EXTINCT GLACIERS OF COLORADO 43 



or more, but in the region most familiar to the writer they seldom extended 

 more than ten or twelve miles from the top of the range. 



The comparative recency of their retreat is attested by the freshness 

 of their work in the upper courses, the fact that the small residual 

 glaciers and neves are still shrinking, and the fact that the streams have 

 deepened their channels but little since the ice disappeared. The subse- 

 quent modification, while plainly apparent, is superficial. The sides 

 of mountain moraines are usually quite steep as a result of rapid 

 building, hence are subject to rapid modification by erosion, and the 

 metamorphic rocks constituting most of the mountain heights are easily 

 disintegrated by atmospheric agencies, so that the rounded surfaces 

 are soon broken up and the polish erased, making the glacial work 

 in the lower portions of the valleys look much older than the upper 

 portions, the apparent difference in age being perhaps greater than 

 the real difference. 



Lakes of various sizes, both rock basin and morainal, abound in the 

 upper portion of the glacial valleys, those formerly existing in the lower 

 portions having been filled or drained by the various agencies always 

 laboring for the destruction of all lakes. 



Roches moutonnees form prominent features of the landscapes and 

 are much fresher in the upper valleys, where the disappearance of the 

 ice has been so recent that vegetation has not yet been able to re-establish 

 itself. Owing to the extreme ease with which gneiss and coarse granite 

 are weathered, polished knobs and scratched boulders are not common. 

 The best examples seen by the writer are in Camp Albion Gulch, north- 

 east of Arapahoe Peak; but why they should be fresher than on the 

 same kind of rocks in the same zone in adjoining gulches has not been 

 satisfactorily explained, unless that glacier has retreated much more 

 recently and rapidly than the others. The lack of morainal matter also 

 suggests rapid retreat. There is no evidence that the polished rocks 

 have been covered so as to prevent weathering. 



The discussion of existing glaciers is not within the scope of this 

 paper, and is therefore left to some future paper. 



Following is a list of publications bearing upon the present subject, 

 for the benefit of those who desire more detailed information: 



