346 



GEOLOGY. 



way. In general, the effect of its presence in keeping down waves 

 overbalances 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 (p. 341). The efficient 

 impact of waves is limited upward by the level of the wave-crests, 

 although the dash of the water produces feebler blows at higher levels. 



Fig. 303. — Diagram illustrating high sea-cliffs. It also shows a submerged terrace, 

 due partly to wave-cutting (wave-cut terrace), and partly to building (wave-built 

 terrace). (Gilbert.) 



Fig. 304. — Diagram showing a low sea-cliff. (Gilbert.) 



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 vertical zone of direct wave-work is therefore limited above by 

 the level of wave-crests, and below by the depth of wave-troughs (nearly). 

 The indirect work of waves is limited only by the height of the shore, 

 for as the zone of excavation is carried landward, masses higher up the 

 slope are undermined and fall. The fallen rock temporarily protects the 

 shore against the waves, but are themselves eventually broken up. 



The pulsating current of the undertow (p. 341) has both an erosive 

 and a transporting function. It carries the detritus of the shore to and 

 fro, and dragging it over the bottom, continues do^vnward the erosion 

 initiated by the breakers. This downward erosion is the necessary 

 concomitant of the shoreward progress of wave-erosion; for, if the 



