STRUCTURE OF BIGHORN MOUNTAINS 543 



are often extensively exhibited. The structure presented locally is usually 

 that of a monocline dipping toward the Plains on the east side and the 

 Bighorn basin on the west. The oldest sedimentary rocks usually are at 

 the base of a ridge facing the interior granite area, and each higher 

 stratum passes beneath a newer one in regular succession outward toward 

 the margin of the uplift. In figure 1 there is given a cross-section show- 

 ing the general structure in the higher portion of the uplift. 



In this section the anticline has steep sides, especially on the west, but 

 in places the steeper dips are on the east side, and in some portions of the 

 uplift they are gentle on both sides. In the northern portion of the area, 

 where the strata cross the arch, the crest of the uplift is nearly flat. 



Figure 1. — Typical Section across the Bighorn Mountains. 



Showing the general structure in the higher portion of the uplift. C, Deadwood for- 

 mation (Middle Cambrian) ; O, Bighorn limestone; LC, Madison limestone (Lower Car- 

 boniferous) ; UC, Amsden and Tensleep formations (Upper Carboniferous) ; R, Red beds ; 

 M, Sundance (Jurassic) and Morrison ; CI, Cloverly sandstone ; Co, Colorado group ; P, 

 Pierre shale; F, Parkman sandstone (Fox Hills?). 



Several great faults break the monoclinal slopes, and in the southern por- 

 tion of the range a great dislocation extends for many miles along the 

 higher portion of the mountain, parallel to the strike of the uplift and 

 near its crest. 



ROCKS 



The sedimentary formations consist of a series of thick sheets of sand- 

 stone, limestone, and shales, all essentially conformable in structure, 

 although lacking some members of the geologic succession. Quaternary 

 deposits of glacial origin lie on the granites in the highlands, and fluvia- 

 tile gravels and sands are on terraces overlapping the older sedimentary 

 rocks on the Plains. The stratigraphy presents many features of simi- 

 larity to the succession of rocks in the Eocky mountains of Colorado and 

 Wyoming and the Black hills, but it possesses numerous distinctive local 

 features. The following is a list of the formations which are exhibited 

 in the uplift, with a generalized statement as to thickness, characteristics, 

 and age. The broader features of distribution are shown in the map 

 (plate 73). 



