THE OCEAN AND ITS WORK 



209 



coves and headlands may also result. Coves are not cut back 

 indefinitely, since after a time the headlands protect them from 

 the full force of the waves and equilibrium is established. When 

 this condition is attained, the headlands and coves are worn back 

 at an equal rate. It is evident from the above that wave action 

 is able to develop small irregularities of coast line, but not great 

 ones. 



Sea Caves and Blowholes. — Caves (Fig. 195) are often developed 

 on rocky shores where the rock is strong enough to form a roof. If 

 the rock is weak, chasms 

 develop. Such chasms or 

 gullies sometimes extend 

 across narrow headlands, 

 converting the outermost 

 parts into islands. Caves 

 occur at the bases of cliffs 

 and are formed in one of 

 several ways, or by a com- 

 bination of them : (1) by the 

 beating of the waves, espe- 

 cially if the water near the 

 shore is neither too deep nor 

 too shallow and if there is a 

 supply of debris which can 

 be used in the work of ex- 

 cavation ; (2) by quarrying 

 along joints. (3) Since the 

 level of underground water 

 near the coast is sea level, 

 solution caves are not un- 

 common at bases of cliffs in 

 limestone strata. Such caves 

 are often enlarged by the 

 waves. (4) If a weak bed of 

 horizontal rock is at sea level and is subjected to the attack of the 

 waves, it affords especially favorable conditions for excavation by 

 waves. In the development of caves hydrostatic pressure and the 

 compression and expansion of the air are important forces. Fingal's 

 Cave has been thus quarried out of the lava of the south shore of the 

 island of Staffa. It extends inland 200 feet, the floor being below 



Fig. 195. — Sea cave, Watermouth, England. 

 The sea worked along some fault or plane of weak- 

 ness in the slate. The enlargement of the cave 

 was assisted by the cleavage planes. (E. A. N. 

 Arber, The Coast Scenery of North Devon.) 



