Fig. 2. PORTION OF THE CALIFORNIA COAST NEAR TAMALPAIS.
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.
Fig. 315.—A recurved spit. Dutch Point, Grand Traverse Bay, Lake Michigan.
Fig. 316.—Cross-section of a bar. (Gilbert.)
The spit, the bar, and the loop.—The disposition of shore-deposits depends largely on the currents at and near shore. If the coast-line is deeply indented, the littoral current usually fails to follow the reëntrants. In holding its course across the mouth of a small bay, a shore-current usually passes into deeper water. Here its velocity is checked because its motion is communicated to the water beneath it, and a larger amount of water being involved in the motion, the motion of each part is diminished. If sediment was being moved along its bottom before the current was checked, some part of it is dropped when and where the current is slackened. It follows that deposition commonly takes place beneath a littoral current as it crosses the mouth of a bay. The belt of deposition is often narrow, and the result is the construction of a ridge beneath the water in the direction of the current. The current would never build the embankment up to the water-level, but when its surface approaches the level of effective agitation, the waves may begin to work on it, as on a barrier, and may build it up to, and even above, the surface of the water. So long as the end of such an embankment is free, it is a spit ([Fig. 315] and [Pl. XXI]). If the spit be lengthened until it crosses, or nearly crosses, the bay, shutting it off from the open water, it becomes a bar. Bars have shut in lakes (ponds) on the coast of Martha’s Vineyard, Mass. ([Fig. 1, Pl. XXII]), and lakes and lagoons at numerous points both on the Atlantic and the Pacific coasts ([Fig. 2, Pl. XXII], Rodeo lagoon). The same phenomena are to be seen along many lake shores. Bars sometimes tie islands to the mainland ([Pl. XXIII, Fig. 1], Nahant, Mass.; [Fig. 2], near Biddeford, Me.). The structure of a bar as seen in cross-section is shown in [Fig. 316].
The construction of a spit has been aptly compared to the construction of a railway embankment across a depression. The material is first carried out from the bordering upland (shallow water) and dumped where the slope to the depression (deep water) begins. The embankment thus begun is extended by the carrying out of new material, which is left at the end of the dump already made.