62 2 THE JOURNAL OF GEOLOGY. 



At and near the centre, superglacial material is not abundant, 

 because melting has not there been sufficient to carry the surface 

 of the ice below the horizon of abundant englacial detritus. 

 Wind-borne fine material might reach the surface of piedmont 

 glaciers in the same way that it reaches the surface of alpine 

 glaciers. 



3. The Continental Ice-Sheets. 



1. Lateral moraines. Wherever the edge of the continental 

 ice-sheet found itself in a region of strong relief, ice-tongues 

 thrust themselves forward into valleys beyond the main body of 

 the ice. In very many respects such tongues of ice corre- 

 sponded to alpine glaciers. Upon their surfaces lateral moraines 

 may have accumulated just as in the case of mountain glaciers 

 today. But the country invaded by the continental ice-sheet of 

 North America was, for the most part, not mountainous. Mar- 

 ginal glaciers of the alpine type must have been restricted to 

 those parts of the ice-sheet which invaded mountain regions, or 

 at any rate to areas of marked relief. As the ice-sheet advanced 

 and thickened, it presently covered the elevations which had 

 earlier occasioned the lobation of its edge. So soon as it covered 

 an elevation, this elevation ceased to give immediate origin to 

 lateral moraines. Since on the whole the relief of the country 

 covered by the North American ice-sheet was not great, there 

 was little chance for the development of extensive lateral 

 moraines upon it. While they doubtless existed on the alpine- 

 glacier-like lobes of the ice-sheet's edge in regions of strong 

 relief, they could have existed for considerable distances back 

 from the margin in but few localities. As the ice invaded regions 

 of slight relief, such as the larger part of the Mississippi basin, 

 it could have acquired little superglacial material. No more 

 than miniature lateral moraines could have come into existence. 

 The same conditions which forbade the development of extensive 

 lateral moraines on the continental ice-sheet, gave the ice little 

 opportunity to acquire englacial material which could subse- 

 quently become superglacial by surface melting. 



