EVOLUTION OF MODERN SURFACE FEATURES 51 



lands near San Francisco, along Salinas Valley (for which it is 

 named) to the Transverse Ranges. Another area of Franciscan base- 

 ment, the central Franciscan area, lies west of Salinia (Reed and 

 Hollister, 1936, pp. 1547-1550). The Transverse Ranges again are 

 largely floored by Nevadan basement, as are the Peninsular Ranges, 

 but another, the southern, Franciscan area lies to the south and 

 west, mostly beneath the sea, and continues far southward along the 

 Pacific side of Baja California. 



The Nevadan basement has been supposed to be the foundation 

 of the Coast Ranges and to underlie the Franciscan rocks; remark- 

 ably enough, however, the base of the Franciscan has never been 

 discovered, and all its contacts with the Nevadan rocks are high- 

 angle, through-going faults. For example, the contact between the 

 Nevadan mass of Salinia and the Franciscan rocks on the northeast 

 is the San Andreas and associated faults. There is thus no evidence 

 of the sort of crustal material on which the Franciscan rocks were 

 laid ; it might have been quite different from the Nevadan basement. 



San Andreas and Other Through-Going Faults. The high-angle, 

 through-going faults thus seem to be a critical feature of Coast 

 Range structure, and many of these faults are remarkable enough of 

 themselves. Instead of their sides merely moving upward or down- 

 ward, one side shifted laterally past the other, and the faults have a 

 large component of sideward movement. For example, along the San 

 Andreas fault, which has been traced the farthest of any, stream 

 courses and stream valleys are offset — a stream draining northeast 

 across the fault turns along the fault line, then resumes its north- 

 eastward course a half a mile or so to the southeast. This faulting 

 has thus altered the geography and surface forms, not merely by 

 raising and lowering blocks of the crust, but by shifting the geo- 

 graphic positions of large sections of the country relative to country 

 on opposite sides; the saying is apt that "San Francisco and Los 

 Angeles are coming closer together," the one city being northeast of 

 and the other southwest of the fault. 



Offset of stream valleys is an obvious late feature, but study of 

 the rocks on the two sides of such faults discloses older and much 

 greater anomalies. Gravels in Tertiary rocks on one side of a fault 

 may contain material whose only possible source is in some area 

 many miles away on the opposite side, suggesting that the source 

 area has moved away from the gravels in the time since they were 



