158 ROLLIN T. CHAM BERLIN 



in Colorado was nearly as widespread and equally as intense as the 

 post-Laramie folding.^ It followed very closely the older Laramie 

 lines of flexure. 



The post-Middle Park folding was followed by a long period 

 of erosion in the Colorado Front Range. Erosion in fact must 

 have been steadily at work on the main anticHnes of the Colorado 

 Rockies since their arching in the Laramide diastrophism. This 

 long continued denudation greatly reduced the country, as is 

 known from the fact that beds regarded as Uinta in age^ were 

 laid down directly upon the Archean granite over wide areas in 

 the Grand River region of Grand County. The broad valleys 

 and basins of the region no doubt received deep fillings of detrital 

 material, as the country was brought toward a common level 

 by degradation of the higher areas and aggradation of the lower. 

 The Upper Eocene beds thus deposited exhibit in places an appre- 

 ciable departure from horizontality, giving rise to a slight waviness 

 in structure, but they seem to have suffered no pronounced folding. 

 This warping of the Upper Eocene beds suggests that in this region 

 there was a mild expression of the mid-Tertiary diastrophism which 

 developed more strongly near the Pacific Coast and elsewhere. 



Peneplanation. — ^After one or more cycles of erosion of whose 

 details little is known, the granite ranges became an imperfect 

 plain, above which unreduced areas remained as scattered monad- 

 nocks from 500 to 2,500 feet in height.^ This peneplain, if such an 

 imperfectly reduced area may be designated by that term, must 

 have extended throughout the region of the Colorado Front Range, 

 for remnants of it are still preserved at many points in different 

 parts of the range. In the neighborhood of the Lyons-Grand 

 River section, whose study constitutes the basis for the investi- 

 gation in hand, these flats form conspicuous portions of the 

 continental divide in Flattop Mountain and some of its immediate 

 neighbors in Rocky Mountain National Park (Fig. 2). The name 

 Flattop is correctly descriptive of an extensive flatfish portion 



' R. C. Hills, "Orographic and Structural Features of Rocky Mountain Geology." 

 Proc. Colo. Sci. Sac, III (1888-90), p. 443. 



^ Geologic Map of Colorado. R. D. George, 1913. 



3 W.M.Davis, "The Colorado FrontRamge," Attn. Assoc. Am.Geog., I (i9ii),3i. 



