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University of California Publications. 



[Geology 



stones, leaf-bearing shales, and conglomerates. These are well 

 exposed in the open cuts opposite the Eailroad Ranch reservoir. 

 Above this are coarse, yellowish-gray, conglomeratic sandstones. 

 Whether they are a part of the San Pablo or belong to the 

 Orindan is not certain. In other words, the contact here be- 

 tween the San Pablo and Orindan is uncertain. There is no 

 tuff between them as there is in the section at Kirker's Creek. 

 The outcrops are in most cases well defined and are destitute of 

 any great amount of soil covering. 



San Ramon Valley Region. — In the hills on either side of 

 San Ramon Valley there are outcrops of San Pablo strata. About 

 six miles southwest of the town of Walnut Creek they appear 

 on the two flanks of a syncline in the ridge extending south- 

 easterly from the town of Lafayette. The Monterey, San Pablo 

 and Orindan formations and the Pinole Tuff have all partici- 

 pated in the folding. A section was made across the strike of 

 the San Pablo in this area. The line of contact between the 

 Monterey and San Pablo could not be definitely determined upon 

 a lithological basis. The Monterey formation in this section has 

 almost exactly the same lithological appearance as the San Pablo. 

 The lower portion of the Monterey sandstone contains a large 

 number of hard cherty shale concretions which are not at all 

 characteristic of the San Pablo. The line of contact has been 

 drawn upon a palaeontological basis entirely and on either side 

 of this line the sandstones of both the San Pablo and Monterey 

 are exactly alike, and no change can be seen in passing from one 

 to the other, except that the San Pablo strata outcrop in rough 

 ragged edges while the Monterey weathers to a soil and gives the 

 hills a smooth rounded outline with few bold outcrops. 



The upper contact with the tuff and Orindan formation is 

 sharp and well defined. The thickness of the San Pablo in this 

 section is about six hundred feet and it is composed almost 

 entirely of a coarse-grained, thick-bedded, yellowish-gray sand- 

 stone which in places is conglomeratic. A few small layers of 

 sandy shale are present but not prominent. No tuffs nor chalky 

 shales were seen. The clip is steep on either side toward the axis 

 of the syncline, but varies somewhat along the line of the strike. 



Farther south near the town of Danville and on the western 



