Clark: The San Lorenzo Series of Middle California 109 



erosion, Lower Miocene deposits were laid across the upturned edges 

 of the Oligocene beds. This is seen at a number of localities to the 

 north of Coalinga at the contact between the Kreyenhagen shales and 

 the "Vaqueros, " reference to which has already been made. It has 

 also been shown that in the Concord Quadrangle, in the region of 

 Mount Diablo, a stratigraphic break occurs between the San Lorenzo 

 beds and those of the Monterey, which is, apparently more than local. 

 In Washington also the Monterey deposits are reported to be uncon- 

 formable on those of the Oligocene. 



Summary op Stratigraphy 



Beds which in this paper are referred to the San Lorenzo (Oligo- 

 cene) are found to the west and to the north of Mount Diablo, Middle 

 California. To the west of the mountain there are three separate 

 areas in which beds of San Lorenzo age are found. The most com- 

 plete section of any of these three areas is found on the sides of the 

 Sobrante anticline (northwest quarter of the Concord anticline). 

 Here three distinct lithologic units are recognized and given local 

 formational names ; beginning with the lowest these are the San 

 Ramon formation, the Kirker tuffs and the Concord formation. An 

 erosional contact was found between the Concord formation and the 

 Kirker tuffs. The maximum thickness of the section in this area is 

 onhy about four hundred feet. The second area, in which the San 

 Lorenzo beds are found, is in the vicinity of the town of Walnut 

 Creek. Here the Oligocene beds are found outcropping on both sides 

 of the San Ramon syncline. On the west side of the syneline, to the 

 south of Walnut Creek, these beds have a thickness of over five hun- 

 dred feet, while on the east side the thickness is in some places not 

 more than fifty feet. The section to the south of the town of Walnut 

 Creek is taken as the type section of the San Ramon formation. It 

 seems probable that beds representing the Kirker tuffs and the Con- 

 cord formation are absent here. 



The third section of the San Lorenzo series, found to the west of 

 Mount Diablo, is that of the Pacheco syncline, Concord Quadrangle ; 

 here, only the San Ramon formation is present, the thickness of which 

 is less than one hundred feet. 



In all the sections of the three areas to the west of Mount Diablo 

 there is good evidence for the unconformable relationship between 

 the San Lorenzo series (Oligocene) and the Monterey (Miocene). 



