306 GEOLOGY 



ting is rapid. The structure of the cliff-rock also influences the 

 slope and configuration of the sea-cliff. 



By working in along the joints of the rock, widening them and 

 quarrying out the intervening blocks, pillars of rock ("chimney- 

 rocks" "pulpit-rocks"), or even considerable islets are sometimes 

 isolated by the waves (Figs. 251 and 252). 



Waves sometimes excavate caves at the bases of cliffs. The 

 bottom and roof of a sea-cave usually have a pronounced inclination 

 landward, and if the cliff is low, the cave may be extended landward 

 until its roof is pierced. Through such an opening in the top of 

 the cliff the water of the incoming waves may be forced in the form 

 of spray. On the New England coast, such holes are sometimes 

 known as "spouting horns." Similar openings may be made, as 

 already pointed out, by the compression or rarefaction of the air 

 in the cave as the wave enters or retreats. The cave, the " spouting 

 horn/' the "pulpit-rock," and other isolated islets, are all closely 

 associated with the sea-cliff in origin. 



The bottom of the sea-cliff is bordered by a submerged platform 

 over which the water is shallow. This platform, or at any rate 

 its landward portion, represents the area over which the water has 

 advanced as the result of w r ave-cutting, and is, therefore, known 

 as the wave-cut terrace. Such a terrace is the necessary accom- 

 paniment of the cliff. Wave-cut terraces may become land by 

 elevation, or by the lowering of the level of the sea (Fig. 253). 

 Elevated sea-cliffs with wave-cut terraces at their bases are among 

 the best evidences of change of relative level between water and 

 land. 



Wave-erosion and horizontal configuration. The structure of the 

 rock along shore has much to do with the horizontal configuration 

 of the wave-shaped coast. In general, waves develop re-entrants 

 in the weaker portions of the shore, leaving the more resistant 

 parts as headlands (Fig. 1, PL XIX). It is to be noted that the 

 resistance of rock to wave-erosion is not determined by its hardness 

 alone. Every division plane, whether due to bedding, to jointing, 

 or to irregular fracture, is a source of weakness to the rock, and 

 rock of great hardness may be so broken as to offer relatively little 

 resistance. A coast which is regular and of equal exposure, but of 



