STUDIES FOR STUDENTS. 52 1 



3. Formations produced by the mechanical action of the 

 edge of the ice. 



1. It is believed that the base of the glaciers was the great 

 seat of action ; that here took place the disrupting of the mate- 

 rial and the larger part of the rubbing, grinding and crushing to 

 which it was subjected in transportation. All of this material, 

 however, did not come to final rest beneath the ice. A portion 

 of it was borne away by the glacial drainage, a portion was 

 thrust up into the ice and borne along to its edge and there 

 deposited as superglacial material, and a portion may have once 

 been uncovered by the retreat of ice and have been subsequently 

 plowed up by a re-advance and so have taken on a new form of 

 aggregation. There is, therefore, a discrimination to be observed 

 between material that was* produced at the base of the glaciers 

 and that which was finally deposited at their base. It is only the 

 latter class that is included here. Of deposits originating under 

 the ice the following sub-classes are distinguished. 



(1). Subglacial sheets of till. — These constitute one form of 

 ground moraines ; the form which is perhaps most commonly 

 recognized. There are to be embraced here those broad sheets 

 of till which were spread out under the ice and left, on its retreat, 

 as a blanket mantling the surface of the land. These sheets are 

 not uniform in thickness nor universal in their presence. In this 

 classification it has been thought best to separate all distinct 

 and special forms of aggregation from the more nearly uniform 

 sheets of till, and to place them in the following sub-class. This 

 is done in the belief that the causes of these special aggrega- 

 tions were somewhat special and peculiar, and that these forms 

 are worthy of distinction for working purposes until their final 

 significance and classificatory value shall be determined. 



(2). Subglacial aggregations of till. — These admit of subdivi- 

 sion into two varieties, between which there is no sharp dividing 

 line and which are perhaps separated from each other genetically 

 only by the degree of their development. These are 



a. Drumlins. 



b. Aggregations not strictly drumloid in form. 



