1907.] CRANDALL— SAN FRANCISCO PENINSULA. 43 



Drainage. — The long, low valley between Buri-Buri ridge and 

 San Bruno ridge is a slight syncline, the form of which is deter- 

 mined by the thrust block on the southwest side. It may have been 

 the formation of this syncline that has controlled the drainage of 

 the stream that once flowed through the basin now occupied by 

 Lake Merced. This lake has been formed by the damming of the 

 stream's mouth by a barrier beach ; the bottom of the lake is now 

 about ten feet below sea level. 



San- Bruno Block. — The physiography of the San Bruno block is 

 apparently older than that of the Merced block and is determined 

 mainly by structure. 



Effect of Structure. — The next feature of note is the San Bruno 

 thrust block. Along the southwestern side of the San Bruno range 

 is a ridge of low hills capped with jasper. In the geological column 

 these are probably within a few hundred feet vertically of the jaspers 

 represented on Buri-Buri ridge. As these jaspers dip toward the 

 southwest while those on Buri-Buri ridge (except near the fault 

 zone) appear to dip toward the northeast there is the suggestion 

 of a pre-Pliocene syncline, whose eroded trough has been the place 

 of deposition of the Merced series. If the San Bruno fault is pre- 

 Pliocene, as it seems to be, then the valley in which the Merced 

 was deposited was an eroded syncline between fault blocks. The 

 greater age of the San Bruno fault is shown by the more mature 

 topography of that range as compared with the Buri-Buri country. 



With one exception the streams of the San Bruno block drain 

 northeast or southwest. In this exceptional case the stream turns 

 at a right angle from a northeast direction and flows southwestward 

 along the northeast side of the San Bruno mountains in a direct 

 line with the Guadalupe Valley. The direction taken by this stream 

 lends color to the suggestion that the Guadalupe \'alley is a minor 

 fault valley like Visitation Valley. 



Drainage. — The anticline in the southwest edge of the San 

 Bruno mountains shows its effect upon the topography at Ocean 

 View where it has broken down. Heading near Ocean View is 

 Islais Creek which runs by an indirect line to empty into the bay 

 south of the Potrero. The drainage of this stream must have been 

 guided by some previous topographic features that are not now 



