356 GEOLOGY. 



If the fall of snow toward the margin of the ice-sheet greatly exceeded 

 that at its center, as it probably did, an infra-marginal bell, rather 

 than the geographic center of the field, may have controlled the mar- 

 ginal movement of the ice. With excess of infra-marginal accumula- 

 tion, the surface slope of the ice would be relatively great from the 

 zone of maximum accumulation to the edge of the ice, but might 

 be very slight, or even nil, within it (Fig. 494). Under these circum- 

 stances, the extension of the ice being due largely to dispersal from 

 the infra-marginal zone, the maximum thickness of the ice-sheets 

 might be notably less than if the geographic center remained the effec- 

 tive dynamic center. 



In an ice-sheet like that which was responsible for the drift of North 

 America, it is probable that all influencing and limiting conditions 

 which may exist in any ice-sheet were found. The varying pressures 



A ! < E 



1 ' i ! i! i! i ! i! i ! i ! i ! i! S 



■ i i i i i i i i i 



Fig. 494. — Diagram to illustrate the surface configuration of a great ice-sheet, accord- 

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

 margins have steep slopes. 



and temperatures to which its various parts were subject tended to 

 produce various degrees of mobility in its mass, and varying degrees 

 of mobility demanded varying degrees of surface slope in order to 

 bring about movement. Could the surface slope necessary for move- 

 ment be determined for any given region, and for any given time during 

 the glacial period, it does not follow that the same slope was necessary 

 for the whole ice-sheet, or even that it was necessary for any particular 

 region, at all stages of its glacial history. Both observation on existing 

 glaciers and ice-sheets, and considerations of a physical nature, make 

 it certain, first, that the angle of slope must have decreased with increas- 

 ing distance from the margin of the ice (that is, with increasing thick- 

 ness of the ice) until, at the center of the field, it approached zero: 

 and second, that at the edge of the ice-sheet, where the ice was thin- 

 nest, the surface slope was greatest. 



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 



