144 Adventures in Scenery 



posited over wide areas to the north and south of San Francisco 

 Bay, but rocks of Tertiary and Quaternary ages occur. Inter- 

 vening periods of withdrawal of the sea and elevation of the 

 land are revealed in the succession of rock formations. The 

 intervals of erosion between formations mark unconformities. 

 Unconformities exist about San Francisco Bay, showing that 

 the sea has repeatedly encroached upon the land and the land 

 has been repeatedly uplifted. Not only have there been re- 

 peated upheavals and subsidences of the land but the formations 

 have been broken by violent disturbances. Rock formations 

 have been broken or "faulted," and by heat and pressure the 

 character of the rocks has been changed (metamorphosed) . 

 Upheaval and subsidence of the land, breaking of the forma- 

 tions by earthquake stresses resulting in vast rock displacements 

 by faulting, and bending, folding, crushing, and crumpling of 

 the rocks, have resulted in the great harbor at San Francisco. 



The dominating feature of San Francisco Bay and harbor 

 is three great fault blocks. These three blocks extend across 

 the region in a northwest-southeast direction. These blocks are 

 segments of the earth's crust that have been formed by break- 

 ing of the crust of the earth along fault lines and the uplifting 

 and tilting of these segments. They are each uplifted on the 

 southwest side and depressed on the northeast side, that is, they 

 are each tilted toward the northeast. The surface of each block 

 slopes more gradually toward the northeast from the crest and 

 abruptly toward the southwest. The three blocks are desig- 

 nated the Montara, the San Francisco-Marin, and the Berkeley. 

 (See Fig. 44.) 



Originally the basin in which San Francisco Bay lies was a 

 river valley through which the drainage of the great interior 

 valley of California reached the sea. The outlet to the sea was 

 where the Golden Gate is now. When the great San Francisco- 

 Marin fault block was uplifted the outflowing waters cut down 

 into the crest of the uplifted block, and continued to flow 

 through the channel, cutting down its bottom as the faulted 



