Vol. 6] Merriam: Virgin Valley and Thousand Creek. 



47 



position of the beds in the western part of the Thousand Creek 

 region could be accounted for only on the assumption of very 

 extensive faulting along Thousand Creek Ridge. The fossil- 

 iferous beds immediately east of this ridge are now at least a 

 thousand feet lower than the uppermost beds in Virgin Valley, 

 while the drop of the mesa cap east of the fault-line along 

 Thousand Creek Ridge to the north amounts only to a little more 

 than four hundred feet, which is not sufficient to account for 

 more than half of the difference in position, even when original 

 slope of the land and possible recent tilting of the whole region 

 to the east are considered. 



The Railroad Ridge mesa, which contains some of the im- 

 portant deposits of the Thousand Creek region, is capped with a 

 basaltic lava which is considered by Professor G. D. Louderback 

 and Mr. E. L. Ickes, who have examined it, as representing the 

 same type of rock as that in the Mesa Basalt. The capping of 

 Railroad Ridge is about four hundred feet lower than that 

 portion of the main mesa to the north, which has been faulted 

 down to the east of the Thousand Creek Ridge fault. There is 

 therefore some reason for considering that Railroad Ridge, and 

 presumably the mammal beds exposed near it, belong to a block 

 which has dropped very far below its original level. 



Mr. Heindl, who has examined the section of Railroad Ridge 

 (see fig. 5, p. 44), finds the uppermost beds composed of very 

 coarse gravels consisting of pebbles of rhyolite, basalt and 

 obsidian, and has suggested that this ridge represents an ancient 

 lava-filled river bed. The course of the ridge runs out from the 

 vicinity of the existing canon of Thousand Creek, and would 

 suggest a drainage passing near the line of the existing canon 

 (see pi. 2). If the river bed was present immediately before the 

 outpouring of the Mesa Basalt and before the later movements 

 along the Thousand Creek fault, this portion of the lava flow 

 might be presumed to have resisted erosion longer than the 

 adjoining portions owing to the original greater thickness of 

 the lava over the channel of the old stream. 



The idea that the Railroad Ridge lava represents the basalt 

 filling of an old river bed also suggests that the Thousand Creek 

 Beds might have accumulated in part from erosion of the western 



