EARLY AND RECENT GEOLOGIC EVENTS 279 



marble, quartzite, argillite, and granite. It is obviously of river origin 

 and was deposited in the valley which the Colorado river had previously 

 eroded in the older detritus. 



Another deposit, probably equivalent in age to the older filling of 

 Grand Wash trough and evidently accumulated before the Colorado 

 river was established in its present course, is found within Detrital- 

 Sacramento valley near the mouth of the Virgin river and consists mainly 

 of soft beds of sand and clay containing gypsum and rock salt. The 

 occurrence of gypsum and salt in Detrital-Sacramento valley at this 

 point is not easy of explanation on the assumption that this valley was 

 excavated by the Colorado river. It is possible, however, that the saline 

 deposits may antedate the formation of the valley, and that the ancient 

 Colorado flowed across them in eroding it, just as the river in its present 

 position crossed them in eroding the recently formed canyons. 



The suggestion that Detrital-Sacramento valley was formed by the 

 Colorado river is strengthened by the observations of Huntington and 

 Goldthwait,* who show that after the first period of faulting of the Colo- 

 rado plateau, represented in the region here described by the displace- 

 ment at Grand Wash fault, which formed Grand Wash trough, and pre- 

 vious to the last period of extensive faulting and the uplift of the Colo- 

 rado plateau, a considerable area north of the Grand canyon in the vicin- 

 ity of Toqueville, Utah, was reduced to a peneplain. The Colorado 

 plateau was undoubtedly much lower than now and the graded plain just 

 described near the mouth of the Grand canyon is probably a part of this 

 peneplain. It is assumed that the planation may have been accom- 

 plished by the Colorado river flowing at that time across the plateau 

 region north of its present course and thence southward through Detrital- 

 Sacramento valley. 



Recent geologic Events 



is trod uctob y statements 



After the formation of the Toqueville peneplain, the second faulting 

 and uplift of the Colorado plateau occurred, with a displacement of 5,000 

 feet or more at Grand Wash fault. Whatever the previous course of the 

 river may have been, this uplift fixed it in its present course by causing 

 it to erode the Grand canyon. West of the plateau it flowed across Grand 

 Wash trough, thence westward through the Virgin range of mountains to 

 the previously formed Detrital-Sacramento valley, which it evidently fol- 



* Ellsworth Huntington and J. W. Goldthwait : The Hurricane fault in the Toqueville 

 district, Utah. Harvard Collection, Museum of Comparative Zoology Bull., vol. 42, 

 1904. 



