354 GEOLOGY. 



reached. Other forces and processes, such as those of aggradation, 

 vulcanism, and diastrophism, are in operation along coasts, and their 

 results are sometimes antagonistic to those of the waves. The hori- 

 zontal configuration of coasts is, therefore, the result of many cooper- 

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

 to note the goal to which the waves are working, even though they 

 are continually defeated in their attempt to reach it. Their immediate 

 goal is an equihbrium of erosion-rate and maturity of configuration; 

 their final goal is the destruction of the land and the deposition of its 

 substance in the sea, that is, in a position nearer the center of gravity of 

 the earth. 



Transportation hy Waves. 



The material eroded from the shore by the waves in the shaping of 

 the cliff and terrace is carried away by the joint action of the waves, 

 undertow, and shore-currents. 



The in-coming wave begins to shift material where it begins to drag 

 bottom, that is, a little outside the fine of breakers. From the line 

 where transportation begins, to the line of breakers, bottom detritus 

 is shifted shoreward by the waves, while the undertow tends to carry 

 it back again. Between the breakers and the shore there is also a ten- 

 dency for the on-shore movement to carry debris to the water's edge, 

 and for the ebbing wave to carry it back again. The result of these 

 opposed tendencies is to keep sediment in transit between the shore 

 and the line of breakers. If the in-coming waves have a direction nor- 

 mal to the shore, the advance and recoil of the water move particles 

 toward and from the shore, but effect no transfer along the shore; but 

 the results which waves normal to the shore would achieve are always 

 modified by other waves and by littoral currents. 



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. Obhque 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 



