20 GEOLOGICAL SURVEY OF THE TEREITORIES. 



tratioQS of this structure are seen where the Big Thompson and Saint 

 Yrain's Creeks emerge into the plains. Great notches are there formed 

 from point to point along the east front of the mountains, from which 

 some of the most important streams, or their branches, emerge into 

 the plains. Cache ^ la Poudre, Big Thompson, Saint Yrain, Fountain 

 Creek at Colorado Springs, and the Arkansas River near Canon City, 

 are excellent examples. This feature in mountain structure is shown in 

 a still more emphatic manner farther to the northward, where the front 

 group flexes to the northwest, leaving, however, somewhat separated 

 the Black Hills of Dakota, the Big Horn range, and the numerous 

 smaller ranges on the Upper Missouri. These ranges, large and small, 

 are all linked together at some point more or less apparent by anticli- 

 nals. By examining a general map of the Western Territories it will be 

 seen that the Black Hills of Dakota are connected with the front range 

 near Fort Laramie, and that the Big Horn Mountains are united farther 

 north at Eed Buttes by a low anticlinal that crosses the interval, re- 

 vealing no rocks older than the Cretaceous. These lines of connection 

 are best shown by colors on a geological map. 



The illustration (section 1) will show quite clearly the dying out in 

 the plains of one of these spurs or ridges. It is also a fine example 

 of an anticlinal. Big Thompson Creek cuts its channel through the 

 south end. The portion thus separated forms a conical hill about 120 

 feet high above the south base, or about 200 feet above the valley of 

 the Big Thompson. The Upper Cretaceous beds pass off in low semicir- 

 cular ridges southeast. The main mass of the hill is triangular in shape, 

 and is composed of the rocks, of various textures, which make up the 

 Dakota group. The character of the group is well shown in this local- 

 ity. The pudding-stones, made up of small rounded pebbles, seldom 

 more than one-fourth of an inch in diameter, smoothly polished, sand- 

 stones and quartzites of almost every texture, with slicken on a marked 

 scale, and with the surface lined with white amorphous quartz. 



The stream, which separates the triangular end of the anticlinal, cuts 

 directly through the ridge at right angles, and exposes in the section, 

 the red beds very distinctly. The trend of the ridges is about 20° west 

 of north, with the uplifted ridges on either side inclining 16° to 20° 

 east and northeast ; on the west side of the ridge, No. 1 presents an 

 almost vertical wall for a mile or more, rising from a few feet to 100 

 or 150 feet in height, resembling the broken wall surrounding some 

 old city. This will always be pointed out to the traveler as one of the 

 curiosities of the region, aside from its geological interest. Just inside 

 is the rather thin group of Jurassic beds ; in the aggregate about 200 

 feet in thickness, made up of irregular thin layers of indurated arena- 

 ceous clay and sandstone, with two or three beds of limestone. Not a 

 trace of a fossil could be found, although a hundred and fifty miles 

 north well-marked Jurassic fossils are abundant ; and twenty-five miles 

 north they occur to some extent. The thinning out of the Jurassic 

 group in its southward extension is well marked, unless we include the 

 red beds among rocks of that age. Below the well-marked Jurassic 

 group the red sandstone appears, forming several small, rather low, 

 ridges, with thin beds of blaish-gray limestone, quite impure, but used 

 for burning into lime. Here and there, in close proximity to these lime- 

 stone layers, we have irregular deposits of gypsum. It is only in the 

 red group that these gypsum-deposits are sufficiently developed for 

 economical use. Gypsum in some form occurs in all the formations 

 above the Carboniferous ; but in the Cretaceous and Tertiary forma- 

 tions it is found mostly in the form of selenite. The origin of the gyp- 

 sum is so weU known that I will not refer to it in this place. It may be 



