THE WORK OF GLACIERS 



147 



to wither back into its cirque and finally to disappear, the character- 

 istic features of the abandoned cirque are slowly obliterated ; land- 

 slides and talus descending from the cliffs are heaped upon the bottom, 

 filling the lakes and covering the bottom; the morainic (p. 159) or 

 rock ridge at its entrance is breached by a gorge cut by the out- 

 flowing stream ; side valleys are developed ; and the resulting topog- 

 raphy presents few features to indicate that it was developed from 

 a cirque. 



Ablation. — The surfaces of glaciers are constantly being lowered by 

 direct evaporation and by melting ; those of the Alpine glaciers are 

 lowered from 18 to 25 feet during the summer months, that of the 

 Mer de Glace having been lowered twenty-four and a half feet in 1842. 

 Since the advance of a glacier depends upon the thickness of its mass, 

 it follows that when ablation is excessive the front will retreat. A 

 retreating glacier is, consequently, thinner and, unless its valley walls 

 are vertical, narrower than when it was advancing. 



Surface of Mountain Glaciers 



The surface of a glacier is usually rough (Fig. 130) as a result of 

 a number of causes. 



(1) Irregularities Due to Tension. — Because of the brittleness 

 of the ice mass, glaciers are broken by cracks called crevasses. Some 

 of these are the result of the more rapid motion of the center than the 

 retarded sides, which 

 produces strains un- 

 der which the ice 

 fractures. The cre- 

 vasses formed in this 

 way are diagonal and 

 extend up the valley 

 (Fig. 136 C, p. 151). 

 When a glacier 

 emerges from a nar- 

 row portion of its 

 valley longitudinal 

 cracks are developed, 

 and the tension on a 

 Curve produces trans- jr IG j^q, — Surface of a glacier showing seracs and 



verse crevasses which crevasses. 



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