308 GEOLOGY 



attempt to reach it. Their immediate goal is maturity of config- 

 uration; their final goal is the destruction of the land and the 

 deposition of its substance in the sea. 



Transportation. The material eroded from the shore by the 

 waves is transported by the joint action of waves, undertow, and 

 shore-currents. 



The incoming wave begins to shift material where it begins to 

 drag bottom. From the line where transportation begins, to the 

 line of breakers, detritus at the bottom is shifted toward the shore 

 by the waves, while the undertow tends to carry it back again. 

 The result of these opposed tendencies is to keep sediment moving 

 back and forth between the shore and the line of breakers. The 

 advance and retreat of waves which come in at right angles to the 

 shore do not move sediment along the shore; but oblique waves 

 and littoral currents shift it along shore. The direction of move- 

 ment is readily inferred from Fig. 244. The direction in which 

 debris is shifted by waves 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 to- 

 gether in assorting the detritus of the shore. If the coarsest parts 

 are beyond the power of all but the strongest waves, they accu- 

 mulate where agitation is great. Less coarse parts are carried 

 farther from the site of greatest agitation, but no materials which 

 are classed as coarse are carried beyond the depth of sensible move- 

 ment. The coarse material which covers the bottom where the 

 agitation of the water at the bottom is effective, constitutes shore 

 drift, whether derived from the shore by the waves, or brought to 

 the sea by streams. 



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. 



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

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



