24 GEOLOGICAL SURVEY OF THE TERRITORIES. 



tertiary, once covered them in their full thickness, but they have all 

 been swept away. Now the more modern tertiary beds jut up against 

 their sides or are deposited high up in the ravines, indicating by their 

 position that this lake existed here after the mountains had attained 

 nearly their present elevation. There is also an anticlinal valley more 

 or less distinct, extending across the intervening country, connecting 

 those elevations with the Black Hills and the Laramie Eange. The fact 

 that nearly all of the ranges, however small they may be or distinct 

 from each other, are connected together by some link however obscure, 

 illustrates the unity and simplicity of the Eocky Mountain system. 



CHAPTER II. 

 FROM FORT FETTERMAN TO SOUTH PASS. 



On the morning of the 20th we left Fort Fetterman for the South Pass 

 by way of the Sweetwater. Our first camp was on Box Elder 

 Creek, twelve and one-third miles up the valley of the North Platte. 

 The lignite tertiary beds are the only rocks exposed, and they incline 

 north and northwest 5° to 15°. They are composed of rusty sands and 

 sandstones, arenaceous clays, with some seams of impure lignite. Some 

 of the sandstones are largely concretionary, and break in pieces readily 

 on exposure. The high ridge to the west of us trends about north- 

 east and southwest. Along the base of the ridge are some terrace-like 

 benches, perhaps broken fragments of higher levels not swept away. 

 They extend at intervals as far as Fort Casper. In some instances 

 they jut up closely and even lap on to the flanks of the ridge 

 They are remnants of the White Eiver group, which once extended un- 

 interruptedly over the whole valley close up to the ridge, but which has 

 been washed away, except these fragments. They now seem to give 

 that beautiful bench-like aspect to the surface, which, contrasting with 

 the rugged features of the ridge, adds interest to the scenery. They 

 nowhere incline more than 5°, and in most instances are horizontal, 

 never conforming with the older formations. It is most interesting, by 

 means of these remnants, to trace the old shore-line of the lake, which 

 can be done as perfectly and as clearly as that of any of the northern 

 lakes of the present time. This hog-back or ridge seems to have formed 

 an effectual barrier to these waters on the north side of the Laramie 

 Mountains. 



The Box Elder Canon affords a good section through the ridge. It is 

 a much more regular gorge than the canon of the La Prele, and is purely 

 one of erosion, and is six hundred to eight hundred feet in depth. It is 

 so narrow that it is difficult to pass through it on foot, and the sides are 

 perpendicular, and sometimes overhanging. The predominating rocks 

 are sandstones, or calcareous sandstones, with some layers of quite fine 

 limestones, but there is a great variety of texture. Some of the layers 

 indicate very quiet deposition for forty or fifty feet, but the greater por- 

 tion of the sandstones is full of irregular layers, showing clearly the 

 nature of the waters at the time. The prevailing color is light yellow, 

 varying to a deep rusty hue, but there is often a tinge of brick red which 

 is marked at some localities. The sandstones are full of beautiful sili- 

 ceous geodes. The crystals of quartz are mostly small, but very clear, 

 and are set in a layer of amorphous silica or chalcedony as a paste. In 

 the cherty limestones are very distinct fossils, as Athyris, Orthoceras, 



