San Francisco Bay and Golden Gate 143 



dred feet the large valleys, as Napa, Santa Rosa, and Santa 

 Clara, would become bays similar to Richardson, San Rafael, 

 and San Pablo. The higher hills and ridges would become 

 islands similar to Angel Island and Goat Island, and high ridges 

 between the present valleys would become promontories similar 

 to Tiburon Peninsula, Hunter Point, and San Pablo Point. 



What is the explanation of San Francisco harbor? During 

 the great lapse of time from Jurassic to Pleistocene the land was 



Photo by G. W. Stose, U. S. Geol. Survey 



FIG. 43. Minutely folded radiolarian chert (Franciscan) Golden Gate 

 Park, San Francisco. 



depressed and covered by the sea several times. Over what is 

 now the Coast ranges sea sediments of great thickness were laid 

 down. Cretaceous deposits north of San Francisco that have 

 not all been removed by erosion reached the astounding depth or 

 thickness of 5 or 6 miles. Erosion since the land was ele- 

 vated has removed much of the Cretaceous formations, and in 

 places the underlying granitic rocks of the basement complex 

 are exposed. Not only Cretaceous formations have been de- 



