LONGSHORE-BARS AND LONGSHORE-TROUGHS.

The submerged longshore-bars and longshore-troughs which skirt the shores off most sandy beaches are described. The troughs which lie landward of the bars are explained as the result of plunging breakers and the longshore currents which are feeders to rip currents. The bars are thought to be partly the result of the excavation of the troughs and partly due to landward migration of sand outside the breakers and seaward migration from the troughs. The depths of the bars and troughs are shown to be related to wave and breaker heights. The elimination of some bars is seen to be the effect of a long continued period of small waves during which the bar moves landward filling the trough. In many areas the deeper bars persist undisturbed through long periods of quiet seas. The analysis of thousands of profiles, mostly taken along California open ocean piers, is the chief basis for the preceding conclusions.