Vol. XI] D1CKERSON—PT. REYES AND SANTA ROSA QUADRANGLES 573 



beds and the tuffs and lavas of Sonoma Mountain. It would 

 appear quite probable that the Point Reyes Triangle was like- 

 wise depressed at the end of the Miocene, as it is difficult to 

 account for the preservation of such a great thickness of the 

 Monterey group within such a limited area in any other way. 



Movements within the Dominant Blocks 



As Anderson indicated in the Geology of the Point Reyes 

 Peninsula, the Monterey strata have been folded in a broad, 

 shallow syncline. The general relations of this structure are 

 shown in the Section E-F which accompanies this report. 

 This folding took place in pre-Merced time, as the Merced 

 probably rests with unconformity upon the Monterey shale 

 at Bolinas Head, near the town of Bolinas, Tamalpais Quad- 

 rangle. 



The writer did not attempt to work out the structure in the 

 Franciscan rocks but it is probable that the dominant folds 

 which Lawson recognizes in the Tamalpais Quadrangle ex- 

 tend in a northwesterly direction. A fault between the 

 Sonoma group at Grand View and the Franciscan rocks is 

 apparently a movement which did not extend to the north- 

 west as no trace of it was found in Burdell Mountain. This 

 fault, however, may be a dominant structure in its southeast- 

 erly extension, but unfortunately a great mass of alluvium 

 prevents us from determining this point. From the general 

 nature of the structure in this part of the Coast Ranges it 

 seems quite probable that movement along this fault at the 

 end of Pliocene time or during the early Pleistocene may have 

 determined the form of Petaluma Valley. The northeastern 

 side of this fault is apparently the downthrown side. How- 

 ever, there are complications within the block, as the basaltic 

 conglomerate near Grand View shows. This basaltic con- 

 glomerate extends beneath the alluvium of Petaluma Valley. 

 The State Highway Engineers in charge of building the 

 Grand View bridge, Mr. Gerlach and Mr. Brown, presented 

 the writer with a section which indicates that these gravels 

 are found at a depth of 105 feet at the east pier of the bridge, 

 which is about 400 feet from the Grand View shore line. 

 Minor faulting or folding may have affected the Merced in 

 the vicinity of the Cinnabar School, near the southern edge 



