966 STUDIES FOR STUDENTS 



of an oscillating movement during the general retreat, were liable 

 to destruction or burial by the next advance phase. So far as 

 buried, they commonly assumed an inter-till position. Those 

 made during the general recession were of course more liable 

 to escape destruction than those made during the advance. 



Deposits made by siibglacial streams. — Subglacial streams, as well 

 as extraglacial, made more or less extensive deposits of stratified 

 drift. These were sometimes concentrated along sharply limited 

 channels (eskers), and sometimes more widely spread. They 

 were sometimes made on the rock surface below all drift, but 

 more commonly on unstratified drift (till) which the ice had 

 already deposited. Because of the ever-changing conditions at 

 the bottom of moving ice, it is probable that the ice frequently 

 came to occupy beds which streams had temporarily commanded. 

 Wherever this happened, the stratified deposits were likely to be 

 destroyed in whole or in part, or buried. In the latter case they 

 became intermorainic, if they rested upon till, or submorainic, 

 if on rock. It is believed that very large numbers of beds of 

 stratified drift, of limited extent, became in this way interbed- 

 ded with till. Subglacial waters which did not organize them- 

 selves into regular systems of drainage, must have done a simi- 

 lar work on a smaller scale. 



Deposits made beneath the ice during its maximum exten- 

 sion in any epoch, and especially near its edge, stood less 

 chance of being buried by later glacial deposits. Stratified 

 deposits made beneath the ice during its retreat, and especially 

 those made near its edge at any stage, were still less likely to 

 attain an inter-till position. 



Deposits made by superglacial and englacial streams. — Theoret- 

 ically, superglacial streams likewise may have made deposits 

 of stratified drift on the ice or in ice valleys. Practically such 

 deposits were probably not made except near the edge of the 

 ice, for nowhere else was there superglacial drift. Even here 

 they were probably not important. Such accumulations of this 

 sort as were made during the final recession of the ice-sheet 

 were delivered on the land as the ice melted, and should remain 



