34 



University of California Publications. 



[Geology 



The thickness of the Virgin Valley Beds, measured from 

 the uppermost strata to the floor of the valley, is about 1500 

 feet. The beds are evidently thinnest around the edge of the 

 valley, and the bottom of the formation is not reached at the 

 point where the measurements were made. Even taking into 

 account a slight dip of the beds from the highest point toward 

 the lowest level, the total thickness may be estimated at over 

 1500 feet. 



Throughout the whole extent of the valley the strata are 

 found to vary only a few degrees from a horizontal position, 

 excepting through landslides. Such variation from the hori- 

 zontal as is shown expresses a gentle syncline with the depression 

 near the middle of the valley. This is presumably due mainly 

 to a slight deformation which has taken place since the principal 

 accumulation occurred. It may be due in a small part to con- 



t 



formation to the form of the trough in which the beds were 

 laid down. 



The Virgin Valley Beds are almost entirely made up of 

 volcanic ash and tuff. At several horizons gravel, sand, clay, 

 lignite, and diatomaceous deposits occur, but are of much less 

 volume than the beds of purely volcanic origin. 



The lowest strata recognized in the valley are ashes and tuffs 

 dipping gently to the west along the western side of Thousand 

 Creek Ridge (pi. 5). These strata are predominatingly white, 

 but range in color through bright red, purple and green. This 

 phase is overlain commonly by a series of dark red beds which 

 have quite an extent along the stream bed in the northern part 

 of the valley. 



The lowest beds of the series rest upon the irregular surface 

 of the Canon Rhyolite. Such evidence as is available suggests 

 unconformity between the lower Virgin Valley Beds and the 

 Canon Rhyolite. 



The white ashes of the lowest portion of the section have 

 probably accumulated quickly. The dark red horizon presum- 

 ably represents a period of slower accumulation with extreme 

 oxidation. The mode of accumulation of the lowest beds 

 examined is not strongly suggested by any evidence obtained 

 thus far, but they are not improbably aeolian. The red beds 

 are possibly aeolian. 



