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University of California Publications. [Geolouy 



tilted and presumably faulted. More or less isolated blocks of 

 it, forming mesas or low-lying outcrops, lie at various levels, 

 surrounded by alluvium, in the basin northeast of Black Moun- 

 tain and between that mountain and the exposure of the Rosa- 

 mond Series north of Barstow. 



The Rosamond in the Barstow syncline has been gently un- 

 warped, presumably without faulting, during this second dia- 

 strophic epoch. 



The hornblende andesite capping the Calico Mountains and 

 uneonformably overlying the beveled surface of the folded Rosa-* 

 mond Series has been faulted against the Rosamond along a 

 nearly perpendicular fault plane. The fault may have been 

 caused by upthrusting. It was noted by Lindgren and Storms. 



The present Sierra Nevada, Tehachapi, San Gabriel, and San 

 Bernardino mountains probably owe much the greater part of 

 their altitude to this second epoch of post-Miocene deformation ; 

 and the eastern Mohave Desert almost certainly received its 

 present orographic features from this diastrophism. The general 

 lines of structure may, however, very likely have come into being 

 as the result of a previous deformation. The great thickness 

 of coarse materials in the Rosamond apparently indicates the 

 presence of mountains at the time of its deposition, as does like- 

 wise the thick coarse deposits of the Fernando on the coastward 

 flanks of the southern Coast Ranges and the San Gabriel Moun- 

 tains. But the bulk of the Fernando is probably younger in 

 age than the Rosamond, and would seem to have been deposited 

 during the period roughly corresponding with the first cycle of 

 post-Miocene erosion in the Mohave Desert. If the drainage 

 of that cycle had an outlet to the ocean the sediment derived 

 from the degradation of the desert ought to have the quality of 

 being recognized by its lithologic characteristics. 



The later movement which has effected the ranges bounding 

 the western Mohave Desert can be recognized directly in their 

 present high altitude above the surrounding country and by 

 their bounding fault-scarps ; and indirectly by the rejuvenation 

 of streams and other erosive agencies. The bounding ranges 

 are not in topographic adjustment with the Mohave Desert. The 

 direct cause of this lack of topographic adjustment was the 

 deformation, while its indirect result was the rejuvenation. 



