MORAINES OF RECESSION 447 
and distribution of this is derived from the continental glacier of Greenland. 
It is there observed that débris prevails in the lower 50 or 75 feet of the 
ice-sheet, and occasionally reaches up to 100, or perhaps even 150 feet. The 
amount of this débris, if it were let down directly upon the glacier’s bottom by 
melting in situ without concentration by ihe forward motion of the ice, would 
be measured by a very few feet, or by a fraction of a foot. The forward 
motion of the ice concentrates this at its edge, so that it may there reach, theo- 
retically, any dimension, entirely without regard to its amount in any given 
vertical section of the ice. The thickness of the deposit formed from the 
englacial drift is quite as much dependent upon the length of time during 
which the edge of the ice remains at one line as upon the amount of drift 
which the ice may carry in any given vertical section. No safe inferences 
from the thickness of deposits of englacial drift can therefore be drawn with 
reference to the amount of englacial material present in any given portion of 
the glacier. If the ice were absolutely stagnant the deposit of englacial drift 
would be precisely that which was held in the ice above the point of deposit. 
If there was any forward motion of the ice while it was being melted away, 
there would necessarily be a concentration. If there be one foot of englacial 
débris in a given section and the ice moves forward 4o feet while the external 
heat causes a retreat of 1 foot, the englacial deposit should be 4o feet deep. 
The thickness of the englacial drift may therefore be quite as much an expres- 
sion of prolonged time as of a large content of débris within the ice. 
Referring to the manner in which the englacial débris becomes 
at length exposed and deposited Professor Chamberlin, contin- 
uing on the next page, says: 
Instead of rising toward the surface of the glacier, it is believed, on the basis 
of observations in Greenland, to pursue a course nearly parallel to the base, 
on the whole, and to come out at the extremity of the glacier. To some slight 
extent it may become superglacial by ablation, but only to a limited degree.: 
Again, describing more specifically the phenomena in Green- 
land, Professor Chamberlin says: 
The débris belts are essentially parallel to the base of the glacier. They 
are chiefly confined to the lower 50 or 75 feet; sometimes they prevail up to 
1oo feet and rarely beyond. I think 150 feet might be named as a rather 
extreme limit. They are more abundant at the sides of the lobes than at the 
center, a fact that is significant in indicating the introduction of a notable 
part of the débris after the lobes were formed. In consonance with this the 
débris appears to be most abundant in the glacier-lobes which descend as 
cataracts or crowd between closely hugging cliffs. If, standing in front of a 
** Glacial Lake Agassiz,” Monograph, pp. 249 and 250. 
