CHAPTER XXXI 

 THE HUMAN OR PRESENT PERIOD 



The end of the glacial period. The termination of the glacial 

 period is usually placed at the time when the ice-sheets disappeared 

 from the lowlands in the middle latitudes of Europe and North 

 America. Notwithstanding this conventional usage, it is to be 

 noted that the ice-sheets had not then completely disappeared, 

 and have not even now, for about 10% of the recently glaciated 

 area of North America (chiefly in Greenland) is still buried in ice. 

 These relics of the last glacial epoch show that the continent has 

 not yet emerged completely from the glacial period. 



Future glaciation. It is not absolutely clear that there may 

 not be another increase of ice before the long series of glacial epochs 

 closes, but the probablities seem to be much against it. The declin- 

 ing series of oscillations already noted seems to have reached its 

 last term, yet the factors that produce glaciation are too complex 

 to warrant more than a comfortable presumption of future immu- 

 nity until another great deformation shall have taken place. 



The end of the deformation period. It is not wholly clear that 

 the deformative period which began in the late Tertiary, and ex- 

 tended through the Pleistocene, is yet completed. We are accus- 

 tomed to regard it as essentially passed, notwithstanding some 

 movements still in progress; and, in the main, this position seems 

 to be justified. 



The movements of post-glacial times, however, are not to be 

 ignored. The northeastern part of North America has been ele- 

 vated relatively since the disappearance of the ice (p. 912). This 

 relative rise is perhaps a reaction from the depression due to the 

 weighting and cooling caused by the ice-sheets. 



A recent movement in the region of the Great Plains seems to 

 be suggested by certain physiographic features. Extensive tracts 



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