958 STUDIES FOR STUDENTS 



DEPOSITS MADE BY EXTRAGLACIAL WATERS DURING THE RETREAT 



OF THE ICE. 



During the retreat of any ice-sheet, disregarding oscilla- 

 tions of its edge, its margin withdrew step by step from the 

 position of extreme advance to its center. When the process 

 of dissolution was complete, each portion of the territory once 

 covered by the ice, had at some stage in the dissolution, found 

 itself in a marginal position. At all stages in its retreat the 

 waters issuing from the edge of the ice were working in the 

 manner already outlined in the preceding paragraphs. Two 

 points of difference only need to be especially noted. In 

 the first place the deposits made by waters issuing from the 

 retreating ice, were laid down on territory which the ice had 

 occupied, and their subjacent stratum was often glacial drift. 

 So far as this was the case, the stratified drift was super- 

 morainic, not extra-morainic. In the second place the edge of 

 the ice in retreat did not give rise to such sharply marked for- 

 mations as the edge of the ice which was stationary. The proc- 

 esses which had given rise to valley trains, overwash plains, 

 kames, etc., while the ice edge was stationary, were still in 

 operation, but the line or zone of activity (the edge of the ice) 

 was continually retreating, so that the foregoing types, more or 

 less dependent on a stationary edge, were rarely well developed. 

 As the ice withrew, therefore, it allowed to be spread over the 

 surface it had earlier occupied, many incipient valley trains, 

 overwash plains, and kames, and a multitude of ill-defined 

 patches of stratified drift, thick and thin, coarse and fine. 

 Wherever the ice halted in its retreat these various types stood 

 chance of better development. 



Such deposits would not cover all the surface discovered by 

 the ice in its retreat, since the issuing waters, thanks to their great 

 mobility, concentrated their activities along those lines which 

 favored their motion. Nevertheless the aggregate area of the 

 deposits made by water outside the ice as it retreated, was great. 



It is to be noted that it was not streams alone which were 

 operative as the ice retreated. As its edge withdrew, lakes and 



