962 STUDIES FOR STUDENTS 



was the principal mode of origin of eskers. Against this view, and 

 against the view that superglacial stream deposits are of conse- 

 quence quantitatively, stand two facts, (i) So far as known the 

 surfaces of ice-sheets are free from drift (apart from wind-blown 

 dust) except for a fraction (and generally a small one) of a mile 

 from their edges ; and ( 2 ) superglacial streams are in general much 

 too swift to deposit drift, or to allow it to accumulate in their chan- 

 nels. The channels of most superglacial streams in North Green- 

 land, eve7i near the edge of the ice zvhere surface debris is abundaiit, are 

 absolutely free from drift. Judging from the force with which 

 they issue from the ice, englacial streams are likewise much too 

 swift to allow of deposition along their channels, as a general rule. 



Such trivial accumulations of drift as may be made in super- 

 glacial or englacial channels would ultimately reach the land 

 surface. During the advance of the ice they would be delivered 

 onto the land, as the ice which sustained them moved forward 

 into the zone of melting. They would then be overridden by 

 its further forward motion. During the retreat of the ice, such 

 deposits, once they reached the land surface, would not be sub- 

 sequently destroyed or overridden by it. 



Summary. — Such are the main phases of water action in con- 

 nection with a single ice-sheet, on the assumption that the edge 

 of the same did not oscillate backward and forward during the 

 period of its advance or retreat. Were this the complete history 

 of an ice-sheet, the stratified deposits, as they now exist, would 

 be (i) in part extraglacial — those made by waters beyond the 

 extreme advance of the ice; (2) in part supermorainic (super- 

 till) — especially those made by extraglacial waters during the 

 retreat of the ice ; and (3) in part submorainic (sub-till) — chiefly 

 those made by extraglacial waters during the advance of the 

 ice, and subsequently buried. The actual relations of the strat- 

 ified drift to the unstratified are, however, far less simple. 



RELATION OF STRATIFIED TO UNSTRATIFIED DRIFT. 



Deposits ?nade by extraglacial zvaters diiriiig the advance of the 

 ice, edge not oscillating. — At all stages of the glacial period, 



