INFLUENCE OF DEBRIS ON FLOW OF GLACIERS. 827 
owing to the more rapid flow of the surface portion, which car- 
ries rt over and beyond the ice previously forming the terminus. 
The more rapid flow of the surface as compared with the basal 
portion, is, no doubt, due, as commonly stated, to an increase in 
friction toward the bottom. The basal portion generally, how- 
ever, contains more englacial débris than the superior portion and 
for this reason would also be retarded. As fresh ice is carried 
beyond the extremity of a glacier, it is more and more exposed 
to conditions which favor melting and thus, if the ice contains 
débris, tends to increase the percentage of foreign material in 
the portion that remains unmelted. The ice thus advanced, 
in its turn, becomes basal and is buried as the ice from above 
continues to descend. 
Even in the case of a glacier composed of clear ice, advanc- 
ing in the manner just cited, upon an unconsolidated gravel bed, 
the basement layer would become charged with gravel as a result 
of the contact and thus caused to stagnate. The ice at the bot- 
tom being densely charged with débris might remain stationary 
until melted and thus protect the gravel below from the erosive 
action of the ice flowing over it. 
Terminal moraines.—In the case of an ice stream which con- 
tains englacial débris, the increase in the rate of melting toward its 
extremity will, as already stated, cause an increase in the percent- 
age of débris in the portion that remains unmelted. As the melt- 
ing of a glacier is mainly superficial, a concentration of englacial 
débris is brought about by the débris first becoming supergla- 
cial and then falling into crevasses and other openings. As the 
percentage of débris increases in the wasting extremity, the flow 
of the ice is retarded, and stagnation finally results. Usually, 
also, in the case of Alpine glaciers, there is a gradual decrease 
in volume and also in gradient toward their extremities, which 
again leads to a decrease in their rate of flow and favors stagna- 
tion. The presence of a large percentage of englacial débris in 
the extremity of a glacier, however, will cause stagnation under 
conditions that would allow a clear ice-stream to flow on. A 
dam of débris-charged ice is thus formed which will check the 
advances of clearer ice from above, and cause it to increase in 
