The southern and southeastern borders of Alaska are exceedingly bold, and present some of the most sublime coast scenery to be found in the world, but to the geographer the greatest interest of this portion of the continental border, as is true also of the entire Pacific coast of North America, centres in its relation to up and down movements of the land.
_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.