THICKNESS OF ICE-SHEETS 625 



dpally to movement 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 con- 

 siderable 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 

 thickness of the ice could be calculated. But it is probable that 

 the centers of the ice-sheet did not remain the effective centers of 

 movement. If the fall of snow toward the margin of the ice-sheet 

 greatly exceeded that at its center, as it probably did, a belt near the 



- I I I I I I I I I I I | | | i--r-- 



I I I I I I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' | ' ! 



Fig. 518. Diagram to illustrate the surface configuration of a great ice-sheet, 

 according to the conception here presented. The central part is relatively flat, and 

 the margins have steep slopes. 



margin, rather than the geographic center of the field, may have 

 Controlled the marginal movement of the ice. With excess of ac- 

 cumulation near the border, the slope of the surface near the edge 

 might be relatively great, while it was slight in the center of the field, 

 as shown by Fig. 518. Under these conditions, the maximum 

 thickness of the ice-sheets might be notably less than if the geo- 

 graphic center remained the effective dynamic center. 



No sufficient data are at hand for determining with accuracy 

 the average slope of such an ice-sheet as that which covered our con- 

 tinent, but something is known of its slope at certain points. Near 

 Baraboo, Wisconsin, 1 the edge of the ice at the time of its maximum 

 extension in that region lay along the side of a bold ridge, the axis 

 of which was nearly parallel to the direction of ice movement. The 

 position of the upper edge of the ice against the slope of the ridge 

 is sharply defined. For the last i> miles, its average slope was 

 about 320 feet per mile. This was at the extreme edge of the ice, 

 where the slope was greatest. In Montana, the slope of the upper 

 surface of the ice for the 25 miles back from its edge has been 

 estimated at 50 feet per mile. 2 



1 Jour. Geol., Vol. Ill, p. 655. 



2 Calhoun, Jour. Geol., Vol. IX, p. 718. 



