- Some rocks of the San Francisco Bay region 

 dating from the Mesozoic Era (250 to 65 million years 

 ago): Franciscan cherts (top left) and pillow basalts 

 (bottom left) of the Marin Headlands, which were 

 originally formed on the ocean floor 150 to 100 mil- 

 lion years ago. (U.S. Geological Survey photographs 

 by Bruce Rogers.) 



Some sedimentary rocks of the San Francisco Bay 

 region dating from the Tertiary Period (65 to 1 .6 mil- 

 lion years ago). Above, deposits of an approximately 

 50-million-year-old submarine landslide exposed near 

 Afio Nuevo on the coast south of San Francisco; left, a 

 beach cobble of the Purisima Formation on the coast 

 south of San Francisco containing abundant fossils, 

 mostly clams, that lived in shallow marine waters a 

 few million years ago and are similar to those living 

 in the region's coastal waters today. (U.S. Geological 

 Survey photographs by Phil Stoffer.) 



10 Oceanography and Geology of the Gulf of the Farallones 



