STUDIES FOR STUDENTS. 627 



margin must be more free from wear than that of a receding 

 margin. 



There must have been another difference between the super- 

 glacial drift acquired by an advancing ice margin, and that 

 acquired by other parts of the ice. The advancing margin of an 

 ice-sheet invades territory the surface of which is likely to be 

 provided with the products of decomposition. It is these prod- 

 ucts of decomposition which in considerable part enter into the 

 composition of the superglacial drift, and into the composition 

 of the englacial drift which is shortly to become superglacial. 

 It follows that the superglacial and englacial-superglacial drift of 

 an advancing ice margin is made up more largely of the products 

 of rock disintegration than the surface drift of any other part of 

 the ice. The drift of a stationary or receding margin constitutes 

 no exception to this general statement, since, as already noted, 

 much of it was originally acquired some distance back from the 

 margin, and from surfaces over which much ice had passed, and 

 from which, therefore, the disintegrated products had been 

 earlier removed. 



It should be further noted that it is not merely an advancing 

 ice-sheet, the superglacial drift of which is largely disintegrated 

 and oxidized, but an advancing margin which is invading terri- 

 tory hitherto unglaciated, or unglaciated for a long period of 

 time. During the forward phase of an oscillatory movement, the 

 edge of the ice may be moving over territory which had been 

 but recently abandoned, and which might therefore be free from 

 the products of rock disintegration. 



Dust might be blown upon an ice-sheet as upon an alpine 

 glacier, but the thickness which it might attain on the former is 

 far greater than on the latter. The ice of a continental glacier 

 is much thicker than the ice of a mountain glacier. It has been 

 much longer in process of accumulation, and presumably contains 

 more dust. Above the zone of wastage, this is chiefly englacial. 

 In the zone of wastage, it gradually becomes superglacial. Since 

 there is a much longer period of surface melting in an ice-sheet 

 than in a mountain glacier, and so a much greater amount of 



