Origin of Eskcrs. — Crosby. 21 
permeable gravel not only co-operates in the vays indicated, to 
favor the melting of its ice floor, but it must tend tO' conserve 
the heat and to prolong the time during which melting can 
take place. 
In view of these considerations, there seems 10 be no escape 
from the conclusion that the ice-floor of a superglacial stream 
will be lowered by the superficial as well as by the basal melt- 
ing of the ice ; that the superficial melting will be more active 
and efficient in proportion to the extent of aggrading of the 
channel and the volume of stagnant water saturating the 
gravel ; and that the stream will be lowered at least as rapidly 
as the interstream surfaces, because, while these are also, ac- 
cording to our initial assumption, covered with drift and thus 
protected from the direct action of the sun's rays, they lack 
the standing water essential to the effective indirect utilization 
of the solar radiation. 
Obviously, the aggrading of the channel may continue dur- 
ing all the time that the deposit is being let down upon the 
ground, and the resulting esker will then conform in elevation 
with the terminal plain, at least as closely as eskers commonly 
do, and show the characteristic lack of sympathy wdth the 
ground topography. Should the stream be diverted to another 
course before the subsidence of its deposits is completed at all 
points, the process will continue without essential change, un- 
less a crevasse should draw ofif the water saturating the gravel, 
the result being one of the eskers falling in whole or in part be- 
low the normal elevation. The exceptionally high points or 
knolls may, according to this hypothesis, represent the de- 
tritus discharged by hanging tributaries into the main canyon 
after the diversion of the headwaters from the latter. All the 
conditions favor vertical rather than lateral melting of the ice ; 
and there is, apparently, no tendency to scatter the deposit iu 
getting it down to the ground. At the same time all observed 
variations in width are fully provided for, and more especially 
the gradual widening which commonly marks the junction with 
the terminal plain. Certainly nothing is more probable than 
the widening of the ice canyon at its mouth in the thin and 
frayed or lobate southern margin of the ice, where its walls are 
bathed by a large body of standing water. Stone's maps'" show 
• V. S. Geol. Surv., Mon., 34. 
