tlie Muddy Mountains, Nevada, etc. 55 



and other structural features will be considered in their 

 relation to separate ranges. 



Intermontane faidts. — At the top of the upper Grand 

 Wash Cliff the Kaibab limestone lies approximately 4,000 

 feet higher than tilted blocks of the same formation on the 

 floor of Grand Wash Valley. The valley represents a 

 zone of large normal faults, all with downthrow on the 

 west. The largest of these displacements, represented in 

 part by the Cliffs, amounts to about 7,000 feet near Colo- 

 rado River but decreases to the north. Since the original 

 faulting, the upper cliff has receded 2 to 6 miles, and the 

 lower cliff has been greatly dissected. The greater re- 

 treat of the upper cliff is due mainly to the weak Supai 

 sandstone at its base ; but it is possible that the faulting 

 occurred in stages separated by long time intervals, and 

 therefore that the upper cliff has suffered much longer 

 erosion than the lower. Lack of coarse materials in the 

 intermontane clays near the foot of the lower cliff sug- 

 gests recurrence of faulting in comparatively recent time. 



Virgin Valley is also an important fault zone. Closely 

 spaced faults parallel to the river and with large down- 

 throw to the west are represented by high scarps on the 

 west side of the Virgin Range. North of St. Thomas out- 

 crops of pre-Cambrian schist end abruptly on the east 

 side of the river, and on the Muddy Mountain side Meso- 

 zoic and Tertiary sediments dip steeply eastward. The 

 total displacement amounts to many thousands of feet, 

 but precise measurement is not practicable. 



Faults bound the Muddy Mountains on the west, at 

 least in part, and it is probable that California Wash is a 

 fault zone similar to Virgin Valley. 



Structure of the Muddy Mountains. 



In the Muddy Mountains there are three well defined 

 structural divisions, separated by lines extending gener- 

 ally east and west. The northern division, between the 

 Arrowhead fault and Muddy Creek, is characterized 

 chiefly by folding, with faulting an important but second- 

 ary factor. The central division consists of Callville 

 Mountain, an irregular remnant of an overthrust block 

 which is bounded as well as cut by large normal faults. 

 In the southern division there is a series of folds with 

 axes extending approximately northeast and southwest, 



