38 



University of California Publications. 



[Geology 



has exposed splendid sections of the region formerly covered 

 by the basalt, no dikes or fissures have been observed through 

 which this lava has come to the surface. Such sources may, 

 however, appear in localities not yet visited. The extent of this 

 flow appears rather remarkable when considered in relation 

 to its thinness. It is difficult to understand how a flow could 

 extend itself so broadly without heaping up more than is 

 indicated in this section. 



Taking into consideration the thinness of the flow and the 

 evenness of the floor upon which the Mesa Basalt was laid 

 down, the present aspect of the table-lands surrounding Virgin 

 Valley may be considered as closely representing the nature of 

 the topography of this region during the latter part of the 

 epoch of deposition of the Virgin Valley Beds. If the lava 

 sheet were removed, the sedimentary beds below would form a 

 nearly level plain reaching well up on the side of the range 

 to the south. Many of the salient features of the topography 

 which existed in early Virgin Valley time would be completely 

 buried, while a few of the highest points would project as islands 

 rising sharply above the surrounding ash accumulation. 



History Subsequent to Outpouring of Mesa Basalt. — No 

 accumulation of sediment has been observed to rest upon the 

 Mesa Basalt. Though such formations may possibly exist in 

 localities that were not visited, the impression received in a 

 general survey of the table-land region is that the basalt sheet 

 was the last deposit laid down in the region anterior to the 

 events that initiated the cycle of erosion during which the 

 present valley was excavated. 



Movements following the outpouring of the basalt sheet are 

 evidenced in the presence of a sharply-marked fault along the 

 line of the scarp following the east face of Thousand Creek 

 Ridge. The basalt cap to the north of Thousand Creek Canon 

 is sharply cut off along the extension of the axis of the ridge, 

 the mesa on the east side of the jog dropping a little over four 

 hundred feet below the level of the mesa to the west. This 

 movement is a late phase of the adjustment of crustal blocks 

 which evidently began moving before the deposition of the 

 Virgin Valley Beds. It is not improbable that a small amount 



