24 The American Geologist. -f^'y- ^^^''^• 
Finally, effective crevassing throughout tl^e zone of esker 
formation and a failure of englacial drift to rise to any con- 
siderable elevation in the ice are always postulated as import- 
ant, if not absolutely essential, elements of the subg'lacial hy- 
pothesis. 
Review of Evidence. 
Direction or Trend of Bskers. — That the typical esker 
tends to conform cloisely in trend with the movement, and es- 
pecially with the latest movement, of the ice-sheet, as recorded 
in the striae, drumlins and bowlder-trains is undoubtedly true. 
The movement of the ice-sheet must have been, in general, 
normal to its margin, or, in other words, in the direction of 
its steepest surface slope, and therefore in the direction which 
superglacial streams would necessarily follow. Eskers de- 
part from this ideal trend no more than might reasonably be 
expected, in the case of a superglacial stream, considering that 
as the ice-sheet becomes thin its surface contours must begin 
to show the influence of the underlying topography, and con- 
sidering further the inevitable inequalities in the distribution 
of the superglacial drift due to and in turn aetermining un- 
equal ablation, as well as the natural tendency of streams to 
meander with diminishing gradient. In brief, the trend of the 
superglacial esker is entirely consistent with the known facts 
and the reasonable probabilities of the case ; and it matters not, 
so far as this feature is concerned, whether the ice was abso- 
lutely stagnant at the time the esker was formed or still re- 
tained a part or the whole of its velocity when in its prime. 
On the other hand, the only factors which may be consid- 
ered as directly influencing the direction of subglacial streams 
and eskers are : the ground topography, which is often contrary 
or indifferent ; crevasses, which, in so far as they exist at all, 
must be chiefly transverse and therefore indifferent ; and the 
movement of the ice, or the differential pressure of the ice 
upon the ground resulting from its movement. That the bas- 
al movement of a sheet of ice thick enough to flow over a deep- 
ly and irregularly corrugated surface of vf^ry low average 
gradient, the gradient being often opposite or transverse to 
the movement, would tend to develop and maintain furrows or 
tunnels in the bottom of the ice is extremely tloubtful. Tun- 
nels meeting the requirements of esker formation would not 
