Origin of Eskers. — Crosby. 2C} 
nels that Stone,* after a careful study, reaches the conclusion 
that through the obstruction of the tunnel, pemaps by the sag- 
ging of its roof, the subglacial stream rose through crevasses 
with its burden of coarse gravel and became, for a longer or 
shorter distance, superglacial. Chamberlin, on the other hand, 
suggests that an ice tunnel became locally roofless, and then 
by lateral recession of its walls, the ice canyon became a lake. 
But it is obvious and generally conceded that superglacial 
streams afford by far the simplest and easiest explanation of 
these and all other lateral expansions of eskers, including the 
not uncommon terminal expansion, where the esker merges 
with the frontal plain or delta. The only ^serious questions 
raised by any one are as to the possibility of persistent super- 
glacial streams with a sufficient volume of formerly 
englacial drift within their reach ; and these difficulties disap- 
pear before the conclusions to which we have been forced that 
effective crevassing was of rare occurrence and high-level en- 
glacial drift abundant. 
Concerning the more usual widenings of the eskers of 
Maine, Stone says,t "When within about 75 miles of the coast, 
every few miles enlargements of the ridges are found which 
have various forms. Sometimes they are little tables only 200 
to 300 feet wide and two or three times as long. These may 
be solid or may contain one or more shallow kettleholes. Here 
and there a hummock appears on top of the osar (esker), ris- 
ing 20 to 40 feet above the rest of the ridge, and at these pin- 
nacles the ridge is generally broader than elsewhere." 
Varying Hight of Eskers. — Woodworth's correlation of 
variations of hight with variations of original breadth of the 
esker deposit and of the ice channel in which it was formed, 
applies in only a limited number of cases. Tt does not, for 
instance, account for the broad and flat cols or depressions, 
or for the occasional high knolls or pinnacles, or for sharp 
crests of rapidly varying hight. Except where Woodworth's 
principle does apply, or masses of ice have been buried by the 
growing esker. or it has suffered erosion, the subglacial hy- 
pothesis calls for substantial uniformity of hight. or at leasi 
for an even and continuous crest line, as well as for a close 
• U. S. Geol. Snrr., Xfon. .34. p. 4.4-4-44-.S. 
t t7. S G. S., Afon. 34, p. 415. 
