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.
[Illustration: FIG. 259.—Map to show the nature of the shore current and the forms which are molded by it.]
At but few places upon a shore will the storm waves beat perpendicularly, and then for but short periods only. The broken wave, as it crawls ever more slowly up the beach, carries the sand with it in a sweeping curve, and by the time gravity has put a stop to its forward movement, it is directed for a brief instant parallel to the shore. Soon, however, the pull of gravity upon it has started the backward journey in a more direct course down the slope of the terrace; and here encountering the next succeeding breaker, a portion of the water and the coarser sand particles with it are again carried forward for a repetition of the zigzag journey. This many times interrupted movement of the sand particles may be observed during a high wind upon any sandy lee shore. The “set” of the water along the shore as a result of its zigzag journeyings is described as the _shore current_ (Fig. 259), and the effect upon sand distribution is the same as though a steady current moved parallel to the shore in the direction of the average trend of the moving particles.