GLACIATION. 247 
ent river valleys. Valleys in granite show best the effects of the carving 
of these local glaciers, since granite is resistant and homogeneous, and these 
valleys present a uniform U-shaped structure. For this reason the valleys 
of Hunter Creek and Roaring Fork offer most complete records. 
About 10 miles above Aspen, and not far from the summit of the 
Sawatch, the valley of the Roaring Fork is wide and shallow, with gently 
sloping walls of granite and a granite floor, polished and carved into irreg- 
ular, rounded forms. Through the middle of this broad glacial valley there 
winds a steep, narrow gorge or canyon, which in places is almost obliterated 
and in other places has steep walls which reach probably 200 feet in height. 
The tops of these canyon walls form part of the general floor of the glacial 
valley. In places this canyon is wanting, the broader valley being locally 
deeper and removing all traces of it, while in other places it becomes quite 
conspicuous. It becomes progressively fainter, however, as the distance 
from the head waters and the depth of the broad glacial valley increase, and 
entirely disappears 9 or 10 miles above Aspen. This gorge is evidently the 
remains of a pre-Glacial canyon of the Roaring Fork, of which only traces 
are left, below the plane of glacial action. Pl. XX XV gives a general view 
of the broad, shallow glacial valley, with the pre-Glacial canyon represented 
by the V-shaped depression on the right. 
Downstream from this bare granite area there is an increasing amount 
of ground moraine, the bowlders being very large, and about 5 or 6 
miles above Aspen comes a frontal moraine which crosses and fills up the 
valley. Just back of this moraine is a little level space, composed of fine 
lake sediments, and capable of some cultivation, and in the middle of this 
is a small pond, about 200 feet across. his is evidently the bed of a 
small lake which was dammed up by the frontal moraine until the stream 
cut through and drained it. 
Below this moraine the bottom of the valley is remarkably smooth 
and level to about a mile above Aspen, where there occurs the rear wall of 
a strongly marked terminal moraine, whose front wall comes quite down 
to Aspen. Between this moraine and the one last described there is 
evidence of a glacial lake which existed immediately after the disappear- 
ance of the Roaring Fork glacier. The valley bottom is a series of broad, 
flat meadows, with patches and islands of coarse morainal material. Most 
of these meadows are wet, and there are also extensive swamps. In 
