Smith.] 



Islands of Southern California. 



227 



erosion, during which the land was more elevated than at present. 

 This period of elevation and erosion was followed by the Pliocene 

 depression, during which deposits of great thickness were laid 

 down in favorable localities, the larger Miocene valleys being filled 

 to a greater or less extent with deposits which have since been 

 re-excavated to a greater or less extent. 



During the post- Miocene erosion interval, it is probable that all 

 the islands then differentiated were mountainous masses belonging 

 to the mainland. Judging from their topography, and the apparent 

 genetic relationships of those of the northern group, the forms then 

 existing probably included all the present islands, except San 

 Nicolas and San Clemente. The latter appears not to have been 

 elevated till the close of the post-Miocene erosion period, or early 

 in the Pliocene depression, and it is probable that the elevation of 

 San Nicolas occurred at about the same time. The disturbance at 

 this time seems to have been general for this whole region, includ- 

 ing both faulting and folding, and leading, not only to the differen- 

 tiation of these two islands, but also, probably, to a greater elevation 

 of all the other islands. Although the forces operative in these 

 movements are believed to have acted intermittently from that time 

 to the present, it is thought that they were mainly effective then, 

 and that any later movements have been of minor importance in 

 relation to the general movements of the California coast, since the 

 highest elevated terraces of San Clemente and the leveled summits 

 of Santa Catalina and Santa Rosa still closely correspond in altitude 

 with the highest terraces on the mainland, and, going further north, 

 with the upper limit of the Pliocene delta deposits along the Ties 

 Pinos Creek.* 



The post-Miocene elevation of the coast was followed by the 

 Pliocene depression, during which the sea stood for a long time 

 some 1,500 feet below its present level, as shown by the highest 

 terraces, the planation of the island summits, and the delta deposits 

 just referred to. Whether this was the full extent of this depres- 

 sion for the southern coast, can not be stated from the evidence at 

 present available. During this depression, at first Santa Catalina, 



*See, Lawson, The Post-Pliocene Diastrophism of the Coast of Southern 

 California, Bull. Dept. Geol., Univ. Calif., Vol. I, No. 4, 1893, p. 153. 



