206 



PHYSICAL GEOLOGY 



Fig. 188. — Chalk cliffs on the coast of France. The waves have cut back the 

 cliffs so rapidly that the streams enter the sea from hanging valleys. 



keep pace with the wearing back of the cliffs and consequently fall 

 over them from hanging valleys (Fig. 188). The wear on granite 

 cliffs, on the other hand, is often so slight that the battering of the 

 waves for a century is scarcely perceptible. Along the coast of Marble- 

 head, Massachusetts, granite, well within reach of the waves, still 



bears glacial striae, 

 showing that thou- 

 sands of years of 

 wave wear have not 

 been effective on this 

 hard rock. Since 

 low-lying, sandy 

 shores are apt to lie in 

 places where sand is 

 accumulating, they 

 usually suffer less 

 than rocky and pre- 

 cipitous shores. On 

 such a coast, how- 

 ever, a slight change 

 in the currents such 

 as that due to un- 

 usual or prolonged 

 storms may cause the shores to be cut away rapidly, as has been true 

 of Coney Island, New York, and along the New Jersey coast, where 

 the former sites of houses and hotels are now covered by the sea. 



Fig. 189. 



Undercutting of massive granite by 

 wave action. 



