PLEISTOCENE HISTORY OF SAN JUAN MOUNTAINS 405 
in large measure for the preservation of the shale upon which it 
rests and for the existence of the peak, the shale in turn has been 
protected from erosion by the underlying Dakota sandstone which 
forms the surface of most of the Uncompahgre plateau and has 
delayed the headward growth of valleys that threaten the Horsefly 
Peak region. Erosion into the Dakota sandstone is still in the stage 
of extreme youth, while nearly all of the overlying Mancos shale 
has been removed. This most remarkable plateau of the San Juan 
district is a very fortunate place for the preservation of a drift 
deposit through long periods of erosion, and it is on just such sur- 
faces that other remnants of this ancient glacial formation are 
most likely to be found. 
Still more striking are the facts which may be deduced from 
the composition of the drift bordering the Uncompahgre Valley. 
The bowlders, as already noted, are almost entirely composed of 
late volcanic rocks with only a very few fragments of quartzitic 
and granitic rock. The latter are believed to have been derived 
from the bowlders of the Telluride conglomerate and their extreme 
rareness is in sharp contrast to the great proportion of similar 
bowlders found in the Uinta and Bighorn drift of the Uncompahgre 
Valley. The quartzites of the two more recent glacial deposits 
are in the main derived from the Uncompahgre quartzite of the 
Uncompahgre canyon south of Ouray, and the absence of bowlders 
from this formation in the drift of the San Juan epoch indicates 
that this formation was not exposed in this part of the range at the 
time of the San Juan glaciation. At the present time the 
Uncompahgre quartzite is found outcropping on the canyon walls, 
2,000 feet above the stream. ‘There must have been a canyon 
deepening at this point of at least that amount since the glaciation 
responsible for drift on Horsefly Peak and beneath Cimmarron 
Ridge. 
Again, the relation of the San Juan drift near Creede to the 
broad upland valleys above the present canyons there suggests the 
extreme age of this earliest of known Pleistocene deposits in the 
western mountains. ‘The relief of the mountains during this stage 
must have been very much less than at the present time and the 
topography must have been characterized by broad, shallow, 
