GLACIAL FEATURES 



293 



typical of a large number which were made in the Peruvian Andes 

 and which have served as the basis of Fig. 195. It will be observed 

 that between 15° and 20° there is a marked change of function and 

 again between +5° and — 5° declivity, giving a double reversed 

 curve. The meaning of the change between 15° and 20° is inferred 

 to be that, with gradients over 20°, snow cannot wholly resist 



300 

 250 

 ZOO 







CURVE OF SNOW 1 



MOTION 









•^ 

















150 

 100 









\ • 



• 













50 

































• 



























• 



• 



• . 













4 











■ • 



-5° 0° 5° 10° 15° 20° Z5° 30° 35° 40" 



Fig. 195 — Curve of snow motion. Based on many observations of snow motion to 

 show minimum thickness of snow required to move on a given gradient. Figures on 

 the left represent thickness of snow in feet. The degrees represent the gradient of the 

 surface. The gradients have been run in sequence down to 0° for the sake of com- 

 pleting the accompanying discussion. Obviously no glacially unmodified valley in a 

 region of mountainous relief would start with so low a gradient, though glacial action 

 would soon bring it into existence. Between -\-5° and — 5° the curve is based on the 

 gradients of nivated surfaces. 



gravity in the presence of diurnal temperature changes across the 

 freezing point and occasional snow or hail storms. With increase 

 of thickness compacting appears to progress so rapidly as to 

 permit the transfer of thrust for short distances before absorp- 

 tion of thrust takes place in the displaced snow. At 250 feet 

 thorough compacting appears to take place, enabling the snow to 

 move out under its own weight on even the faintest slopes ; while, 



