Woodwortli.] 208 r^>b. 7, 
course in tlie ice-sheet; its variable crest-line, now liigl), now 
low, indicates an ice-channel, wide liere and narrow there. 
Since the maximuni change in elevation coincides in positions 
with the maxinuini change in direction, the niaxiniuin changes in 
width of the channel, it may be assumed, occurred at points of 
maximum change of direction. In this respect, the esker pre- 
sents conditions similar to those in river bends. The meanders 
of a stream in the ice may be modified by crevasses and forward 
motion, the first tending to divert, the second to maintain the 
stream in a given course. Corrasion and liquefaction would 
accentuate meanders at bends. Differential forward motion of 
the base of the ice-sheet divided by the subglacial channel would 
produce pinches and swells in the tunnel. Deposits made in 
channels so distorted would be constructionally wide and narrow, 
and ultimately high and low, upon the readjustment of their sides 
to gravity. Some slight motion of the ice against the sand-plain 
below Auburndale has been noticed by Professor Davis. It 
does not appear, however, from the succession of high and low 
points in this esker that shearing of the ice-sheet has here led to 
the lines above indicated. 
Significance of the Cross-section of Eskers. 
The cross-section of a typical esker tends to have the form of 
an isosceles triangle whose sides incline about 30° to the horizon. 
The altitude seldom exceeds one hundred feet, and is commonly 
not more than one quarter of that height. The base is at least 
one and a half times the altitude. The cross-section tends to 
uniformity. This remarkable feature indicates the control of 
some agency resident in the ice-sheet, in which eskers arise, and 
a satisfactory explanation of the cause of this limitation in cross- 
section must throw light on the question as to where in the ice 
eskers were deposited. 
There seems nothing in the nature of existing superglacial 
drainage lines to restrict the deposits of a stream to a narrow and 
somewhat continuous channel. The conditions on the surface 
favor frequent perturbations of the drainage, and detritus accu- 
mulated on the surface of a glacier is ajjt to undergo more than 
one change of position and arrangement of its particles by reason 
