628 THE JOURNAL OF GEOLOGY. 



surface melting, more dust must finally rest on the surface of an 

 ice-sheet than on the surface of a lesser body of ice. Unless 

 carried away by surface drainage, superglacial material having an 

 eolian origin should appear at the upper (inner) margin of the 

 zone of wastage, and should increase to the very edge of the ice. 

 It should be most abundant in this position, since most ice has 

 here been melted, leaving its dust behind. 



To the fine material which was left on the surface by the 

 melting of the upper ice, was added such as blew upon the sur- 

 face within the zone of wastage, and which was never englacial. 

 Like the former, this latter material must have been most abun- 

 dant at the extreme edge of the ice. 



It is not definitely known how important (quantitatively) 

 wind drift was on the North American ice-sheet. But it is 

 believed that its amount was far greater than has been commonly 

 recognized. 



DEPOSITION OF SUPERGLACIAL MATERIAL. 



If the margin of an ice-sheet thins to an edge, the englacial 

 drift must ultimately become either superglacial or subglacial. 

 Either basal melting will bring it to the bottom of the ice, or 

 surface melting will bring it to the top. There is no third 

 alternative. If the drift rise or sink through the ice, the case is 

 in no wise altered, so far as the final result is concerned. If an 

 ice-sheet terminates with an abrupt front, it is manifest that some 

 englacial material may be deposited from its englacial position. 

 Since the ice-sheet is believed to have thinned virtually to an 

 edge, it is clear that essentially all the englacial material was 

 either superglacial or subglacial in its final deposition. 



While the superglacial drift is being carried forward on the 

 surface of the ice, the edge of the ice is being continually melted 

 back. When it is melted back as rapidly as it advances, the 

 edge of the ice is stationary in position. When the rate of edge 

 melting exceeds that of forward flowage, the edge recedes. 

 When the wastage falls short of the advance, the edge moves 

 forward. In any case the edge of an ice-sheet is being melted 



