Vol. 6] Jones: The Geology of the Sargent Oil Field. 



67 



and form prominent benches on the hill slopes. In the canon 

 to the east of the section ridge they form three vertical walls. 

 The sandstone layers are in some cases impregnated with the 

 residues from evaporated petroleum which has seeped up through 

 the underlying shales. Many of these seepages are quite active. 



The basal conglomerates (Member A), of the San Pablo 

 formation occupy a considerable area over the region and 

 considerable patches of these beds are of frequent occurrence. 

 The coarse conglomerate layers stand out prominently wherever 

 this member was seen. At the east end of La Brea canon the 

 conglomerates which here rest on serpentine and which contain 

 large amounts of that rock and fragments of Franciscan rocks 

 with a scattering of shale, are overlapped by the B member of 

 the San Pablo formation. Here the B member loses its char- 

 acteristic color and appearance owing to the absorption of 

 petroliferous material from active oil seepages. 



Members B and C. — These two members of the San Pablo 

 formation are separated more or less arbitrarily to better 

 emphasize structure in mapping. Taken together these beds 

 vary in thickness from 300 to 1000 feet and consist of a 

 gradational series of light azure blue sandstones, quite coarse 

 at the base and gradually grading into a fine-grained hard shale 

 which has an almost conchoidal fracture near the top. This 

 blue color, as is well known, is one of the characteristic lithologic 

 features of the San Pablo formation over wide areas throughout 

 the central Coast Ranges of California. These beds are best 

 exposed on the south slope of La Brea canon and north of 

 Pescadero Creek in the central part of the area mapped, though 

 they do not form prominent outcrops at any point. 



The nature of these sediments may best be seen in the lower 

 or coarser parts of the members. The rock here is more or 

 less friable and is seen to consist of round grains of white sand 

 covered with a film of pasty light blue material. 



Member I). — This member consists of a fine to medium 

 grained sandstone varying in thickness from 300 to 600 feet. 

 The thickness of this division of the San Pablo formation is 

 fairly constant over wide areas in contradistinction to the beds 

 below it. The lower beds are believed to be of continental 



