58 ■ DESCRIPTIVE GEOLOGY. 



Dakota Cretaceous. The Red Beds are cliaracterized by heavy, coarse 

 sandstones, of an intensely deep red color at the base, shading off into 

 light yellowish-red near the top, both the Triassic and Jurassic carrying 

 the lime-beds well developed, as limestone and gypsum. In the Triassic, 

 calcareous layers, more or less mixed with sand, would appear to occupy 

 a thickness of 50 feet ; both the thin beds of limestone, which charac- 

 terize the Red Beds in other places, being well developed, rising above 

 tl^G sandstone in low, regular ridges. The gypsum deposit, a nearly pure 

 white bed, here attains a thickness of at least 25 feet, interstratified in dark 

 red sandstone. In the Jurassic, the light gray cherty limestone, which is 

 a persistent and well-marked feature in the horizon, crops out in a bed 

 nearly 10 feet in width, both underlaid and overlaid by fine, light marls. 



The line of separation between Triassic and Jurassic, at best an arbi- 

 trary one, based chiefly upon lithological distinctions, is, at the entrance to 

 the Big Thompson Canon, somewhat difficult to follow, as the marly beds 

 show a considerable vertical expansion, passing down gradually into the 

 Red Beds. 



Cretaceous Plains op Colorado. — The clays of the Fort Pierre 

 division of the Colorado group become more and more arenaceous toward 

 the top, and, by imperceptible gradations, pass up into well-defined sand- 

 stones of Upper Cretaceous strata. This great development of sandstone 

 embraces both the Fox Plill and Laramie divisions. It covers by far the 

 greater part of the area east of the Colorado Range, extending beyond 

 the limits of our exploration both eastward and southward, forming the 

 characteristic feature of the Great Plains. The lower member, the Fox 

 Plill sandstone, lies directly upon the Fort Pierre clays, whose upper beds, 

 flattening out to the eastward, are rarely seen inclined at a higher angle 

 than 5° or 6°, with an average dip of 2° to 4'^. The junction between 

 the two formations is generally marked by a low ridge, 2 or 3 feet in height, 

 presenting a mural face toward the mountains, but sloping off gently east- 

 ward. This wall, which constitutes a somewhat striking physical feature, 

 is formed by the soft clays having been eroded into shallow longitudinal 

 valleys, and irregular depressions and basins, partially filled in with detrital 

 accumulations, leaving the overlying harder sandstone at a slightly higher 



