THE PLEISTOCENE OR GLACIAL PERIOD 861 



not to be forgotten that this does not preclude the belief that at 

 various times and places, in the course of the ice period, icebergs 

 may have been formed, or that locally and temporarily they played 

 an important role. It does not preclude the idea that, contem- 

 poraneously with the production of the great body of the drift by 

 glacier ice, the sea may have been at work on some parts of the 

 present land area, modifying the deposits made by ice and ice 

 drainage. Indeed, there is abundant evidence that such was the 

 fact, for some regions, now covered by drift, stood lower than now, 

 relative to sea level, when the drift was deposited, or since. The 

 glacial theory does not deny that rivers produced by the melting 

 of the ice were an important factor in transporting and depositing 

 drift, bath within and without the ice-covered territory. It does 

 not deny that lakes formed in one way and another through the 

 influence of ice, were locally important in determining the character 

 and disposition of the drift. Not only does the glacier theory deny 

 none of these things, but it distinctly affirms that rivers, lakes, the 

 sea, icebergs, and pan-ice must have co-operated with glacier ice in 

 the production of the drift, each in its appropriate way and measure, 

 and that after the disappearance of the ice and the ice-water, 

 the wind had some effect on the drift before it was clothed with 

 vegetation. 



The Development and the Thickness of the Ice-sheets 

 The development of glaciers from snow-fields has been discussed 

 already (p. 229), but a few words with reference especially to the 

 development of the ice-sheets of our continent, are here added. 



If the expansion of the ice-sheets was due principally to move- 

 ment from a center or centers, the ice at these centers must have 

 been prodigiously thick, for in the course of its progress it en- 

 countered and passed over hills, and even mountains, of consider- 

 able height. In the vicinity of elevations which it covered, its 

 thickness must have been at least as great as the height of these 

 elevations above their bases. 



If the centers of the North American ice-sheets remained the 

 centers of movement throughout the glacial period, and if the degree 

 of surface slope necessary for movement were known, the maximum 



