EXTINCT AND EXISTING GLACIERS OF COLORADO 47 



cially a decrease in the mean summer temperature, may produce a 

 glacier without the climate becoming severe. A marked increase in 

 the snowfall on our mountains would at once initiate another extension 

 of glaciers down the valleys. As it is, they are slowly retreating and 

 a very sHght decrease in the snowfall would soon destroy them. 

 Only observations carried on a great many years can really deter- 

 mine whether or not the present recession is permanent, or only 

 represents a short cycle of disadvantageous conditions. 



Limiting Conditions. — The reasons there are glaciers or neve on 

 some of our high mountains are various and topographic. Although 

 Pike's Peak, for instance, is higher than Arapahoe and her adjoining 

 peaks, on the latter the glaciers have been much more important, 

 because of differences in topography, probably causing greater pre- 

 cipitation on Arapahoe, though records are lacking. The general 

 limiting factors are: {a) Size of catchment area, (h) Direction of 

 exposure of places into which the wind drifts the snow with reference 

 to protection from the sun. {c) Precipitation, {d) Temperature. 

 {e) Character of the ground in the direction of the prevailing winds. 



Altitude. — The altitude at which conditions permitted the forma- 

 tion of glaciers in the Rocky Mountains differed greatly. In the 

 Big Horn Mountains, Wyoming, an altitude from 9,500 to 11,500 feet 

 was necessary to initiate glaciers, and they extended in some instances 

 down to 6,500 feet, the maximum depth of ice being 1,500 feet.^ In 

 Wasatch Mountains, Utah, the necessary altitude was from 8,000 

 to 9,000 feet, and many extended down to from 5,000 to 6,000 feet, 

 the largest ones and those reaching the lowest altitudes in their exten- 

 sion being on the west side of the range.^ In the Uinta Mountains, 

 Utah, most of the catchment areas were 10,000 feet or more above the 

 sea, but a few were between 9,000 and 10,000 feet. The longest was 

 27§ miles in length and several exceeded 20 miles, the lowest altitude 

 reached being 6,600 feet.^ 



In Colorado probably few glaciers originated at altitudes less than 



' Salisbtiry, R. D., "Glacial Geology," Geo/ogy of the Big Horn Mountains, U.S. Geol. Surv., Prof. Paper, 

 No. 51, pp. 72-73, IQ06. 



' Salisbury, R. D., "Glacial Work in the Western Mountains in 1901," Journ. Geol., Vol. IX, pp. 725-27, 

 1901. 



' Sausbury, op. oil., pp. 728-29. 



