74 University of California Publications in Geology [Vol. 11 



SAN LOEENZO SEEIES OF THE SOBEANTE ANTICLINE 



What is probably the most complete section of the San Lorenzo 

 series of the Concord Quadrangle is found on the flanks of the 

 Sobrante anticline in the northwest quarter of the Concord Quad- 

 rangle. The strata exposed in the center of this fold are of Tejon 

 age; immediately above, on both sides of the anticline, are the Oligo- 

 cene beds. 



In this section there is apparently an angular unconformity 

 between the upper shales of the Tejon (Upper Eocene) and the coarse 

 basal sandstones of the San Lorenzo, as shown by the fact that the 

 shales, in certain localities, are dipping more steeply than the sand- 

 stones. It is difficult, however, to find good contacts and therefore 

 this difference in dip might possibly be due to the buckling of the 

 shales, rather than to an unconformity. 



Fig. 1. A north-south section through the Sobrante antic-line (AA on plate 3) 

 taken in part from Lawson 's geological map of Concord Quadrangle, San Fran- 

 cisco Folio, no. 193, TJ. S. Geol. Surv., 1914. Ttj, Tejon; Tsr, San Ramon; Tk, 

 Kirker tuff ; Tc, Concord formation ; Tm, Monterey ; Tb, Briones ; Tsp, San Pablo ; 

 Tor, Orinda. 



The San Lorenzo of this section is divisible into at least three 

 distinct lithologic units, to which local formational names have been 

 given. The lowest of these is the San Ramon formation ; above it are 

 the Kirker tuff's and above them, the Concord formation. The maxi- 

 mum thickness of the San Lorenzo beds in this section is about four 

 hundred feet. 



San Eamon Formation 



The San Ramon formation, as measured to the west of Bear Creek 

 on the south side of the anticline, has a thickness, of from forty to fifty 

 feet. It consists, for the most part, of medium-fine gray sandstones 

 which at the base are massive, rather coarse and somewhat calcareous. 

 Fossils are found in these basal beds in abundance at a number of 

 localities. 00 Toward the top of the formation, the beds are not so 



eo The principal fossil localities in the basal beds of the San Eamon formation 

 of this section are University of California localities 14, 1173, 2954, 2955. Locality 

 3081, one of the most important localities in this section, is possibly fifty feet 

 above the base. 



