MORAINES OF RECESSION 451 
It appears to be a plain inference, even in the case of the 
extremely slow motion of the Greenland cap, that however 
slowly the upper layers of the ice move, the débris-laden bottom 
layers move still more slowly. Even in a fiord tongue which 
moves 50 or 100 feet a day, if the ice is 1000 to 1500 feet or 
more deep the extreme bottom layers may move quite slowly. 
The principles involved in basal clogging have been well 
brought out by Professor I. C. Russell. He reduces them to 
this proposition: ‘The rate of flow of glacial ice, under given 
conditions, will depend upon the percentage of débris com- 
mingled with it, and be least where the percentage is greatest. 
For our present purpose this law may be advantageously restated 
Be a 
in terms of drift transportation and deposition rather than ice- 
motion, thus: Whether the basal layers of a glacier will absorb 
or deposit débris at a given place or pass over without doing 
either, depends on their carrying an underload, an overload, or 
just an even full load at the existing velocity and pressure. 
Under deep ice, however, the building up of ridges or promi- 
nences like terminal moraines, by the clogging of débris-laden 
bottom layers, would seem to be impossible, because the tendency 
there is to wear down and abrade every prominence of the land 
that is overridden. Clogging under deep ice probably occurs to 
some extent, but only by the shearing off of thin bottom layers 
which do not remain as subglacial prominences. Hollows of the 
land surface would tend to be filled up by clogging, and a mass 
of débris once dropped in such a place would tend to stay there 
unless the peculiar and rare conditions which lead the ice to 
scoop out basins came into play. While the general truth of 
Professor Russell’s proposition is plain, and the principle stated 
is one of great value, it may be doubted whether it can have such 
a function as he supposes when he suggests that it was the deter- 
mining cause of the moraines of recession and their peculiar 
distribution.? 
t“The Influence of Débris on the Flow of Glaciers.” Jour. GEOL., Vol. III, 
No. 7, pp. 823-832. 
In a footnote to his article (page 831) Professor Russell makes the following 
