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



49 



deposition, it would be necessary to suppose the Thousand Creek 

 Beds formed from sediment obtained during the cutting of the 

 lower or later portion of Virgin Valley. 



The possibilities as to age of the Thousand Creek Beds with 

 relation to the Virgin Valley Beds appear to be as follows : 

 (1) They represent a portion of the Virgin Valley Beds faulted 

 down into their present position. (2) They are younger than 

 the Virgin Valley Beds, but older than the Mesa Basalt, and have 

 been moved down by faulting. (3) They represent an accumu- 

 lation formed of the older wash derived from the post-Mesa 

 Basalt erosion of the existing valleys of Virgin Creek and Beet 

 Creek, or other similar drainage, and have since their accumu- 

 lation been dropped by faulting. (4) They represent an accumu- 

 lation of sediment laid down during the period of erosion of the 

 lower or younger portions of these valleys. (5) They are not a 

 stratigraphic unit, and may be partly of the age of the Virgin 

 Valley Beds and partly later. 



Without more detailed geologic information than it has been 

 possible to obtain, it is not entirely clear as to which of these 

 possibilities corresponds to the actual history. 



The first possibility has much in its favor, viz., that the 

 Thousand Creek Beds represent a series of deposits which arc 

 comparable to the late Virgin Valley Beds and have been faulted 

 down to their present position. 



The second case suggests a situation which is a possibility, 

 though the evidence does not seem to indicate definitely that this 

 has been the mode of accumulation of these beds. 



According to the third and fourth possibilities, viz., that the 

 Thousand Creek Beds represent an accumulation of wash carried 

 out in the excavation of Virgin Valley and other valleys of 

 approximately the same age, it must be presumed that the 

 beginning excavation of Virgin Valley occurred a considerable 

 time before the close of the Tertiary, as the Thousand Creek 

 fauna antedates the end of the Tertiary. It would then be neces- 

 sary to consider that the comparatively thin sheet of Mesa 

 Basalt has been able to protect the Virgin Valley Beds beneath 

 it from erosion through the whole of the Pleistocene and a part 

 of Pliocene time, unless some later formation has in turn pro- 



