Fig. 279.—Uplifted sea cliffs and terraces on the coast of Russell Fjord, Alaska (after Tarr and Martin).
Fig. 280.—Diagrams to show how excessive sinking upon the sea floor will cause the shore to migrate landward as it is uplifted.
Fig. 281.—A drowned river mouth, or estuary upon a coastal plain.
As was noted in our study of earthquakes, the recent instrumental records of distant earthquakes tell us that the movements upon the sea floor are many times larger than those upon the continents, and that while the mountainous coasts are generally rising, the deeps of the sea are sinking. The effect of this over-balance of sinking, or resultant shrinking of the earth’s shell, may be to compress the mountain district and so cause the shore line to move landward at the same time that it moves upward ([Fig. 280]).
The sunk or embayed coast.—When now, upon the other hand, a section of the coast line sinks with reference to the sea, the water invades all the near-shore valleys, thus “drowning” them and yielding the drowned river mouth or estuary. If the relief of the shore was slight, as it generally is upon a coastal plain, slight depression only will produce broad estuaries, such as Chesapeake Bay at the drowned mouth of the Susquehanna ([Fig. 281]).
If, on the other hand, the relief of the shore is strong and the subsidence is large, the entire coast line will be transformed into an archipelago of steep-walled rocky islets which rise abruptly from the sea ([Figs. 282] and [284]). A plateau which is intersected by deep and steep-walled valleys of U-section ([p. 341]) under large submergence yields the fjords so characteristic of Scandinavia or Alaska. A ragged coast line, fringed with islands as a result of submergence, is described as an embayed coast.
