STUDIES FOR STUDENTS. 63 I 



glacial character, but are necessarily converted into subglacial 

 drift. 1 



Such superglacial drift as was deposited by the melting of 

 the ice at the time of its maximum extension, and later, after 

 the recession began, would not become subglacial except by a 

 subsequent advance of the ice. But oscillations of the ice-edge 

 are probably frequent, even during the recession of a continental 

 glacier, and it is only the superglacial drift left by the ice in any 

 given locality at the time of its final withdrawal from that local- 

 ity, which can properly be classed as superglacial drift. In view 

 of this fact, and in view of the further fact that receding ice has 

 little superglacial drift to deposit, it would seem that there was 

 little chance for the presence of much superglacial drift in such 

 a region as that from which our continental ice-sheet withdrew, 

 unless, indeed, there was actual transfer of drift from a basal to 

 a superglacial position. Not only would the amount of super- 

 glacial drift deposited by the receding ice be slight, compared with 

 the deposits of an advanced or advancing margin, but it would 

 be composed of fresher material, which had been subjected to 

 more wear. It would, therefore, be less distinct from the sub- 

 glacial drift, so far as these characteristics are concerned, than 

 the superglacial drift deposited by the extreme margin of the 

 ice. 



It is no part of the purpose of this paper to discuss the criteria 

 for the recognition of superglacial drift in glaciated regions. 

 This may be the subject of a later paper. But it may not be use- 

 less to point out that such superglacial material as was deposited 

 by the ice at and near the limit of its maximum advance, may 

 have been largely acquired by the advancing margin of the ice 

 while working over territory which had not been glaciated hith- 

 erto. So far forth, it may have been largely composed of oxi- 

 dized, and disintegrated materials. Here, and here only, it is 



I This statement leaves out of consideration the effect of possible unequal rates of 

 advance and recession of the ice. Such inequality might slightly change the result. 

 It also leaves out of consideration the drift which is thought by some to rise through 

 the ice during its motion. 



