TERRACE GRAVEL OF COW CREEK 267 



relation of these gravels to the old drift, as well as their stratification, 

 suggests that they may represent a combination of outwash deposits from 

 the ice which deposited the early drift below Cimarron ridge as moraine 

 and from the retreating Cow Creek glacier. 



A second gravel-covered terrace occurs below the first one in Cow 

 creek. On the southwest side of the creek, as shown on the mesa about 

 i miles from the mouth (plate 25, figures 1 and 2), the vertical distance 

 between the two terraces is about 100 feet, while on the northeast side 300 

 feet intervenes between the two. Intermediate terraces occur at a number 

 of levels, but they are poorly preserved and merge one into another, so that 

 correlations are impossible. Eemnants of this lower level are preserved 

 as benches or isolated mesas on both sides of Cow creek and the Uncom- 

 pahgre river and are striking topographic features. 



The highest gravel-covered terrace has not been identified west of the 

 Uncompahgre river in the vicinity of Dallas creek, but about 6 miles to 

 the north, just beyond some low hills due to an intrusive porphyry, it is 

 well preserved. Close to the river at this point the line between the 

 highest and intermediate terraces is sharp, but in the direction of Horse- 

 fly peak it soon becomes indistinct, and the two surfaces appear to blend 

 into one another. The terraces bordering directly on the Cow Creek 

 drainage are covered by gravels composed almost entirely of late volcanic 

 material, those of the lower levels being well rounded and water-worn, 

 and, like the ones of the upper level, covered with fine, red soil. The 

 terrace gravels of the Uncompahgre drainage belonging to the lower level 

 accord closely with those of Cow creek in elevation, but the materials are 

 different, consisting of massive late volcanic rocks, porphyries possibly 

 derived from the Eocene conglomerate, and examples of nearly all the 

 sedimentary rocks of the upper Uncompahgre, including Algonkian 

 quartzite. 



INTERGLACIAL EROSION 



It is clear that a long period of erosion occurred between the times 

 of deposition of the older drift and the fresher and younger material. 

 During this period the mesas west of Cow creek were developed, the 

 surface of the one about 4 miles above the mouth of the creek being a 

 remnant of the slopes still preserved between Cow creek and Cimarron 

 ridge, while the other somewhat lower mesas correspond to the level of the 

 lower terraces (plate 25, figures 1 and 2). Evidence has been found in 

 the higher mountain region to the south that the canyon of Cow creek was 

 probably developed at this time, as the old graded slopes west of Cimarron 

 ridge may be traced more or less continuously all the way to the head of 

 Cow creek ; it is below these that the present canyon lies. Taking this 

 XXII— Bull. Geol. Soc. Am., Vol. 17. 1005 



