630 THE JOURNAL OF GEOLOGY. 



•glacial till). In position it would be beneath the subglacial drift 

 deposited by the same ice-sheet. 



At every stage in the advance of the ice-sheet, there would 

 be a narrow margin of ice covering the superglacial deposits 

 made at the edge of the ice just before. Such superglacial 

 deposits might for a time retain their superglacial characteristics, 

 even though buried by the ice. So long as this remained true, 

 their classification might be open to question. But it is not 

 apprehended that this condition of things commonly existed for 

 any considerable distance back from the ice's edge. This state- 

 ment, which is believed to be true as a general statement, is not 

 to be construed to mean that superglacial material, deposited by 

 an advancing ice-sheet, may not in exceptional cases be buried 

 by the subglacial deposits of the ice at a later and more advanced 

 .stage of its development, without being in any way changed, be- 

 yond being compacted by the pressure of the over-riding ice. 



The ice at any stage whatsoever in the period of its advance, 

 must have worked over, or in exceptional cases buried, all the 

 superglacial material which had been deposited up to that time. 

 What had been superglacial material thus became subglacial, as 

 the ice advanced. However great the amount of superglacial 

 material acquired and deposited by the advancing edge of a con- 

 tinental ice-sheet as it passes over rough surfaces, essentially all 

 of it must subsequently be reworked beneath the ice, must lose 

 its superglacial characteristics in the process, and must have im- 

 pressed upon it the features which ice impresses upon materials 

 worked over beneath itself. That is, all superglacial and engla- 

 cial-superglacial drift deposited by an advancing ice-sheet must 

 be transformed later from superglacial into subglacial drift. It 

 has already been shown that it is the advancing margin of ah 

 ice-sheet which is most favorably circumstanced for carrying a 

 heavy load of superglacial material. The conditions of glacier 

 motion and melting determine the continuous deposition of this 

 material, while the ice is still advancing. It follows that the 

 heaviest deposits of superglacial material which an ice-sheet can 

 make at any period of its history, cannot retain their super- 



