BASE LEVELS OF EEOSION. 207 
duction of low plains and hills for a number of miles back from the stream. 
North of the Caiion of Desolation and south of the Uinta Mountains, another 
local base level of erosion is found, so near to the general surface of the 
country that we find a district of valleys and low hills stretching back from 
Green River, up the Uinta to the west, and White River to the east, for many 
miles. North of the Uinta Mountains a third local base level of erosion is seen, 
but its influence on the topographic features is confined to a small area of 
two or three hundred square miles. Going up the chief lateral streams of 
the Colorado, we find one or more of these local base levels of erosion, where 
the streams course through valleys. 
Where these local base levels of erosion exist, forming valley and hill 
regions, the streams no longer cut their channels deeper, and the waters of 
the streams, running at a low angle, course slowly along and are not able to 
carry away the products of surface wash, and these are deposited along the 
flood-plains, in part, and in the valleys, among hills, and on the gentler slopes. 
This results in a redistribution of the material in irregular beds and aggre 
gations. 
In this region, there are occasional local storms of great violence. 
Such storms may occur in any particular district only at intervals of many 
years, possibly centuries. When such a one does occur, it reopens great 
numbers of channels that have been filled by the ordinary wash of rains, 
and often cuts a new channel through beds which have accumulated in the 
manner above described. The structure of these beds is weU exposed, and 
we find beds of clay, beds of sand, and beds of gravel occurring in a very 
irregular way, due to the vicissitudes of local wash, and, where the progress 
of erosion has been more or less by undermining, larger fragments or boul 
ders are found, and these boulders are sometimes mixed with clay, and some 
times with sand and gravel, and where thin sheets of eruptive rocks have 
been torn to pieces, more or less by undermining, (for such is the usual way 
in this country,) the beds appear to contain erratics, and in fact some of the 
rocks are erratics, for in the various changes in the levels produced they have 
often been transported many miles, not by sudden and rapid excursions, but 
moved a little from time to time. 
Again, the beds from which they were derived, doubtless, in many cases 
