CHAPTER IV 



THE GLACIAL PERIOD 



REAT glaciers came out of the north in 

 recent geological time covering all the 

 northern part of North America (Fig. 

 38), Europe, and Asia, glaciers compara- 

 ble to the great ice cap that now covers 

 all the antarctic lands or to the one 

 that still over-rides Greenland. The 

 glacial period probably came on gradu- 

 ally. The snow accumulated in the north more rapidly during 

 the long winters than the summer's diminishing heat could 

 melt it. It grew deeper and deeper. The chill of the great 

 snowfields was felt in the lands south of them where the climate 

 became more rigorous and the snows began to pile up ; thus the 

 snowfields became more extensive. In the far north the snow 

 banked up miles deep until the lower layers under the terrific 

 pressure were transformed into ice and were squeezed out, 

 moving southward over once fertile lands and transforming 

 them into arctic desolation. This same process is going on now 

 on a small scale in the high mountains all over the world where 

 lakes of snow in the upper valleys outlet by rivers of ice that 

 slowly push their way down the slopes (Fig. 10). 



What agencies brought on the glacial period has been a 

 matter of interesting conjecture. The uplift of the northern 

 part of the continent to Alpine height, a shift of the warm ocean 

 currents so they did not flow into the northern oceans, a change 

 in the position of the earth's axis so that winter came in the 

 northern hemisphere when the earth in its orbit was farthest from 

 the sun instead of nearest as now, a change of eccentricity of the 

 earth's orbit, an increase in the amount of carbon dioxide in the 



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