22 
The American Geologist. J"'^'- i^°-- 
that this terminal widening is a particularly characteristic 
feature of the great esker systems of Maine; and some even, 
of the subglacial tunnels of the Malaspina glacier become, at 
the last, canyons with rapidly diverging walls. The constant 
movement and re-adjustment of the deposit during the settling 
process keeps it loose and permeable, and enables the water to 
wash out some of the filling of the coarsest gravels and thus 
give rise to the open-work gravel specially noted by Davis. 
In the quite exceptional case where crevasses or other ac- 
cidents drain the esker channel before its subsidence is com- 
pleted, the differential melting is likely to be reversed, the in- 
terstream surfaces going most rapidly because less protected 
from the sun's heat, the esker deposit is left on a ridge of ice, 
and, sliding down on both sides, gives rise to a double esker, 
of which I have noted several good examples in the vicinity 
of Boston.* The quite common failure of the esker to 
connect properly with its terminal plain, a weak place or break 
intervening, may be in part attributed to the deposition of the 
head of the plain over the sloping margin of the ice and the 
subsequent melting of the latter. And the very numerous in- 
stances of plains of all the various types without associated 
eskers or feeder channels are simply the cases where the super- 
ficial stream was diverted before its channel was base-levelled, 
and consequently before it was aggraded to any important ex- 
tent. Finally, the lateral position characteristic of valley esk- 
ers follows naturally from the tendency of the ice, before it 
ceased flowing, to become concentrated in the valleys or along 
the lines of freest movement, giving rise to what were virtu- 
ally valley glaciers with arched profiles in what may still have 
been at the surface a continuous ice-sheet, and possiblv with 
lines of shearing between the free-moving ice and the rel- 
atively stagnant ice of the uplands. When the ice becomes whol- 
ly stagnant, these lateral lines of weakness and the arched pro- 
file still remain to influence the courses of superglacial streams. 
Subglacial Hypothesis. 
As formulated by Davis, t this explanation of eskers 
also assumes a stagnant and decayed marginal zone of the ice- 
sheet. The water resulting from the basal melting of the ice, 
• Occas. Papers, Boston Soc. Nat. Hist., iv, pp. 278-284. 
+ Proc. Boston Soc. Nat. Hist., vol. xxv, pp. 477-t79. 
