36 CRANDALL— SAN FRANCISCO PENINSULA. 



[January 4, 



first of these has resulted in the letting down of the sandstones on 

 the southwest side of the hills known as University Mound, and the 

 formation of Visitation Valley. At the entrance of the valley are 

 two little jasper remnants that have been faulted down. The line 

 of faulting is approximately parallel to Visitation avenue, passing 

 between the two ridges of jasper in the northeastern end of the 

 valley. 



Guadalupe Valley is parallel to this line but since it is entirely 

 in San Bruno sandstone it is not certain whether or not its struc- 

 ture is the same as that of Visitation Valley. The dips of the beds 

 on the northeast side of the San Bruno mountains are very much 

 steeper than those on the southwest side of the ridge. This leads 

 to the belief that the Guadalupe is a fault valley, though definite 

 proof has not been found. 



Wood Gulch Fault. — In the Merced series about one mile north 

 of Mussel Rock there is another minor fault having a downthrow 

 of some eight hundred feet on the southwest side. There are other 

 small faults in the Merced beds with displacements of twenty-five 

 feet and more, but none of them are important. 



The deformation of the Merced beds shows that the San Andreas 

 faults are of post-Pliocene age but the age of the faults across the 

 city of San Francisco from the Presidio to the Portrero (Lone Mt. 

 fault) and at San Bruno Mountain may or may not be of the same 

 age. The greater maturity of the topography in the city hills and 

 on San Bruno Mountain suggests, however, that these faults are 

 both older than the San Andreas-Portola fault. 



Folding. — This area has been folded into three blocks which are 

 considered in the order of their importance. 



Merced Block. — The area occupied by the Merced beds between 

 Buri-Buri Ridge and San Bruno Mountain is in the form of a slight 

 syncline ; this is formed as the result of a block tilted toward the 

 northeast by a thrust fault on the southwest side. 



San Bruno Block. — Between San Bruno fault and Lone Moun- 

 tain fault is the anticlinal fold, already noted and parallel to the 

 San Bruno fault. Otherwise the whole block is simply tilted toward 

 the northeast, the dips becoming steeper shortly before reaching the 

 Lone Mountain fault. 



