INFLUENCE OF DEBRIS ON FLOW OF GLACIERS. 831 
this may cause the clear ice above a débris-charged ice-dam to 
melt away, and form a new terminus which would in turn become 
congested and undergo a similar process once more. 
An explanation is, then, suggested of the varying behavior 
exhibited by the extremities of glaciers, which is independent of 
fluctuation of climate. Two glaciers supplied in their névé 
regions with the same amount of snow, and alike in all respects 
except in the percentage of débris carried by them, would have 
the débris concentrated in their extremities at different rates 
and hence form débris-charged ice-dams at different periods, and 
consequently be checked and advance or retreat at different 
times and at different intervals. If in the case of two glaciers 
the amount of débris carried was the same, but other conditions 
varied, the fluctuation of their extremities would again vary. So 
diverse are the conditions controlling the flow of glaciers, that in 
no two instances could their fluctuations in length, due to the 
influence of débris, be expected to occur synchronously.* 
Drumlins.—In the case of a mass of englacial débris, densest 
at its center and gradually becoming less and less abundant in 
all directions, it is evident that glacial motion will be least at 
its center and increase in all directions until the normal flow of 
clear ice under the conditions present will be reached. If the 
central portion of such a mass is sufficiently charged with débris,. 
glacial flow will there cease and the stagnant portion be carried 
along, for a time at least, as englacial bowlders are carried. If 
such a stagnant nucleus should be situated at the base of a gla- 
cier, however, it would retain its position and the clearer ice 
above and on either side would flow past it. The “plucking” of 
débris from such a stagnant mass might lead to its removal, but 
if the advancing ice contained rock fragments, these on coming 
* The considerations offered above, lead to the suggestion that a series of terminal 
moraines in a formerly glaciated valley, or a similar succession of ridges left by a con- 
tinental glacier, are not necessarily evidence of repeated climatic oscillations, but may 
have been formed during a uniform and continuous meteorological change favorable 
to glacial recession. That is, a débris-charged glacier may retreat for a time, then 
halt, and again retreat, owing to its terminus becoming congested with foreign mate- 
rial, in response to a climatic change which would cause a glacier composed of clear 
ice, to recede continuously and without halts. 
