DENVER & RIO GRANDE WESTERN ROUTE. 99 
outline and has a nearly flat floor. From the train it looks like a 
very small feature, but its walls are probably several hundred feet 
high, and it is not less than half a mile across. (See fig. 23.) To 
such a semicircular hollow as that on Mount Harvard or the one on 
the Continental Divide west of Salida (Pl. XLV, B) geologists 
have applied the French term “ cirque,’ meaning circle. It was 
produced by a small glacier that was formed in a ravine far up on 
the mountain slope. 
As far as milepost 246 the valley has a general width of 5 to 8 
miles, but on looking ahead the traveler may see that it becomes 
narrower and finally seems to close in completely. The old and 
rather broad valley doubtless continued to the head of the stream 
near ‘Tennessee Pass, but a little distance above Riverside it is so 
Bese fe 
Figure 23.—Great cirque on Mount Harvard. 
much filled with gravel and boulders that it is scarcely recognizable. 
Near this constricted part of the valley large boulders abound, form- 
ing a fan similar to the boulder fans observed at the mouths of the 
canyons below. The change in the character of the valley is due to 
the fact that in the Great Ice Age, when glaciers were active, they 
formed mainly on the mountain slopes at or above an altitude of 
11,000 feet and flowed down the side canyons or gulches for 
distances that depended on the grade of the canyon and the size 
of the glacier. In the Arkansas Valley below Riverside the glaciers 
that headed in the Sawatch Range reached only to the mouths of 
i the side canyons, but farther north the altitude of 
Riverside. the valley is so great that they not only reached the 
pcration 8,374 feet. mouths of the rock-bound canyons but pushed out 
ead sm into the river and filled the main valley with the 
rocky débris that they had carried on their surfaces or that had been 
embedded in them. This condition prevails above Riverside, and for 
this reason the valley is much narrower here than it is lower down. 
The glacial material brought down from the mountains crowded the 
river to the east side of the valley and even forced it over on the 
granite of the east wall, as it did in the other canyons below. The 
large blocks of rock that were derived from this granite were carried 
down the canyon and for some distance out on the flat valley floor. 
