THE OCEAN AND ITS WORK 



207 



Influence of Joints and Other Planes on Erosion. — The profile 

 of a cliff is largely determined by the nature and trend of the divi- 

 sional planes of the rock of which it is composed (Fig. 189), especially 

 of the stratification planes and joints. 



If stratified rock is not strongly jointed and dips toward the sea (Fig. 190^), the cliff 

 formed will incline in the same direction. In such a case the wave moves up the slope 

 with little resistance, since an overhanging cliff is absent. When the strata dip gently 



Fig. 190. — A y cliff formed in seaward-dipping strata without strong joints. 

 B, cliff formed in strongly jointed seaward-dipping strata. 



Fig. 191. — Ay cliff formed in landward-dipping strata without strong joints. 

 By cliff formed in strongly jointed, landward-dipping strata. 



sE. 



si 



y_ 



-X 



si. 



Ai I 1 1 ) Ml 



I 'I I ! I I (If 



X 



ZLL 



1 i 1 1 1 ' 1 1 I I 



A 



B 



Fig. 192. — Ay profile of a cliff formed in horizontal strata without strong joints. 

 By profile of a cliff formed in strata with strong joints. 



Figs. 190-192. — Diagrams showing the profiles of cliffs formed by wave erosion. 



towards the sea and a porous stratum rests on an impervious one, landslides may occur, 

 when the porous stratum is undermined. If the strata are inclined towards the land 

 (Fig. 191 A) overhanging cliffs will be formed, since as one layer is worn back another 

 equally overhanging, is exposed. It is on such cliffs that the waves are most effective. 

 If the strata are horizontal the base of the cliff is excavated, but as the upper part is in 

 the form of a stair (Fig. 192 A) y the waves have little effect. It should be borne in mind 

 in this connection that if the joints of the rock are better developed than the stratifi- 

 cation planes, the profile of the cliff will depend largely upon their direction, so that 



CLET.AND GEOL. — 1 4 



