12 GEOLOGICAL SURVEY OF THE TERRITOEIES. 



of grass; liere and there in the valley of the creek, or some side ravine, 

 the rocks are exposed, revealing their age to be upper tertiary. Cream-col- 

 ored marls and sometimes deep, ferruginous sands are seen, hut the pre- 

 vailing color of the rock material is light drab-yellow. The valley of the 

 Lodge Pole Creek will average about six miles in width from bluff to 

 bluff. The surface of the highlands is covered to a greater or less ex- 

 tent with gravel and stray boulders of moderate size, varying from that 

 of a pin's head to a foot in diameter. Masses of carboniferous lime- 

 stone are very abundant, showing that these beds were very accessible 

 to the waters during the later drift period. Some of these limestone 

 masses are quite full of fossils, as crinoidal stems, Athyris subtilita, 

 Orthis, and Ckonetes. A careful examination of the stray boulders scat- 

 tered upon the plains will enable one to determine with a great degree 

 of certainty what formations are revealed along the flanks of the moan- 

 tains. I take the position that these superficial deposits are the result of 

 forces acting from the mountains toward the plains, and that in sweep- 

 ing down from the flanks across the upturned edges of the beds -of 

 different geological periods, as there exposed, they carry portions of each 

 formation with them, and strew them over the plains. Wherever a form- 

 ation is well developed and exposed, there the more fragments of it will 

 predominate. When the red sandstones are largely exposed, then the 

 drift will be filled with fragments of red sandstone, and the same is in- 

 variably the case with the carboniferous strata. 



The Laramie Mountains, or Black Hills, as they are usually called, 

 form one of the shore lines of a great freshwater lake, which covered 

 an enormous area on the eastern slope of the Rocky Mountains during 

 the middle and upper tertiary epoch. When the waters were drained 

 away from this lake basin, a vast thickness of clays, marls, sands, and 

 sandstones was left high on the sides of the mountains, sometimes 

 reaching nearly to the crest or divide. In many localities the beds have 

 not been subjected to as much denudation as in others, and in that case 

 they jut up against the mountain sides so as to conceal all the older un- 

 changed strata, and not unfrequently concealing the metamorphic rocks 

 over large areas. 



Along the immediate line of the Union Pacific Railroad, the tertiary 

 beds form a sort of bench, which rises gradually from Cheyenne nearly 

 to Granite Station. The tertiary beds are stripped off only to a moderate 

 extent, revealing a bed or two of carboniferous limestone. A vertical 

 section would show the upper tertiary deposits resting directly, though 

 unconformably, on the carboniferous limestones, and the latter lying 

 on the granites. But on either side, north or south, not only the car- 

 boniferous rocks are exposed, but the red beds and, perhaps, the triassic 

 or cretaceous. All along the flanks of the mountains, from Granite Caiion 

 Station northward to the northern boundary of this lake basin, we know 

 that formations of the age of carboniferous, triassic, Jurassic, cretaceous, 

 and perhaps lignite tertiary, exist, whether exposed by the denudation of 

 the White River tertiary beds or not. It is proper, therefore, to color all 

 these formations on a geological map by bands or zones along the sides 

 of mountain ranges. Though if a map were constructed on a large scale 

 and the geology colored in detail upon it, these bands would be some- 

 what interrupted here and there by the concealment of one or more of 

 the formations, by modern tertiary, or drift deposits. The valleys of 

 the little streams, as they extend down into the plains from the moun- 

 tains, are usually quite rugged at first, but become less so until the sides 

 are rounded and grass-covered. But along the immediate base of the 

 mountains there is often a valley at right angles to the /alleys of the 



