202 ELIOT BLACKWELDER 



be likely to have coarse gravel upon its surface. The surface 

 underlying the Eocene formations, which is gravel-strewn, has 

 already been described as being rugged. 



In Yellowstone Park the formation of the peneplain may have 

 been prevented, or if it was made, it may have been overwhelmed, 

 by the volcanic eruptions which students of that district assign to 

 the Pliocene and later epochs. 



Turning to other parts of the Rocky Mountains, I find elevated 

 peneplains widespread. In the Bighorn Range^ there seems to 

 be a similar high plateau surmounted by an axial mountain range, 

 which, prior to the cirque-making by Quaternary glaciers, had a 

 subdued post-mature topography. There also the eroded surface 

 trunkates the various rock structures indiscriminately. In an- 

 other place^ I have described the peneplain which is well preserved 

 in the Sherman Mountains of southeastern Wyoming. Other cases 

 of somewhat similar characteristics have been reported from Colo- 

 rado, Montana, Idaho, Washington, and other western states. 

 These peneplains have been assigned to the Eocene, Miocene, and 

 Pliocene,^ but they are so similar in their characteristics and rela- 

 tions as to suggest that they may be only local remnants of a once 

 widespread old-age surface, subsequently more or less warped. 



The age of the Wind River summit peneplain is debatable, but, 

 by a process of finessing, it may be worked out with some degree 

 of assurance. It will be admitted by all that, since it trunkates 

 the structures produced by the folding at the close of the Creta- 

 ceous, it must be of Cenozoic age. The stage of topographic old 

 age, when planation is being accomplished, does not permit the 

 accumulation of thick sedimentary deposits in the same district 

 because the two processes are mutually exclusive. This appears 

 to be true whether the work is done by wind or by streams. For 

 that reason it is hardly conceivable that the peneplain could have 

 been developed while the Eocene and Oligocene strata were being 

 deposited to thicknesses of several thousand feet in various parts 



' See Plates VIII A, XXVII A, XXX, and XXI A, in "Geology of the Bighorn 



Mountains," by N. H. Darton, U.S. Geol. Survey, Prof. Paper jz, 1906. 

 ^ Jour. Geol., XVII (1909), 429-44. ^ gee p. 194. 



