4 DESCRIPTIVE GEOLOGY. 



hills and knolls and depressed basins and valleys, wliicli evince a consid- 

 erable amount of denudation in recent times. They are entirely destitute 

 of any growth of trees, except in a few favorable localities, such as the 

 more sheltered positions on the Chalk Bluffs, and along some of the larger 

 water- courses, as the Cache la Poudre and Chugwater. 



The characteristic features of the plain country extend, as already sug- 

 gested, close up to the base of the Colorado Range, whose foot-hills rise up 

 in a very abrupt and marked manner, presenting an exceedingly diversified 

 physical and geological aspect of ujDturned sedimentary beds lying between 

 the monotonous nearly level country to the eastward and the rugged central 

 mass of the range. 



Before proceeding with a description of the foot-hills and their relations 

 to the later formations, a short discussion will be given of the leading topo- 

 graphical and geological features of the main mountain-range, or rather that 

 portion of it that falls within the limits of our map. 



Physical Desceiption of the Laramie Hills. — Several designations 

 have been given to the different groups of elevation making up the great 

 mountain-chain which forms the eastern rim of the Cordilleras, and extends 

 from New Mexico to tlie northern limits of the United States. 



To that relatively depressed portion lying mostly within the limits of the 

 41st and 42d parallels of north latitude, the name Larainie Plills has been 

 applied. Their natural boundary to the north is the Laramie River, which, 

 after running northward through the Laramie Plains, turns somewhat 

 abruptly, and breaks through the chain a few miles beyond the line of the 

 map of the Fortieth Parallel Survey, while, to the southward, the North 

 Fork of the Cache la Poudre may be considered their limit, giving them a 

 length of about 80 miles and a width varying from 14 to 18 miles. 



In their topographical features, the Laramie Hills present marked con- 

 trasts to the rugged groups both to the north and south, forming, as they 

 do, a low mass between two higher members of the great chain. Their sum; 

 mit forms, in its general outline, a broad, undulating plateau, with rounded, 

 easy-sloping ridges falling away gradually to the eastward; the higher por- 

 tion of the hills being invariably the western edge. 



The average altitude of this broad summit varies from 7,800 to 8,300 



