Vol. XI] DICKERSON—PT. REYES AX D SANTA ROSA QUADRANGLES 535 



from their mouths as a result of the general subsidence of 

 this region. In most of the streams in the middle portion 

 of the block, wide valley stages are characteristic of their 

 middle courses. Owing to initial differences the streams in 

 the block have been differently affected by the same general 

 movement within the San Francisco-Marin Block during 

 Pleistocene time. Professor Holway concluded that the 

 Lower Russian River was a part of an old stream which 

 existed when the whole region was near sea level, and that 

 Mark West Creek was the direct upstream extension of the 

 Lower River. A glance at the general index map in this 

 publication (See Plate XX) indicates a peculiar condition of 

 Lower Russian River. This portion of the stream is in- 

 trenched in Franciscan rocks, but a study of the distribution 

 of the Merced strata in this region and the Pleistocene plain 

 which truncates them near Freestone, indicates that the 

 Pleistocene Russian River attained the essentials of its present 

 course as a consequent East- West stream upon a low coastal 

 plain which was developed upon Merced strata. Merced 

 strata occur at the town of Plantation, a few miles north of 

 Russian River Canyon and at Freestone, only a few miles 

 south of Russian River. The immediate region bordering 

 the Lower Russian River is wholly composed of Franciscan 

 rocks and when one stands upon hills bordering the canyon 

 of Russian River, an old plateau surface is clearly apparent, 

 but this surface is not a perfectly even one and considerable 

 elevations rise above it. The distribution of Merced strata 

 as stated above shows that this old surface was once covered 

 by the muds and sands of a Merced sea. The Pleistocene 

 surface, which is very evident two miles south of the main 

 stream around Freestone (See Plate XXVI) can be traced a 

 few miles north of Freestone to the vicinity of Occidental, 

 where Osmont reports the presence of boring mollusks. To 

 summarize briefly the evidence, a consequent Mark West Rus- 

 sian River of Pleistocene age developed upon a wide low 

 coastal plain. After a slight uplift this stream became in- 

 trenched in this plain and since the Merced strata were soft, 

 cut rapidly downward until the old pre-Merced erosion sur- 

 face which truncated Franciscan rocks was reached. This 

 surface acted as a temporary base level for a time and the 



