48 



University of California Publications. 



[Geology 



fault block previous to the outpouring of the Mesa Basalt. If 

 extensive movement occurred along the Thousand Creek fault 

 line before the outflow of the Mesa Basalt, accumulation may 

 have taken place to the east of this line. During the time of such 

 accumulation probably no deposits would be formed over the 

 Virgin Valley region. Unless the whole region were reduced to 

 the same level following such differential movement, one would 

 expect to find the Mesa Basalt accumulating to much greater 

 thickness east of the fault line, which is not clearly shown. A 

 movement in pre-Mesa Basalt time, such as is suggested here, 

 would presumably not result in more than a relative thickening 

 of the beds to the east of the fault line, and possibly in a tem- 

 porary interruption of sedimentation over the block west of the 

 fault line. 



Another possible explanation of the Railroad Ridge gravels, 

 and of the Thousand Creek Beds in part or as a whole, is that 

 they have been derived from the wash of Thousand Creek or 

 other similar streams during the cutting of Virgin Valley. As 

 a rough estimate, we may consider that at least ten cubic miles 

 of rock have been carried out of Virgin Valley since the initia- 

 tion of the cutting of the present valley. As nearly as one may 

 judge, the distance to which this material could have been 

 carried was short, and it could have been deposited over only 

 a small area. It is therefore not improbable that some part of 

 this material may have been deposited on the east side of 

 Thousand Creek Ridge, particularly after the faulting move- 

 ments occurred along the line of this ridge. 



According to the hypothesis just suggested, it would be neces- 

 sary to consider either that the Thousand Creek Beds have been 

 lowered by faulting since their deposition, or that they were 

 accumulated very late in the history of the cutting of Virgin 

 Valley. The beds forming Railroad Ridge are now far below 

 the top of the mesa in Virgin Valley, and we can hardly imagine 

 them as derived from the first sediment washed out in the cutting 

 of the uppermost strata of the Virgin Valley Beds a few miles 

 away and deposited in their present position. Without con- 

 sidering that differential movement has changed the position of 

 these beds in relation to the Virgin Valley Beds since their 



