OCEANS AND LAKES 243 



rocks capable of standing in vertical faces, commonly slope 

 sharply back toward the land?) The rapid weathering of 

 sea cliffs is favored by the absence of protecting talus (Why 

 absent ?) and often of vegetation, and by the frequently wet 

 condition of the rocks due to the spray. The active issu- 

 ance of ground water as seepage and springs near the level of 

 the sea often helps to undermine sea cliffs. 



FIG. 260. Sea cliffs on the northern coast of France. 



Sea caves, stacks, natural bridges. The enlargement by 

 the waves of a joint or other opening in the face of a sea cliff 

 may result in a sea cave (Fig. 262), provided the overlying 

 rock is strong enough to form a roof. Occasionally a sea 

 cave is worn back and up to the surface of the ground some 

 distance back from the cliff. Again, a fissure or joint may 

 form an opening between the inner end of a sea cave and the 

 surface of the ground. Storm waves sometimes drive spray 

 and water up through such openings, which are then called 

 blowholes. 



Taking advantage of joint systems, waves sometimes 

 quarry out the rocks about a section of a cliff, leaving it as an 



