TERTIARY FAUNAS OF THE PACIFIC COAST 



243 



in the southern Coast Ranges, especially, are largely derived from 

 granitic rocks and are usually coarser at the base, becoming finer and 

 finer toward the top, possibly indi- 

 cating a subsidence greater than the 

 concomitant sedimentation. Excep- 

 tions to the rule of coarse basal 

 sediments are not uncommon, how- 

 ever, and in the Santa Cruz 

 Mountains and also in eastern 

 Monterey County, Cal., the uncon- 

 formable deposition of fine shale 

 directly upon older rocks is a well- 

 marked phenomenon. This, of 

 course, indicated a sudden and 

 rather deep submergence of the 

 areas in question at the initi ation 

 of the upper Miocene. Conditions 

 favoring the life of diatoms, so 

 marked in the Monterey, continued 

 over part of the Monterey diato- 

 maceous shale territory during the 

 upper Miocene (Santa Margarita 

 and Fernando formations). The 

 areas of maximum deposition 

 during the period were apparently 

 on the southwestern side of the San fig. 4.— Map showing hypothetical 



Joaquin Valley in western Fresno distribution of land and water on the 



County and in central Ventura ^^^'^" ^°^^' ^""'^s "PP^'^ ^^'°""' 



time. 



County, Cal., where thicknesses of 



over 8,000 feet of sediments, belonging largely to the upper Miocene, 



occur. 



EROSION AND VOLCANIC ACTIVITY 



The peneplanation of the Klamath Mountains and the Sierra 

 Nevada was probably completed during the upper Miocene, the 

 detrital material from these land areas forming the great deposits in 

 the San Joaquin and Sacramento valleys and the coastal belt of 

 northern California. Erosion was practically continuous in these 



