CHAPTER VII 

 ICE SHEETS AND GLACIERS 



The Conditions of Maximum Erosion 



I HAVE investigated glacial conditions in many 

 countries both in the northern and southern hemi- 

 spheres, and my studies have led me to certain perhaps 

 novel conclusions with regard to the real meaning of 

 the onset of an ice age. Any one who has done much 

 flying knows that there is an environment in many ways 

 suited for an ice age within a mile or two of any 

 temperate city. It occurs, of course, vertically over- 

 head. It is profitable, therefore, to look upon the 

 approach of an ice age as essentially due to the lower- 

 ing of certain isothermal zones or layers from an 

 overhead position until such zones intersect more or 

 less of the land surface concerned.^ These zones have a 

 slight slope from the equator to the Pole. If we travel 

 from New York to the North Pole or from Sydney to 

 the South Pole, we reach latitudes where this Ice Age 

 layer comes down to sea level, at about 60° of latitude 

 in both cases. This layer corresponds essentially to the 

 permanent snow line. No part of the world perhaps is 

 better suited for a study of the optimum temperature 



1 Fluctuations in the amount of snowfall are of secondary im- 

 portance. 



