340 ELIOT BLACKW ELDER 



along the structural lines established at the close of the Cretaceous, 

 but in part at variance with them. It is probable that this mid- 

 Tertiary disturbance involved also a general elevation which left 

 the region above base-level, and therefore subject chiefly to erosion 

 rather than to sedimentation. 



From the mid-Tertiary revolution down to the present time, 

 the history of western Wyoming is a chronicle of denudation. It 

 is believed that at all times the master streams of the region con- 

 trolled the general shaping of topography, although the effects of 

 stream erosion were modified in important details by mountain 

 glaciers, by the wind, and by slumping. The progress of general 

 stream erosion was evidently not steady, but was interrupted by 

 successive elevatory movements or other changes which compelled 

 the readjustment of stream activities. In the earliest of the erosion 

 cycles of which good evidence still remains, the region seems to 

 have been generally reduced to a peneplain even where the rocks 

 were tolerably resistant. This plain is believed to have been estab- 

 lished during the Pliocene epoch. 



Later cycles followed each other with more rapidity — or more 

 probably we read the later chapters in more minute detail. They 

 were apparently caused in large measure by general uplifts of the 

 Rocky Mountain region, although they may well have been influ- 

 enced in noteworthy degree by climatic changes, river piracy, and 

 other modifying causes. While these erosion cycles and inter- 

 ruptions were in progress, there occurred certainly two, and almost 

 surely three, distinct advances of glacial ice in the mountains, fol- 

 lowed by as many retreats and perhaps complete disappearances. 

 Between the glacial stages the streams continued their work and 

 sank their valleys through the moraines and into the underlying 

 rocks. Since the latest glacial advance only slight topographic 

 changes have been made. 



