STUDIES FOR STUDENTS. 629 



off continually. In the first case, the extreme edge is in the 

 same place from year to year, but the ice which is at the extreme 

 edge this year is not the same ice which was in the correspond- 

 ing position last year. That ice has been melted. The ice 

 which is now at the edge was then back from the margin the 

 distance of one year's melting. All the drift covering carried 

 by that ice which has been melted during the year has been 

 dropped, and dropped on the surface over which the ice lay 

 when it melted. The superglacial drift on the ice which is now 

 at the front, is the superglacial drift which last year was back 

 from the edge the distance of one year's melting, together with 

 such material as was then englacial, but which surface melting 

 has meantime allowed to become superglacial. 



While the edge of the ice remains stationary in position, all 

 deposits of superglacial drift must take place in a narrow belt at 

 its edge. These deposits would tend to build up a marginal 

 ridge, or dump moraine. For reasons already given, a stationary 

 ice margin must have less surface drift than an advancing mar- 

 gin, other conditions being equal. 



The case is somewhat altered if the edge of the ice be 

 advancing. If the ice moves forward 500 feet per year, while it 

 is melted back 400 feet, it makes a net advance of 100 feet. 

 But the 400 feet which were at the front last year are gone, and 

 the superglacial material which this 400 feet of ice carried has 

 been deposited where the ice which carried it melted, that is, on 

 ground now occupied by the ice. The ice has already worked over 

 in part, and buried in part, the superglacial material deposited from 

 the 400 feet of ice which have been melted off during the year. 

 When the ice has advanced still further, it will have covered 

 the particular superglacial drift referred to more deeply, and 

 will have modified it more completely. At no considerable dis- 

 tance from the margin, the larger part of it would probably have 

 lost every trace of its earlier superglacial character, by having 

 been worked over beneath the ice, and so converted into sub- 

 glacial drift. Such part as did not suffer this fate might be 

 buried, and, in genesis, would be superglacial material (super- 



