One important geological significance of the deep sea deposits is the proof which they furnish that, from at least as far back as the beginning of the Paleozoic era, fully twenty-five million years ago, to the present time, the two great deep ocean basins—the Atlantic and the Pacific—have maintained essentially the same positions on the earth. This is proved by the fact that nowhere, on any continent among the rocks of all ages, as old at least as the early Paleozoic, do we find any really typical deep-sea deposits. There is then no evidence that a deep sea ever spread over any considerable part of any continent, and this in spite of the fact that marine deposits of shallow water origin furnish abundant evidence of former sea extensions. The shallow seas have at various times spread over large portions of the continents.
On many rocky coasts the waves are incessantly pounding and wearing away the rocks. In such places the sea, like a mighty horizontal saw, is cutting into the borders of the lands. The finer materials produced by the grinding up of the rocks are carried seaward by the undertow. But, if the land remains stationary with reference to the sea, this landward cutting by the waves reaches a limit. Since even big waves have very little effect in water 100 or 200 feet deep, a shelf is cut by the waves and this shelf, not many miles wide, is covered by shallow water. The finer ground-up rock materials carried out by the undertow are dumped just beyond the edge of the shelf which is thus built out seaward as a terrace. In traveling over this shelf and terrace, the waves, due to friction, lose their power. With gradually sinking land, a much wider shelf may be cut, because the power of the waves is then allowed to continue.
It might be of interest to cite a few cases of relatively rapid coast destruction by the waves which have come under human observation. A remarkable example is the island of Heligoland on which is (or was) located the powerful German fort which guards the entrance to the Kiel Canal. In the year 800 A. D. this island had 120 miles of shore line; in 1300 it had 45 miles of shore; in 1649 only 8 miles; and in 1900 but 3 miles of shore line remained. In southeastern England “whole farms and villages have been washed away in the last few centuries, the sea cliffs retreating from 7 to 15 feet a year.” A church located a mile from the sea shore near the mouth of the Thames river, in the sixteenth century, now stands on a cliff overlooking the sea. An island in Chesapeake Bay covered over 400 acres in 1848, and the waves have since reduced it to about fifty acres. Study showed that the relatively soft unconsolidated strata of the Nashaquitsa Cliffs on the island of Martha’s Vineyard, were cut back at the rate of 51/2 feet per year, between 1846 and 1886.
If part of the relatively smooth sea bottom should be raised into land, the resulting shore line would of course, be regular and free from indentations or sharp embayments. Examples of such coast which are very young are at Cape Nome, Alaska; the northern coast of Spain; and the west coast of northern South America. Soon, however, such a shore line is attacked, and, either where the waves are greatest or the rocks are weakest, indentations will result and the whole coast is gradually eaten back until the power of the waves is largely spent in traveling across the shallow water shelf. Sand bars are then built across the mouths of the bays or indentations which later the rivers gradually fill up with sediment. The result is a relatively straight or regular old shore line. The coast of Texas has about reached this stage.
If a portion of the relatively rugged land surface should become submerged under the sea, a very irregular, deeply indented shore line would result, due to the entrance of tidewater into the valleys. The deeply indented coast of Maine is a fine example of a very irregular youthful shore line produced by geologically recent sinking of a rugged, hilly region so that tidewater backs for miles into the lower reaches of the river valleys. The promontories and islands are undergoing rapid wear, and the development of bars across the inlets has scarcely begun. Other excellent examples are the coasts of Norway and southern Alaska. Such a coast is then attacked by the ocean waves and the promontories are cut back until the broad shallow water shelf is formed, after which sand bars are built across the remaining embayments and the shore line becomes relatively regular.
It is, then, a remarkable fact that, whether shore lines originate by emergence of sea bottom, or by sinking of land, there is a very strong tendency on the part of nature to develop regular shore lines. It should be stated that the principles of wave work and shore-form development just outlined apply almost equally well to lakes, especially large ones.
Before leaving this subject of shore-line development, mention should be made of the fact that bars and beaches are often built part way or wholly across embayments of the coast with surprising rapidity. To illustrate, Sandy Hook, New Jersey, is advancing northward, while Rockaway Beach, New York, is extending westward, the tendency being to close up the entrance to New York harbor and to make the line of seashore more nearly regular. Records show that Rockaway Beach actually advanced westward more than three miles between the years 1835 and 1908.