Vol. XI] DICKERSON—PT. REYES AND SANTA ROSA QUADRANGLES Zfij 



the triangle. The comparatively straight Inverness Riclge 

 parallels the San Andreas Rift. Short, rapid streams descend 

 from the high northeastern scarp of this ridge to Tomales 

 Bay or the Rift Zone in the vicinity of Olema. The south- 

 western slopes of this ridge are cut by many deeply in- 

 trenched streams, which very evidently at one time had far 

 longer courses than is at present indicated, as Drakes Estero 

 is clearly a drowned stream valley, due to the last general 

 subsidence in this region. This southwestern slope of Inver- 

 ness Ridge has been further chiseled into fine, wave-cut ter- 

 races of several different elevations. According to F. M. An- 

 derson the most distinct terraces occur between 600 and 700 

 feet elevation and at 200 feet elevation. In addition, there 

 are other less distinct shelves, which can be clearly seen from 

 certain points of view. Whether or not these shelves are 

 correlatives with the small plateau near Inverness is exceed- 

 ingly difficult to decide, since this plateau is so eroded that its 

 exact character was not evident to the writer. Other in- 

 distinct plateaus occur in the western shores of Tomales Bay 

 in the vicinity of Tomales Point, but much detailed work 

 would be necessary to decide their origin. Likewise these 

 titan steps in the southwestern side of Inverness Ridge do 

 not have correlatives in the northeastern shores of Tomales 

 Bay in the mainland mass. The Pleistocene beds of the Mill- 

 erton or Tomales formations which are found in the headlands 

 on the eastern shores of Tomales Bay, are apparently unre- 

 lated either to these wave-cut terraces of Point Reyes Penin- 

 sula or to the indefinite 400 foot terrace of the mainland 

 mass. This lack of synchrony very clearly indicates that the 

 Point Reyes Triangle has moved upward or downward at 

 times quite independently of the movements of the mainland. 

 The Point Reyes Triangle is then clearly recognized as a 

 definite orographic block whose history is quite different from 

 that of the mainland. 



Other differences between these two regions also occur. 

 According to Lawson 27 in the San Francisco Folio : 



The vegetation on the west side of the San Andreas Rift valley is 

 radically different from that on the east side. From Bolinas Lagoon 

 northward, the eastern slope of the main ridge of the Point Reyes 

 Peninsula is covered with a forest which though not continuous, is tairly 



27 Lawson, A. C, San Francisco Folio. U. S. G. S., No. 193. p. 3, 1914. 



