GLACIERS 193 



Long-lived accumulations of snow constitute snow fields (Figs. 

 199 and 200). 



Snow fields become ice fields by the same processes which 

 transform many snow banks into ice banks each winter. The 

 bottom snow is compressed by the weight of that above and 

 becomes more and more compact, the result being much as 

 when snow is packed into an icelike mass in making snow- 





FIG. 200. Snow fields of Monte Rosa, Switzerland. (R. T. Chamberlin.) 



balls. Water from rains and from surface melting during the 

 warmer periods sinks into the snow beneath, and when it freezes 

 helps to cement the mass. Still other processes aid in the 

 change, and the originally loose snow passes by degrees into 

 compact ice. 



Formation of glaciers. When the ice has formed in suf- 

 ficient quantity, it begins to spread from the place of origin. 

 If formed on plains or plateaus, ice fields are thickest at or 

 near their centers, thinning more or less regularly to the mar- 

 gins, where wastage balances snowfall. In such situations the 

 ice accordingly moves slowly under its own weight in all direc- 

 tions from the center. If formed in and about the heads of 

 mountain valleys, snow fields and ice fields acquire a slow 

 movement down valley. When ice fields start to move, they 

 become glaciers. 



