THE GLACIER'S SURFACE FEATURES 



397 



debris, sand, or dirt cone (Fig. 425). The novice in glacier study 

 is apt to assume that these black cones contain only dirt, but is 

 rudely awakened to the reality when he attempts to kick them to 

 pieces. Both glacier tubs and debris cones may assume large 

 dimensions ; as, for example, in Alaska, where they may be properly 

 described as lakes and hills. 



A patch of hard and dense snow which is less easily melted 

 than that upon which it rests may lead to the formation of snow 

 cones upon the glacier surface similar in size and shape to the 

 better known debris cones. Such cones of snow have, with 

 doubtful propriety, been designated " penitents," for it is pretty 

 clear that the interesting bowed snow figures, which really re- 

 semble penitents and which were first described from the southern 

 Andes under the name of nieves penitentes, are of somewhat dif- 

 ferent character. 



One further ice feature shaped by differential melting around 

 rock particles remains to be mentioned. Wherever the seasonal 

 snowfalls of the n6ve are exposed in crevasses, they are generally 

 found to be separated by layers of dirt, and lines of pebbles simi- 

 larly separate those ice layers which are revealed at the foot 

 of the glacier. In either case, if the sun's rays can reach these 

 layers in an opened crevasse, the half-buried 

 rock fragments are warmed by the sun upon 

 their exposed surfaces and slowly melt their 

 way down the ice surface, thus removing from 

 it a thin layer of snow or ice and causing that 

 part above the pebble layer to project like 

 a cornice. This process will go on until the 

 overhanging cornice protects the pebbles from 

 any further warming by the sun, but each 

 lower pebble layer that is reached by the sun 

 will produce an additional cornice, so that 

 the original surface may at the bottom have 

 been retired by the process a number of inches, 

 are described as glacier cornices (Fig. 426). 



Glacier drainage. Already in the early morning of every 

 warm summer day, active melting has begun upon the surface of 

 the Swiss glaciers. Rills of icy water soon make their way along 

 depressions upon the surface, and are joined to one another so that 



FIG. 426. Schematic 

 diagram to show the 

 manner of formation 

 of glacier cornices. 



These features 



