78 GEOLOGICAL SURVEY OF THE TERRITORIES. 



stones, underlaid by well known upper cretaceous clays, outcropping on 

 the left hand, and the rather curious, mound-like ridges of the dark plastic 

 clays of the lower cretaceous, on the right. Along the flanks of the 

 mountains it is quite seldom that all the formations are clearly shown. 

 Sometimes portions have been removed by erosion or concealed by drift 

 deposits ; or all the beds are so crushed together that only two or three of 

 them are exposed to the eye. At this locality for a short distance, 

 perhaps one or two miles, the lower cretaceous clays are remarkably 

 well shown, lying on the sides of the mountains; while the Jurassic, red- 

 beds, and carboniferous are scarcely seen or are concealed entirely. We 

 know that they must exist here in their full development, for just west 

 of Rattlesnake Creek all these formations are very clearly shown in 

 lofty, well-defined ridges,*inclining at an angle of 20° to 40°. These facts 

 show the importance of careful detailed study of the geology of all the 

 mountain districts, and the necessity also for the construction of a suita- 

 ble topographical map on a sufficiently large scale to show these changes 

 clearly. 



Near Fort Halleck the carboniferous limestones rise high up on the 

 sides of the mountains, resting upon the granite. All the more recent 

 beds are worn away or so concealed as not to be visible. East of 

 Fort Halleck the ridges of sandstone bend off to the northwest, while 

 to the east and southeast the sedimentary rocks jut up against the sides 

 of the foot-hills of the mountains. 



From Elk Mountain to Little Laramie River there are no remarkable 

 exhibitions of the uplifted sedimentary rocks. For the most part the 

 foot-hills near the base of the mountains are quite smoothly rounded 

 off, and covered with grass ; and only here and there are any of the beds 

 exposed. High up in the mountains, toward the sources of the Big and 

 Little Laramie rivers, are some fine exposures of the entire series of sed- 

 imentary beds ; but these will be described more fully in a subsequent 

 portion of this report. 



Along the flanks of Elk Mountain there is exposed, in one or two 

 localities, a vertical ridge of gray sandstone and quartzite, which I have 

 regarded as No. 1, or lower cretaceous. The rock does not differ in tex- 

 ture from the same formations as seen along the eastern slope of the 

 Laramie range from Red Buttes to Cache a la Poudre and southward ; 

 but in this instance it forms, for a short distance, a high vertical wall, 

 pressing up close to the granite nucleus, while the carboniferous lime- 

 stones are carried up on the tops of the granite to the summit of the 

 mountain. 



Our camp on the Medicine Bow Creek was a pleasant one, with an 

 abundance of timber, and the greatest supply of grass for our animals. 

 The bottoms are wide and of inexhaustible fertility. The valleys of all 

 the little branches furnish the most extensive and valuable pastoral 

 districts for stock. Beaver dams occur everywhere, and sometimes flood 

 a space half a mile in width. In the branches of the Medicine Bow the 

 plastic clays of the lower cretaceous prevail, in a horizontal position. All 

 the formations seem to jut up against the mountain sides with very 

 little inclination. The river emerges from a sort of jog in the range, 

 ten miles or more south of Elk Mountain, and the cretaceous beds, nearly 

 horizontal, extend down into the plains in long, grassy, bench-like ridges. 

 On the Wagon Hound Creek are some beds of sandstone in a nearly 

 vertical position, or inclining at a high angle from the mountains, which 

 are intercalated with beds of clay and coal seams, evidently the beds of 

 passage to the tertiary. 



After passing the Medicine Bow Creek eastward, the country assumes 



