Vol. XI | DICKERSON—PT. REYES AND SANTA ROSA QUADRANGLES 535 



the area of patches of the softer and later series through 

 which the Middle River has cut down to its present position." 

 The occurrence of Merced fossils at Plantation a few miles 

 north of the mouth of the Russian River, also at Freestone, a 

 short distance south of the Lower River, indicates to the 

 writer that the intervening area was once covered by Merced 

 sandstones and shales and probably a thin veneer of marine 

 Pleistocene terrace material as well. Subsequent erosion has 

 removed these incoherent materials along the present course 

 of Russian River and has left for present topography the 

 old Miocene or early Pliocene peneplain cut in Franciscan 

 rocks upon which the sands and gravels of the Merced sea 

 were deposited. In brief, the Lower River as well as the 

 Middle River is superimposed. Hoi way described accurately 

 the peculiar relations of Walker and San Antonio creeks in 

 the monograph, "The Physiographically Unfinished Entrances 

 of San Francisco Bay" and his hypothesis that they were 

 once one stream which drained to Tomales Bay is confirmed 

 by these studies. The writer thinks that the tilting of the 

 mainland block is the cause of this "broken-backed" stream 

 and that the waters of the middle and upper portions of the 

 Pleistocene Walker Creek were spilled out by way of one of 

 the headwater tributaries when the tilting was sufficient to 

 overcome the grade of the stream. This will be discussed 

 further in the latter portion of this paper. 



Geology 



The oldest rocks in this general region are limestone, 

 quartzite, and schists of possible Paleozoic age which occur 

 only in Point Reyes Peninsula. These rocks are remnants 

 of the roof of a great granitic batholith of probable Mesozoic 

 age and they occur as inclusions in the granite of the Point 

 Reyes Peninsula. Rocks of the Franciscan group of possible 

 Jurassic age are restricted to the east side of the San Andreas 

 Rift and Tomales Bay. For many miles east of Tomales Bay, 

 the Franciscan is either the surficial rock or it forms a base 

 upon which the later rocks rest. Another interesting group 

 from a distributional point of view is the Monterey of Middle 

 or Lower Miocene age. This, like the granites and lime- 

 stones, is restricted to the Point Reyes Triangle and no in- 



