The work of the sea is confined to the coast-line, which it cuts back by the impact of its waves and currents. Speaking broadly, the waves do but little effective work below the limits of low tide, and advance by undermining and cutting down the cliffs which form the coast. The result of the work is to form a platform covered by shallow water, which is called a plain of marine denudation. As observed in actual cases, these platforms are narrow; for so long as the sea-level remains constant with reference to the land, there is a limit to the effective assault of the waves upon the shore. The water covering the platform is very shallow, and only in exceptional cases do the waves have sufficient power to overcome the friction of a wide platform. The materials removed from the land are piled up at the seaward foot of the platform and extend it in that direction.

An example of a plain of marine denudation is found on the north coast of Spain, where there is a broad platform between the mountains and the sea, almost perfectly flat. This plain has been uplifted above the sea-level and has been but little dissected by the subaerial agents. Narrower platforms, still in process of extension, may be observed on most rocky and precipitous coasts, as those of Scotland, Ireland, and France. Along a slowly sinking coast the platforms may be cut back much farther, for the deepening water prevents the loss of wave power by the friction on a shoal bottom. If, on the other hand, the coast rises at intervals, a series of terrace-like platforms will be cut.

As we shall see in the following section, plains may be produced by the work of the subaerial agencies, and it is often important to distinguish between the plains of submarine and those of subaerial origin. This distinction cannot always be made with certainty, but not unfrequently the plain shows unmistakable signs of the manner in which it was made. In the plain of marine denudation the sediments formed from the waste of the land will be deposited upon the seaward portion of the platform, or upon a lower level of previous formation. Further, this sediment will show by its character that it actually was derived from the material cut away by smoothing the plain, and the whole of it, even its bottom layers, will be of marine origin. In such a plain the advancing sea must have obliterated the stream valleys which had been excavated when the region was land. This obliteration will be performed partly by shaving down the divides, or watersheds, between the streams and partly by filling up the valleys with sediment.

When the region is once more uplifted above the level of the sea, an entirely new system of drainage will be established upon it, determined by the slopes of the overlying cover of newly deposited sediments, and having no reference to the structure and arrangement of the underlying older rocks. These newly established streams may, if the upheaval of the country gives them sufficient fall, cut down through the newer sediments. Indeed, the latter may eventually be swept away entirely by the various subaerial agencies, but the stream courses, which were determined originally by the slopes of that newer sediment, will show little or no adjustment to the structure of the underlying older rocks.

These criteria are useful in identifying those plains which were smoothed by the action of the sea; but when the processes of sub-aerial denudation have completely dissected the elevated area, all such evidences may be removed and the origin of the plain may become quite indeterminable.

The Subaerial Agents are those which operate over the entire surface of the land. Their tendency is, in the first instance, to carve out valleys and leave relative eminences standing, and thus to increase the irregularity, or relief, of the land. This, however, is merely a temporary stage, and if time enough be granted, these agencies will sweep away the irregularities and plane the entire region down to base-level.

Rivers cut down and.deepen their channels so long as then-beds have sufficient slope and fall. The banks also are undermined, as the current swings from side to side, and frequently fall, thus widening the channel. The sides of the trench, unless removed by other agencies, will be as steep as the nature of the rock material will allow. Unassisted river action will, therefore, cut nearly vertical trenches, which are continually deepened, until the base-level is reached. Examples of such river-cut trenches are the Au Sable Chasm (see Fig. 58, p. 142) and the inner gorge of the Grand Canon of the Colorado.

The trench-like valley, with nearly vertical sides, is, however, not the usual form of river valley. The atmospheric agencies, the undermining and sapping of springs, landslips, and the like, are continually wearing away the sides of the excavation, the waste thus produced being readily carried away by the stream. As the upper part of each hillside and cliff is that which has been longest exposed to the denuding agencies, the valley will be widened at the top more than at the bottom, and will gradually become widely open, unless the alternation of hard and soft strata be such as to favour the retention of the cliff-like form by undermining. A system of river valleys is normally accordant, the tributaries entering the main stream at grade, and each valley is winding, with projecting spurs from the sides, and of V-shaped cross-section.

The rapidity with which the deep and narrow trench is widened into the broad, gently sloping valley will depend upon two sets of conditions, (1) Upon the climate, which is as much as to say the intensity with which the denuding forces operate. Canons and narrow gorges are much more frequent in arid regions than in those of abundant rainfall. (2) Upon the resistant power of the rocks. If the valley sides are composed of rocks which yield readily to weathering, the trench will be speedily broadened, while if the rocks offer great resistance to chemical and mechanical disintegration, the gorge-like form will be retained very much longer. This is illustrated by almost any considerable stream, such as the Delaware or the Potomac. In certain places the valley is widely open, while in other parts of the course are deep gorges, as at the Delaware Water Gap and Harper's Ferry. The gorges occur in the places where the stream cuts across hard, resistant rocks, and the open valleys are found where it intersects softer and more destructible rocks.

Rivers also produce changes in topography by constructional processes, as in their flood plains and terraces, processes which are most notable in the lower parts of the course, and which gain increased efficiency through a subsidence of the region.

Degradation is most rapid on the hillsides which border river valleys, because of the removal of waste by the rivers. Away from the streams the denudation of the country is much slower, because the waste is less readily removed. Those points will longest remain standing above the general level which are composed of the hardest rocks and are farthest removed from the principal lines of drainage.

A glaciated region has a topography marked by rounded, flowing outlines, with smoothed, polished and striated rocks in the central zone, where erosion was most active, and with lines of moraine, sheets of drift and overwash plains, eskers and drumlins in the peripheral zone, where denudation was feeblest and deposition more important. Glacially excavated valleys are over deepened, U-shaped in section, with the projecting spurs truncated, or entirely removed, and the tributary valleys are not graded to the main trunk, but left hanging on the retreat of the ice. Great numbers of lakes are characteristic of such regions.

The subaerial agencies act with the greater efficiency the more elevated the region upon which they operate. Consequently, so long as the region be not again elevated, denudation operates at a continually diminishing rate. The strong relief of hill and valley is carved out with comparative rapidity, but the more nearly the country is reduced to base-level, the more slowly does degradation proceed,, and the final stages of base-levelling must be exceedingly slow. Nevertheless, if no renewed upheaval takes place, the loftiest and most rugged land surface must be eventually cut down to that level. The universal and permanent base-level is, of course, the sea; but other local and temporary base-levels may for a time control the development of certain areas. Tributaries cannot cut below the main stream into which they flow; a lake forms the base-level for the streams which supply it, until the lake is removed by draining away or being billed with sediment. Regions, like the Great Basin, whose drainage finds no outlet, may have base-levels either above or below the level of the sea; e.g. the surface of the Dead Sea of Palestine is 1308 feet below the Mediterranean.

It is perhaps a question whether any large region has ever remained stationary for a sufficiently long time to be absolutely base-levelled. On the other hand, there is abundant evidence to show that such areas have been worn down to a low-lying, featureless surface, with only occasional low protuberances rising above the general level. Such a surface is called a peneplain, and represents what is usually the final stage of a cycle of denudation. Here and there an isolated peak may remain high enough to deserve the name of mountain, which owes its preservation to the exceptionally resistant nature of the rocks of which it is composed, or to its exceptionally favourable position with reference to the drainage lines. A renewed upheaval of the peneplain will begin another cycle of denudation, revivifying and rejuvenating all the destructive agencies, and valleys and hills will be carved out of the approximately level surface. In a peneplain dissected by the revived streams the sky-line of the ridges is notably even, and all the heights rise to nearly the same level. Differences of level are, however, frequently produced by a warping process, which may accompany the upheaval, raising some portions of the peneplain to greater heights than others.

Excellent examples of reelevated and subsequently dissected peneplains are the uplands of southern New England and the highlands of New Jersey.

Fig. 231. - Peneplain, with residual mountain, Southern California. (Photograph by H. W. Fairbanks).

In topography climatic differences are very obvious, because in each climate the dominant subaerial agents are characteristic, and the modifying effects of vegetation are likewise dependent upon climatic factors. In the polar lands destructive work is accomplished chiefly by the activities of frost and ice, while in temperate lands with normal rainfall rivers and rain are the principal agents. In arid regions changes of temperature and wind are the most active processes, though the scanty vegetation gives to the rare but violent rains an unusual effectiveness. All of these climatic differences are reflected in characteristic topographical forms.