518 RALPH ARNOLD 
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 which 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 
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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 
Fic. 2.—Map showing hypothetical a characteristic marine fauna are 
distribution of land and water on the found at many localities throughout 
Pacific Coast during Oligocene time. 
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 
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