350 



GEOLOGY. 



The principles involved in the development of the sea-cliff are appli- 

 cable to any broad stretch of water. 



The height of the cliff depends on the height of the land on which 

 the sea is advancing. Its slope may be steep or gentle (compare Figs. 

 303 to 306), according to the nature of the material of which it is com- 

 posed and the rapidity of the cutting. Rapid cutting tends to produce 

 steep cliffs and slow cutting gentle ones, for in the latter case weather- 

 ing is more important relative to the cutting, and at sea-level (low 

 altitudes) weathering generally tends to reduce the angle of slope. 

 In general, the more resistant the material the steeper the slope of the 

 cHff. Incoherent materials, such as sand and clay, are not likely to form 

 steep cliffs; but if the cutting be very rapid, bold faces may be developed 

 even in such materials (Fig. 307) . If beds of slight resistance at sea- 

 level underlie beds of greater resistance, the development of steep cliffs 

 is favored. The structure of the cliff-rock also has an influence on the 

 slope. The rock may be massive or bedded. If bedded, the beds may 

 be horizontal, or they may dip at any angle, in any direction. The 

 rock, whether stratified or not, may be abundantly or sparsely jointed. 

 All these structures influence the slope and configuration of the sea- 

 cliff (see Figs. 305 to 308). 



Chimney-rocks, etc. — 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 some- 

 times isolated by the waves. This is 

 most readily accomplished where the 

 joints converge back from the shore. A 

 well-known example of this sort is the 

 ''Old Man of Hoy" (Fig. 309) on the 

 coast of the Orkneys. A pulpit-rock or 

 other island, or any jutting point of rock 

 may be pierced, giving an arch or 

 bridge. La Roche Percee, a steep-faced 

 isle near Gaspe Harbor, is an example. 



Sea-caves. — Waves sometimes ex- 

 cavate caves at the bases of cliffs. 

 This is especially hkely to occur where 

 the rock is much jointed and where the joints are not continued 



Fig. 309.— "Old Man of Hoy.' 

 (Geikie.) 



