208 



PHYSICAL GEOLOGY 



an overhanging cliff (Figs. 190 B, 193) will be the result of joints inclining inland; a 

 sloping cliff, of joints that incline toward the sea (Fig. 191 B) ; and a vertical cliff, of 

 vertical joints (Fig. 192 B). 



In overhanging cliffs the dismemberment sometimes begins at the 

 top of the cliff, where the agents are not the waves, but the rain, frost, 



etc. In such cases the work 

 of the sea consists largely in 

 keeping the base of the cliff 

 free from talus. The height 

 of a sea cliff depends, to some 

 extent, upon the rapidity 

 of marine erosion, since if 

 weathering is more rapid 

 than the work of the ocean, 

 talus will accumulate at its 

 base and protect the shore. 

 In this connection the im- 

 portance of springs and seep- 

 age from underground water 

 should not be overlooked, 

 for they often assist in under- 

 mining cliffs. Loose material, 

 such as sand or glacial deposits, will not form cliffs unless the erosion 

 is very rapid. 



Coves and Headlands. — The irregularities which result from 

 marine erosion may in general be classed as headlands and crescent- 

 shaped beaches called coves, and are brought about (1) by the un- 

 equal resistance of the rock, the softer being cut away more rapidly 

 than the harder. Such 

 a condition results 

 when vertical or 

 steeply dipping strata, 

 composed of hard and 

 soft beds, lie at right, 

 or at considerable 

 angles to the coast 

 (Fig. 194), and a 

 similar shore line is produced when a rock is much more jointed or 

 fractured in one portion than in another. (2) Where the force 

 of the waves is greater on certain parts of shores than on others, 



Fig. 193. — The effect of marine erosion on 

 strongly jointed beds. Nantucket, Massachu- 

 setts. (Photo. S. Powers.) 



Fig. 194. — Block diagram showing coves formed in 

 weak strata, a harder stratum and a lava dike projecting 

 as headlands. 



