SHORE DEPOSITS 183 



Since the conditions of erosion along coasts are constantly, 

 it" slowly, changing, maturity is constantly being approached, 

 but rarely reached. Other forces and processes, such as those of 

 aggradation, vulcanism, and diastrophism, are in operation along 

 coasts, and their results may antagonize the waves. The horizon- 

 tal configuration of coasts is, therefore, the result of many co- 

 operating forces, of which waves are but one. It is, nevertheless, 

 important to note the end toward which waves are working, 

 t-vi'ii though they are continually defeated in their attempt to reach 

 it. Their immediate goal is maturity of configuration; their final 

 goal is the destruction of the land and the deposition of its sub- 

 stance in the sea. 



Transportation. Material eroded from the shore by waves is 

 transported by the joint action of waves, undertow, and shore- 

 currents. The incoming wave begins to shift material where it 

 bc-gins 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 movements is to keep sediment moving 

 to and from the shore in shallow water. Waves which come in at 

 right angles to the shore, and the undertow resulting, do not move 

 sediment along the shore; but oblique waves and littoral currents 

 do. 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 (Fig. 

 178). Waves of storms, rather than those of prevailing winds, 

 determine the direction of greatest transportation. 



Waves, undertow, and littoral currents work together in assort- 

 ing the detritus of the shore. If the coarsest parts are beyond the 

 power of all but the strongest waves, they accumulate 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 movement. The 

 coarse materials which cover the bottom where agitation of 

 the water at the bottom is effective, and which are shifted about 

 by waves, etc., constitute shore drift. 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 zone of shore drift, and clears only after 

 the waves have died away. 



