382 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
topographic independence, as recently noted by Stone (’99). He 
finds in Maine, where exists the finest and most extensive develop¬ 
ment of eskers on this continent, that while they freely climb slopes 
and cross ridges from one hundred to two hundred, and more rarely 
three hundred and four hundred feet in heisrht above the ground 
on which they rest to the northward, they are constrained to follow 
still deeper valleys or wind around still higher hills, usually seek¬ 
ing the lowest gap in a ridge or water parting. Valley eskers are 
very rarely found in the true axis of the valley or resting on the 
lowest part of the till or bed rock profile, even where, as often hap¬ 
pens, the channel of the modern stream is remote from or well above 
the real bottom of the valley. But the normal position of the esker 
is lateral or along one side of the vallev, and even the base of the 
esker is not infrequently well above, sometimes a hundred feet or 
more above, the level of the modern flood plain, which may in turn 
overlie a great depth of modified drift. The indifference of eskers 
to the contours of the surfaces on which thev rest is further seen in 
the fact that they may appear first on one side of a valley and then on 
the other, but rather rarelv on both sides simultaneous!v ; and the 
same continuous esker, broken only by the modern stream, may 
cross and recross the valley from side to side. 
Eskers seldom, if ever, occupy channels in either the bed rock or 
till, which may reasonably be regarded as due to the erosive action 
of the streams that formed the eskers. And in general, distinct 
evidence of erosion by esker streams is wanting, except perhaps in 
the notching of the protruding drumlins or other nunataks of the 
waning ice sheet. 
Eskers commonly terminate southward in the delta and over-wash 
plains formed along the southern margin of the ice sheet, and to 
these they hold the relation of tributaries or feeders ; and an approx¬ 
imate agreement in height of these terminal deposits with the proxi¬ 
mal portions of the eskers has often been noted. Also, eskers com¬ 
monly widen as they approach the plains and merge gradually with 
the latter. 
