OCEANS AND LAKES 



245 



of the sea. Extensive peneplains have been developed re- 

 peatedly in the past, but, so far as known, wave-cut submarine 



FIG. 262. Sea caves on the southern coast of California. (Fairbanks, 



U.S. Geol. Suru.) 



When the upper cave was cut it stood in the same relation to sea level 

 that the lower one now does. Since it was formed the land has therefore 

 been elevated with reference to the level of the ocean. 



plains of great extent have not been formed. This is because, 

 as already indicated, waves drag bottom across the sub- 

 marine flat which they cut, and so become weaker as the flat 

 becomes wider. The 

 gradual subsidence of a 

 coast and marginal sea 

 bottom aids in the ex- 

 tension of a wave-cut 

 plain by gradually in- 

 creasing the depth of the 

 water upon it, and so 

 maintaining the vigor of 

 the waves at the shore. 

 Gradual emergence, on 

 the other hand, opposes 

 the formation of an exten- 



FIG. 263. Stacks on the west coast 

 of France. 



B- * E. GEOL. 



14 



