eroded across it.
The lower Mississippi is the trunk in which three large rivers join; the chief figures (approximate only) regarding them are as follows:—
| Drainage Area
The small proportion of total water volume supplied from the great Missouri basin is due to the light precipitation in that region. The The Lower Mississippi River. lower Mississippi receives no large tributary from the east, but two important ones come from the west; the Arkansas drainage area being a little less than that of the Ohio, and the basin of the Red River of Louisiana being about half as large. The great river thus constituted drains an area of about 1,250,000 sq. m., or about one-third of the United States; and discharges 75,000 cub. yds. of water per second, or 785,190,000,000 cubic yds. per annum, which corresponds roughly to one quarter of the total precipitation on its drainage basin. Its load of land waste (see I. C. Russell, Rivers of North America) is as follows:—
|In suspension||6,718,694,400||cub. ft. or||241||ft. deep over||1||sq. m.|
|Swept along bottom||750,000,000||””||26||””||1||”|
Average annual removal of waste from entire basin, 1 in. or 1 ft. in 4000 years.
The head of the coastal plain embayment is near the junction of the Ohio and the Mississippi. Thence southward for 560 m. the great river flows through the semi-consolidated strata of the plain, in which it has eroded a valley, 40 or 50 m. wide, and 29,700 sq. m. in area, enclosed by bluffs one or two hundred feet high in the northern part, generally decreasing to the southward, but with local increase of height associated with a decrease in flood plain breadth on the eastern side where the Grand Gulf cuesta is traversed. This valley in the coastal plain, with the much narrower rock-walled valley of the upper river in the prairie states, is the true valley of the Mississippi river; but in popular phrase the “Mississippi Valley” is taken to include a large central part of the Mississippi drainage basin. The valley floor is covered with a flood plain of fine silt, having a southward slope of only half a foot to a mile. The length of the river itself, from the Ohio mouth to the Gulf, is, owing to its windings, about 1060 m.; its mean fall is about 3 in. in a mile. On account of the rapid deposition of sediment near the main channel at times of overflow, the flood plain, as is normally the case on mature valley floors, has a lateral slope of as much as 5, 10, or even 12 ft. in the first mile from the river; but this soon decreases to a less amount. Hence at a short distance from the river the flood plain is often swampy, unless its surface is there aggraded by the tributary streams: for this reason Louisiana, Arkansas and Mississippi rank next after Florida in swamp area.
The great river receives an abundant load of silt from its tributaries, and takes up and lays down silt from its own bed and banks with every change of velocity. The swiftest current tends, by reason of centrifugal force, to follow the outer side of every significant curve in the channel; hence the concave bank, against which the rapid current sweeps, is worn away; thus any chance irregularity is exaggerated, and in time a series of large serpentine or meanders is developed, the most symmetrical examples at present being those near Greenville, Miss. The growth of the meanders tends to give the river continually increasing length; but this tendency is counteracted by the sudden occurrence of cut-offs from time to time, so that a fairly constant length is maintained.
The floods of the Mississippi usually occur in spring or summer; Owing to the great size of the drainage basin, it seldom happens that the three upper tributaries are in flood at the same time; the coincident occurrence of floods in only two tributaries is of serious import in the lower river, which rises 30, 40, or occasionally 50 ft. The abundant records by the Mississippi River Commission and the United States Weather Bureau (by which accurate and extremely useful predictions of floods in the lower river course are made, on the basis of the observed rise in the tributaries) demonstrate a number of interesting features, of which the chief are as follows: the fall of the river is significantly steepened and its velocity is accelerated down stream from the point of highest rise; conversely, the fall and the velocity are both diminished up stream from the same point.
The load of silt borne down stream by the river finally, after many halts on the way, reaches the waters of the Gulf, where the decrease of velocity, aided by the salinity of the sea water, causes the formation of a remarkable delta, leaving less aggraded areas as shallow lakes (Lake Pontchartrain on the east, and Grand Lake on the west of the river). The ordinary triangular form of deltas, due to the smoothing of the delta front by sea action, is here wanting, because of the weakness of sea action in comparison with the strength of the current in each of the four distributaries or “passes” into which the river divides near its mouth. (See Mississippi River.)
After constriction from the Mississippi embayment to 250 m. in
western Louisiana, the coastal plain continues south-westward
The Texas Coastal Plain.
with this breadth until it narrows to about 130 m. in
southern Texas near the crossing of the Colorado river
(of Texas); but it again widens to 300 m. at the
national boundary as a joint effect of embayment up the
valley of the Rio Grande and of the seaward advance of this river's
rounded delta front: these several changes take place in a distance
of about 500 m., and hence include a region of over 100,000 sq.
m.—less than half of the large state of Texas. A belted arrangement of
reliefs and soils, resulting from differential erosion on strata of unlike
composition and resistance, characterizes almost the entire area
of the coastal plain. Most of the plain is treeless prairie, but the
sandier belts are forested; two of them are known as “cross timbers,”
because their trend is transverse to the general course of the main
consequent rivers. An inland extension from the coastal plain in
north-central Texas leads to a large cuesta known as Grand Prairie
(not structurally included in the coastal plain), upheld at altitudes
of 1200 or 1300 ft. by a resistant Cretaceous limestone, which dips
gently seaward; its scalloped inland-facing escarpment overlooks
a denuded central prairie region of irregular structure and form;
its gentle coast ward slope (16 ft. to a mile) is dissected by many
branching consequent streams; in its southern part, as it approaches
the Colorado river the cuesta is dissected into a belt of discontinuous
hills. The western cross timbers follow a sandy belt along the inner
base of the ragged escarpment of Grand Prairie; the eastern cross
timbers follow another sandy belt in the lowland between the eastern
slope of Grand Prairie and the pale western escarpment of the next
eastward and lower Black Prairie cuesta. This cuesta is supported
at an altitude of 700 ft. or less by a chalk formation, which gives
an infacing slope some 200 ft. in height, while its gently undulating
or “rolling” seaward slope (2 or 3 ft. in a mile), covered with marly
strata and rich black soil, determines an important cotton district.
Then comes the East Texas timber belt, broad in the north-east,
narrowing to a point before reaching the Rio Grande, a low and
thoroughly dissected cuesta of sandy Eocene strata; and this is
followed by the Coast Prairie, a very young plain, with a seaward
slope of less than 2 ft. in a mile, its smooth surface interrupted
only by the still more nearly level flood plains of the shallow, consequent
river valleys. Near the Colorado river the dissected cuesta
of the Grand Prairie passes southward, by a change to a more nearly
horizontal structure, into the dissected Edwards plateau (to be
referred to again as part of the Great Plains), which terminates in a
maturely dissected fault scarp, 300 or 400 ft. in height, the northern
boundary of the Rio Grande embayment. From the Colorado
to the Rio Grande, the Black Prairie, the timber belt and the Coast
Prairie merge in a vast plain, little differentiated, overgrown with
“chaparral” (shrub-like trees, often thorny), widening eastward in
the Rio Grande delta, and extending southward into Mexico.
Although the Coast Prairie is a sea bottom of very modern uplift, it appears already to have suffered a slight movement of depression, for its small rivers all enter embayments; the larger rivers, however, seem to have counteracted the encroachment of the sea on the land by a sufficiently active delta building, with a resulting forward growth of the land into the sea. The Mississippi has already been mentioned as rapidly building forward its digitate delta; the Rio Grande, next in size, has built its delta about 50 m. forward from the general coast-line, but this river being much smaller than the Mississippi, its delta front is rounded by seashore agencies. In front of the Brazos and the Colorado, the largest of the Texan rivers, the coast-line is very gently bowed forward, as if by delta growth, and the sea touches the mainland in a nearly straight shore line. Nearly all the rest of the coast is fringed by off-shore reefs, built up by waves from the very shallow sea bottom; in virtue of weak tides, the reefs continue in long unbroken stretches between the few inlets.
The Great Plains.—A broad stretch of country under laid by nearly horizontal strata extends westward from the 97th meridian to the base of the Rocky Mountains, a distance of from 300 to 500 m., and northward from the Mexican boundary far into Canada. This is the province of the Great Plains. Although the altitude of plains increases gradually from 600 or 1200 ft. on the east to 4000, 5000 or 6000 ft. near the mountains, the local relief is generally small; the sub-arid climate excludes tree growth and opens far-reaching views. The plains are by no means a simple unit; they are of diverse structure and of various stages of erosional development; they are occasionally interrupted by buttes and escarpments; they are frequently broken by valleys: yet on the whole a broadly extended surface of moderate relief so often prevails that the name, Great Plains, for the region as a whole is well deserved. The western boundary of the plains is usually well defined by the abrupt ascent of the mountains. The eastern boundary of the plains is more climatic than topographic. The line of 20 in. of annual rainfall trends a little east of northward near the 97th meridian, and if a boundary must be drawn where nature presents only a gradual transition, this rainfall line may be taken to divide the drier plains from the moister prairies. The plains may be described in northern, intermediate, central and southern sections, in relation to certain peculiar features.
The northern section of the Great Plains, north of latitude 44°, including eastern Montana, north-eastern Wyoming and most of the Dakotas, is a moderately dissected peneplain, one of the best examples of its class. The strata here are Cretaceous or early Tertiary, lying nearly horizontal. The surface is shown to be a plain of degradation by a gradual ascent here and there to the crest of a ragged escarpment, the cuesta-remnant of a resistant stratum; and by the presence of lava-capped mesas and dike-ridges, surmounting the general level by 500 ft. or more and manifestly demonstrating the widespread erosion of the surrounding plains. All these reliefs are more plentiful towards the mountains in central Montana. The peneplain is no longer in the cycle of erosion that witnessed its production; it appears to have suffered a regional elevation, for the rivers—the upper Missouri and its branches—no longer flow on the surface of the plain, but in well graded, maturely opened valleys, several hundred feet below the general level. A significant exception to the rule of mature valleys occurs, however, in the case of the Missouri, the largest river, which is broken by several falls on hard sandstones about 50 m. east of the mountains. This peculiar feature is explained as the result of displacement of the river from a better graded preglacial valley by the Pleistocene ice-sheet, which here overspread the plains from the moderately elevated Canadian highlands far on the north-east, instead of from the much higher mountains near by on the west. The present altitude of the plains near the mountain base is 4000 ft.
The northern plains are interrupted by several small mountain areas. The Black Hills, chiefly in western South Dakota, are the largest group: they rise like a large island from the sea, occupying an oval area of about 100 m. north-south by 50 m. east-west, reaching an altitude in Harney Peak of 7216 ft., and an effective relief over the plains of 2000 or 3000 ft. This mountain mass is of flat-arched, dome-like structure, now well dissected by radiating consequent streams, so that the weaker uppermost strata have been erode down to the level of the plains where their upturned edges are evenly truncated, and the next following harder strata have been sufficient eroded to disclose the core of underlying crystalline rocks in about half of the domed area.
In the intermediate section of the plains, between latitudes 44° and 42°, including southern South Dakota and northern Nebraska, the erosion of certain large districts is peculiarly elaborate, giving rise to a minutely dissected form, known as “bad lands,” with a relief of a few hundred feet. This is due to several causes: first, the dry climate, which prevents the growth of a grassy turf; next, the fine texture of the Tertiary strata in the bad land districts; and consequently the success with which every little rill, at times of rain, carves its own little valley. Travel across the bad lands is very fatiguing because of the many small ascents and descents; and it is from this that their name, “mauvaises terres pour traverse,” was given by the early French voyageurs.
The central section of the Great Plains, between latitudes 42° and 36°, occupying eastern Colorado and western Kansas, is, briefly stated, for the most part a dissected fluviatile plain; that is, this section was once smoothly covered with a gently sloping plain of gravel and sand that had been spread far forward on a broad denuded area as a piedmont deposit by the rivers which issued from the mountains; and since then it has been more or less dissected by the erosion of valleys. The central section of the plains thus presents a marked contrast to the northern section; for while the northern section owes its smoothness to the removal of local gravels and sands from a formerly uneven surface by the action of degrading river sand their inflowing tributaries, the southern section owes its smoothness to the deposition of imported gravels and sands upon a previously uneven surface by the action of aggrading rivers and their outgoing distributaries. The two sections are also unlike in that residual eminences still here and there surmount the peneplain of the northern section, while the fluviatile plain of the central section completely buried the pre-existent relief. Exception to this statement must be made in the south-west, close to the mountains in southern Colorado, where some lava-capped mesas (Mesa de Maya, Raton Mesa) stand several thousand feet above the general plain level, and thus testify to the widespread erosion of this region before it was aggraded.
The southern section of the Great Plains, between latitudes 35½° and 29½°, lies in eastern Texas and eastern New Mexico; like the central section it is for the most part a dissected fluviatile plain, but the lower lands which surround it on all sides place it in so strong relief that it stands up as a table-land, known from the time of Mexican occupation as the Llano Estacado. It measures roughly 150 m. east-west and 400 m. north-south, but it is of very irregular outline, narrowing to the south. Its altitude is 5500 ft. at the highest western point, nearest the mountains whence its gravels were supplied; and thence it slopes south-eastward at a decreasing rate, first about 12 ft., then about 7 ft. in a mile, to its eastern and southern borders, where it is 2000 ft. in altitude: like the High Plains farther north, it is extraordinarily smooth; it is very dry, except for occasional shallow and temporary water sheets after rains. The Llano is separated from the plains on the north by the mature consequent valley of the Canadian river, and from the mountains on the west by the broad and probably mature valley of the Pecos river. On the east it is strongly undercut by the retrogressive erosion of the headwaters of the Red, Brazos and Colorado rivers of Texas, and presents a ragged escarpment, 500 to 800 ft. high, overlooking the central denuded area of that state; and there, between the Brazos and Colorado rivers, occurs a series of isolated outliers capped by a limestone which underlies both the Llano on the west and the Grand Prairies cuesta on the east. The southern and narrow part of the table-land, called the Edwards Plateau, is more dissected than the rest, and falls off to the south in a frayed-out fault scarp, as already mentioned, overlooking the coastal plain of the Rio Grande embayment. The central denuded area, east of the Llano, resembles the east-central section of the plains in exposing older rocks; between these two similar areas, in the space limited by the Canadian and Red rivers, rise the subdued forms of the Wichita Mountains in Oklahoma, the westernmost member of the Ouachita system.
The Cordilleran Region.—From the western border of the Great Plains to the Pacific coast, there is a vast elevated area, occupied by mountains, plateaus and intermont plains. The intermont plains are at all altitudes from sea-level to 4000 ft.; the plateaus from 5000 to 10,000 ft.; and the mountains from 8000 to 14,000 ft. The higher mountains are barren from the cold of altitude; the timber line in Colorado stands at 11,000 to 12,000 ft.
The chief provinces of the Cordilleran region are: The Rocky Mountain system and its basins, from northern New Mexico northward, including all the mountains from the front ranges bordering on the plains to the Uinta and Wasatch ranges in Utah; the Pacific ranges including the Sierra Nevada of California, the Cascade range of Oregon and Washington, and the Coast range along the Pacific nearly to the southern end of California; and a great intermediate area, including in the north the Columbian lava plains and in the south the large province of the Basin ranges, which extends into Mexico and widens from the centre southward, so as to meet the Great Plains in eastern New Mexico, and to extend to the Pacific coast in southern California. There is also a province of plateaus between the central part of the Basin ranges and the southern part of the Rocky Mountains. An important geological characteristic of most of the Cordilleran region is that the Carboniferous strata, which in western Europe and the eastern United States contain many coal seams, are represented in the western United States by a marine limestone; and that the important unconformity which in Europe and the eastern United States separates the Palaeozoic and Mesozoic eras does not occur in the western United States, where the formations over a great area follow in conformable sequence from early Palaeozoic through the Mesozoic.
The Rocky Mountains begin in northern Mexico, where the axial crystalline rocks rise to 12,000 ft. between the horizontal structures The Rocky Mountains. of the plains on the east and the plateaus on the west. The upturned stratified formations wrap around the flanks of the range, with ridges and valleys formed on their eroded edges and drained southward by the Pecos river to the Rio Grande and the Gulf of Mexico. The mountains rapidly grow wider and higher northward, by taking on new complications of structure and by including large basins between the axes of uplift, until in northern Colorado and Utah a complex of ranges has a breadth of 300 m., and in Colorado alone there are 40 summits over 14,000 ft. in altitude, though none rises to 14,500. Then turning more to the north-west through Wyoming, the ranges decrease in breadth and height; in Montana their breadth is not more than 150 m., and only seven summits exceed 11,000 ft. (one reaching 12,834).
As far north as the gorge of the Missouri river in Montana, the Front range, facing the Great Plains, is a rather simple uplift, usually formed by upturning the flanking strata, less often by a fracture. Along the eastern side of the Front Range in Colorado most of the upturned stratified formations have been so well worn down that, except for a few low piedmont ridges, their even surface may now be included with that of the plains, and the crystalline core of the range is exposed almost to the mountain base. Here the streams that drain the higher areas descend to the plains through narrow canyons in the mountain border, impassable for ordinary roads and difficult of entrance even by railways; a well-known example is the gorge of Clear Creek east of the Georgetown mining district. The crystalline highlands thereabouts, at altitudes of 8000 to 10,000 ft., are of so moderate a relief as to suggest that the mass had stood much lower in a former cycle of erosion and had then been worn down to rounded hills; and that since uplift to the present altitude the revived streams of the current cycle of erosion have not entrenched themselves deep enough to develop strong relief. This idea is confirmed 80 m. farther south, where Pike's Peak (14,108 ft.), a conspicuous landmark far out on the plains, has every appearance of being a huge monadnock, surmounting a rough peneplain of 10,000 ft. in general elevation. The idea is still better confirmed farther north in Wyoming, where the Laramie Range, flanked with upturned strata on the east and west, is for the most part a broad upland at altitudes of 7000 or 8000 ft., with no strong surmounting summits and as yet no deep carved valleys. Here the first of the Pacific railways chose its pass. When the summit is reached, the traveller is tempted to ask, “Where are the mountains?” so small is the relief of the upland surface. This low range turns westward in a curve through the Rattlesnake Mountains towards the high Wind River Mountains (Gannett Peak, 13,775 ft.), an anticlinal range within the body of the mountain system, with flanking strata rising well on the slopes. Flanking strata are even better exhibited in the Bighorn Mountains, the front range of northern Wyoming, crescentic in outline and convex to the north-east, like the Laramie Range, but much higher; here heavy sheets of limestone arch far up towards the range crest, and are deeply notched where consequent streams have cut down their gorges.
Farther north in Montana, beyond the gorge of the Missouri river, the structure of the Front Range is altogether different; it is here the carved residual of a great mass of moderately bent Palaeozoic strata, overthrust eastward upon the Mesozoic strata of the plains; instead of exposing the oldest rocks along the axis and the youngest rocks low down on the flanks, the younger rocks of the northern range follow its axis, and the oldest rocks outcrop along its eastern flanks, where they override the much younger strata of the plains; the harder strata, instead of lapping on the mountain flanks in great slab-like masses, as in the Bighorns, form out-facing scarps, which retreat into the mountain interior where they are cut down by outflowing streams.
The structure of the inner ranges is so variable as to elude simple description; but mention should be made of the Uinta range of broad anticlinal structure in north-east Utah, with east-west trend, as if corresponding to the east-west Rattlesnake Mountains, already named. The Wasatch Range, trending north-south in central Utah, is peculiar in possessing large east-west folds, which are seen in cross-section in the dissected western face of the range, because the whole mass is there squarely cut off by a great north-south fault with down-throw to the Basin Range province, the fault face being elaborately carved.
Volcanic action has been restricted in the Rocky Mountains proper. West Spanish Peak (13,620 ft.), in the Front Range of southern Colorado, may be mentioned as a fine example of a deeply dissected volcano, originally of greater height, with many unusually strong radiating dike-ridges near its denuded flanks. In north-western Wyoming there are extensive and heavy lava sheets, uplifted and dissected, and crowned with a few dissected volcanoes. It is in association with this field of extinct volcanic activity that a remarkable group of geysers and hot springs has been developed, from which the Yellowstone river, a branch of the Missouri, flows north-eastward, and the Snake river, a branch of the Columbia, flows south-westward. The geyser district is held as a national domain, the Yellowstone Park.
Travellers whose idea of picturesqueness is based upon the abnormally sharpened peaks of the ice-sculptured Alps are disappointed with the scenery of the central and southern ranges of the Rocky Mountains. It is true that many of these ranges are characterized by the rounded tops and the rather evenly slanting, waste-covered slopes which normally result from the long-continued action of the ordinary agencies of erosion; that they bear little snow in summer and are practically wanting in glaciers; that forests are often scanty on the middle and lower slopes, the more so because of devastation by fires; and that the general impression of great altitude is much weakened because the mountains are seen from a base which itself is 5000 or 6000 ft. above sea-level. Nevertheless the mountains are of especial interest to the physiographer who wishes to make a comparative study of land forms as affected by normal and by glacial sculpture, in order to give due attention to “process” as well as to “structure and stage” in the analysis and description of mountain topography. A journey along the range from south to north reveals most strikingly a gradual increase in the share of sculpture due to Pleistocene glaciers. In New Mexico, if glaciers were formed at all in the high valleys, they were so small as not greatly to modify the more normal forms. In central Colorado and Wyoming, where the mountains are higher and the Pleistocene glaciers were larger, the valley heads were hollowed out in well-formed cirques, often holding small lakes; and the mountain valleys were enlarged into U-shaped troughs as far down as the ice reached, with hanging lateral valleys on the way. Different stages of cirque development, with accompanying transformation of mountain shape, are finely illustrated in several ranges around the headwaters of the Arkansas river in central Colorado, where the highest summit of the Rocky Mountains is found (Mt Massive, 14,424 ft., in the Sawatch range); and perhaps even better in the Bighorn range of Wyoming. In this central region, however, it is only by way of exception that the cirques were so far enlarged by retrogressive glacial erosion as to sharpen the preglacial dome-like summits into acute peaks; and in no case did glacial action here extend down to the plains at the eastern base of the mountains; but the widened, trough-like glaciated valleys frequently descend to the level of the elevated intermont basins, where moraines were deployed forward on the basin floor. The finest examples of this kind are the moraines about Jackson Lake on the basin floor east of the Teton Range (Grand Teton, 13,747 ft.), a superb north-south range which lies close to the meridional boundary line between Wyoming and Idaho. Farther north in Montana, in spite of a decrease of height, there are to-day a few small glaciers with snowfields of good size; and here the effects of sculpture by the much larger Pleistocene glaciers are seen in forms of almost alpine strength.
The intermont basins which so strongly characterize the Rocky Mountain system are areas which have been less uplifted than the enclosing ranges, and have therefore usually become the depositories of waste from the surrounding mountains.
Some of the most important basins may be mentioned. San Luis “Valley” is an oval basin about 60 m. long near the southern end of the mountain system in New Mexico and Colorado; its level, treeless floor, at an altitude of 7000 ft.. is as yet hardly trenched by the Rio Grande, which escapes through an impassable canyon southward on its way to the Gulf of Mexico. The much smaller basin of the upper Arkansas river in Colorado is well known because the Royal Gorge, a very narrow cleft by which the river escapes through the Front Range to the plains, is followed by a railroad at river-level. South Park, directly west of Pike's Peak, is one of the highest basins (nearly 10,000 ft.), and gains its name from the scattered, park-like growth of large pine trees; it is drained chiefly by the South Platte river (Missouri-Mississippi system), through a deep gorge in the dissected mass of the plateau-like Front Range. The Laramie Plains and the Green river basin, essentially a single structural basin between the east-west ranges of Rattlesnake Mountains on the north and the Uinta Range on the south, measuring roughly 260 m. east-west by 100 m. north-south, is the largest intermont basin; it is well known from being traversed through its greatest length by the Union Pacific railway. Its eastern part is drained north-eastward through a gorge that separates the Laramie and Rattlesnake (Front) ranges by the North Platte river to the Missouri-Mississippi; its western part, where the basin floor is much dissected, often assuming a bad-land expression, is drained southward by the Green river, through a deep canyon in the Uinta Range to the Colorado river and then to the Pacific. The Bighorn basin has a moderately dissected floor, drained north-eastward by Bighorn river through a deep canyon in the range of the same name to the Missouri. Several smaller basins occur in Montana, all somewhat dissected and drained through narrow gorges and canyons by members of the Missouri system.
The Plateau province, next west of the southern Rocky Mountains, is characterized for the most part by large-textured forms, developed The Plateau Province. on a great thickness of nearly horizontal Palaeozoic, Mesozoic and Tertiary formations, and by a dry climate. The province was uplifted and divided into great blocks by faults or monoclinal flexures and thus exposed to long-lasting denudation in a mid-Tertiary cycle of erosion; and then broadly elevated again, with renewed movement on some of the fault lines; thus was introduced in late Tertiary time the current cycle of erosion in which the deep canyons of the region have been trenched. The results of the first cycle of erosion are seen in the widespread exposure of the resistant Carboniferous limestone as a broad platform in the south-western area of greater uplift through central Arizona, where the higher formations were worn away; and in the development of a series of huge, south-facing, retreating escarpments of irregular outline on the edges of the higher formations farther north. Each escarpment stands forth where a resistant formation overlies a weaker one; each escarpment is separated from the next higher one by a broad step of weaker strata. A wonderful series of these forms occurs in southern Utah, where in passing northward from the Carboniferous platform one ascends in succession the Vermilion Cliffs (Triassic sandstones), the White Cliffs (Jurassic sandstones, of remarkably cross-bedded structure, interpreted the dunes of an ancient desert), and finally the Pink Cliffs (Eocene strata of fluviatile and lacustrine origin) of the high, forested plateaus. Associated with these irregular escarpments are occasional rectilinear ridges, the work of extensive erosion on monoclinal structures, of which Echo Cliffs, east of the Painted Desert (so called from its many-coloured sandstones and clays), is a good example.
With the renewal of uplift by which the earlier cycle of erosion was interrupted and the present cycle introduced, inequalities of surface due to renewed faulting were again introduced; these still appear as cliffs, of more nearly rectilinear front than the retreating escarpments formed in the previous cycle. These cliffs are peculiar in gradually passing from one formation to another, and in having a height dependent on the displacement of the fault rather than on the structures in the fault face; they are already somewhat battered and dissected by erosion. The most important line of cliffs of this class is associated with the western and southern boundary of the plateau province, where it was uplifted from the lower ground. The few rivers of the region must have reached the quiescence of old age in the earlier cycle, but were revived by uplift to a vigorous youth in the current cycle; and it is to this newly introduced cycle of physiographic evolution that the deep canyons of the Plateau province are due. Thus the Virgin river, a northern branch of the Colorado, has cut a vertical slit, 1000 ft. deep, hardly wider at the top than at the bottom, in the heavy Triassic sandstones of southern Utah; but the most famous example is the Grand Canyon (q.v.) of Arizona, eroded by the Colorado river across the uplifted platform of Carboniferous limestone.
During the current cycle of erosion, several of the faults, whose scarps had been worn away in the previous cycle, have been brought to light again as topographic features by the removal of the weak strata along one side of the fault line, leaving the harder strata on the other side in relief; such scarps are known as “fault-line scarps,” in distinction from the original “fault scarps.” They are peculiar in having their altitude dependent on the depth of revived erosion, instead of the amount of faulting, and they are sometimes “topographically reversed,” in that the revived scarp overlooks a lowland worn on a weak formation in the upheaved fault-block. Another consequence of revived erosion is seen in the occurrence of great landslides, where the removal of weak (Permian) clays has sapped the face of the Vermilion Cliffs (Triassic sandstone), so that huge slices of the cliff face have slid down and forward a mile or two, all shattered into a confused tumult of forms for a score or more of miles along the cliff base.
Volcanic features occur in abundance in the Plateau province. Some of the high plateaus in the north are capped with remnants of heavy lava flows of early eruption. A group of large volcanoes occurs on the limestone platform south of the Grand Canyon, culminating in Mt San Francisco (12,794 ft.), a moderately dissected cone, and associated with many more recent smaller cones and fresh looking lava flows. Mt Taylor in western New Mexico is of similar age, but here dissection seems to have advanced farther, probably because of the weaker nature of the underlying rocks, with the result of removing the smaller cones and exposing many lava conduits or pipes in the form of volcanic necks or buttes. The Henry Mountains in south-western Utah are peculiar in owing their relief to the doming or blistering up of the plateau strata by the underground intrusion of large bodies or “cisterns” (laccolites) of lava, now more or less exposed by erosion.
The lava plains of the Columbia basin are among the most extensive volcanic outpourings in the world. They cover 200,000 sq. m. or more in south-eastern Washington, eastern Oregon and south-western Idaho, and are known to be 4000 ft. deep in some river gorges. The lava completely buries the pre-existent land forms over most of its extent. The earlier supposition that these vast lava flows came chiefly from fissure eruptions has been made doubtful by the later discovery of flat-sloping volcanic cones from which much lava seems to have been poured out in a very liquid state. Some of the flows are still so young as to preserve their scoriaceous surface; here the “shore-line” of the lava contours evenly around the spurs and enters, bay-like, into the valleys of the enclosing mountains, occasionally isolating an outlying mass. Other parts of the lava flood are much older and have been more or less deformed and eroded. Thus the uplifted, dislocated and dissected lava sheets of the Yellowstone National Park in the Rocky Mountains on the east (about the headwaters of the Snake river) are associated with the older lavas of the Columbian plains.
The Columbia river has entrenched itself in a canyon-like valley around the northern and western side of the lava plains; Snake river has cut a deeper canyon farther south-east where the plains are higher and has disclosed the many lava sheets which build up the plains, occasionally revealing a buried mountain in which the superposed river has cut an even narrower canyon. One of the most remarkable features of this province is seen in the temporary course taken by the Columbia river across the plains, while its canyon was obstructed by Pleistocene glaciers that came from the Cascade Mountains on the north-west. The river followed the temporary course long enough to erode a deep gorge, known as “Grande Coulee,” along part of its length.
The lava plains are treeless and for the most part too dry for agriculture; but they support many cattle and horses. Along parts of their eastern border, where the rainfall is a little increased by the approach of the westerly winds to the Rocky Mountains, there is a belt of very deep, impalpably fine soil, supposed to be a dust deposit brought from the drier parts of the plains farther west; excellent crops of wheat are here raised.
The large province of the Basin ranges, an arid region throughout, even though it reaches the sea in southern California, involves some The Basin Range Province. novel problems in its description. It is characterized by numerous disconnected mountain ranges trending north and south, from 30 to 100 m. in length, the higher ranges reaching altitudes of 8000 or 10,000 ft., separated by broad, intermont desert plains or basins at altitudes varying from sea-level (or a little less) in the south-west, to 4000 or 5000 ft. farther inland. Many of the intermont plains—these chiefly in the north—appear to be heavily aggraded with mountain waste; while others—these chiefly in the south—are rock-floored and thinly veneered with alluvium. The origin of these forms is still in discussion; but the following interpretation is well supported. The ranges are primarily the result of faulting and uplifting of large blocks of the earth's crust. The structure of the region previous to faulting was dependent on long antecedent processes of accumulation and deformation and the surface of the region then was dependent on the amount of erosion suffered in the prefaulting cycle. When the region was broken into fault blocks and the blocks were uplifted and tilted, the back slope of each block was a part of the previously eroded surface and the face of the block was a surface of fracture; the present form of the higher blocks is more or less affected by erosion since faulting, while many of the lower blocks have been buried under the waste of the higher ones. In the north, where dislocations have invaded the field of the horizontal Columbian lavas, as in south-eastern Oregon and north-eastern California, the blocks are monoclinal in structure as well as in attitude; here the amount of dissection is relatively moderate, for some of the fault faces are described as ravined but not yet deeply dissected; hence these dislocations appear to be of recent date. In western Utah and through most of Nevada many of the blocks exhibit deformed structures, involving folds and faults of relatively ancient (Jurassic) date; so ancient that the mountains then formed by the folding were worn down to the lowland stage of old age before the block-faulting occurred. When this old-mountain lowland was broken into blocks and the blocks were tilted, their attitude, but not their structure, was monoclinal; and in this new attitude they have been so maturely re-dissected in the new cycle of erosion upon which they have now entered as to have gained elaborately carved forms in which the initial form of the uplifted blocks can hardly be perceived; yet at least some of them still retain along one side the highly significant feature of a relatively simple base-line, transecting hard and soft structures alike, and thus indicating the faulted margin of a tilted block. Here the less uplifted blocks are now heavily aggraded with waste from the dissected ranges: the waste takes the form of huge alluvial fans, formed chiefly by occasional boulder-bearing floods from the mountains; each fan heads in a ravine at the mountain base, and becomes laterally confluent with adjacent fans as it stretches several miles forward, with decreasing slope and increasing fineness of material.
In the southern part of the Basin Range province the ranges are well dissected and some of the intermont depressions have rock floors with gentle, centripetal slopes; hence it is suggested that the time since the last dislocation in this part of the province is relatively remote; that erosion in the current cycle has here advanced much farther than in the central or northern parts of the province; and that, either by outwash to the sea or by exportation of wind-borne dust, the depressions—perhaps aggraded for a time in the earlier stages of the cycle—have now been so deeply worn down as to degrade the lower and weaker parts of the tilted blocks to an evenly sloping surface, leaving the higher and harder parts still in relief as residual ranges. If this be true, the southern district will furnish a good illustration of an advanced stage of the cycle of arid erosion, in which the exportation of waste from enclosed depressions by the wind has played an important part. In such case the washing of the centripetal slopes of the depressions by occasional “sheet-floods” (widespreading sheets of turbid running water, supplied by heavy short-lived rains) has been efficient in keeping the rock floor at even grade toward a central basin, where the finest waste is collected while waiting to be removed by the winds.
Only a small part of the Basin Range province is drained to the sea. A few intermont areas in the north-west part of the province have outlet westward by Klamath river through the Cascade range and by Pitt river (upper part of the Sacramento) through the Sierra Nevada: a few basins in the south-east have outlet by the Rio Grande to the Gulf of Mexico; a much larger but still narrow medial area is drained south-westward by the Colorado to the head of the Gulf of California, where this large and very turbid river has formed an extensive delta, north of which the former head of the gulf is now cut off from the sea and laid bare by evaporation as a plain below sea-level. It is here that an irrigation project, involving the diversion of some of the river water to the low plain, led to disaster in 1904, when the flooded river washed away the canal gates at the intake and overflowed the plain, drowning the newly established farms, compelling a railway to shift its track, and forming a lake (Salton Sea) which would require years of evaporation to remove (see Colorado River). Many streams descend from the ravines only to wither away on the desert basin floors before uniting in a trunk river along the axis of a depression; others succeed in uniting in the winter season, when evaporation is much reduced, and then their trunk flows for a few score miles, only to disappear by “sinking” (evaporating) farther on. A few of the large streams may, when in flood, spread out in a temporary shallow sheet on a dead level of clay, or playa, in a basin centre, but the sheet of water vanishes in the warm season and the stream shrinks far up its course, the absolutely barren clay floor of the playa, impassable when wet, becomes firm enough for crossing when dry. One of the south-western basins, with its floor below sea-level, has a plain of salt in its centre. A few of the basins are occupied by lakes without outlet, of which Great Salt Lake (q.v.), in north-west Utah, is the largest. Several smaller lakes occur in the basins of western Nevada, next east of the Sierra Nevada. During Pleistocene times all these lacustrine basins were occupied by lakes of much greater depth and larger size; the outlines of the eastern (Lake Bonneville) and the western (Lake Lahontan) water bodies are well recorded by shore lines and deltas on the enclosing slopes, hundreds of feet above the present lake surfaces; the abandoned shore lines, as studied by G. K. Gilbert and I. C. Russell, have yielded evidence of past climatic changes second in importance only to those of the Pleistocene glaciated areas. The duration of the Pleistocene lakes was, however, brief as compared with the time since the dislocation of the faulted blocks, as is shown by the small dimensions of the lacustrine beaches compared to the great volume of the ravine-heading fans on which the beaches often lie.
Strong mountain ranges follow the trend of the Pacific coast, 150 or 200 m. inland. The Cascade Range enters from Canada, trending The Pacific Ranges. southward across the international boundary through Washington and Oregon to latitude 41°; the Sierra Nevada extends thence south-eastward through California to latitude 55°. The lower coast ranges, nearer the ocean, continue a little farther southward than the Sierra Nevada, before giving way to that part of the Basin Range province which reaches the Pacific in southernmost California.
The Cascade Range is in essence a maturely dissected highland, composed in part of upwarped Columbian lavas, in part of older rocks, and crowned with several dissected volcanoes, of which the chief are (beginning in the north) Mts Baker (10,827 ft.), Rainier (14,363 ft.), Adams (12,470 ft.) and Hood (11,225 ft.); the first three in Washington, the last in northern Oregon. These bear snowfields and glaciers; while the dissected highlands, with ridges of very irregular arrangement, are everywhere sculptured in a fashion that strongly suggests the work of numerous local Pleistocene glaciers as an important supplement to preglacial erosion. Lake Chelan, long and narrow, deep set between spurless ridges with hanging lateral valleys, and evidently of glacial origin, ornaments one of the eastern valleys. The range is squarely transected by the Columbia river, which bears every appearance of antecedent origin: the cascades in the river gorge are caused by a sub-recent landslide of great size from the mountain walls. Klamath river, draining several lakes in the north-west part of the Basin Range province and traversing the Cascade Range to the Pacific, is apparently also an antecedent river.
The Cascade Mountains present a marked example of the effect of relief and aspect on rainfall; they rise across the path of the prevailing westerly winds not far inland from a great ocean; hence they receive an abundant rainfall (80 in. or more, annually) on the westward or windward slope, and there they are heavily forested; but the rainfall is light on the eastward slope and the piedmont district is dry; hence the forests thin out on that side of the range and treeless lava plains follow next eastward.
The Sierra Nevada may be described, in a very general way, as a great mountain block, largely composed of granite and deformed metamorphosed rocks, reduced to moderate relief in an earlier (Cretaceous and Tertiary?) cycle of erosion, sub-recently elevated with a slant to the west, and in this position sub-maturely dissected. The region was by no means a peneplain before its slanting uplift; its surface then was hilly and in the south mountainous; in its central and still more in its northern part it was overspread with lavas which flowed westward along the broad open valleys from many vents in the eastern part: near the northern end of the range, eruptions have continued in the present cycle, forming many cones and young lava flows. The tilting of the mountain mass was presumably not a simple or a single movement; it was probably slow, for Pitt river (headwaters of the Sacramento) traverses the northern part of the range in antecedent fashion; the tilting involved the subdivision of the great block into smaller ones, in the northern half of the range at least; Lake Tahoe (altitude 6225 ft.) near the range crest is explained as occupying a depression between two block fragments; and farther north similar depressions now appear as aggraded highland “meadows.” The tilting of the great block resulted in presenting a strong slope to the east, facing the deserts of the Basin Range province and in large measure determining their aridity; and a long moderate slope to the west. The altitudes along the upraised edge of the block, or range crest, are approximately 5000 ft. in the north and 11,000 ft. in the south. The mountains in the southern part of the block, which had been reduced to subdued forms in the former cycle of erosion, were thus given a conspicuous height, forming the “High Sierra,” and greatly sharpened by revived erosion, normal and glacial. In this way Mt Whitney (14,502 ft.) came to be the highest summit in the United States (excluding Alaska). The displacement of the mountain block may still be in progress, for severe earthquakes have happened in the depression next east of the range; that of Owen's Valley in 1870 was strong enough to have been very destructive had there been anything in the desert valley to destroy. In the new altitude of the mountain mass, its steep eastern face has been deeply carved with short canyons; and on the western slope an excellent beginning of dissection has been made in the erosion of many narrow valleys, whose greatest depth lies between their headwaters which still flow on the highland surface, and their mouths at the low western base of the range. The highlands and uplands between the chief valleys are but moderately dissected; many small side streams still flow on the highland, and descend by steeply incised gorges to the valleys of the larger rivers. Some of the chief valleys are not cut in the floors of the old valleys of the former cycle, because the rivers were displaced from their former courses by lava flows, which now stand up as table mountains. Glacial erosion has been potent in excavating great cirques and small rock-basins, especially among the higher southern surmounting summits, many of which have been thus somewhat reduced in height while gaining an Alpine sharpness of form; some of the short and steep canyons in the eastern slope have been converted into typical glacial troughs, and huge moraines have been laid on the desert floor below them. Some of the western valleys have also in part of their length been converted into U-shaped troughs; the famous Yosemite Valley, eroded in massive granite, with side cliffs 1000 or 2000 ft. in height, and the smaller Hetch-Hetchy Valley not far away, are regarded by some observers as owing their peculiar forms to glacial modifications of normal preglacial valleys.
The western slope of the Sierra Nevada bears fine forests similar to those of the Cascade Range and of the Coast Range, but of more open growth, and with the redwood exchanged for groves of “big trees” (Sequoia gigantea) of which the tallest examples reach 325 ft. The higher summits in the south are above the tree line and expose great areas of bare rock: mountaineering is here a delightful summer recreation, with camps in the highland forests and ascents to the lofty peaks. Gold occurs in quartz veins traversing various formations (some as young as Jurassic), and also in gravels, which were for the most part deposited previous to the uplift of the Sierra “block.” Some of the gravels then occurred as piedmont deposits along the western border of the old mountains; these gravels are now more or less dissected by new-cut valleys. Other auriferous gravels are buried under the upland lava flows, and are now reached by tunnels driven in beneath the rim of the table mountains. The reputed discovery of traces of early man in the lava-covered gravels has not been authenticated.
The northernmost part of the coast ranges, in Washington, is often given independent rank as the Olympic Range (Mt Olympus, 8150 ft.); it is a picturesque mountain group, bearing snowfields and glaciers, and suggestive of the dome-like uplift of a previously worn-down mass; but it is now so maturely dissected as to make the suggested origin uncertain. Farther south, through Oregon and northern California, many members of the coast ranges resemble the Cascades and the Sierra in offering well-attested examples of the uplift of masses of disordered structure, that had been reduced to a tame surface by the erosion of an earlier cycle, and that are now again more or less dissected.
Several of the ranges ascend abruptly from the sea; their base is cut back in high cliffs; the Sierra Santa Lucia, south of San Francisco, is a range of this kind; its seaward slope is almost uninhabitable. Elsewhere moderate re-entrants between the ranges have a continuous beach, concave seaward; such re-entrants afford imperfect harbourage for vessels; Monterey Bay is the most pronounced example of this kind. On still other parts of the coast a recent small elevator movement has exposed part of the former sea bottom in a narrow coastal plain, of which some typical harbour less examples are found in Oregon. Most of the recent movements appear to have been upward, for the coast presents few embayments such as would result from the depression and partial submergence of a dissected mountain range; but three important exceptions must be made to this rule.
In the north, the Strait of Juan de Fuca and the intricately branching waterways of Puget Sound between the Cascade and the Olympic ranges occupy trough-like depressions which were filled by extensive glaciers in Pleistocene times; and thus mark the beginning of the great stretch of fiorded coast which extends northward to Alaska. The waterways here afford excellent harbours. The second important embayment is the estuary of the Columbia river; but the occurrence of shoals at the mouth decreases the use that might otherwise be made of the river by ocean-going vessels. More important is San Francisco Bay, situated about midway on the Pacific coast of the United States, the result of a moderate depression whereby a transverse valley, formerly followed by Sacramento river through the outermost of the Coast ranges, has been converted into a narrow strait—the “Golden Gate”—and a wider intermont longitudinal valley has been flooded, forming the expansion of the inner bay.
The Coast Range is heavily forested in the north, where rainfall is abundant in all seasons; but its lower ranges and valleys have a scanty tree growth in the south, where the rainfall is very light: here grow redwoods (Sequoia sempervirens) and live oaks (Quercus agrifolia). The chief metalliferous deposits of the range are of mercury at New Almaden, not far south of San Francisco. The open valleys between the spaced ranges offer many tempting sites for settlement, but in the south irrigation is needed for cultivation.
The belt of relative depression between the inner Pacific ranges and the Coast range is divided by the fine volcano Mt Shasta (14,380 ft.) in northern California into unlike portions. To the north, the floor of the depression is for the most part above base level, and hence is dissected by open valleys, partly longitudinal, partly transverse, among hills of moderate relief. This district was originally for the most part forested, but is now coming to be cleared and farmed.
South of Mt Shasta, the “Valley of California” is an admirable example of an aggraded intermont depression, about 400 m. long and from 30 to 70 m. wide. The floor of this depression being below baselevel, it has necessarily come to be the seat of the mountain waste brought down by the many streams from the newly uplifted Sierra Nevada on the east and the coast ranges on the west; each stream forms an alluvial fan of very gentle slope; the fans all become laterally confluent, and incline very gently forward to meet in a nearly level axial belt, where the trunk rivers-the Sacramento from the north and the San Joaquin from the south-east-wander in braided courses; their tendency to aggravation having been increased in the last half century by the gravels from gold washing; their waters entering San Francisco Bay. Kings river, rising in the high southern Siena near Mt Whitney, has built its fan rather actively, and obstructed the discharge from the part of the valley next farther south, which has thus come to be overflowed by the shallow waters of Tulare Lake, of fiat, reedy, uncertain borders. A little north of the centre of the valley rise the Marysville Buttes, the remains of a maturely dissected volcano (2128 ft.). Elsewhere the floor of the valley is a featureless, treeless plain. (W. M. D.)