STUDIES FOR STUDENTS. 62$ 



sons for believing that the marginal part of an advancing ice- 

 sheet is more favorably situated for acquiring englacial material 

 than any other. If the marginal part of an advancing ice-sheet 

 acquired more englacial material than any other part, and if a 

 larger proportion of that which it acquired became superglacial, 

 it will be seen that this part of the ice had great advantage over 

 other parts in the matter of superglacial drift. Apart from all 

 considerations of topography, surface material, thinness of ice, 

 and rate of surface melting, an advancing ice margin has an 

 advantage over a receding margin in the acquisition of material, 

 because of its greater vigor of movement. 



Not only must englacial-superglacial material be most abun- 

 dant at and near the margin of the ice, but, under most condi- 

 tions, it must be more abundant at the margin of an ice-sheet 

 during its earliest advance than at any other time, since it is the 

 first advancing margin which in general finds the roughest topog- 

 raphy, and the most loose material ready for removal. A quali- 

 fication to the first part of this statement, and a partial excep- 

 tion to the last, should be stated. If the interval since the 

 ice has retreated from a given region be long, a rough topography 

 may have been developed since the earlier passage of the ice. In 

 this event, the first advancing margin might have no advantage 

 over the second in securing englacial drift which may become 

 superglacial. When the ice re-advances over a surface from 

 which it has receded, it may find a large supply of loose material 

 in the form of drift, ready for removal. 



In summation it may be said that the advancing margin of 

 a continental ice-sheet must have been more abundantly supplied 

 with superglacial material than the receding, because ( 1 ) the 

 motion was more vigorous, favoring the acquisition of more 

 englacial material capable of becoming superglacial; (2) the 

 surface over which the advancing margin spread was, on the 

 whole, better supplied with material which might become super- 

 glacial, either directly or indirectly; (3) the topography of the 

 country as the ice invaded it, was such as to allow it to acquire 

 material more readily than at any other time. The first of 



