Smith.] 



Islands of Southern California. 



207 



already described. Thus, while the platform itself is due to marine 

 abrasion, its outer scarp presents slopes characteristic of subaerial 

 erosion, modified to a greater or less extent by deposition through 

 marine action. This is believed to have been the condition in gen- 

 eral, also, throughout the post-Pliocene elevation of the coast; that 

 is, that the off-shore slopes, as successive elevations of the land 

 occurred, were largely those characteristic of subaerial degradation, 

 and therefore greater in the more resistant than in the less resistant 

 rocks. 



Where young subaerial topography is found along a coast line, 

 gentle slopes and slightly dissected surfaces are ordinarily pre- 

 sented, whether the rocks are hard or soft. Exceptionally, such 

 forms may present high, abrupt slopes to wave action, as in the 

 case of a fault scarp facing the ocean. Old subaerial forms are also 

 characterized by gentle slopes and moderate contrasts. Such 

 forms along a coast line present a variable, though always moder- 

 ate, angle of slope to wave action. 



Mature subaerial forms present the greatest contrasts, such 

 topography, in rocks of varying degrees of resistance, being gener- 

 ally rugged, as a whole. Where the rocks are hard it will be 

 characterized by a generally bold shore line, with high, precipitous 

 cliffs, though softer rocks will give gentler slopes and lower cliffs. 

 If gentle subaerial slopes with little dissection (and therefore young) 

 are found in the more resistant rocks along such a coast, they can 

 be ascribed only to accident {e.g., faulting). Along the California 

 coastal region, the harder rocks present normally steep and well- 

 dissected subaerial slopes, while the softer ones show slopes of very 

 moderate angle, and usually much less dissected. The gentle 

 slopes found on the slightly dissected ocean facing of San Clemente 

 Island, in rather resistant volcanics, are therefore abnormal and 

 accidental, being due to faulting. 



With only slight original elevation above sea level, mature 

 forms in rocks of any degree of resistance would differ to a greater 

 or less extent from forms developed in the same rocks with greater 

 original elevation. The matured forms considered in this paper are 

 those resulting from a considerable original elevation, as exempli- 

 fied in the Coast Ranges of California. 



