l8 * The American Geologist. J^'^'' i902. 
ward sloping valleys, or, less rigidly, in the detrital cone 
formed by the stream itself as it escapes from the ice. What- 
ever the character of the control, it determines for each super- 
glacial stream a base level, towards which it must approximate, 
but below which it cannot cut its channel by merely mechanical 
erosive action. The stream discharging across the ragged, 
southern edge of the ice is but the trunk or main stem of a 
system, deriving both water and detritus from many tribu- 
taries 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 theii 
relations to the main ice canyon. But as the floor of the latter 
approximates the base level, the higher gradient will be trans- 
ferred to the tributaries, which must then, in large part, dis- 
charge their accumulated burdens of coarse detritus into the 
main channel and thus lead to its rapid clogging and aggrad- 
ing. 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 
superglacial stream will be the surface of the ground on which' 
the ice rests ; and then the clogging and aggrading will take 
place 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, especially con- 
cerned with the fate of the esker which finds itself at the end 
of the base-levelling 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 grad- 
ually clogs and aggrades the channel of the superglacial 
stream, will have sufficient depth and volume to form a typical 
esker, or an esker approximating in bight the sand plain to 
