290 University of California Publications in Geology [Vol. 8 



who is at present engaged in mapping the Rock Creek Quadrangle, 

 placed his data at the writer's disposal and his kindly co-operation 

 and guidance in the field were very helpful. The different formations 

 and groups found in this field were outlined by him before the writer 

 visited the locality. Dr. Ralph Arnold 1 identified the first fossils 

 obtained from this locality and recognized the Martinez age of the 

 strata which contained them. 



GENERAL STATEMENT OF STRATIGRAPHY AND STRUCTURE 



The area to be discussed in this paper is located in the northern 

 half of the Rock Creek Quadrangle, on the southern edge of the 

 Mohave Desert, or of that portion of it known as Antelope Valley (see 

 fig. 1). The field which was particularly studied is on the northern 

 flanks of the San Gabriel Range, about twenty-five miles north of 

 Pomona. The main drainage lines of this region are Rock Creek and 

 Little Rock Creek. The sources of Rock Creek and of the South 

 Pork of Rock Creek are in the summit regions of the San Gabriel 

 Mountains, which attain a height along the east-west line varying from 

 8000 feet to 9389 feet, the elevation of North Baldy, the highest peak 

 in the Rock Creek Quadrangle. When this region was visited in 

 December, 1913, all the mountains above 6000 feet in elevation were 

 covered with snow. The melting snows supply Rock Creek with water 

 throughout the year. The South Pork of Rock Creek is in reality the 

 main stream, and it appears to be consequent. That portion of Rock 

 Creek south of Pinyon Ridge is a steam whose course was determined 

 by a minor fault of the San Andreas rift. The main portion of Rock 

 Creek is an antecedent stream and cuts through the extension of 

 Pinyon Ridge and the ridge between the Mohave Desert on the north 

 and the rather wide, level Rock Creek Valley on the south. 



The section most carefully examined is that in the vicinity of Rock 

 Creek. A brief reconnaissance was made into the area in the north- 

 west corner of the quadrangle. The region studied is a portion of the 

 San Andreas rift zone. Rift features such as kernbuts, kerncols and 

 fault-sag ponds are beautifully exemplified in this area. Pour approxi- 

 mately parallel northwest-southeast fault lines traverse this area, and 

 minor parallel faults are also found. 



i Arnold, Ralph, The Tertiary and Quaternary Peetens of California, U. S. 

 Geological Survey, Professional Paper no. 47, p. 11, 1906. 



