406 WALLACE W. ATWOOD AND KIRTLEY F. MATHER 
mature valleys with low interstream ridges in marked contrast to 
the rugged peaks, the sharp divides, and the narrow canyons of the 
present time. The physiographic development of the great scenic 
features of the San Juan area is therefore largely the work of inter- 
and post-glacial times. 
The valley deepening in the San Juan Mountains since the San 
Juan glacial epoch must be reckoned in thousands of feet. The 
Uncompahgre interglacial epoch between the San Juan and Bighorn 
glaciations must have been several times as long as the Animas 
interglacial epoch. It is probable that the long period of erosion 
following this earliest glaciation was not a time of continuous valley 
deepening. In addition to the depositional interval during which 
certain high mesas were mantled with bowlders there may have been 
other epochs of glaciation which intervened between the glaciations 
of the San Juan and Bighorn epochs but which have left no per- 
manent record. ‘The Uncompahgre interglacial epoch may not be 
in a strict sense an interglacial epoch but may include one or more 
glacial epochs as well. 
Comparing the deposits of these three known glacial times in 
the San Juan district it is at once seen that the earliest is in striking 
contrast to the two younger ones in point of area covered and dis- 
tance from the mountain front attained by the ice. The glacier 
which extended north from the range to some distance beyond 
Horsefly Peak should probably not be called a valley glacier. The 
San Juan ice on the south side of the range was not restricted to a 
valley, for it extended some distance beyond the mountain front as 
a great piedmont glacier. The wide area covered by ice at this 
time seems to indicate a period of far more extensive glaciation than 
that of the two epochs of valley glaciers, and it is probable that 
during San Juan glacial epoch much of the mountain country was 
covered by an ice-cap, while piedmont glaciers deployed over the 
surrounding plateau lands. 
Relation of glacial epochs to physiographic stages in the history of 
the San Juan mountains.—The physiographic stages in the late 
history of the San Juan mountains have been worked out in some 
detail* on the south and southwest sides, and in part on the north 
«W. W. Atwood, Jour. Geol., XIX (1911), 449-53. 
