THE WORK OF THE OCEAN. 



347 



land were merely planed away to the level of the wave-troughs, the 

 incoming waves would break where shoal water was first reached, and 

 become ineffective at the water margin. The rate of erosion by the 

 undertow becomes less and less as the surface it affects is lowered. 

 Littoral currents do little erosive work beyond that inflicted on the 

 material which they transport. 



The general result of wave-erosion is the advance of the sea on the 

 land, the rate of advance being determined chiefly by the nature of the 



.'■^*^^^^^m^-vL'' 



Fig. 305.— Steep cliff developed by waves. Allen Point, Grand Island, 

 ' Lake Champlain. (Perry.) 



material attacked and the strength of the waves. Numerous as examples 

 are of the retreat of coast -lines before the advance of the sea, it is not to 

 be understood that the advance of the sea on the land is universal or 

 uninterrupted. Numerous instances may be cited of the encroach- 

 ment of the land on the sea. At Long Branch the advance of the sea, in 

 spite of elaborate breakwaters, has been so rapid in recent years as to 

 menace important buildings, while a few miles to the north and south, 



