3904 WALLACE W. ATWOOD AND KIRTLEY F. MATHER 
Uinta epoch. ‘These glaciers formed in the basins near the north 
base of Mount Sneffels. The terminal moraines of these glaciers 
are conspicuous masses of drift in the valley and upstream from 
these moraines there are other glacial deposits. 
The extreme recency of the Uinta glaciation is shown in many 
ways. The materials of the moraines are fresh and unaltered, 
retaining in many cases polished surfaces and striae. Post-Uinta 
weathering and stream erosion are of very limited amount as shown 
not only by the glaciated rock surfaces so numerous in the upper 
courses of the streams but also by the character of the glacial 
débris itself. The streams are still engaged in the task of cutting 
channels through the drift and clearing it away from their courses, 
while downstream from the terminal moraines the outwash terraces 
are never more than a few feet above the present stream channels. 
Most conspicuous of all, as noted by many observers in the western 
mountains, is the slight modification which the later glacial deposits 
have undergone. ‘Typical knob and kettle topographies are present 
in the drift deposits at many places and small lakes occupy many 
of the undrained depressions. 
The Animas interglacial interval.—The time interval immediately 
preceding the epoch of. glaciation which has just been described 
was a time of active erosion In the San Juan area. The streams 
were vigorously engaged in the work of lowering their channels, 
and canyons were cut beneath the level of the broad valley floors 
which had received the deposits of the next earlier or Bighorn 
glacial epoch. The Animas River lowered its channel during this 
time by more than 300 feet, near the city of Durango, while the 
valley deepening in the Uncompahgre Valley was even greater. 
This change in work from that of lateral planation in progress 
during the Bighorn glacial epoch, to that of active downward 
cutting, may be attributed to the clearing of the waters following 
the melting away of the Bighorn ice, but it is probable that this inter- 
val of canyon cutting is an evidence of mountain growth during or 
immediately following the Bighorn epoch. A slight renewal of the 
domal uplift movements that seem to have affected this region 
several times during Tertiary and Quaternary times would have 
rejuvenated streams radiating from the central portion of the 
