CROSBY: ORIGIN OF ESKERS. 
405 
and they are superposed directly upon the underlying bed rock. 
They are rarely, if ever, covered by till, or even sprinkled with 
bowlders, except such as might readily be supposed to slide or fall 
into a superglacial channel from the bordering slopes of ice. This 
relation is, in every particular, strictly normal for superglacial fluvial 
deposits, but the subglacial fluvial deposits could not possibly escape 
being covered by till and angular bowlders on the melting of the 
ice, except on the supposition that the englacial drift was so strictly 
limited to the basal portion of the ice during the esker-forniing 
period as hardly to warrant its classification as englacial, which 
would make the maximum elevation of drift in the Pleistocene ice 
sheet distinctly less than in the Malaspina glacier and many of the 
Greenland glaciers. If the normal esker is of subglacial origin, then 
the englacial drift was indeed scanty and confined to very low levels 
in the ice; and the subglacial stream was deprived of one important 
and necessary source of detritus for aggrading its bed and building 
its terminal plain. The only alternative, apparently, is to suppose 
that the subglacial stream always held tenaciously to its course 
until finally, through the general process of ablation, superficial and 
basal, its tunnel became roofless at all points. This would mean, for 
one thing, that every esker was formed in part, or at the last, in 
€arth-bottomed canyons open to the sky, which is impossible wher¬ 
ever the grade rises southward, as it practically does for nearly every 
true esker in some part of its course. The subglacial esker must 
remain under cover, under a roof of ice thick enough to hold the 
subglacial stream to a channel which, regardless of the ground 
topography, crossed directly or obliquely ridges hundreds of feet 
in height, until it is finished; and then, as a necessary corollary, the 
stream which made it must completely abandon its channel, perhaps 
scores of miles in length, before the esker is uncovered at any point 
south of which it rises to a higher elevation. How the subglacial 
stream is to be diverted while the ice is still several hundred feet 
thick above it, and how it fails, in general at least, to build an esker 
in its new channel, are points which have not, perhaps, been duly 
considered. 
Several writers have emphasized, perhaps unduly in some cases, 
the brevity of the time required for the formation of an extensive 
sand plain and its tributary esker, a few years or even a single sea¬ 
son being considered sufficient in most cases. On the other hand, 
