If the in-coming wave is oblique to the shore, it shifts material in its own direction. The transfer by undertow, taken alone, would be sensibly normal to the shore, but the effect of the oblique waves is to slightly modify this direction. There is thus a slow transportation along shore, even in the absence of steady currents. A great amount of transportation would be effected in this way, though it would be carried on at a slow rate. Oblique waves also tend to develop a definite shore-current (p. 342) which affects both the amount and direction of the transportation. Any particle in suspension, or in motion on the bottom as the result of the wave or undertow, is shifted along shore by the littoral current, which affects the same water (Fig. 300). By the coöperation 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 coöperation 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 débris is shifted by wave- and shore-current 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 determine 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 bottom 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) vigorous wave-action, (2) prolonged transportation, and (3) a moderate volume of sediment.[164] The effect of these several conditions will be readily understood. Extensive transportation of shore drift of a given degree of coarseness 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._[165]