DE]!«TER & RIO GRANDE WESTERIST 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 (PI. 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 



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 gi-ade 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 

 the side canyons, but farther north the altitude of 

 the valley is so great that they not only reached the 

 Elevation 8,374 feet, mouths of i\iQ rock-bouud cauyous but pushed out 



Denver 248 miles. . , . -, nt^ -, ^ "• n 



into the river and filled the main valley with the 

 rocky debris 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. 



Riverside. 



