To gain some conception of the importance of wave cutting as an eroding process, we may consider the late history of Heligoland, a sandstone island off the mouth of the Elbe in the North Sea ([Fig. 256]). From a periphery of 120 miles, which it possessed in the ninth century of the Christian era, the island has reduced its outline to 45 miles in the fourteenth century, 8 miles in the seventeenth, and to about 3 miles at the beginning of the twentieth century. The German government, which recently acquired this little remnant from England, has expended large sums of money in an effort to save this last relic.
Fig. 257.—Cut and built terrace with bowlder pavement shaped by waves on a steep shore formed of loose materials.
The cut and built terrace on a steep shore of loose materials.—In materials which lack the coherence of firm rock, no vertical cliff can form; for as fast as undermined by the waves the loose materials slide down and assume a surface of practically constant slope—the “angle of repose” of the materials ([Fig. 257]). The terrace below this sloping cliff will not differ in shape from that cut upon a rocky shore; but whenever the materials of the shore include disseminated blocks too large for the waves to handle, they collect upon the terrace near where they have been exhumed, thus forming what has been called a “bowlder pavement” ([Fig. 258]).
Fig. 258.—Sloping cliff and terrace with bowlder pavement exposed at low tide upon the shore at Scituate, Massachusetts.
The edge of the cut and built terrace is, as already mentioned, maintained at the depth of wave base. If one will study the submerged contours of any of our inland lakes, it will be found that these basins are surrounded by a gently sloping marginal shelf,—the cut and built terrace,—and that the depth of this shelf at its outer edge is proportioned to the size of the lake. Upon Lake Mendota at Madison, Wisconsin, the large storm waves have a length of about twenty feet, which is the depth of the outer edge of the shore terraces ([Fig. 267], [p. 242]). The shelf surrounding the continents has, with few local exceptions, a uniform depth of 100 fathoms, or about the wave base of the heaviest storm waves.
The work of the shore current.—In describing the formation of the built terrace, it was stated that the greater part of the rock material quarried upon headlands by the waves is diverted from the offshore terrace. This diversion is the work of the shore current produced by the wave.
