Vol. XI] DICKERSON—PT. REYES AND SANTA ROSA QUADRANGLES 529 



the Tertiary formations, but no attempt was made to differ- 

 entiate the formations within the Franciscan group. The 

 northeni quarter of the Point Reyes Quadrangle is reconnais- 

 sance mapping only, as the topographic sheet was not avail- 

 able in the field. A small amount of detailed mapping was 

 done upon the Pleistocene formations in Tomales Bay. The 

 writer is indebted to Mr. F. M. Anderson 1 for the excellent 

 mapping of the rest of this quadrangle. Owing to different 

 scales, some of the contact lines are not exact; nevertheless, 

 the map shows the approximate distribution of the forma- 

 tions correctly. 



Outline of Results 



The area under discussion is immediately north of the San 

 Francisco Bay Region, or it may be regarded as a northern 

 part of this terrain. It is cut on the west by Tomales Bay 

 which was determined by that notable feature, the San 

 Andreas Rift. Long continued faulting in this zone has 

 caused a separation of the Point Reyes Triangle, a northern 

 extension of Professor Lawson's 2 Montara Block from the 

 eastern shore of Tomales Bay, the mainland. One of the 

 conclusions of this paper is that this condition is an old one 

 and that the Triangle and the mainland have been subjected 

 to different sets of movements during the geologic past as 

 well as the present. The exact nature of these motions is 

 not entirely understood, but some progress has been made in 

 discovering their character. The Pleistocene formations 

 which are exposed in the headlands on the eastern side of 

 Tomales Bay prove, both by their lithology and fauna, that 

 this long inlet existed during their deposition and that earth- 

 quake movements were prevalent then. The absence of these 

 sediments from the western shores of the bay and the pres- 

 ence of fine terraces in the shales and sandstones of the Point 

 Reyes Peninsula, indicate that the Triangle moved inde- 

 pendently of the mainland, as successive titan steps (marine 

 terraces) were cut during periods of standstill. These steps 

 do not correspond to those of the mainland nor to the slotted- 



i Anderson, F. M., The Geology of Point Reyes Peninsula, Univ. Calif. Publ., Bull. 



DeP 2 t Lawson. V A 1 : C \ /sinVrSco Folio. No. 193, U. S. Geological Survey, p. 16. 

 1914. 



