448 FPRANIG BURSEBY TAYLOR 
glacial lobe, the dirt bands are traced, many will be found disappearing at the 
cataracts, or the embossments of the bottom, or at the spurs on the sides. 
The general impression produced by such conclusions as these 
is that an ice-sheet probably carries somewhat less englacial drift 
than the tongues that, like those of Greenland so far described, 
branch off from the main sheet and descend several miles and 
2000 or 3000 feet down ravines or constricted valleys. The 
tongues certainly have better opportunities to gather débris than 
the bottom layers of the main cap. And the force of this 
impression is greatly increased when we think of an ice-cap that 
deployed over so smooth a plain as did, for the most part, the 
Laurentide glacier in the area here considered. 
Mr. Upham inclines to the opinion that ‘‘englacial drift was 
carried up through the lower quarter or third part of the ice- 
sheet, where, as in Manitoba, it was probably a mile thick.” 
But, as Professor Chamberlin has said, there seems to be no 
reason to suppose that the thickness of the bottom débris- 
laden layers bears a fixed ratio to the total thickness of the 
ice. Indeed, from the very fact that the upper part of a glacier 
moves forward faster than its lower layers, it follows that the 
bottom layers cannot rise beyond a very limited extent, except 
by overthrust in consequence of flow over high points or emboss- 
ments that project upward into the ice. In passing over high 
obstructions high englacial drift may be introduced. But the 
amount of such drift appears to be really insignificant, and it 
even then becomes superglacial only after it has moved with the 
ice far enough forward into the peripheral zone of ablation 
to have had all the ice that overlies it melted off, or until it 
reaches the very edge and is thrust upward over a moraine or 
other obstruction. 
Those who have not seen glaciers have often been much 
*“ Recent Glacial Studies in Greenland,” Bull. G.S. A., Vol. VI, 1895, page 205. 
Contrast with these ideas the opinion of Mr. UPHAM, where he says, speaking of the 
great Leaf Hills moraine in Minnesota, that “perhaps not more than fifty or even 
twenty-five years [were occupied] for amassing these morainic hills 100 to 350 feet 
high on a belt 3 to 5 miles wide!” (‘Glacial Lake Agassiz,’ Mon., p. 242). 
2“ Sublacustrine Till,” W. UPHAM, Am. Geol., Vol. XVII, June 1896, pp. 374-375- 
