442 ELIOT BLACKWELDER 
cipitous cliffs. The gradients of the streams are so steepened that 
bowldery rapids replace the grass-grown channels characteristic of 
the Leslie stage. Some of these canyons are three to four hundred 
feet deep and wholly impassable except for one on foot. Although 
cut chiefly in granite, they are apt to be narrowest where they pass 
through the upturned Carboniferous limestone. As would be expected, 
these younger vallevs have been considerably widened in the soft 
Tertiary strata farther east. There again flat bottoms have been 
developed, and ranches are scattered along the streams. ‘The sides of 
the valleys are steep, however, and tributaries coming in from the 
abandoned upper reaches, which I have interpreted as belonging to 
the Leslie cycle, descend the slopes abruptly. 
In the Laramie basin we find no trace of the canyon cycle. I have 
suggested that the backward cutting of the canyons from the lower 
course of the Laramie River has not yet progressed as far as the 
Laramie basin. If this view is correct, the Laramie basin is still in 
the Leslie cycle, and its rivers have suffered no marked disturbance 
for a peiod of time sufficiently long to enable them to plane off broad 
flat valleys. Rejuvenation such as is indicated by the canyons has 
ordinarily been ascribed to diastrophic disturbances. More recently 
Johnson’, Davis,? and others have emphasized the competence of 
climatic changes to produce the same results. In this particular case 
the cause has not been worked out. It can probably be done best by 
a broad study of the whole of the Great Plains and eastern Rocky 
Mountain region. 
As to the age of the canyon cycle some inferences may be mace. 
Assuming that the Sherman peneplain is post-Miocene, a considerable 
length of time must be allowed in addition for the excavation of the 
open Leslie valleys, unless these are to be correlated with the Sherman 
cycle itself. On this line of.estimation the canyons should be no older 
than Pliocene. Approached from the other direction, it may be said 
that the canyons represent a very youthful stage of the present cycle 
of erosion. In other parts of the West it is known that valleys of 
comparable development have been made since the early part of the 
« W. D. Johnson, ‘‘The High Plains and Their Utilization,” U.S. G.S. Ann. Rep., 
XXI, Part 4, pp. 630, 631. 
2 W. M. Davis, ‘“‘Explorations in Turkestan,” ae Institution Publication 
No. 26, pp. 203-6. 
