294 University of California Publications in Geology [Vol. 8 



The occurrence of conglomerate and lignite strata is especially note- 

 worthy in this section as indicating deposition along a shore or upon 

 land. 



The predominantly sandstone terrane has an estimated thickness of 

 about two thousand to twenty-five hundred feet. 



The predominantly shale member of the Martinez Group is char- 

 acteristically exposed in the canon of the South Fork of Rock Creek. 

 It consists of about twenty-five hundred feet of alternating strata of 

 brown sandstone, "coffee-ground" shale and impure gray limestone. 

 These alternating strata vary from one to twenty-four inches in 

 thickness with the shales forming the main bulk of the exposures. 

 There are approximately one thousand to fifteen hundred feet of these 

 shales exposed in the middle portion of the section on the South Fork 

 of Rock Creek. They have a strike about N 40° W and dip from 45° 

 to 60° S. A brown sandstone stratum of two hundred feet in thick- 

 ness is found at the top of this member in fault contact with the 

 Basement Complex and a Pleistocene formation. The lowest member 

 exposed is found along Rock Creek at the confluence of the main 

 stream with the South Fork. This consists of about three hundred 

 feet of "coffee-ground" shale. These strata are in fault contact with 

 the granitoid rocks of Pinyon Ridge on the northeast. A northeast- 

 southwest cross-fault a mile northwest of the mouth of the South 

 Pork brings them in contact with the Pleistocene formation and the 

 sandstone member of the Martinez Group. Thus we see that this 

 member is bounded on all sides by faults and, on this account its 

 stratigraphie relations to the other terranes are uncertain. Dr. Noble 

 found float boulders containing the characteristic Martinez fossil 

 Turritella pachecoensis in a stream along the contact between the 

 Miocene ( ?) formation and this member. It is uncertain whether these 

 boulders came from the uppermost sandstone stratum of the Martinez 

 or were boulders which were previously in the Miocene (?) con- 

 glomerate. Dr. Noble and the writer found boulders containing T. 

 pachecoensis embedded in the gray sandstone matrix of the con- 

 glomerate. Careful search did not result in finding any fossils in 

 place in this member. The sandstone, the shale, and the limestone 

 of this member appear to be lithologically identical with rocks of 

 corresponding texture in the predominantly sandstone member of 

 proven Martinez age. 



