POST-CRETACEOUS HISTORY OF WESTERN WYOMING 217 



In short, it appears that the Quaternary has been characterized 

 by widespread changes of level, which in turn have induced the 

 erosion of deep valleys, and that these valleys have been widened 

 in proportion to the weakness of the strata in which they were 

 carved (Figs. 28 and 29). Although there is an advantage in 

 stating the history in these simple terms in order to display the 

 general conditions, it is appropriate to say that the events of the 

 Quaternary have been much more varied and complex than this 

 generalized statement implies. There is good evidence that the 

 uplift or other interruption did not take place once for all at the 

 beginning of the period, nor uniformly throughout the period, but 

 at intervals scattered through a long time. Denudation has been 

 accented by these disturbances, and doubtless also by climatic 

 changes, of which mention has not yet been made. Furthermore, 

 the work of denudation has been done, not only by streams, but 

 in minor degree also by winds, glaciers, avalanches, ground-water, 

 and unaided gravity. This complex chapter of the history may 

 now be analyzed ; and, since the changes produced by the different 

 agencies are not in all cases readily correlated, I find it best 

 to treat them separately and in the order of their relative im- 

 portance. 



