Geologic map of 

 San Francisco Bay 

 region, showing 

 the San Andreas 

 Fault, related 

 faults, and regional 

 rock types. - 



* 



. 



100 MILES 



400 KILOMETERS 



Ofl 



Lassen Volcanic Region 



Rockland Ash erupted 

 -600,000 years ago 



Quaternary (less than 1 S million 



years old) deposits of the marine 



and coastal Merced Formation in 



an eroded seacliff at Fort Funston 



Beach in San Francisco (U.S. 



Geological Survey photograph by 



Phil Stoffer). The white band in 



the middle of this approximately 



150-foot cliff is the Rockland Ash, 



a layer of volcanic ash from a 



powerful eruption that occurred 



in the Lassen area of northern 



California about 600.000 years 



ago. That eruption was 50 times 



larger than the 1 980 eruption of 



Mount St. Helens, Washington, and because the winds at the time of the 



eruption were blowing southward, ash several inches thick was deposited as 



far south as the San Francisco Bay area. Map shows known distribution of the 



Rockland Ash, and inset shows particles from it magnified about 70 times. 



PACIFIC 

 OCEAN 



Farallon 

 Islands 



EH 



EH 



EXPLANATION 

 I | Artificial fill 

 I I Quaternary sediments 



Tertiary 

 Volcanic rocks 



Marine sedimentary 

 rocks 



Nonmarine sedi- 

 mentary rocks 



Mesozoic 



Ophiolite (sea-floor) 

 rocks 



Sandstone, shale, 

 conglomerate 



Chert 

 Granite 

 Melange 

 Limestone 



Fault, dashed where 

 approximate 



. v 



N 



5 MILES 



5 KILOMETERS 



Regional Setting 11 



