398 



EARTH FEATURES AND THEIR MEANING 



FIG. 427. Superglacial stream upon the Great 

 Aletsch glacier. 



they sometimes form brooks of considerable size (Fig. 427). Such 

 streams continue their serpentine courses until these are inter- 

 sected by a crevasse down which the waters plunge in a whirling 

 vortex which soon develops a vertical shaft of circular section 

 within the ice. Such shafts with their descending columns of 



whirling water are the 

 well-known' moulins, 

 or " mills," which 

 may be detected from 

 a distance by their 

 gurgling sounds. The 

 first plunge of the 

 water may not reach 

 to the bottom of the 

 glacier, in which case 

 the stream finds a 

 passageway below the 

 surface but above the 

 floor until another 

 crevasse is encountered and a new plunge made, here perhaps to 

 the bottom. Once upon the valley floor the stream is joined by 

 others, and pursues its course within an ice tunnel of its own 

 making (Fig. 421, p. 394) until it issues at the glacier front. 



The coarser of the rock debris which was gathered up by the 

 stream upon the glacier surface is deposited within the tunnel in 

 imperfect assortment (gravel and sand), while all finer material 

 and that lifted from the floor (rock flour) is retained in suspension 

 and gives to the escaping stream its opaque white appearance. 

 This glacier milk may generally be traced far down the valleys or 

 out upon the foreland, and is often the traveler's first indication 

 that a range which he is approaching supports glaciers. 



Deposits within the vacated valley. For every excavation 

 of the higher portions of the upland through glacial sculpture, 

 there is a corresponding deposit of the excavated materials in 

 lower levels. So far as these materials are deposited directly by 

 the ice, they form the lateral, medial, ground, and terminal moraines 

 .already described. A considerable proportion of them are, how- 

 ever, deposited by the water outside the terminal moraine*; but 

 ias with the shrinking glacier the ice front retires in halting move- 



