302 



GEOLOGY 



general, the effect of its presence in keeping down waves over- 

 balances its effect as an agent of erosion. 



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 



Fig. 246. Showing blocks similar to those of Fig. 245, reduced and rounded 

 by wave-action. Shore of Lake Champlain. The rock is Utica shale. 

 (Perry.) 



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 temporarily protects the shore against the waves (Fig. 

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



