590 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



may have been effective as well. The marine terrace which 

 was developed during Pleistocene time possibly extended to 

 near the present eastern border of Santa Rosa Valley. When 

 the San Francisco-Marin and Berkeley Hills blocks were 

 given an eastward tilt this eastern border began to receive 

 some drainage from the west. That is, when the backs of 

 Pleistocene Salmon and Tomales creeks were broken, depo- 

 sition from the west as well as the east began in the ponded 

 area along the former border of an earlier Pleistocene plain. 



The velocity of the swift upper portion of Pleistocene 

 Salmon Creek (Santa Rosa Creek) was checked, with the 

 consequent rapid building of great alluvial fans and a south- 

 east tributary of the Pleistocene Russian River, which was 

 strong enough to maintain a course across the uplifted block 

 captured this drainage. The writer is not satisfied that this 

 is a complete explanation, but it is partially correct at least. 



Lower Petaluma Valley may have been aided by move- 

 ments along the fault just southeast of Burdell Mountain, 

 but this valley may have had a history somewhat similar to 

 Santa Rosa Valley as outlined above. A tributary of the 

 Pleistocene Sacramento River captured this drainage and 

 gradually robbed the headwaters of Pleistocene Tomales and 

 Walker creeks. These two valleys are apparently compara- 

 tively young, geologically speaking, and developed after the 

 tilting of the mainland block, but before the last event in the 

 Pleistocene, the subsidence which gave rise to San Francisco 

 Bay and the narrow flooded stream valleys of Russian River 

 and its associated streams. 



Tertiary Geologic History 



Since no Eocene rocks were recognized in this area the 

 events of this period are not well known. Both Martinez, 

 Lower Eocene, and Tejon, Upper Eocene, rocks occur on 

 Carquinez Straits, a few miles southeast, and Tejon rocks are 

 known in the vicinity of Carneros Creek in the Napa Quad- 

 rangle east of Petaluma. The nature of some of the sedi- 

 ments in the Martinez indicates that they were deposited upon 

 the outer edge of the continental shelf. The faunas obtained 

 from these beds developed in such a habitat. It appears 

 probable a large portion of the sites of the Petaluma and 



