PLEISTOCENE HISTORY OF SAN JUAN MOUNTAINS 399 
the mesa surfaces are cut into and below the elevations now 
mantled with the drift of that, the oldest known, glacial epoch. 
The bowlder-gravels capping the mesas may be found to be, in parts 
outwash materials from the early glaciers, and, in part, remnants of 
a widespread deposit made by the larger streams during and perhaps 
immediately following the San Juan glacial epoch. They may 
belong in part to the Uncompahgre interglacial epoch. 
Some time after the retreat of the San Juan ice the streams 
began to erode their channels vigorously in the mountainous portion 
of the area. Much of the material removed from the upper courses 
of the streams was deposited as great alluvial fans on the bordering 
plateaus. This process of canyon deepening in the mountains con- 
tinued until much of the San Juan area was skirted by a piedmont 
alluvial plain sloping gently away from the central mountain region. 
The dissection of these bowlder-capped mesas began as the 
effects of rejuvenation advanced upstream through the bordering 
plateau country, and continued until the inauguration of the Bighorn 
glacial epoch, when the streams again became heavily loaded with 
débris and began to aggrade their valleys. 
The distribution of the deposits that have been interpreted as 
belonging to the San Juan glacial epoch is such as to indicate with 
certainty that the great canyons through which the glaciers of the 
two later epochs moved did not exist as deep troughs at the time of 
the earliest known or San Juan epoch. Deposition of bowlder- 
gravels may have characterized the earlier part of the Uncompahgre 
interval, but it is evident that before the close of the interval great 
canyons had-been developed. ‘This interval was certainly much 
longer than the Animas interglacial interval and it may have been 
marked by many changes in physiographic conditions which have 
not as yet been determined. 
The valley forms suggest that the streams had reached a tem- 
porary base level and had somewhat widened their valleys by 
lateral planation in the lower country before the formation of the 
Bighorn glaciers. The rock benches which are now capped with 
Bighorn moraine and outwash were then the floors of the valleys. 
The San Juan glacial epoch—Evidence bearing on a third and 
much earlier glacial epoch has now been secured from three widely 
