28 CRANDALL— SAN FRANCISCO PENINSULA. [January 4,. 



shown upon a map, and for this purpose a colored map, with nec- 

 essary legend, is appended. 



Merced Series. — The series that deserves first mention here is 

 the Pliocene, Pleistocene and the derivative sand dunes. These 

 cover the largest part of the area mapped, and because of the wide- 

 spread distribution of the seolian sand, give the area the appear- 

 ance of hills almost buried by sand. 



The Merced series proper is confined to the long low valley 

 lying between the Buri-Buri ridge and San Bruno mountains. The 

 series lies upon the old Franciscan rocks near Mussel Rock and from 

 there rises to the top of Buri-Buri ridge, follows southeast along the 

 trend of the ridge to about two miles northwest of Millbrae, where 

 it drops into the low hills and appears on the low lying hill to 

 about two or three miles southeast of Millbrae. The complete south- 

 east extent of this series is not shown on the map. Below Millbrae 

 the streams have cut through this series exposing the underlying 

 rocks showing that it appears only as a thin edge and that it is 

 probably chiefly wind-blown material. 



A point of interest in this connection is the exposure of the 

 Franciscan rocks on the fronts of the steep hills west of Millbrae 

 and of Baden. In the stream bottoms that run between these 

 hills and in the circular-shaped basin behind them the Merced beds 

 are in place. 



Going northward along seven mile beach nearly the entire thick- 

 ness of the series is exposed tilted toward the northeast. Just 

 south of the Life Saving Station the beds disappear under the Pleis- 

 tocene formation. The northeast line of the contact with Lake 

 Merced to South San Francisco is not exposed, the rest of the east- 

 ern contact from South San Francisco to south of Millbrae, is de- 

 termined by the marsh line. The obscuring of this contact along San 

 Bruno mountains is caused by the Pleistocene materials which have 

 been blown up against San Bruno mountains about to the five 

 hundred foot contour in places. North of San Bruno mountains 

 these sands cover most of the city proper, obscuring the underlying 

 geologic structure in many places. This formation merges into the 

 recent deposits along the flats. 



San Bruno Sandstone. — The formation of next importance in 



