Changes in the Coast-Line due to Oscillation of the Land.—Land areas are exposed to the erosive action of wind, rain, streams, etc., and are sculptured by these agencies into valleys, cañons, peaks, ridges, and other familiar topographic forms. The various processes by which land areas are modified lead in general to a roughening of the surface. As an extreme illustration, a high plateau becomes dissected by streams so as to form an intricate system of rugged mountain ridges and peaks, with deep, steep-sided valleys between. The degree of this roughening depends principally on the elevation of the land, together with contrasts in the resistance of the rocks due mainly to variation in hardness, climatic conditions, etc., but in general
one may say the higher the land is raised above the sea the more rugged will be its topography as the process of wearing away progresses. It is to be remembered in this connection, however, that land areas pass through a somewhat definite series of changes, from topographic youth to topographic old age, each stage being accompanied by changes in the relief. It is during topographic maturity that the greatest roughness of the surface of a land area is produced.
Land areas are continually wasting away, owing especially to the attacks of the streams, and the material removed is deposited in the sea. The débris brought from the continents by streams is laid down in shallow water—about the shores of North America almost entirely on the surface of the continental shelf—and in this region of deposition the hollows are filled and a generally smooth surface given to the sea-floor.
The topography of the land, for the reason stated above, is nearly everywhere uneven; while the topography of the sea-floor is characterized by uniformity. We can easily predict, therefore, the general character of the changes in a coast which would result from either a subsidence of the land, thus allowing the sea to encroach upon it, or of an elevation, which would expose a portion of the sea-bottom, thereby increasing the area of the land. A subsidence of the land adjacent to the sea permits an extension of the waters landward; the sea will enter the valleys so as to form estuaries, bays, straits, etc., while the high land between the partially water-filled depressions will rise above the water-level and appear as peninsulas, capes, and islands. A bold, deeply sculptured coast when depressed will give origin to an intricate, and what may be termed a ragged shore-line; while a lower region crossed by large river-valleys would be changed to a system of broad estuaries.
An upward movement in the earth's crust along the ocean's shore would expose a portion of the sea-floor and add a strip of generally level country to the previous land area. The boundary between the old and new topography in such an instance would be the upraised coast-line with
its sea-cliffs, wave-cut caves, terraces, beaches, and other characteristic features of coast topography.
There are thus two strongly contrasted types of coast scenery, produced by oscillations of the earth's crust where ocean and continents meet. In each class there is a wide range in details, which vary in harmony with the amount the land rises or falls in reference to sea-level.
When one has these general laws in mind a map of the coast-line of North America acquires great significance.
From about the latitude of New York southward to Central America many comparatively small oscillations of the land have occurred in recent geological time, and what was formerly a portion of the continental shelf is now exposed and forms a coastal plain. This plain, in general from 50 to 100 miles broad, slopes gently seaward, and its continuation under the sea forms the present continental shelf (Fig. 2). Evidently a slight up or down movement or a gentle tilting of this partially submerged plain in an east and west direction would cause a marked advance or recession of the sea. Each time the sea advanced the country submerged would be smoothed over by the action of the waves and currents and a sheet of sediment laid down upon it; and each time the sea receded the emerged land would be trenched by the rivers flowing across it. The records show that many such changes have occurred.
The Gulf border of Mexico and Texas, composed of soft marine sediments, forms a gently sloping plain bordered on the west by a roughened upland, and illustrates the general feature of a recently emerged coastal plain (Fig. 9). The same is true also of the entire coast from Texas to New York, but it happens that a recent movement through this region was of such a nature as to allow the sea to encroach on the land, and the previously excavated stream valleys are now, in part, occupied by the sea. This feature is most marked from the Carolinas to New York (Fig. 6), where there are several great estuaries and drowned river-valleys which extend far into the land. The best examples are Albemarle Sound and Chesapeake and Delaware Bays. The James River channel is submerged as far as Richmond,