POST-CRETACEOUS HISTORY OF THE MOUNTAINS OF 

 CENTRAL WESTERN WYOMING 



ELIOT BLACKWELDER 



University of Wisconsin 



PART III 



Quaternary cycles of stream erosion. — ^Although in the higher 

 mountains stream erosion was superseded at intervals by glaciation, 

 over most of the district the Quaternary period was a time of con- 

 tinuous action by running water and its accessory agencies. The 

 erosion history was accented, however, by several changes which 

 rejuvenated the streams from time to time, and thus prevented 

 the completion of the successive erosion cycles. Conceivable 

 causes of such interruptions are: (a) a series of general uplifts; 

 {h) the readjustment of stream grades on account of the; removal 

 of obstructions in the channels of the master rivers, or because 

 of piracy in their lower courses; and (c) notable changes of climate. 



Of these causative changes, those which were climatic should 

 have produced more or less harmonious results simultaneously over 

 the entire district. The same should have been true of the gentle 

 elevatory movements, if they were uniform over wide areas. The 

 two factors should differ in their results in that climatic changes 

 would doubtless make themselves felt all over the district at the 

 same time, and streams would then respond either by trenching 

 their fioodplains or by aggrading them. The changes would 

 apparently take place simultaneously throughout the entire graded 

 profiles of all the streams. The elevatory movements, however, if 

 equal everywhere, would cause rejuvenation first in the lower 

 courses of the streams, or, in this instance, outside the district. 

 The phenomena due to elevation or depression would then migrate 

 up the individual streams; in such cases the new cycle features of 

 the lower courses would be distinctly older than those of the upper 

 courses at any given time. If the surface of the district, instead of 



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