266 GEOLOGY OF THE DENVER BASIN. 



fragments of silicified wood and of rhyolite from the Arkansas divide 

 region. The coarse grain of the loess composing the western edge of 

 Capitol Hill is due to the fact that it covers a portion of the ancient 

 channel of Cherry Creek. 



UPLAND DRIFT. 



Large quantities of erratic bowlders of more or less rounded form are 

 found above the Level of 5,800 feet along the foothills and on the mesas 

 running out from them, as well as on isolated buttes in the plains that exceed 

 this elevation, that are scarcely to be distinguished from the glacio-natant 

 drift, except by their elevated position, by the absence of the calcareous 

 coating, and l>v a generally fresher and less decomposed surface. 



The material of which this drift is composed shows that it is mainly 

 derived from the Archean areas of the mountain region, and along the foot- 

 hills it is often accompanied by finer ddbris and transported soil. It may 

 be seen to best advantage on the mesas near Colorado Springs and Palmer 

 Lake, and also about Cheyenne. Similar material is also found in the 

 valleys between the Dakota hogback and the Archean foothills. To this 

 class of deposits the name of Upland drift has been provisionally assigned 

 by Mr. ( lannon, though it is by no means certain that the material is in all 

 cases of contemporaneous deposition and constitutes a well-defined formation. 

 More detailed study will be required to elucidate this point. 1 



MODERN EROSION EPOCH. 



The change from a period of deposition to one of erosion and removal 

 may be assumed to have been caused by a change in the general slope of 

 the region, or by a relative elevation of the region nearer the sources of the 

 streams in the mountains. Data are as yet wanting for determining the 

 nature and amount of this elevation, but that some changes of level have 

 taken place in recent times is rendered probable by the occurrence of faults 

 in the ancient river drift and loess. 



The writer is inclined to believe that the material found ou the higher mesas of the plains area 

 is a residuary formation resulting from the disintegration in place of an nnstratified conglomerate, 

 similar in origin to the Wyoming conglomerate of the Uinta Mountains, which at some period of 

 general floods, probably before the ancient erosion epoch, spread as sheets of coarse gravel and 

 bowlders over the general surface of the country to a considerable distance from the mountains, 

 varying greatly in thickness, however, according to the configuration of the surface. 



