6" TRA TIFIED DRIFT 967 



at the surface to the present time, so far as the ice of that epoch 

 was concerned. Such deposits as were made by superglacial 

 streams during the advance of the ice must likewise have been 

 delivered on tjie land surface, but would have been subsequently 

 destroyed or buried, becoming in the latter case, submorainic. 

 This would be likely to be the fate of all such superglacial 

 gravels as reached the edge of the ice up to the time of its 

 maximum advance. 



Streams descending from the surface of the ice into crevasses 

 also must have carried down sand and gravel where such mate- 

 rials existed on the ice. These deposits may have been made 

 on the rock which underlies the drift, or they may have been 

 made on stratified or unstratified drift already deposited. In 

 either case they were liable to be covered by till, thus reaching 

 an inter-till or sub-till position. 



Englacial streams probably do little depositing, but it is 

 altogether conceivable that they might accumulate such trivial 

 pockets of sand and gravel as are found not infrequently in the 

 midst of till. The inter-till position would be the result of sub- 

 sequent burial after the stratified material reached a resting 

 place. 



Complexity of relations. — From the foregoing it becomes clear 

 that there are diverse ways by which stratified drift, arising in 

 connection with an ice-sheet, may come to be interbedded with 

 till, when due recognition is made of all the halts and oscilla- 

 tions to which the edge of a continental glacier may have been 

 subject during both its advance and retreat. 



It is evident that the stratified drift and the unstratified 

 drift had abundant opportunity to be associated in all relation- 

 ships and in all degrees of intimacy. It is evident that stratified 

 drift may alternate with unstratified many times in a formation 

 of drift deposited during a single ice epoch. The extent of 

 individual beds of stratified drift, either beneath the till or inter- 

 bedded with it, may not be great, though their aggregate area 

 and their aggregate volume is very considerable. It is to be 

 borne in mind that the ice, in many places, doubtless destroyed 



