626 THE JOURNAL OF GEOLOGY. 



these conditions applies with equal force to a first or a later ice 

 advance. The second and third apply with greater force to the 

 first advance of the ice, unless the interval between the first and 

 second was sufficiently long to develop a rough and essentially 

 non-glaciated topography. The conditions most favorable for 

 the acquisition of superglacial material are, (i) a rough country, 

 with (2) much loose material upon its surface, affected (3) by 

 an advancing ice margin. Whenever the edge of the ice was 

 stationary, the elevations which were yielding superglacial 

 material, or englacial material which stood a chance of becoming 

 superglacial, were elevations which had already been worked 

 over by the advancing ice, so that they were less productive than 

 when the ice first reached them. When the ice was retreating, 

 its surface would have had still less superglacial material than 

 when stationary. 



As noted in connection with alpine glaciers, the drift which 

 has been superglacial from the outset should differ from the 

 englacial-superglacial drift by showing less wear. The difference 

 might be great or slight, depending in part upon the duration of 

 the englacial history of the englacial-superglacial drift. 



From what has been said it is clear that the englacial-super- 

 glacial drift acquired by the thin margin of an advancing ice- 

 sheet had a shorter englacial history than that acquired by any 

 other part of the ice-sheet. It should, therefore, show less wear 

 than the corresponding drift picked up by the ice back from the 

 margin. The amount of material which was superglacial from 

 the outset must also have been greatest at the margin of the ice 

 during its advance, and this had little or no opportunity of 

 suffering wear. The aggregate of superglacial drift at the mar- 

 gin of an advancing ice-sheet must, therefore, be much more free 

 from wear than that of any part of the ice back from the mar- 

 gin. Since the superglacial drift of a receding ice margin may 

 be mainly or wholly englacial-superglacial, and since it may 

 have been mainly or wholly acquired by ice of considerable 

 thickness, attaining its superior position only after a long engla- 

 cial course, it follows that the surface drift of an advancing ice 



