862 GEOLOGY 



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

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

 centers of movement, and that the surface slope necessary for move- 

 ment was variable. 



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 margin, 

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

 the marginal movement of the ice. With excess of accumulation 

 near the border, the slope of the surface near the edge might be rela- 

 tively great, while it was slight, or even nil in the center of the field, 

 as shown by Fig. 574. Under these circumstances, the maximum 



i ' , ' , ' . ' . ' . ' . ' . ' i ' . i ' . ' . ' . ' i ' . ' , ' , ' , 



Fig. 574. 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. 



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

 graphic center remained the effective dynamic center. The former 

 view is illustrated by Fig. 574. 



No sufficient data are at hand for determining with accuracy 

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

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

 Near Baraboo, Wis., 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 ri 

 is sharply defined. For the last 1% miles, its average slope was 

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

 where the slope was at a maximum. In Montana, the slope of the 

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

 estimated at 50 feet per mile. 2 Half of this seems like a very Imv 

 slope. A surface with a slope of 25 feet per mile would Seem to the 



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

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



