THE WORK OF THE OCEAN. 355 



shifted along shore by the httoral current, which affects the same 

 water (Fig. 300). By the cooperation of wave- and shore-current, 

 more and heavier material can be moved than by either alone, 

 and the direction of movement is more nearly parallel to the shore 

 than that of the wave. Similarly, by the cooperation of undertow and 

 shore- current, more and heavier material can be moved than by 

 either alone. The direction of movement is readily inferred from Fig. 

 300. The direction in which debris is shifted by wave- and shore-cur- 

 rent is modified by the undertow, and the direction which would result 

 from undertow and current is modified by the wave. It is often the 

 waves of storms, rather than those of the prevailing winds, which deter- 

 mine the direction of greatest shore transportation. 



The waves, the undertow, and the littoral currents work together 

 in assorting the detritus of the shore. The coarsest parts may be beyond 

 the power of all but the strongest waves. They accumulate where 

 agitation is great. Less coarse parts are shifted farther from the site 

 of greatest agitation, but no materials which are classed as coarse are 

 carried beyond the depth of sensible movement. The coarse material 

 which covers the bottom where the agitation of the water at the bot- 

 tom is effective, constitutes shore drift. 



Shore drift is not all derived from the shore by the cutting of the 

 waves. A part of it is brought to the sea by streams and mingled 

 with that eroded from the cliffs. The material which is fine enough to 

 be held in suspension is measurably independent of depth. This is 

 shown during storms when the water becomes turbid far beyond the 

 line of breakers, and clears only after the waves have died away. 



This sorting of shore drift, effected while it is in transportation, 

 is often very perfect. The conditions favoring assortment are (1) vig- 

 orous wave-action, (2) prolonged transportation, and (3) a moderate 

 volimie of sediment.^ The effect of these several conditions mil be 

 readily understood. 



Extensive transportation of shore drift of a given degree of coarse- 

 ness 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? 

 The beach. — The zone occupied by the shore drift in transit is the 



^ Willis. Jour, of Geol., Vol. I, p. 481. 



' See Gilbert. Topographic Features of Lake Shores, 5th Ann. Rept. U. S. Geol. Surv. 



