Vol. XI] D1CKERSON—PT. REYES AND SANTA ROSA QUADRANGLES 575 



apparently the waters of the Merced sea covered completely, 

 all the region west of the northeastern border of Petaluma 

 and Santa Rosa valleys. Little or no Merced strata occur 

 east of the projection of the Tolay fault in the Petaluma 

 Quadrangle, and it seems probable that the Berkeley Hills 

 Block was somewhat upthrust into the zone of erosion during 

 this time, as much of the lava and volcanic ash deposits of 

 Sonoma Mountain was laid down upon an erosion surface 

 cut across the rocks of the Petaluma formation of upper- 

 Miocene age. During a portion of Pliocene time, a distinct 

 divide cut off Sonoma Mountain and the Berkeley Hills from 

 the ocean, as the Orinda and Siesta formations of the Berk- 

 eley Hills are in a large part composed of lacustrine de- 

 posits. The Merced-Pliocene was shown to be the equivalent 

 of the Sonoma group and in turn of the Orinda beds of the 

 Berkeley Hills. Within the San Francisco-Marin Block, the 

 uplift during post-Pliocene time was not apparently uniform, 

 as Merced strata are now lacking in the vicinity of Tamal- 

 pais. Further north of Tamalpais, in and around Petaluma. 

 residuals of Merced strata occur, and as one goes further 

 northward towards Freestone the thickness increases. Of 

 course it is barely possible that the Tamalpais mass may have 

 been an extension of a Pliocene San Francisco Peninsula, and 

 therefore may not have been the site of Merced deposition. 

 An alternative explanation, that Merced strata once covered 

 Tamalpais but have been removed completely owing to a 

 greater uplift of this portion of the block, is probably correct, 

 as Merced strata once covered its northern flanks. This is 

 clearly shown by the occurrence of Merced beneath the cap- 

 ping of the basalt on Burdell Mountain, which has preserved 

 this small remnant to the present day. 



During the Pleistocene the Point Reyes Triangle underwent 

 movements which are not recognized in the San Francisco- 

 Marin Block. This block was apparently successively elevated 

 during this time, with periods of standstill long enough for 

 the sea to chop out a fine series of marine terraces on its 

 western side. These terraces do not have their correlatives 

 on the northeast side of Tomales Bay, showing these orogenic 

 blocks moved independently. 



