22 W. UPHAM — DRUMLINS AND MARGINAL MORAINES. 



brought into it from below by upward movements due to faster flow of 

 the central and upper glacial currents than of those retarded by friction 

 on the ground. The thinned border of the ice-sheet upon the belt hav- 

 ing a remaining thickness of less than 1,000 feet would therefore become 

 covered with drift, as Russell has described the borders of the INIalaspina 

 glacier or ice-slieet, which stretches from the Mount Saint Elias range to 

 the ocean. 



At many times the general recession of the ice-sheet was temporarily 

 interrupted. The return of a prevailingly cold climate for several dec- 

 ades of years, or occasionally, as we may suppose, for a century or more, 

 brought increased snowfall, which sufficed to hold the ice boundary 

 nearl}'- stationary, perhaps frequently first having pushed it again a con- 

 siderable distance forward. The thick ice lying far back from the border 

 may then have flowed over its previously thin and drift-covered outer 

 belt, aiding with the new snowfall to envelop the once superglacial drift 

 stratum within the ice-sheet. With the increased thickness and steeper 

 gradient of the outer part of the ice-sheet while the recession of its bound- 

 ary was slackened, wholly stopped, or changed to a re-advance, due mainly 

 to very abundant snowfalls, much drift which had been formerly exposed 

 on the ice surface would become again englacial, so that a stratum of 

 drift several feet thick might be enclosed in the ice at an altitude increas- 

 ing inward from less than 50 feet to 500 feet or more. 



The upper current of the thickened ice above the englacial bed of drift 

 would move faster than that drift, which in like manner would outstrip 

 the lower current of the ice in contact with the ground. Close to the 

 glacial boundary, whether it halted and even re-advanced or merely its 

 retreat was much slackened but did not entirely cease, the upper part of 

 the ice must have descended over the lower part. This diff'erential and 

 shearing movement, as I think, gathered the stratum of englacial drift 

 into the great lenticular masses or sometimes longer ridges of the drum- 

 lins, thinly underlain by ice and overridden by the upi)er ice flowing 

 downward to the boundary and bringing with it the formerly higher part 

 of the drift stratum to be added to these growing drift accumulations. 

 The courses of the glacial currents and their convergences to the i)laces 

 occu})ied b}" the drumlins were apj)arently not determined so much by 

 the topography of the underlying land as by the contour of the ice sur- 

 face, which under its ablation had become sculptured into valleys, hills, 

 ridges, and peaks, the isolation of the elevations by deep intervening 

 hollows l)eing doubtless most conspicuous near the ice margin. 



Variability in the rate and manner of departure of the ice-sheet may 

 well account for the geographic distribution of the drumlins, as certain 

 areas of their abundance, neighboring tracts where they are more scat- 



