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University of California Publications in Geology [Vol. 8 



South Side op Mount Diablo 

 STEATIGEAPHY AND LITHOLOGY 



The San Pablo Group on the south side of the Mount Diablo 

 anticline has a maximum thickness of about twenty-six hundred feet, 

 as contrasted with the six hundred feet of strata on the north side. 

 Here, as is also true of the group north of the mountain, the beds 

 vary as one follows them along the strike, and only a generalized 

 statement of the section can be given. 



All of the formations on the south side of the mountain dip very 

 steeply ; in some places the beds are vertical, and at points along the 

 strike they are even overturned. In the vicinity of Shell Ridge to 

 the east of the town of "Walnut Creek, the Mount Diablo anticline has 

 been overturned toward the southwest and on the south side the San 

 Pablo dips to the north at an angle of about 60°, resting on the Pinole 

 Tuff, which is younger, and is in turn overlain by the Monterey, which 

 is older. The general strike of the beds on the south side of the moun- 

 tain is about North 50° West. 



Lithology. — The lower fifteen hundred feet of the San Pablo, as 

 measured in the vicinity of Tassajero and Sycamore creeks, are 

 chiefly sandstones. At the base of the section Tassajero Creek cuts 

 through a heavy conglomerate which cannot be followed for any great 

 distance either to the north or to the south. Above these basal con- 

 glomerates are about seven hundred and fifty to eight hundred feet 

 of alternating layers of coarse, fossiliferous sandstones, massive, non- 

 fossiliferous sandstones and thin layers of conglomerate. On the north 

 side of Tassajero Creek, and stratigraphieally about seven hundred 

 and fifty feet from the base of the group, there is a thin band of 

 greenish-yellow shale. There is an irregular contact between this 

 shale and the coarse, conglomeratic sandstone above. Above the sand- 

 stone are about eight hundred feet of medium fine, shaly, gray-brown 

 sandstone. 



Above the medium fine, shaly, gray-brown sandstone just men- 

 tioned, and about fifteen hundred and fifty feet above the base, is the 

 most persistent shale member of the group ; its thickness is about 

 twenty feet and it is continuous for over ten miles. In some localities 

 it contains many impressions of leaves and rushes, suggesting estuarine, 

 brackish, or fresh-water origin. No shells were found, but a study 

 of the fauna above and below the shale strongly suggests that it comes 



