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.
The sand beach.—The first effect of the shore current is to deposit some portion of the sand within the first slight recess upon the shore in the lee of the cliff. The earlier deposits near the cliff gradually force the shore current farther from the shore and so lay down a sand selvage to the shore, which is shaped in the form of an arc or crescent and known as a beach ([Fig. 259] and [Fig. 260]).
Fig. 260.—Crescent-shaped beach formed in the lee of a headland. Santa Catalina Island, California (after a photograph by Fairbanks).
Fig. 261.—Cross section of a beach pebble.
The shingle beach.—With heavy storms and an exceptional reach of the waves, the shore currents are competent to move, not the sand alone, but pebbles, the area of whose broader surface may be as great as the palm of one’s hand. Such rock fragments are shaped by the continued wear against their neighbors under the restless breakers, until they have a lenticular or watch-shaped form ([Fig. 261]). Such beach pebbles are described as shingle, and they are usually built up into distinct ridges upon the shore, which, under the fury of the high breakers, may be piled several feet above the level of quiet water ([Fig. 262]). Such storm beaches have a gentle forward slope graded by the shore current, but a steep backward slope on the angle of repose. Most storm beaches have been largely shaped by the last great storm, such as comes only at intervals of a number of years.
Fig. 262.—Storm beach of coarse shingle about four feet in height at the base of Burnt Bluff on the northeast shore of Green Bay, Lake Michigan.