blackwelder: post-cretaceous history 445 



GEOLOGY. — Post-Cretaceous history of the mountains of central 

 western Wyoming.^ Eliot Blackwelder, University of 

 Wisconsin. 



The field includes the Teton, Gros Ventre and Wind River 

 Ranges with their environs. A continuous plain of aggradation 

 about the close of the Cretaceous period was deformed by fold- 

 ing and overthrusting. Orogenic activity then ceased and the 

 corrugated surface was sculptured into hills, mountains, and 

 plains. The Wasatch and later Eocene and Oligocene conti- 

 nental deposits were then strewn by rivers and allied agencies 

 upon the denuded outcrops of weak strata rather than in struc- 

 tural basins. Meanwhile, especially late in this sedimentary 

 epoch, explosive volcanic eruptions added thick deposits of ash 

 and breccia. 



At a later time, probably in the jNIiocene, very gentle folding 

 and normal faulting terminated the sedimentation and left new 

 relief features, such as the great scarp of the Tetons, which were 

 not accordant with those made at the close of the Cretaceous. 



The subsequent erosion of the district produced a peneplain 

 on even the hardest rocks. This plain was probably the char- 

 acteristic feature of the Pliocene, but it is now represented only 

 by high-level remnants in the Wind River and perhaps other 

 ranges. 



During the Quaternary, wide-spread elevatory movements with- 

 out notable warping or faulting but perhaps with climatic changes 

 induced the dissection of the peneplain by streams, wind and 

 glaciers — in the order of their quantitative importance. The 

 land forms, thus left, were controlled by the structure and rela- 

 tive resistance of the rocks below. Thus, while broad plains 

 were excavated in the Tertiary clays, only narrow canyons were 

 carved in the Pre-Cambrian gneiss. 



Four distinct cycles of erosion later than the peneplain are 

 distinguished and with them are associated three stages of Alpine 

 glaciation. The oldest glacial deposits are the most widespread 

 but have lost nearly all traces of glacial topography. The two 



1 Published by permission of the Director, U. S. Geological Survey. (To be 

 printed in full in the Journal of Geology.) 



