TYPICAL EXAMPLES OF GLACIAL CONSTRUCTION. 81 



In the morainic till, if I rightly understand its derivation, we have 

 accumulations predominently supplied, especially on their surface, from 

 the up2)er part of the englacial drift. Its bowlders have travelled with- 

 out attrition and are deposited usually in large numbers. A narrow belt 

 aggregates as many unworn rock fragments of large and small size as 

 belong in the veneer of englacial material falling from the departing ice 

 upon the surface of a very wide adjoining tract. 



When the returning warm climate at the end of the Glacial period 

 melted away the ice, its ablation exposed the formerly englacial drift 

 upon its surface, as the border of the Malaspina glacier or ice-sheet in 

 Alaska is found by Russell to be covered thickly by drift which supports 

 a growing forest. The recession of the Pleistocene ice-sheet, however, 

 during the time including the formation of its complex series of moraines 

 in the northern United States and southern Canada, although often 

 interrupted and demonstrably attended by many irregularities and ine- 

 qualities in the rate of retreat and in the extent and duration of the 

 oscillations of its frequent pauses and re-advances when the moraines 

 were formed, was yet probably so rapid that no vegetation gained a 

 foothold on its drift-covered frontal slope. Rains falling and rills and 

 larger streams flowing on the freshly revealed drift of the melting ice 

 surface gathered portions of it and deposited them in ice-walled channels 

 and at the mouths of the glacial rivers as eskers and kames, and on the 

 adjoining land as plains of gravel and sand, and beyond these as beds 

 of fine silt, loess, ^nd clay, laid down in lakes or more commonly in very 

 gentle river floods spreading across wide valleys. Besides the moraines, 

 kames, and eskers, the valley drift also, in its great volume and diverse 

 phases, thus seems to have been chiefly supplied from drift that was 

 enveloped in the ice-sheet at the time of its final melting. 



Relations of the englacial Drift to subglacial Till. 



The superficial portion of the general sheet of till which appears to 

 have fallen from the ice-sheet at the time of its departure is commonly 

 no more than a few feet thick ; but it is very unequally distributed, 

 varying from almost nothing to depths of forty feet on some tracts to 

 which glacial currents carried this englacial drift in exceptional amount. 

 The lower and usually much greater part of the till was deposited be- 

 neath the ice-sheet, but for almost the entire drift-bearing area, excepting 

 only near its boundaries, I think that this deposition took place mainly, 

 or at least in large measure, during the time of the recession of the ice- 

 sheet. According to this view, the drift which became then massed in 

 the ground moraine had up to that time been englacial, being gradually 



