Origin of Eskers. — Crosby. 37 
ablation or to subglacial streams through the erosive action 
of the stream itself on the roof and sides of its tunnel. As- 
suming, in view of the preceding discussions, that the englacial 
drift was sufficiently abundant and extended to a sufficient 
bight in the ice to meet the requirements of esker and plain 
formation through the agency of superglacial streams, and 
recalling that all drift set free by superficial ablation is, vir- 
tually, within easy reach of the superglacial streams and avail- 
able for the aggrading of their beds and deltas , we may now 
give our attention particularly to the subglacial streams. 
The subglacial drainage could derive but little detritus 
from the englacial drift without such an enlargement of the 
tunnel as would cause its collapse , and it is, therefore, practi- 
cally limited to the subglacial drift or ground moraine; for 
crevasses are probably wanting, and even if ihey were not, 
to depend upon detritus washed into them from the surface of 
the ice-sheet would be, as we have already noted, to grant a 
greater volume and bight of englacial drift ihan subglacial- 
ists have heretofore been willing to allow. What opportunity 
has the subglacial stream to erode the ground moraine. On 
the first obstruction of its mouth, by standing water or ter- 
minal deposit, it must begin to aggrade its bed and erosion here 
almost or wholly ceases. We have already noted the absence 
of tributary streams, and if such existed, their beds also, at 
least in their lower courses, would necessarily be aggraded 
with that of the main river. On either side of the tunnels 
the ice rests heavily upon the ground, and the marginal lakes 
of valley glaciers as well as crevas-ses filled with water, show 
that the basal contact must usually be iso tight as to permit a 
movement of water only by seepage, which could, at the best, 
effect the removal of only the finest or clayey part of the drift. 
The absence of an adequate available supply of material is, 
to my mind, the most serious of all the objections to the sub- 
glacial theory of eskers. Anyone who considers the great ex- 
tent and depth of many of our delta plains, and the vast vol- 
umes of material required to form them and their tributary 
eskers, will not doubt that, if derived from the subglacial drift, 
the latter should show extensive erosion over the areas to the 
northward ; but we look in vain for evidence of such erosion, 
although its record should be verv distinct in the cases where 
