CHAPTER XXII 

 THE CONTINENTAL GLACIERS OF THE " ICE AGE " 



Earlier cycles of glaciation. Our study of the rocks compos- 

 ing the outermost shell of the lithosphere tells us that in at least 

 three widely separated periods of its history the earth has passed 

 through cycles of glaciation during which considerable portions 

 of its surface have been submerged beneath continental glaciers. 

 The latest of these occurred in the yesterday of geology and has 



FIG. 322. Map of the globe showing the areas which were covered by the con- 

 tinental glaciers of the so-called "ice-age" of the Pleistocene period. The arrows 

 show the directions of the centrifugal air currents in the fixed anticyclones above the 

 glaciers. 



often been referred to as the " ice age," because until quite re- 

 cently it was supposed to be the only one of which a record was 

 preserved. 



This latest ice age represents four complete cycles of glaciation, 

 for it is believed that the continental ice developed and then 

 completely disappeared during a period of mild climate before the 

 next glacier had formed in its place, and that this alternation of 

 climates was no less than three times repeated, making four cycles 

 in all. At nearly or quite the same time ice masses developed in 



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