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



stream no longer cut downward but swung from side to side, 

 making a wide valley in the sandstones of the Merced. Later 

 in the Pleistocene, the San Francisco-Marin and the Berkeley 

 Hills blocks were uplifted together and tilted from the west 

 to the east. Since the Mark West Russian River had de- 

 veloped a strong system of tributaries in the mountains north- 

 east of Santa Rosa, its drainage area was the largest of any 

 of the consequent streams within these two blocks. Owing 

 to its greater volume this stream succeeded in maintaining its 

 course against the uptilting of this block during Pleistocene 

 time. Owing to the tilting of this block Santa Rosa Valley 

 became a basin of accumulation and it is the writer's opinion 

 that Laguna de Santa Rosa is ponded as a specific result of 

 the West-East tilt. In other words, there is a neutral zone 

 within this tilted block in which we have a balanced condition 

 as respects drainage. 



A part of the story was derived from the study of some 

 of the smaller streams south of Russian River. Salmon 

 Creek has a lower course intrenched in Franciscan rocks, and 

 a middle and upper course which is in a notably wide valley 

 whose surrounding hills are composed of Merced strata and 

 whose valley bottom is underlain at no great depth by rocks 

 of the Franciscan group. A connecting link between the 

 mouth of this stream and its upper and middle course was 

 found about two miles down stream from Bodega town. As 

 one looks across Salmon Creek from this place to the north, 

 well developed stream terraces, a couple of hundred feet above 

 the bottom of the gorge of Salmon Creek are seen. The 

 gorge of Salmon Creek is cut in Franciscan rocks and this 

 stream terrace is likewise composed of Franciscan rocks 

 until its northern boundary is reached, where Merced rocks 

 are encountered at once. This terrace then, is a remnant of 

 a temporary base level, the pre-Merced erosion surface, which 

 was developed upon Franciscan rocks and has been exhumed 

 by Salmon Creek during Pleistocene and Recent times. 

 Salmon Creek was not sufficiently powerful to maintain its 

 course across the entire block. During early Pleistocene time 

 Salmon Creek may have drained the area directly east of 

 Freestone in the vicinity of Sebastopol and Santa Rosa, but 

 the tilting in later Pleistocene time probably diverted the 



