58 University of California Publications in Geology [Vol. 7 



Pinole Tuff 



Lying unconformably above the Upper Division of the San 

 Pablo is a series of tuff beds. This unconformity is shown by 

 a marked, irregular contact with as much as ten to fifteen feet 

 relief, and a slight difference in dip and strike. 



The thickness of these tuffs is about one hundred and fifty 

 feet. The basal bed of this series, as seen west of Kirker Creek, 

 is a layer of a white, massive tuff, three to four feet thick. The 

 beds immediately above the basal white layer are made up of 

 large angular fragments of light-gray to bluish pumice, which 

 are included in a matrix of rather ashy, reddish-brown material. 

 This ashy matrix gives a noticeably red color to the beds. Scat- 

 tered through these tuffs are angular fragments of volcanic rock, 

 which are undoubtedly volcanic ejectments thrown out with the 

 ash. These beds are massive and stand out as prominent layers 

 several feet in thickness. 



In the vicinity of Markley Canon, at the base of the Pinole 

 Tuff, is a layer of conglomerate, which contains subangular water- 

 worn boulders of basalt, two to three feet in diameter. These 

 coarse boulder-beds are overlain by coarse, thick layers of red- 

 dish tuff, which weathers out in prominent wall-like outcrops. 



One mile and a quarter to the west of Kirker Creek a basalt 

 flow about four feet in thickness was found in these basal beds. 

 This is situated only a few feet above the contact, with the tuff 

 above and below. The rock shows a distinct flow structure, and 

 the upper surface is quite vesicular. 



The upper members of the tuff beds lack the reddish color. 

 In some places they are nearly pure white, and have lenses of 

 cross-bedded gravels and sands mixed irregularly with them. 

 Turner determined these tuff beds to be rhyolitic in composi- 

 tion. 



The tuff beds above the San Pablo at Kirker Pass have been 

 correlated provisionally with the Pinole Tuff seen on San Pablo 

 Bay, that to the north of Carquinez Straits, and that in the 

 vicinity of the town of Walnut Creek, first, because of their 

 stratigraphic position ; second, because of their lithologic and 



