136 University of California Publications in Geology [Vol. 13 



This limited known occurrence of the Briones deposits and the fact 

 that in all other regions in California the Briones formation has not 

 been recognized between the Monterey and the Santa Margarita, 

 indicate that the Briones sea was probably an embayment of small 

 extent. 



DESCRIPTION OF THE FORMATION 



The Briones formation shows a wide range in thickness. The 

 minimum noted is on the south side of Mt. Diablo in the vicinity of 

 Sycamore Creek, on the east side of the Walnut Creek syncline. Here 

 the thickness is about 450 feet. Ten miles to the north along the strike 

 the thickness of the formation has increased to over 1000 feet. Five 

 miles to the west on the west side of this syncline, the thickness of the 

 Briones averages about 1100 feet. On progressing northwesterly 

 toward San Pablo Bay, the deposits gradually increase in thickness 

 and attain a maximum of over 2300 feet. 



One of the most typical exposures of the Briones formation is found 

 on the southwest side of Mt. Diablo. Here the basal beds consist of 

 some 75 to 150 feet of gray, fairly well indurated, medium-grained, 

 slightly fossiliferous sandstones. Overlying these sandstones are about 

 75 feet of hard, massive, firmly cemented, coarse-grained sandstones, 

 which in places are finely conglomeratic. These beds contain a great 

 number of fossils and the shells are packed so closely together that 

 they make veritable shell beds. The strata overlying these shell beds 

 vary in thickness from 300 to 800 feet and consist of yellow, sandy 

 shales alternating with fine-grained sandstones. Certain members are 

 relatively resistant to erosion and appear as small longitudinal ridges 

 on the sides of the hills. Near the top of the formation the deposits 

 become more fine-grained and shaly in texture. 



In the Contra Costa Hills and regions to the south, except in the 

 territory near San Pablo Bay, the exposures of the Briones formation 

 are very similar to those found on the southwest side of Mt. Diablo. 

 The most striking features are the hard, massive, extremely fos- 

 siliferous, reef-like sandstones near the base of the formation, which, 

 due to their resistant nature and generally high angle of dip, weather 

 in bold relief. 



In the northwestern portion of the Contra Costa Hills, progressing 

 northward from Walnut Creek to San Pablo Bay, the Briones deposits 

 gradually thicken, and become more fine-grained in texture. Even the 



