20 



Physiography 



STATUTE MILES 

 CONTOUR lNTERVAL-500 FEET 



Figure 20. Areas of most abundant coastal landslides (cross-hatched areas) identified mostly from observations aboard 

 ship. Section 7 mostly from field studies by Roth (1959). 



gion, that near the center of the Palos Verdes 

 Hills section, is neither steep nor much ex- 

 posed; however, the Miocene shales of which 

 it is composed dip toward the ocean and con- 

 tain beds of bentonite which is slippery when 

 wet. Most of them are rock slides or debris 

 slides, according to Sharpe's (1938) system 

 of classification. 



Direct erosion at the shore cuts only a 

 slot into the base of the sea chff", so that for 

 high sea cliffs the bulk of the rock is eroded 

 indirectly by landsliding. This is analogous 

 to erosion by youthful streams (Fig. 21) in 



which direct erosion by the stream is re- 

 stricted to a downward direction and the 

 bulk of the material that was removed to 

 form the valley reached the stream by land- 

 sliding or slope wash. In addition to land- 

 sliding, much of the sea cliff" face is removed 

 by spray that is thrown against the cliff. 

 During storms when great sheets of water 

 drain down the face of the cliff", the runoff" 

 must be many times that from rain, and 

 thus it must produce many times as much 

 erosion. 



In spite of the numerous methods by 



