i 7 8 



WORK OF THE OCEAN 



exert a pressure of three tons per square foot are not infrequent. 

 Exceptional storm-waves have moved blocks of rock exceeding 100 

 tons in weight. Waves are most efficient on bold coasts bordered 

 by broad expanses of deep water, for here their force is expended 

 almost wholly near the water line; where shallow water borders the 

 land, the force of the waves is expended over a greater area. 



The direct effect of wave-erosion is restricted to a zone which 

 is narrow both horizontally and vertically. There is no impact 

 of breakers at levels lower than the troughs of the waves, though 

 erosion may extend down to the limit of effective agitation. The 

 upper limit of effective wave-action is the level of the wave-crests. 

 The rise and fall of the water during the flow and ebb of the tides 

 gives the waves a greater vertical range than wind-waves alone 

 would have. The indirect work of waves is limited only by the 

 height of the shore, for as the zone of excavation is carried land- 

 ward, masses higher up the slope are undermined and fall. The 

 fallen rock protects the shore against the waves temporarily (Fig. 

 179), but the fallen masses are themselves broken up eventually. 



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 material attacked and the strength of the waves. Though 

 examples of the retreat of coast-lines before the advance of the 



Fig. 181 Fig. 182 



Fig. 181. High sea cliffs, and a submerged terrace, due partly to wave-cutting 

 and partly to building. 



Fig. 182. A low sea cliff. 



sea are numerous, the advance is not universal or uninterrupted. 

 On the contrary, the land encroaches on the sea in some places, and 

 the two things may go on side by side. At Long Branch, N. J., 

 advance of the sea has been so rapid in recent times as to menace 

 important buildings, while a few miles to north and south, land is 

 advancing into the sea by the deposition of shore drift. The low 

 coast of the Middle Netherlands has retreated two miles or more in 

 historic times, but the land has advanced at other points in the 



