San Francisco Bay and Golden Gate 



145 



block was uplifted. The channel of the ancient stream is 

 thought to have been between Angel Island and Tiburon 

 Peninsula, where now, in Raccoon Strait, there is a deep chan- 

 nel. It is thought that this river was a superimposed stream, 

 that is, its course was established when the region was covered 

 with soft formations of Pliocene age, which have since been 

 eroded away. Outside of the Golden Gate is a submerged em- 

 bankment composed of fine silt which it is thought may rep- 

 resent a delta of the ancient stream which flowed through the 



FIG. 44. Generalized cross section diagram of the three great fault blocks 

 in the San Francisco Bay district. The arrow SW indicates the directions of 

 crustal movement from northeast to southwest. The other arrows indicate 

 upward movement of the fault blocks. 



Golden Gate before the region was depressed. The embank- 

 ment extends beneath shallow water for 30 miles to the Farallone 

 Islands. Fine silt is carried through the Bay of San Francisco 

 during flood seasons, and the embankment has been probably in 

 part thus built up. 



It should be borne in mind that the uplifting of the great 

 fault blocks was not a sudden or cataclysmic upheaval but went 

 on slowly and for a long time, so that the river continued to 

 flow across the crest of the uplifted side of the block and cut 

 its channel into the hard rock. The depth of the present chan- 

 nel, known as the Golden Gate, is more than 400 feet. San 

 Francisco Bay lies chiefly on the northeastern slope of the San 



