Extensive transportation of shore drift of a given degree of coarseness is favored by (1) strong waves and undertow, (2) continuous currents, and (3) shallow water, deepening but gradually off shore.
Deposition by Waves, Undertow, and Shore-currents.[165]
Fig. 312.—Cross-section of the beach. (Gilbert.)
The beach.—The zone occupied by the shore drift in transit is the beach. The lower margin is beneath the water, a little beyond the line where the great storm-waves break. Its upper margin is at the level reached by storm-waves, and is usually a few feet above the level of still water. To the beach, material is brought from seaward by the in-coming waves, and from it detritus is carried out by the undertow. The cross-section of a beach is shown in [Fig. 312]. In horizontal position the beach follows the general boundary between water and land, though it does not conform to its minor irregularities ([Fig. 313]). The beach or barrier ridge often causes the deflection of the lower courses of streams descending to it ([Pl. XXI]).
Fig. 313.—A lake-beach (barrier); Griffin’s Bay, Lake Ontario.
PLATE XXI.
U. S. Geol. Surv.