Vol. XI] DICKERSON—PT. REYES AND SANTA ROSA QUADRANGLES 504 



Point Reyes quadrangles was once covered by the sediments 

 of the Martinez and Tejon seas, but these were subsequently 

 eroded. Lawson holds that Point Reyes Peninsula may have 

 been under erosion during the Cretaceous and Eocene, as a 

 long time interval would be required to remove the Fran- 

 ciscan rocks from their granite basement. Cretaceous time is 

 quite sufficient for erosion to have accomplished this result. 



No Oligocene rocks were discovered in the environs of 

 Petaluma, but rocks of this age are present a few miles east, 

 in a limited area on Carneros Creek, and in the Contra Costa 

 hills. These rocks could have been deposited over the Point 

 Reyes Triangle and the San Francisco-Marin Block and then 

 eroded during Epi-Oligocene time. 



As the areal mapping shows, rocks of Monterey Miocene 

 were deposited upon the Point Reyes Triangle, but they are 

 absent from the San Francisco-Marin Block. However, in- 

 dications of their presence in the Berkeley Hills Block were 

 found. It appears probable that they were removed during 

 Epi-Miocene time from the San Francisco-Marin Block. Dur- 

 ing this period the Point Reyes Triangle and the Berkeley 

 Hills Block were relatively lower than the San Francisco- 

 Marin Block, faulting being active along the San Andreas 

 Rift and Tolay Fault. 



Records of Upper-Miocene time in the Point Reyes Tri- 

 angle and the San Francisco-Marin Block are completely ob- 

 literated, but it appears probable from a study of the Peta- 

 luma formation (Upper Miocene) that the San Francisco- 

 Marin Block was uplifted sufficiently high to form a bar- 

 rier between the ocean and Petaluma Lake, which probably 

 drained out southward into an arm of the San Pablo sea. 

 During Epi-Miocene time both the Berkeley Hills and the 

 San Francisco-Marin blocks were upthrust into the zone of 

 active erosion and an extensive peneplain was developed. 



After this long period of vigorous erosion, the two outer 

 blocks were lowered beneath the sea. Sands and muds were 

 deposited unconformably across the older rocks. During this 

 time great lava floods swept downward from the northeast 

 and actually entered the Merced sea. Volcanoes in the same 

 region also threw forth great volumes of ashes which were 

 deposited in the sea and upon the shore, which was located 



