Page:EB1911 - Volume 27.djvu/636

PHYSICAL GEOGRAPHY] 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

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,