ICE SHEETS AND GLACIERS 



the South Island of New Zealand. Here the conditions 

 of a plentiful snowfall are combined with a suitable 

 position of the isothermal layer of J2° F. It is this 

 last control, the environment where freeze-and-thaw 

 action is naturally most active, which is all-important 

 in eroding a landscape by the action of ice. Each time 

 the temperature sinks below freezing point (32° F.) 

 a wedge of ice is formed in every crevice of rock. This 

 may crack off a fragment of rock, which falls away 

 when thaw sets in. When this is repeated day and 

 night during many months, the amount of erosion must 

 be tremendous. The actual wearing away of a valley 

 floor by a glacier is, no doubt, more striking and on 

 a larger scale. But it only affects a relatively small por- 

 tion of the topography, whereas nivation (freeze and 

 thaw) attacks the whole landscape. So also the river 

 only cuts out its bed, but the rains and rills wear away 

 fragments from an infinitely greater area. 



A good deal of rather ill-defined evidence in the Ant- 

 arctic indicates that movement of the ice (and conse- 

 quently glacial erosion) is much more active in the 

 summer months than in the winter. Even Cape Adare 

 (in latitude 71°) has only one month above freezing 

 point, while at Cape Evans (latitude 78°) the hottest 

 month has an average temperature of only 25° F. 

 The writer has formed the opinion that temperatures 

 fluctuating around 32° F. are the most favorable for 

 pronounced glacial erosion, but we have no accurate 

 measurements in Antarctica on this question of seasonal 

 variation in erosion. However, in Greenland Ryder 



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