364 



FIRST YEAR SCIENCE 



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

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

 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. The exact manner of glacial move- 

 ment, however, is not fully understood. 



SNOW FIELD AT THE HEAD OF A GLACIER. 



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 

 accumulating masses of snow and ice, hundreds of feet 

 thick, are formed. The ice then slowly flows dowji a val- 

 ley till a point is reached where the melting at the end is 

 equal to the forward movement. An ice stream of this 



