80 



University of California Publications. 



[Geology 



Age. — It seems likely that this fault is the northward exten- 

 sion of the San Bruno fault, which Lawson thinks was formed 

 about the time of the tilting' of the Merced. 



GEOMORPHOLOGY. 



From the summit of Mt. St. Helena a magnificent view is 

 obtained of most of the area embraced in this paper. On clear 

 days the Sierras may be seen in the bazy distance beyond the 

 great Sacramento Valley, while to the west steamers may be 

 sighted on the waters of the Pacific. To the North Cobb Moun- 

 tain shuts off the view, but southward is the beautiful Napa 

 Valley stretching away toward San Francisco Bay, with Mt. 

 Diablo plainly discernible in the distance. 



Tlte Elevated Coastal Peneplain. — An interesting feature is 

 the very even, almost horizontal crest line of many of these 

 ridges. The one immediately adjacent to the coast appears 

 to have a very uniform elevation in the neighborhood of 

 800 feet for many miles south of the Russian River. The 

 break where the latter cuts through scarcely makes any vis- 

 ible impression on this coast line. The ridge is composed entirely 

 of Franciscan strata, and has resisted erosion sufficiently to 

 retain a suggestion of its old topographic form. It is evidently 

 an old elevated and dissected peneplain. 



Wide, Flat-bottomed Valleys of Erosion. — One of the most 

 striking peculiarities of the topography is the succession of par- 

 allel ridges extending in a roughly northwest and southeast direc- 

 tion, and separated by wide, flat-bottomed valleys. 



Synclinal Valleys and Subsequent Drainage. — Between this 

 coastal ridge and Mt. St. Helena the hills are lower and not so 

 even in crest-line, being composed of hard lavas alternating with 

 soft tuffs and sandstones. This very fact has contributed to 

 the rapid maturing of the topography, causing the streams to 

 become largely subsequent in their nature, and to seek out the 

 synclinal and widen them as they approached base level, into 

 large, flat-bottomed valleys, such as Santa Rosa and Sonoma, 

 with well defined terraces up to 350 feet above the present flood 

 plain. 



