THE PLEISTOCENE OR GLACIAL PERIOD. 377 



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 melted from beneath. They would then 

 be over-ridden by its further forward motion. During the retreat of 

 the ice, such deposits, once they reached the land surface, would not 

 be subsequently destroyed or overridden by it. 



Relations of Stratified to Unstratified Drift. 1 



The general relations of the stratified to the unstratified drift have 

 already been indicated in a general way. These relations may be 

 understood, when it is remembered (1) that the edge of each ice-sheet 

 probably oscillated back and forth, more or less, during both its advance 

 and its retreat, (2) that there were several ice-sheets over large parts 

 of the area affected by drift, and (3) that stratified drift was being 

 deposited at all stages of every ice-sheet, at points (a) beneath the 

 ice, (b) at its edge, and (c) beyond it. 



On the basis of position, existing stratified drift deposits may be 

 classified as follows: 



1. Extraglacial deposits, made by the waters of any glacial epoch 

 if they deposited beyond the farthest limit of the ice. 



2. Supermorainic deposits, made chiefly during the final retreat of 

 the ice from the locality where they occur, but sometimes by extra- 

 glacial streams or lakes of an epoch later than that when the subjacent 

 till was deposited. Locally, too, stratified deposits of an early stage 

 of a glacial epoch, lying on till, may have failed to be buried by the sub- 

 sequent passage of the ice over them, and so remain at the surface. 

 In origin, supermorainic deposits of stratified drift were for the most 

 part extraglacial (including marginal), so far as the ice-sheet calling 

 them into existence was concerned. Less commonly they were sub- 

 glacial, and failed to be covered, and less commonly still (if ever) 

 superglacial. 



3. Submorainic (basal) deposits were made chiefly by extraglacial 

 waters in advance of the first ice which affected the region where they 

 occur. They were subsequently overridden by the ice and buried 

 by its deposits. Submorainic deposits, however, may have arisen in 

 other ways. Subglacial waters may have made deposits of stratified 



J Jour. of Geol., Vol. IV, pp. 948-970. 



