CROSBY : ORIGIN OF ESKERS. 
391 
from many tributaries as well as by the ablation of its banks. In 
fact, it holds so far essentially the same relation to the drift-covered 
ice that an ordinary stream does to the drift-covered bed rock. The 
tributaries making large angles with the trunk channel, or not 
approximately agreeing with it in direction, will not share its high 
gradient; and in consequence their channels will not be deepened so 
rapidly, but will become hanging valleys in their relations to the main 
ice canyon. But as the floor of the latter approximates the base level, 
the higher gradient will be transferred to the tributaries, which 
must then, in large j^art, discharge their accumulated burdens of 
coarse detritus into the main channel and thus lead to its rapid 
clogging and aggrading. Only unimportant accumulations will 
remain in the channels of the approximately right-angled tributaries, 
and we are thus able to explain the practical absence of right-angled 
branching in eskers. In special cases the base level of the super¬ 
glacial stream will be the surface of the ground on which the ice 
rests; and then the clogging and aggrading will take j^lace in earth- 
bottomed canyons, the slow melting of the ice walls of which will 
complete the process and leave the eskers as we now see them. 
There is apparently no reason why, in the absence of a frontal 
barrier, existing independently of the glacial stream or through its 
agency, this simple explanation may not fairly be postulated. We 
are, however, specially concerned with the fate of the esker which 
finds itself at the end of the base-leveling process on a foundation of 
ice twenty, fifty, or possibly one hundred feet thick. 
To begin with, it is, perhaps, improbable that the coarse detritus, 
which lags behind as the grade diminishes and gradually clogs and 
aggrades the channel of the superglacial stream, will have sufficient 
depth and volume to form a typical esker, or an esker approximat¬ 
ing in height the sand plain to which it is tributary, as long as the 
floor of the ice gorge is nowhere below the base level or the level of 
the frontal barrier. Down to this level the gorge has been formed 
by the mechanical erosion or corrasion, as well as by the chemical 
erosion or melting of the ice; and below this level, corrasion is 
surely inoperative. But here Ui^ham’s suggestion of a farther deep¬ 
ening of the gorge by melting alone, intervenes; and I hope to show 
that it will continue with increased rather than diminished efficiency. 
Basal melting of the ice sheet must be in constant progress, sum¬ 
mer and winter, during this stage, and is possibly an important 
