the Muddy Mountains, Nevada, etc. ttS 



structural origin, permits easy passage between the 

 Grand Wash and Virgin valleys. The highest part of 

 the Muddy Range is Callville Mountain, an abrupt mass 

 shaped roughly like a large letter ^^C with the open- 

 ing toward the south. The general elevation of the 

 mountain above sea-level is about 3,000 feet, but many 

 peaks rise to an elevation of 4,000 feet or higher. 

 Muddy Peaks, the highest points, are nearly 5,800 feet 

 above sea-level. Both north and south of Callville 

 Mountain the average elevation is slightly more than 

 2,000 feet, and abrupt ridges rise several hundred feet 

 higher. Everyw^here the surface is nearly destitute of 

 vegetation and has been carved into extremely rugged 

 forms. 



The valleys of Grand Wash, Virgin River, and Cali- 

 fornia Wash are each several miles in width, and have a 

 total area approximately equal to that of the intervening 

 ranges. The stream courses occupy comparatively nar- 

 row inner valleys, from which there is a gradual rise 

 toward the mountain walls on a series of broad terraces, 

 partly dissected into badlands. Colorado River has cut 

 canyons through the ranges, but its valley is wide and 

 comparatively open in crossing the intermontane troughs. 

 South of the Muddy Range the Colorado is less than 700 

 feet above sea-level. 



Rock Series and Formations. 



West of the Grand Wash Cliffs sedimentary formations 

 are separated into two especially distinct groups. The 

 first is a thick series resting on pre-Cambrian schists and 

 ending upward with Mesozoic sediments. This group 

 clearly corresponds to the series exposed in the Grand 

 Canyon and on adjacent plateaus, and, as in the case of 

 that series, the limestones, sandstones, and shales belong- 

 ing to many periods have remarkable conformity among 

 themselves. The second group has a distinctly younger 

 aspect, and is separated from the older series by an un- 

 conformity -of the first order. Below this break the 

 youngest rocks exposed are not higher in the time scale 

 than middle Mesozoic, whereas the oldest rocks of the 

 younger series are probably later than middle Tertiary. 

 Other unconformities within the younger group indicate 

 continued unrest and an incomplete sedimentary record 

 since the disturbances which first tilted the older rocks. 



