236 RALPH ARNOLD 



counties in southern California, has been doubtfully referred to the 

 Oligocene and the map made to agree with this correlation; but it is 

 possible this formation is Eocene. 



Certain marine shales and sands underlying the lower Miocene 

 beds in western Fresno and Kern County may also belong to the 

 Oligocene. If so they imply that an arm of the sea remained in the 

 San Joaquin Valley following the post-Eocene elevation that excluded 

 marine conditions from much of the coastal belt of western America. 



The total thickness of the Oligocene over the region where it has 

 been recognized varies from over 1,000 feet in Washington to 2,300 ± 

 feet in the Santa Cruz Mountains. The Sespe formation of Ventura 

 and Santa Barbara counties, which has been tentatively correlated 

 with the Oligocene, attains a maximum thickness of about 4,300 feet. 



CONDITIONS OF EROSION AND DEPOSITION 



With the close of the Arago stage (Eocene)' the Klamath Moun- 

 tains and Coast Ranges of Oregon and California were uplifted to a 

 moderate elevation and subjected to extensive erosion, in some 

 localities completely removing the sediments deposited during the 

 Eocene. With the possible exception of an area in Ventura County 

 in southern California no mountains of strong relief contributed 

 directly to the Oligocene sediments. In eastern Washington the 

 great lakes which prevailed during the Eocene were elevated and the 

 sediments which had been deposited in them were folded and eroded, 

 the resulting detritus in addition to large quantities of volcanic 

 ejectamenta being collected in bodies of freshwater in eastern Oregon 

 farther south. It is thus known that with the elevation of this northern 

 country volcanic activity still continued although on an insignificant 

 scale as compared with the periods preceding and following the 

 Oligocene. In California there is no evidence of volcanism in the 

 Oligocene period. 



FAUNA AND CLIMATE OF THE OLIGOCENE 



What little is definitely known concerning the faunas of the 

 Oligocene as a whole indicates their closer affiliation to the Miocene 

 than to the Eocene. The fauna from the Oligocene of the Santa 

 Cruz Mountains (San Lorenzo formation) and a similar fauna from 



I J. S. Diller, Roseburg Folio. ' 



