THE NATURE OF THE ENGLACIAL DRIFT 

 OF THE MISSISSIPPI BASm. 



It is of some importance, both to the practical work of the 

 field and the theoretical deductions of the study, to determine the 

 nature and amount of the drift that was carried forward in the 

 body of the ancient continental glaciers, and brought out on 

 their terminal slopes and at length deposited at their frontal 

 edges, and to distinguish it from that which was pushed or 

 dragged or rolled along at the bottom of the ice.' It may be 

 helpful to indulge in a speculative discussion at the outset to 

 prepare the way for the specific evidence and the inferences to 

 which it leads. 



Whenever a prominence of rock is overridden and enveloped 

 by a glacier of the free-moving continental type, one of two 

 things takes place ; either that part of the ice which passes over 

 the summit of the prominence flows down its lee slope, carrying 

 whatever debris it dislodges down to the rear base, and thence 

 onward along the bottom of the ice, or else the currents which 

 pass on either side of the prominence close in behind it before 

 the corresponding current which passes over the summit reaches 

 the point of their junction, in which case the summit current is 

 forced to pass off more nearly horizontally into the body of the 

 ice, carrying with it whatsoever debris it has dislodged from the 

 summit of the prominence and embodied within its base. The 

 law of the phenomena appears to be that whenever the height 

 of the prominence is less than one-half the base, measured trans- 

 versely to the movement of the ice, the summit current will fol- 

 low down the lee slope ; but whenever the height of the promi- 



' Debris, which may be imbedded in the basal layer of the ice during some part 

 of its transportation, but which is brought down to the bottom and subjected to basal 

 action in the latter part of its course, and ultimately becomes a part of the basal 

 deposit, is not here included in the englacial drift. 



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