3o8 ELIOT BLACKW ELDER 



being uniformly raised (relatively), was warped, then the read- 

 justments of stream action and their results must have been more 

 complex. Rejuvenation due to the removal of falls and rapids 

 or to stream-piracy would be likely to cause changes in one valley 

 system which would have no counterpart in the adjacent drainage 

 basins. 



Climatic change may well explain some of the minor rejuvena- 

 tions, but apparently is incompetent to account for the cutting 

 of valleys hundreds or even thousands of feet deep. Inasmuch as 

 it is not only possible, but fairly probable, that several or even 

 all of these changes have been at work, it is clear that the problem 

 to be dealt with is by no means simple; it should be approached 

 in a spirit of caution and without an expectation of solving it com- 

 pletely except after more detailed field studies. It is an observed 

 fact that the topographic features in different parts of the district 

 here considered, and even in entirely different drainage basins, are 

 so similar in aspect, in order, and in relations as to suggest that 

 each has passed through a similar history, owing to the operation 

 of some widespread influence. These facts suggest either climatic 

 changes or regional uplifts. There is no question as to the compe- 

 tence of uplifts to induce dissection to proportionate depths, but 

 climatic changes competent to bring about the incision of valleys 

 to depths of hundreds or even several thousands of feet, as is indi- 

 cated by the canyons in the ranges of Wyoming, would have to be 

 extraordinary in magnitude. For that reason the hypothesis of 

 changes of level seems to me the more favored one, although it 

 should not be inferred that the other influences are excluded from 

 participation in a minor degree. 



The work of interpreting the erosion history of such a district 

 involves largely the recognizing and discriminating of surfaces pro- 

 duced during successive cycles and correlating these in chronological 

 order. Although it is often possible to recognize mature and even 

 young topographic forms of preceding cycles of erosion, the most 

 readily recognized features are the broad graded surfaces or flood 

 plains which normally become wider and wider as the cycle pro- 

 ceeds. In Wyoming, however, a careful discrimination must be 

 made between remnants of old river-graded surfaces, and those 



