THE MUDDY MOUNTAIN OVERTHRUST IN NEVADA 69 



stone are included. The breccia is well cemented with calcium 

 carbonate, and the base is smoothly polished. Above the zone of 

 typical breccia the limestone is intensely shattered through a thick- 

 ness ranging from 100 to 500 feet. The shattered condition of the 

 rock is responsible for the development of numerous grotesque forms 

 by the agents of erosion. 



There can be no reasonable doubt that the formation beneath 

 the overthrust is the Jurassic sandstone, corresponding to the 

 LaPlata group. No fossils have been found in this formation, but 

 its lithologic features are very distinctive. The only other for- 

 mation in the region that in any way resembles it is the Supai; 



Fig. 5. — View to the northwest across Muddy Peaks Basin, showing remnants 

 of dark Paleozoic limestone capping hills of Jurassic sandstone. 



and to anyone who has studied the two formations carefully the 

 differences are sufficiently apparent to prevent confusion. A thick- 

 ness of approximately 700 feet of the sandstone is exposed in Muddy 

 Peaks Basin, and throughout this thickness the rock has large scale 

 cross-bedding, both straight and tangential; it has alternating 

 laminae with coarse and fine grains ; and it shows an irregular blend- 

 ing of red coloring with gray. All of these features are typical of 

 the Jurassic sandstone. 



Extent of the overthrust. — It is quite certain that the entire Call- 

 ville Mountain mass is included in the overthrust block, although 

 the actual surface of thrusting is exposed only in the "window" of 

 Muddy Peaks Basin. The mountain is a unit in stratigraphy, and 

 in all parts the lower beds show the intense shattering which is a 



