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



in blocks of Pleistocene in the Tomales Bay trench of the 

 San Andreas Rift. The mainland, the San Francisco-Marin 

 and Berkeley Hills, blocks of Lawson, as will be shown later, 

 has been subjected to a peculiar hinge-like movement by 

 rotation along an axis situated somewhere in the area east 

 of Santa Rosa and Petaluma valleys. This mainland-block's 

 uptilted edge is exposed along the shores of Tomales Bay and 

 the Pacific Ocean. Apparently warping or differential tilting 

 has also affected it. The peculiar type of stream drainage 

 within the area affected by these fault blocks, so well described 

 by Holway, 3 is a result of this type of movement. The northern 

 extension of the Hayward Rift Zone was recognized on the 

 western flanks of Sonoma Mountain and proofs of recent ac- 

 tivity were established through the recognition of fault sag 

 ponds, fault shelves, minor drainage modifications, and similar 

 features. The occurrence of estuarine fossils of upper Mio- 

 cene age in the clays and sands of the Petaluma formation 

 fixes the age of a freshwater fauna which is also found in 

 these beds. A study of the Petaluma formation adds an in- 

 teresting chapter to the history of this region during upper 

 San Pablo time and indicates that the San Pablo sea did not 

 extend to the northward in this region. A revision of the 

 stratigraphy of the beds in Sonoma Mountain which yielded 

 the remains of the Pliocene horse, Neohipparion gidleyi Mer- 

 riam, indicates that this horizon is in the Sonoma group. 

 The Sonoma group is correlated with the marine Merced 

 group by excellent stratigraphy in this region. Neohipparion 

 is a form which occurs in the Orinda formation of the 

 Berkeley Hills and the Etchegoin of the San Joaquin Valley. 

 Upon this basis and stratigraphic studies, the Merced group 

 is broadly correlated with the Sonoma group, Pinole tuff, 

 Orinda of the Berkeley Hills and Etchegoin formation of the 

 Great Valley. 



The investigation of the Pleistocene deposits of Tomales 

 Bay led to the recognition of two distinct formations of this 

 period. Small estuarine faunas were obtained from the up- 

 per beds of the Tomales formation, while the lower beds of 

 the Millerton formation yielded several species which are now 

 found only in the latitude of San Diego. In other words, 



3 Holway, R. S., Physiographically unfinished entrances to San Francisco Bay, Univ. 

 Calif. Publ., Geog., vol. 1, No. 3, pp. 90-95, 1914. 



