i6o ROLLIN T. CHAMBERLIN 



The age of this peneplain was not determined. No ready means 

 of dating it was found in the time at disposal, and exhaustive search 

 was not felt to be vital to the immediate problem in hand, though 

 admittedly very important in the history of the range and that of 

 the general region. Perhaps something may be learned from a 

 comparison with more favorably situated peneplains in adjoining 

 states. The pronounced peneplain upon the Wind River Range 

 of Wyoming has been assigned to the middle of the Miocene by 

 C. L. Baker. ^ Baker believes that this peneplanation was inter- 

 rupted, probably later in the Miocene, by an upHft, both regional 

 and orogenic, along the hnes of the earlier Laramide movement. 

 Westgate and Branson, however, did not feel justified in dating 

 the peneplain more closely than mid-Tertiary.^ Blackwelder, 

 on the other hand, beHeves that the Wind River summit peneplain 

 is of Pliocene age, for which he gives a well considered argument.^ 



In the Laramie region of southeastern Wyoming, Blackwelder 

 has described Pole Mountain and the Medicine Bow plateau 

 surface as remnants of a peneplain developed by the Eocene 

 cycle of denudation, while the Sherman peneplain, at somewhat 

 lower elevation, represents in his opinion a Pliocene date.'' These 

 two erosion levels, if such they be, are not far apart near the 

 Wyoming-Colorado line, indicating that but little uplift took place 

 between these stages; but southward in the Front Range of 

 Colorado the relief of the country becomes steadily greater. In 

 the Long's Peak region extensive flats are found east of the main 

 range in the neighborhood of 8,000 feet, which is not far from the 

 Sherman level in the Sherman quadrangle, but the summit pene- 

 plain on the continental divide is here at an altitude of approxi- 

 mately 12.000 feet. The Great Plains abut the foothills in the 

 Sherman region at an elevation of about 7,000 feet, ignoring the 



' Charles Laurence Baker, "Notes on the Cenozoic History of Central Wyoming," 

 Bull. Geol. Soc. Amer., XXIII (1912), 73. 



^ L. G. Westgate and E. B. Branson, "The Later Cenozoic History of the Wind 

 River Mountains, Wyoming," Jour. Geol., XXI (1913), 144-47. 



3 Eliot Blackwelder, "Post-Cretaceous History of the Mountains of Central 

 Western Wyoming," Jour. Geol., XXIII (1915), 193-207. 



■» Eliot Blackwelder, "Cenozoic History of the Laramie Region," Jour. Geol., 



XVII (1909), 429-44- 



