TERTIARY FAUNAS OF THE PACIFIC COAST 



235 



THE OLIGOCENE PERIOD 

 THE OLIGOCENE A PERIOD OF ELEVATION 



The Oligocene on the Pacific Coast was primarily a period of eleva- 

 tion and erosion over many areas v^hich are now land. As indicated 

 by the fine character of most of 

 the sediment deposited during the 

 period, the relief was not strong, 

 except in a few regions. Outside 

 the Washington - Oregon province 

 there are few evidences of the period, 

 except a more or less marked un- 

 conformity between the Eocene and 

 lower Miocene, and these for the 

 most part are on the extreme con- 

 tinental border or along the edges 

 of the provinces of persistent sub- 

 sidence. The extreme localization 

 of the post-Eocene movements is 

 well shown in the southwestern San 

 Joaquin Valley where the lower 

 Miocene and Eocene are apparently 

 conformable and again occur within 

 a distance of a quarter of a mile 

 separated by a profound angular 

 unconformity. Strata of undoubted 

 Oligocene age consisting largely of 

 sandy to clayey shales and carrying 

 a characteristic marine fauna are 

 found at many localities throughout 

 the Puget Sound and northwestern 

 Oregon areas and an isolated occurrence of similar beds is found 

 in the Santa Cruz Mountains, a short distance south of San 

 Francisco. Wherever their relations are known these beds lie 

 conformable with the Eocene below and lower Miocene above; they 

 therefore mark areas of persistent subsidence. A characteristic 

 reddish to lavender formation (the Sespe), consisting of sandstone, 

 shale, and some conglomerate found in Ventura and Los Angeles 



Oliqocene 



Fig. 2. — Map showing hypothetical 

 distribution of land and water on the 

 Pacific Coast during OHgocene time. 



