PLEISTOCENE GEOLOGY OF THE SAWATCH RANGE 703 



above the river flat, and slope upward toward the base of the moun- 

 tain at an angle of 2-4°. They have plane surfaces and uniform 

 slopes, and, although deeply cut by erosion, are evidently remnants 

 of a plain which sloped somewhat continuously from the base of the 

 mountains on either side of the valley to an axial trough somewhere 

 near the middle of the valley. 



In constitution these terraces are composed of well-rounded, 

 water-worn gravels, coarser toward the base of the mountains and 

 up the Arkansas valley, and finer away from the mountains and 

 down the valley. In general, the gravels in any particular place 

 seem to correspond to the rocks found in the mountains just above. 

 For the most part these beds are uncemented, although locally the 

 gravels have been cemented into a friable conglomerate by lime car- 

 bonate. 



Mr. Emmons, in his monograph on the Leadville. district,^ refers 

 to the terrace deposits as being of lacustrine origin, and argues that 

 the slope of the terraces away from the mountains is due to subsequent 

 tilting. The correspondence in the slope of the terraces on the two 

 sides of the valley, and the lack of any observed deformation in the 

 beds, do not seem to favor the view that the beds have been tilted 

 since deposition, while the coarseness and the imperfect stratification 

 of the gravels, and the absence of any observable delta stuctures, 

 seems to the writers to make the Lake Bed hypothesis untenable. 

 Although locally there may have been small lakes in which deposits 

 of gravel were laid down, by far the greatest portion of these deposits 

 are referable to river work. With this hypothesis the slope of the 

 terraces is in harmony. These deposits must at one time have been 

 considerably more extensive than now, for the Arkansas River has 

 cut a very considerable valley in the former plain of aggradation. 



For the construction of the plains from which these terraces were 

 developed there must have been a great amount of detritus supphed 

 to the streams. In following the high terraces from the river to the 

 mountains on either side we found in the three places which were most 

 carefully studied, that their upper edges were in contact with bodies 

 of older drift. The earher glaciers, of considerably greater size than 

 the later ones, may have supphed great quantities of debris to the 



I Monograph XII, U. S. Geological Survey, pp. 41, 71, 72. 



