STUDIES FOR STUDENTS. 623 



As the ice of the continental glacier closed round nunataks, 

 the lateral moraines which had come into existence became 

 medial ; but medial moraines, arising by the coalescence of 

 lateral moraines, could not have been more extensive than the 

 latter. After the ice overtopped the nunataks which gave rise 

 to lateral moraines, these same elevations might still yield en- 

 glacial material to the over-riding ice. Later, part of this en- 

 glacial material, perhaps became superglacial, by having the sur- 

 face of the ice brought down to its horizon as the result of surface 

 ablation. On reaching the surface, this material might assume the 

 form of a medial moraine, as in alpine glaciers. As on mountain 

 glaciers, lateral or medial moraines on a continental ice-sheet 

 might readily lose their distinctive character during the melting 

 of the surface ice. As in the case of alpine glaciers, the pro- 

 portion of englacial material in a continental ice-sheet which 

 must become superglacial as the result of surface melting would 

 depend, 1), upon the position of the englacial material in the 

 ice; and, 2), upon the relative rates of basal and super- 

 ficial melting. The higher the elevations from which the engla- 

 cial material was derived, the nearer will it be to the upper 

 surface of the ice at the beginning of its history. The nearer it 

 is to the upper surface of the ice, the better its chance of becom- 

 ing superglacial. The amount of englacial-superglacial material 

 would therefore be greatest in a region of strong relief, and 

 especially in a country where the relief was great, relative to the 

 thickness of the ice. A region of 2,000 feet relief, beneath an 

 ice-sheet 5,000 feet thick, might yield little englacial material 

 which would ultimately become superglacial. If the ice over 

 the same region were but 2,500 feet thick, a much larger pro- 

 portion of its englacial drift would be likely to reach the surface 

 of the ice. Since the ice-sheet was always thinnest at its mar- 

 gin, the relief of any given region was always greater, relative to 

 the thickness of the overlying ice, when the marginal part of the 

 ice overlay it, than at any other time. Advance of the ice means 

 the thickening of the ice at all points back of the margin. With 

 increasing thickness of the ice, a less and less proportion of the 



