250 GEOLOGY 



into the ice. The depth to which it penetrates is undetermined 

 by observation, but it doubtless goes down to the zone of constant 

 temperature in all cases, and still lower in some cases, as where 

 there are crevasses, and where the temperature is as high as 32 

 Fahr. all the way to the bottom. At considerable distances above 

 the line of perennial snow there is little water either from melting 

 or from rain, and hence relatively slight penetration. Below the 

 line of perennial snow, and for a short distance above it, there is 

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

 often 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. 

 Exceptionally it follows definite englacial channels, as shown by 

 springs or streams issuing from the ice at some point above its 

 bottom. Oftener it descends or moves forward through the irreg- 

 ular openings which the accidents of motion have developed. If 

 it reaches a level where the temperature is below its freezing-point, 

 it congeals. Otherwise it remains in cavities or descends to the 

 bottom. The water produced by melting within the glacier prob- 

 ably 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 River is already a con- 

 siderable stream where it issues from beneath the Rhone Glacier. 

 In the glaciers of high latitudes, subglacial 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 luvn 

 superglacial, 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, forming glacio-fluvial deposits. 



