THE LOCAL ORIGLN OF GLACIAL DRIFT 429 



expense of others. The preceding series would then be brought 

 to some such terms as the following : 



From formation 1 - - - - - - 6 \ = i\ % + 



From formation 2 - - - - - 6 A 4 - = 6 % -f- 



From formation 3 - - - - - if =23' % + 



From formation 4 - - - - e~4 =69 % + 



Thus the constant tendency of the drift to lodge, after being 

 once started, tends still further to diminish the quantity of drift 

 from any formation after the formation is passed. 



The effectiveness of the tendency to lodgment of drift near 

 its source is dependent on several conditions. One of them is 

 the rate and steadiness of movement, and another the topog- 

 raphy of the surface. The edge of the ice is not believed to 

 have moved forward at equal rates, either during its advance or 

 during its retreat. Whenever the edge of the ice, after a given 

 advance, remained approximately constant in position for a 

 period of years, all the drift brought to the edge of the ice dur- 

 ing the halt accumulates beneath it. It presently accumulates 

 in sufficient quantity to form a submarginal ridge or barrier, and 

 when the ice is again affected by movement sufficient to carry its 

 edge farther forward, it is obliged to override or carry forward 

 this submarginal accumulation. Judging from the phenomena of 

 North Greenland, such material was more largely overriden than 

 urged along. Thus the drift gathered toward the center of the 

 ice field is lodged in exceptional quantity wherever the edge of 

 the ice was for a time nearly stationary, and the ice which passed 

 on ove*- the drift which was lodged proceeded to gather a new 

 load made up chiefly of material derived from the surface outside 

 the position of the preceding halt. This tends to emphasize the 

 local tacies of the drift. 



These considerations in themselves would be quite sufficient, 

 as the last set of fractions shows, to explain the great predomi- 

 nance of relatively local material in the drift of any given region, 

 but other factors serve to emphasize the point still further. 



The top of the ice-sheet moves forward faster than the bot- 

 tom. One reason for this is that the lower part, which is more 



