DERDLTARV EAU NAS OR Maile PACTEIC COAST 519 
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 
tJ. 5. Diller, Roseburg Folio. 
