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CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



Petaluma and Santa Rosa quadrangles. As explained above, 

 it is the writer's opinion that the Monterey formation, which 

 is probably beneath the Petaluma formation in this region, 

 was preserved from erosion by being dropped below base- 

 level along the Tolay fault at the end of the Miocene or dur- 

 ing Epi-Miocene time. As will be seen from the geological 

 map, the structures within the Petaluma formation are, as a 

 rule, small, and exploitation for oil in this region would be 

 attended with considerable expense. The best location is the 

 anticline north of the Eureka School along the road between 

 the Eureka School and Adobe Fort. 



Water Resources 

 artesian 



The synclinal structure at the head of Tolay Creek is a 

 fold in the Petaluma formation and it appears quite possible 

 that moderately deep wells sunk in this syncline might obtain 

 water, since this formation has abundant coarse sandstones to 

 act as water reservoirs, and good stiff impervious clays over- 

 lying these sandstones, acting as a cap rock. It is possible 

 that water might be obtained from the same synclines about 

 two miles northwest of Adobe Fort in the Santa Rosa Quad- 

 rangle. The possibilities for artesian water in the Petaluma 

 Quadrangle are very slight, as most of this region is under- 

 lain at no great depth by the non-water-bearing rocks of the 

 Franciscan group. This group has been tested in many places 

 within the Santa Rosa Quadrangle and practically all the 

 wells sunk are failures. The same statement applies to the 

 southwestern corner of the Santa Rosa Quadrangle and most 

 of the Point Reyes Quadrangle as well. Suitable structures 

 in Sonoma and the Mayacamas Mountains were not recog- 

 nized. Time did not permit a thorough exploration of the 

 artesian possibilities of Santa Rosa Valley. 



One of the interesting results of the investigation of the 

 Merced deposits around Petaluma was the recognition of the 

 intimate relations between ground-water and the unconsoli- 

 dated sandstone of this Merced group. The Merced sand- 

 stones in and around Petaluma vary from 50 to 200 feet in 



