THE WORK OF SNOW AND ICE. 281 



surface water penetrates is undetermined, but it doubtless varies greatly, 

 not only in different glaciers, but in different parts of the same glacier, 

 and in the same part at different limes. Above the line of perennial 

 snow there is little water either from melting or from rain, and hence 

 relatively shght penetration. Below the Hne of perennial snow there is 

 much melting and much rain, and here it is probable that the water 

 sometimes, perhaps usually, penetrates to the bottom of the ice during 

 the melting season, even independently of crevasses. 



Once within the glacier, the course of the water is variable. Excep- 

 tionally it follows definite englacial channels, as shown by springs or 

 streams issuing from the ice at some point above its bottom (Fig. 251). 

 Oftener it descends or moves forward through the irregular openings 

 which the accidents of motion have developed. If it reaches a level 

 where the temperature is below its freezing-point, it congeals. Other- 

 wise it remains in cavities or descends to the bottom. The water pro- 

 duced by melting within the glacier probably follows a similar course. 

 So far as these waters descend to the bottom, they join those produced 

 by basal melting and issue from the glacier with them. In alpine 

 glaciers the waters beneath the ice often unite in a common stream in 

 the axis of the valley, and hollow out a tunnel. Thus the Rhone is 

 already a considerable stream where it issues from beneath the Rhone 

 glacier. In the glaciers of high latitudes, sub-glacial tunnels are less 

 common and the drainage is in streams along the sides of the glaciers 

 or through the debris beneath and about them. 



At the end of the glacier, all waters, whether they have been super- 

 glacial, englacial or subglacial, unite to bear away the silt, sand, gravel, 

 and even small bowlders set free from the ice, and to spread them in 

 belts along the border of the ice or in trains stretching down the valleys 

 below. These are the most common of the glacio-fluvial deposits. 



THE WORK OF GLACIERS. 



Erosion and transportation. 



The work accompHshed by glaciers is distinctive, for while like 

 rivers, they abrade the valleys through which they pass, carry forward 

 the material which they remove from the surface, and wear, grind, 

 and ultimately deposit it, and while their work therefore includes 

 erosion, transportation, and deposition, their method is peculiar. 



