THE PLEISTOCENE OR GLACIAL PERIOD. 355 



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, modi- 

 fying 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 trans- 

 porting and depositing drift, both 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 cooperated 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 its appropriate effect on the drift before it became clothed 

 with vegetation. 



The Development and the Thickness of the Ice-sheets. 



The development of glaciers from snow-fields has been discussed 

 in Volume I, but a few words with reference especially to the develop- 

 ment and thickness 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 encountered 

 and passed over hills, and even mountains, of considerable 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 such elevations were remote from the center of movement, 

 the ice must have been still thicker at those centers, to afford the 

 necessary "head." 



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

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

 surface slope necessary for movement were known, the maximum 

 thickness of the ice could be calculated. It is probable, however, 

 both that the centers of the ice-sheet did not remain the effective cen- 

 ters of movement, and that the surface slope necessary for movement 

 was variable. 



