Vol. XI] DICKERSON—PT. REYES AND SANTA ROSA QUADRANGLES 5J1 



its southwest corner, in an area which is largely covered by 

 Merced strata. Dr. Stock has supplied the note given below : 



Remains of the ground sloth Mylodon harlani were found in the Santa 

 Rosa Quadrangle approximately seven miles northwest of Petaluma and 

 three-quarters of a mile southeast of William McGrew's Ranch house, in 

 bluish clay overlain by gravels of Pleistocene age. These deposits rest 

 unconformably upon greenish-colored sandstones presumably of Francis- 

 can age. The bones were exposed in a gully about 100 yards north of 

 the road leading to the McGrew house and one-half mile west of main 

 road connecting Stony Point with Cherry. The fossil material consists 

 of parts of a pelvis sacrum, and lumbar and caudal vertebrae. 



A narrow Pleistocene marine terrace was observed at Dil- 

 lon's Beach, on the Pacific Ocean between Tomales and Bo- 

 dega bays, about 50 feet in elevation at its old cliff. This 

 shelf, which is now being rapidly eroded, was cut in mica- 

 ceous gray Franciscan sandstone, interbedded with thin strata 

 of lignite and carbonaceous shale. The deposits which cover 

 a portion of this wave-cut shelf consist of incoherent tan- 

 colored sandstone, 10 to 30 feet in thickness. The Franciscan 

 rocks in Hog Island are covered by a similar deposit. These 

 last two are probably referable to the Tomales formation. 



General impressions obtained very largely from physio- 

 graphic studies on the tilted mainland block indicate that the 

 time interval required to strip the Merced strata from a con- 

 siderable area was a long one. This data will be presented 

 again in connection with a discussion of the physiography. 



Structure and Stratigraphy 



The dominant features of the area under discussion are, as 

 previously pointed out, the Point Reyes Triangle, the San 

 Andreas Rift Zone, the San Francisco-Marin Block, and the 

 Berkeley Hills Block. The Point Reyes Triangle and the 

 orogenic block on the northeast side of Tomales Bay are re- 

 spectively recognized by Lawson 25 as extensions of the Mon- 

 tara and San Francisco-Marin blocks. 



The Berkeley Hills Block which Lawson recognizes in the 

 San Francisco Bay Region, appears in the vicinity of Peta- 

 luma and Sonoma Mountain as a dominant orographic feat- 

 ure. Concerning the relationship of the blocks in this area, 

 Lawson, after pointing out the geological differences between 



2S Lawson, A. C, San Francisco Folio, U. S. Geological Survey, p. 15, 1914. 



