Vol. 6] Jones: The Geology of the Sargent Oil Field. 



57 



its lower reaches and northeast-southwest where it turns and 

 merges into the Santa Clara Valley. The slopes of the hills 

 in the Santa Cruz Mountains are unusually steep, occasionally 

 obtaining an incline of forty-five degrees. See pi. 13, fig. 1. 



The Pajaro Valley is really composed of three broad 

 alluvial plains. The lowest of these plains reaches down to 

 the Bay of Monterey. To the east of this, and separated from 

 it by a low range of hills, is a circular valley in which is situated 

 the town of Aromas. Over these two plains the river meanders 

 in sweeping curves. Still farther east is the Chittenden Valley. 

 (See pi. 13, fig. 2.) This valley shows signs of recent meander- 

 ing but recent uplift has forced the river to the south. Southeast 

 of here lies the San Benito Valley, a fiat, fertile plain with the 

 San Benito River flowing in sweeping curves but intrenched in 

 its own flood-plain to a depth of 25 or more feet. Through 

 the barriers separating the three lower plains the Pajaro River 

 has cut sharp canons. The divide between the San Francisco 

 Bay and the Monterey Bay drainage area is a very gradual 

 rise in the floor of the Santa Clara Valley, near the town of 

 Morgan Hill. 



SUMMARY STATEMENT OF THE GEOLOGY. 



In general the geological features of the area studied com- 

 prise a basement complex of altered sediments invaded by a 

 plutonic mass upon which rests the Franciscan series. The latter 

 has been intruded by peridotites now altered to serpentine. The 

 Shasta-Chico rocks are absent and the oldest rocks resting on the 

 Franciscan are of Miocene age. The San Pablo series rests 

 unconformably upon the Miocene and the Merced uneonformably 

 upon the San Pablo. Above the Merced is a thick series of fresh- 

 water beds with occasional intercalations of marine strata. The 

 lower Miocene formation is the source of the oil of the district 

 and the oil is reached by borings along an anticline to the 

 north of Chittenden. The most interesting and important 

 feature of the field is the clearly exposed unconformable relation 

 of the San Pablo to the Miocene. 



