THE BUILDING OF THE COLORADO ROCKIES 157 



of the Upper Wasatch, and is represented, according to Veatch, 

 by a break which amounts to perhaps 5,000 feet of strata.'^ But 

 the older, post-Laramie break amounts to over 20,000 feet, and 

 Veatch has expressed the opinion that the folding following the 

 Lower Wasatch should perhaps be regarded as of much less relative 

 significance than this comparison of figures might indicate, because 

 the movements of the second disturbance were along the lines of 

 weakness produced by the first. As to correlation, Veatch is 

 conservative, placing the 6,000 feet of strata between these two 

 breaks in the debatable ground between the known Cretaceous 

 and Tertiary. 



Along the western margin of the Rocky Mountain belt in the 

 Colorado section here studied, the Grand Hogback upturn involves 

 the Wasatch beds as fully as the Cretaceous. This may be observed 

 to the west of Glenwood Springs, both north and south of the 

 Grand River. At Piceance Gap to the northwest both the Wasatch 

 and Green River beds are included in the monoclinal fold according 

 to Gale.^ Farther west in the Uinta Basin of Utah the Wasatch 

 has been folded with the Cretaceous.^ Whether this flexing on the 

 western borders of the Rocky belt is to be correlated with the 

 Middle Park folding within the range is uncertain. Irrespective 

 of this it is pecuHar in manifesting certain characteristics of basin- 

 like subsidence and downwarping. 



R. C. Hills has described this early Tertiary folding exhibited 

 in certain portions of the Sangre de Cristo Range as post-Bridger, 

 because the Huerfano beds were involved.^ To this period of fold- 

 ing he has also assigned the flexing of the Grand Hogback, as well 

 as rather widespread folding on the east flank of the front range. ^ 

 Hills is the authority for the belief that the post-Bridger folding 



'A. C. Veatch, U.S. Geol. Survey, Prof. Paper 56 {1907), p. 75. 



^ Hoyt S. Gale, "Coal Fields of Northwestern Colorado and Northeastern Utah, " 

 U.S. Geol. Survey, Bull. 415 (1910), PI. XIII. 



3 C. T. Lupton, "The Blacktail Mountain Coal Field, Wasatch Count}', Utah," 

 U.S. Geol. Survey, Bull. 471 (1912), PI. LXII. 



''R. C. Hills, U. S. Geol. Survey Geol. Atlas, Walsenherg Folio, No. 68 (1900) 

 PP- 2-3. 



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

 Proc. Colo. Sci. Soc, HI (1888-90), 408-19. 



