108 



PHYSIOGRAPHY 



summer. Some of the surface water sinks into the ice, but some of it 

 forms little streams which flow on the ice until they reach a crevasse 

 or the edge of the glacier. These streams make little valleys in the 



ice (Fig. Ill), and so help to make 

 its surface rough. 



3. The stony and earthy debris 

 which many valley glaciers carry 

 on their surfaces also makes them 

 uneven. Large stones protect the 

 ice beneath from melting, and 

 therefore come to stand on pillars 

 of ice (Fig. 1, PL XXX, p.101), after 

 the surface about them is melted 

 away. Quantities of debris of any 

 sort have the same effect, by 

 shielding the ice beneath from the 

 sun's rays. Small stones on the 

 surface of the ice have the opposite 

 effect. Rock absorbs heat better 

 than the ice does, and thin pieces 

 of rock are warmed through, and 

 melt their way down into the ice more rapidly than the sun melts 

 the surface about them. 



Fig. 111. Valley of a superglacial 

 stream in the Bighorn Mountains. 



Movement 



Waste and supply of ice. The ice of a glacier is continually 

 wasting, (1) by melting at the surface, especially in summer, (2) by 

 melting below the surface, and (3) by evaporation. But in spite 

 of this constant waste, many glaciers remain about the same size 

 year after year. This shows that there must be some source of 

 supply to replace the waste. The supply is from the snow-fields, 

 from which the ice creeps down the valleys until it reaches a place 

 so low and so warm that the melting at the end balances the for- 

 ward motion. 



The movement of a glacier is so slow that it cannot be seen. 

 It was first known by observing (1) that the ends of glaciers were 

 sometimes farther down the valleys than they were at earlier times, 



