28 TJic American Geologist. "^^'^'- ^^*'-- 
able hight in tlic ice, and over a wide area, may be asserted 
with renewed confidence. Granted a waning ice-sheet free 
from crevasses and well supplied with superglacial drift over 
a broad marginal zone, and the conditions are ripe for the 
development, through the agency of superglacial streams, of 
eskers and esker systems equal in length and continuity to any 
which have been described. 
The difficulties of the subglacial hypothesis are here rel- 
atively, much more serious, since it requires us to postulate 
and maintain continuous ice tunnels from five miles or less 
to one hundred and fifty miles or more in length. In fact, the 
formation, either before or after the ice has ceased to move, 
of subglacial streams and tunnels one hundred to one hundred 
and fifty miles long, extending back to points where the thick- 
ness and pressure of the ice must be very great and crevasses 
are practically impossible, can be accepted only as a last resort, 
or when the failure of the alternative explanation has been 
demonstrated. 
Varying Width of Eskers. — The expansion of supergla- 
cial streams to form lakes of greater or less breadth is normal, 
and the most extreme variations in the breadth of eskers pre- 
sent absolutely no difficulty ; while, as the product of subgla- 
cial streams they are simply inexplicable, requiring, according 
to Stone, tunnels of all widths up to three-fourths of a mile. 
The broad eskers described by Stone,* in which a medial ridge 
of coarse gravel is flanked on either side by and merges with a 
plain of finer gravel and sand, the whole being, often, confinea 
to one side of a valley, by an ice border, and, like true eskers, 
crossing ridges from one valley to another, indicate, as recog- 
nized by Stone, the formation of a normal esker in a narrow 
channel, followed bv a considerable expansion of the channel, 
perniitting the deposition of the finer material of the bordering 
plain. This broadening of the channel might, in the case of 
a superglacial stream, be attributed to the melting back of the 
ice walls, or better to localized surface ablation due possibly 
to water saturating drift which has accumulated on the ice 
through previous ablation. So great is the diffixulty of ex- 
plaining these broad deposits by deposition in subglacial chan- 
* r. S. Geol. Surv.. Mon. S4-. pp. 440 444. 
