42 



GRAND ALL— SAN FRANCLSCO PENINSULA. 



[January 4. 



Merced Block. — Without considering the region south of San 

 Andreas Lake in the jMerced series, there is a tilted block, with 

 terraces both older and newer than the faulting. This may be 

 regarded as one large block that has many minor faults or smaller 

 blocks of the same type as the large one. The largest of these 

 faults, the one at Wood's Gulch, has determined the stream drain- 

 age at that point. Otherwise the drainage of the whole block is 

 toward the northeast, or down the slope of the tilt. These streams 

 have their heads in circular canyons, and which must represent a 

 drainage system older than the Merced formation. The Franciscan 

 series, forming the northeast front of the Buri-Buri ridge, are the 

 cliffs of the pre-Pliocene beach, and the fact that the Merced series 

 rests on the tops of these hills with almost a flat area shows that 





Fig. 20. 

 A typical form of topography in the beds of the Merced series. 



this is a pre-Pliocene terrace. The presence of the Merced series 

 in place in the creeks and between hills of Franciscan sandstone 

 indicates that the present drainage is controlled by the older drain- 

 age, the existing streams running down the submerged valleys of 

 pre-Pliocene times. 



