THE WORK OF GLACIERS 



149 



Fig. 133. — Denver Glacier, Alaska, showing an ice- 

 fall, feeding grounds, and lateral and medial moraines. 

 (Photo. F. B. Sayre.) 



of glaciers become irregular in other ways besides fracturing. Water 

 from the melting ice forms rivulets, which erode and melt channels in 

 the ice. When such a 

 stream reaches a cre- 

 vasse it plunges down, 

 forming a circular 

 shaft called a moulin 

 (French for mill). As 

 the ice moves on, this 

 opening is closed and 

 a new one formed in 

 its place. Thus a 

 series of inactive 

 moulins, in various 

 stages of preserva- 

 tion, are left extend- 

 ing down the glacier 

 from the active one. 

 The active moulin, however, may be said to remain stationary or 

 confined to narrow limits, and may, in time, excavate potholes 



(p. 93) many feet in depth in the rock 

 beneath the glacier (Fig. 134). 



Since the ice of a glacier varies in com- 

 pactness it melts unevenly, and this also 

 tends to produce a rough surface. 



The surface of a glacier is also roughened 

 by the irregular melting of the ice, due to 

 the accumulation of debris. If a fragment 

 of rock which has fallen on the ice is too 

 thick to be heated through by the sun it 

 will protect the ice beneath from melting. 

 Because of this it may in time stand on an 

 ice pillar several feet in height, forming an 

 ice table (Fig. 135). After a time the pillar 

 may become so high that the sun will be able 

 to melt it. The protecting cap of rock will 

 then be undermined and will slide off", on 

 the south side in the northern hemisphere, 

 and will then be ready to cause the forma- 

 tion of another column. 



Fig. 134. — A giant pot- 

 hole formed in the bed of a 

 glacier by the water, sand, 

 and gravel carried through a 

 crevasse. Near Christiania. 

 (After A. Geikie.) 



