PLEISTOCENE HISTORY OF SAN JUAN MOUNTAINS 307 
deposit although it has lost its morainal topography. Since this 
deposit is on a rock bench several hundred feet above the valley 
flat and downstream from the Uinta terminal moraine has lead to 
the interpretation that it is of Bighorn glacial age. Directly across 
the valley on the hills east of Dallas there are other drift deposits 
with weak morainal topography which have also been interpreted 
as remnants of the Bighorn drift. Downstream from these patches 
of Bighorn moraine there are several remnants of the outwash 
materials from the ice of this epoch. ‘These outwash gravels cap 
rock benches at elevations of 300 to 500 feet above the present 
channel of the Uncompahgre River. 
Downstream from the Uinta moraines in the headwaters of 
Dallas Creek there are remnants of an older series of moraines 
which have been interpreted as of Bighorn glacial age (see Fig. 4). 
Associated with these deposits there are still remaining some out- 
wash materials which are believed to have been deposited at the 
time the moraines were laid down. 
Although the glaciers of the Bighorn epoch were as a rule 
slightly longer than those of the Uinta epoch, in no case did they 
extend beyond the foothill zone surrounding the range or override 
the walls of the canyons very far so as to spread out over the 
bordering lands. The Bighorn epoch glaciers were all of the valley 
type. The valley trains of the Bighorn epoch were of greater extent 
than those of the later epoch, and the gravel-capped terraces which 
are remnants of the Bighorn valley floors may be traced for several 
miles down each of the larger streams radiating from the range. 
The amount of valley cutting beneath the level upon which the 
Bighorn deposits were laid down was much greater than that which 
has been accomplished since the last retreat of the ice. The length 
of the Animas interglacial interval, which immediately preceded the 
Uinta glaciation, was certainly several times as long as the time 
since the last melting away of the ice. 
The Uncompahgre interglacial interval—Adjacent to the range 
on the south there are extensive bowlder-capped mesas high above 
the outwash material of the Bighorn epoch. The Florida and 
Fort Lewis are good examples of such mesas. Similar gravel- 
