GEOLOGICAL SUEVEY OF THE TERRITORIES. 13 



streams and parallel with, the mountain ranges, and evidently scooped 

 out Tby forces acting from them. From the head of Crow Creek to the 

 Chugwater, there is a well-marked illustration of this type of valley. 

 It averages from five to ten miles in width, and the surface is gently 

 rolling, t and usually covered with grass. The little streams as they pass 

 across it do not cut deep channels. The eastern side is a high, abrupt, 

 irregular wall of White Eiver tertiary beds, oftentimes so eroded as 

 to present in some degree the architectural appearance of the " bad lands." 

 Through this wall the little streams have cut their channels, and flow 

 down through the plains in valleys with more or less bluff like hills on 

 either side, from one hundred and fifty to two hundred feet in height. 

 Where these parallel valleys occur at the foot of the mountains, the 

 changed as well as unchanged rocks have suffered great erosion. Here 

 and there they are omitted for some reason, and again appear in their 

 full proportions. Immediately north of Horse Creek there is a remnant 

 of the main "hog-back," or ridge remaining, composed of the triassic and 

 carboniferous beds, extending for about five miles, which is divided into 

 three parts by the channels of streams flowing through it at right angles 

 from the mountains. Its trend is nearly north and south, and its dip 

 east 5 and immediately west the granites and gneiss rise gradually 

 toward the crest of the range. This fragment of the main ridge shows 

 that but for erosion it would have been continuous all along the flanks 

 of the mountains. 



An interesting question arises as to the manner in which these parallel 

 valleys have been scooped out. That it must have occurred after the 

 deposition of the latest tertiary beds is evident, from the fact that the 

 streams which form the outlets have cut their way through them. As 

 I have before stated, the mountains formed the western shore of the 

 great fresh-water lakes of the middle and upper tertiary periods. As 

 the mountains were slowly elevated, so that the waters receded, there 

 was a depression at the immediate base of the mountains, of greater or 

 less depth, that received the drainage. The water- course would be 

 gradually formed for the principal streams and their branches. The 

 waters in the parallel valleys formed a sort of lake-like expansion of 

 the little streams, and the waters of the lake performed their work of 

 erosion at the same time that the streams wore their channels through 

 the plains. It is probable that since the close of the tertiary period, 

 and the commencement of the present era, the climate of the west has 

 been much colder; that ice and snow accumulated on the mountain 

 ranges in vast quantities ; and that the quantity of water to produce 

 the results which we find indicated by erosion and in the drift was far 

 greater than at present. It may be that ice was not the most important 

 agency, and though the evidence is clear that it performed an active 

 part, yet water was the principal agent, and the present existence of an 

 occasional moderate- sized boulder in the plains, too large to be trans- 

 ported by water alone, indicates that an iceberg was now and then 

 drifted out on the waters to the plains. The grooves, scratches, and 

 smooth sides of the mountain valleys in Colorado and other portions of 

 the Eoeky Mountains point to tbe same conclusion. 



From Horse Creek we proceeded northward to the Chugwater, nine 

 miles distant. The road is a perfectly smooth one for wagons. On our 

 right are continuous walls of White Eiver tertiary, pierced here and there 

 by some little branch. On the left the granite rocks are seen in long, 

 irregular ranges, rising very gradually nearly to the summit. The un- 

 changed beds seem to have been worn away to the level of the valley and 

 the edges covered with a deposit of drift. Here and there, however, they 



