GLACIERS 287 



the cold season, a thick covering of snow and ice will in 

 time be formed. The ice is due to the pressure exerted 

 on the lower layers by the weight of the snow above and 

 to the freezing of the percolating water which comes from 

 the summer melting of the upper snow layers. 



Although ice in small pieces is brittle, in great masses 

 it acts somewhat like a thick and viscid liquid. It con- 

 forms itself to the surface upon which it lies, and under 



SNOW FIELDS AT THE HEAD OF A GLACIER 



the pull of gravity or pressure from an accumulating mass 

 behind, slowly moves forward, resembling in some ways 

 thick tar creeping down an incline or spreading out when 

 heaped into a pile. Such a moving mass of ice is called a 

 glacier. The exact manner of glacial movement, however, 

 is not fully understood. 



In mountain regions where the snow holds over through 

 the summer, the wind-drifts and the snow-slides carry 

 great quantities of snow into the upper valleys, until ever 



