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 5-1/2 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.