LARAMIE REGION, WYOMING A439 
this idea with the existence of flat plateaus east of the Sherman uplift. 
Their sides are relatively steep, and the valleys ascribed to the Leslie 
cycle are rather sharply sunk beneath them. Such a hypothesis 
should have consideration in future study of the district, but the weight 
of evidence seems to me favorable to the view that the Leslie valleys 
have actually been excavated in a surface which was even less rough— 
the Sherman peneplain. If the latter hypothesis is correct, then we 
have evidence of a cycle of erosion initiated by the rejuvenation of the 
streams after the completion of the Sherman peneplain. 
This change in the activity of the streams may have been produced 
by actual uplift, or by a climatic variation. The sediments corre- 
Fic. 7.—Sketch (from a photograph) of a flat-bottomed open valley (Leslie nee) 
near the crest of the Laramie range. 
sponding to these erosion cycles have not been recognized, and it is 
therefore difficult to get any data bearing on the climatic changes. 
Uplifts are known to have occurred at intervals during the Tertiary 
and Quaternary periods, and the-occurrence of one in this district 
is not improbable. 
East of the uplift again we find broad flat-bottomed valleys sunk 
beneath the plateau surface which has been interpreted as the Sherman 
plain, but themselves trenched by canyons which are obviously younger. 
The largest of these broad depressions has been excavated parallel to 
the Sherman arch east of the limestone hog-backs, apparently 
because the soft Brule clay is exposed there. In the building of the 
