STRUCTURE OF GLACIER ICE 



139 



water produced 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 some alpine glaciers, the 

 waters beneath the ice unite in 

 a common stream in the axis of the 

 valley, and hollow out a tunnel 

 in the bottom of the ice. The 

 Rhone River is already a consider- 

 able stream where it issues from 

 beneath the glacier. In high lati- 

 tudes, subglacial tunnels are not 

 common, and the drainage is in 

 streams along the sides of the gla- 

 ciers, or through the debris beneath 

 and about them. 



At the end of the glacier, all 

 waters, whether they have been 

 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 val- 

 leys below, forming glacio-fluwal 

 deposits. 



The structure and the motion of 

 glacier ice have been the subject of 

 much discussion. Though univer- 

 sal agreement concerning them has 

 not been reached, a brief outline of one of the current views is added. 

 Mention also is made of other views, some of which are still held 

 by various geologists. 



THE STRUCTURE OF GLACIER ICE 



The key to the structure and motion of glacier ice is based on the view that a 

 glacier is a mass of crystalline rock of the purest and simplest type known. It is 

 made of a single simple mineral, ice, which is always crystalline. It differs from 

 other rock chiefly in that its one mineral is liquefied at a low temperature. 



The dri'clofni->it of icr from snow. The fundamental conception of a 

 glacier is best developed by tracing the growth of its constituent crystals. When 



Fig. 141. A part of the vertical 

 side of a North Greenland glacier. 

 The vertical or even overhanging 

 faces are in some cases more than 

 100 feet high. 



