=Wave-built terraces.=—Under the influence of off-shore currents, littoral currents may be drawn from the coast-line. If such a current continues as a well-defined surface-current, it builds a spit, but if it spreads, it tends to build a terrace. The accumulation then is not at the end of a beach, as in the case of a spit, but on its side, and the result of the deposition is to carry the beach seaward. The undertow abets the process. The widened beach is a _wave-built terrace_. The wave-built terrace often borders the wave-cut terrace along its seaward margin (Figs. 303 and 318). With the help of waves, the surface of the terrace may be built up into land by the expansion of the crest of the beach. Terrace-cutting and terrace-building are both involved in the development of the continental shelves. Beach ridges, spits, bars, etc., like sea-cliffs and wave-cut terraces, are often preserved after the relative level of sea and land has changed. If the shore has risen, relatively or absolutely, these features are relied on as evidences of the change. If shore features be submerged instead of elevated, they furnish less accessible, though not less real, evidence of the change of level. Similar features about lakes have a like significance, but in this case it is often demonstrable that it is the water rather than the land which has changed its level. _Effect of Shore-deposition on Coastal Configuration._ The tendency of shore-deposition is to cut off bays and to straighten and simplify the shore-lines. This is abundantly illustrated along the Atlantic and Gulf coasts of the United States (see Fig. 319 and Pl. XXII). It is to be noted, however, that in the simplification of the shore-line through deposition, the initial stages often result in great irregularity (Fig. 320 and Pl. XXIII). In some cases, the irregularities are not temporary. Thus deltas (p. 198), though not wholly the work of sea- (or lake-) water, often constitute irregularities of a more or less permanent nature. This is the case where they project beyond the general trend of the coast-line. Where, on the other hand, they are built at the heads of bays, they tend to simplify the coast-line by obliterating the indentation. The delta at the head of the Gulf of California is an example. So too is the delta of the Mississippi, the real head of which is far above the present debouchure of the stream. The form of the delta in ground-plan depends on the horizontal configuration of the coast where it is developed, on the strength of the waves and shore-currents, and on their relation to the amount of detritus contributed by the stream concerned. Good illustrations are furnished by the Gulf of Mexico where the deltas of the Mississippi and Rio Grande are in contrast.