[Illustration: +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). [Illustration: +Fig.+ 313.—A lake-beach (barrier); Griffin’s Bay, Lake Ontario.] [Illustration: PLATE XXI. NEW JERSEY U. S. Geol. Surv.] [Illustration: PLATE XXII. Fig. 1. PORTION OF SOUTH COAST OF MARTHAS VINEYARD, MASSACHUSETTS. U. S. Geol. Surv. Fig. 2. PORTION OF THE CALIFORNIA COAST NEAR TAMALPAIS. U S. Geol. Surv.] [Illustration: +Fig.+ 314.—Section of a barrier. (Gilbert.)] =The barrier.=—When the agitation of the water along shore becomes insufficient to carry the material, it is dropped. In its deposition it assumes various forms. Where the bottom of the lake or sea near shore has a very gentle inclination, the in-coming waves break some distance from the shore-line, and it is here that the most violent agitation occurs when the waves are strong. To this line of breakers, material is shifted from both directions: from shore by undertow, and from seaward by the waves. Accumulating here, it builds up a low ridge. This is a _barrier_ (Fig. 314). If it is built up above the surface of the water by storm-waves, it may shut in a lagoon behind it, and this may ultimately be filled by sediment washed down from the land. At one stage in the filling, the lagoon becomes a marsh.[166] In the part which the barrier plays in the history of a coast, it is identical with the beach. [Illustration: +Fig.+ 315.—A recurved spit. Dutch Point, Grand Traverse Bay, Lake Michigan.] [Illustration: +Fig.+ 316.—Cross-section of a bar. (Gilbert.)]