580 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



del Creek, flowing in a northwesterly direction, cuts deeply 

 into a low basaltic ridge two miles southeast of Melitta. It 

 is very possible that this stream has followed the course of 

 a fault line as being the easier way, although a comparatively 

 wide, undrained valley one-half mile to the north would seem 

 the logical way. The writer was not able to spend sufficient 

 time upon this interesting problem to work out the details, 

 but it is quite apparent that the canyon course of Sonoma 

 Creek south of Kenwood is clearly antecedent to the fault 

 movements which gave rise to Kenwood valley. Since a 

 northern branch of this stream was well developed before 

 the faulting, this northern branch was able to maintain its 

 course against the upthrust on the southwestern side of Ken- 

 wood valley. Likewise Kenwood valley did not drop rapidly 

 enough to divert the drainage out by way of Santa Rosa 

 Creek or north by way of Beltaine Pass, which the Southern 

 Pacific Railroad uses. The northern block, the Mayacamas 

 Mountains, was uplifted with considerable rapidity. The 

 streams draining that block were greatly accelerated and 

 hence could carry a great load of debris which aided in filling 

 Kenwood valley graben. Rincon valley is apparently also 

 due to faulting. 



Sonoma Mountain 



The main mass of Sonoma Mountain is essentially a lava 

 plateau composed of nearly horizontally bedded basalts and 

 tuffs of the Sonoma group. (See Plate XXX, Figures 1 and 

 2.) Its northern flanks, Bennett Mountain and Taylor Moun- 

 tain, have evidently been subjected to considerable faulting 

 at the end of, or during, Pliocene time. Faulting along the 

 Hayward Rift has not affected the main mass of Sonoma 

 Mountain or its principal streams essentially, but minor 

 modifications in the drainage in the southwest flanks in the 

 vicinity of the Petaluma Reservoir and Roger's Creek are 

 results of this recently developed line of weakness. (See 

 Plate XXIX, Figure 1 ; Plate XXXVI, Figures 1 and 2 ; Plate 

 XXXVII, Figures 1 and 2.) The drainage of Sonoma Moun- 

 tain is distributed in three directions : Matanzas Creek and its 

 South Fork, and the south tributaries of Santa Rosa Creek 

 drain the vicinity of Taylor and Bennett mountains. Graham 



