48 DESCRIPTIVE GEOLOGY. 



Chugwater Canon, the Carboniferous limestone presents a very marked 

 appearance, rising almost out of the plain, for several hundred feet, in a 

 nearly vertical position, its beds considerably metamorphosed. On the 

 summit of the ridge occurs a limestone bed, altered into a fair variety of 

 white marble. 



A short distance south of the Chugwater, the Palseozoic and Mesozoic 

 rocks become depressed, and disappear entirely beneath Tertiary gravels 

 that have been referred to the Wyoming Conglomerate of the Bridger 

 basin. 



On the Chugwater, some 4 or 5 miles below where it cuts through the 

 wall of Dakota sandstone, already mentioned, occurs a very considerable 

 development of all three subdivisions of the Mesozoic rocks. Unfortunately, 

 but little time was allowed for the examination of this group ; it would 

 appear, however, to derive its chief interest from the fact that, so far as 

 known, it forms within the belt of our exploration the only occurrence east 

 of the Laramie Hills of an outcrop of pre-Tertiary rocks, isolated from the 

 main ridges of uplifted beds along the flanks of the range. Where they 

 first lise above the surrounding Tertiary beds, the stream has cut a channel 

 through the dark ferruginous shales of the Fort Benton division of the Col- 

 orado Group, beyond which the stream runs through a narrow passage in 

 the Dakota sandstones. On the north side of the river, the Dakota beds 

 form quite a high ridge, dipping at an angle of 25° to 30° to the south- 

 ward, the entire series of rocks trending approximately east and west. North 

 of the Dakota ridge occurs a narrow valley of the Jurassic marls, with the 

 characteristic low ridge of Jurassic limestone in the middle, in turn under- 

 laid on the north side of the valley by the Eed Beds, which appear in a low 

 bench, the bright red soil marking the outlines of the formation for long dis- 

 tances. . 



So far as observed, this group of upturned beds only extends for 3 or 4 

 miles,, beyond which the Niobrara Tertiary covers all the older rocks, and 

 the Chugwater widens out into a broad open valley, with high bluffs and 

 walls of horizontal sandstone. 



Returning to the foot-hill ridges, the entire series are again seen coming 

 to the surface just above the northern tributary of Horse Creek. From 



