Origin of Eskcrs. — Crosby. 17 
flow of subglacial waters did not exist before the ice became 
stagnant, the only explanation that suggests itself being that 
while the ice was in motion it would cause the drift to rise and 
fill any channel opened above it, a principle which is not opera- 
tive beneath alpine glaciers (and possibly not beneath the Mal- 
aspina glacier) because they have long since 'Swept their chan- 
nels free of drift. 
Coiiiparisoii of Hypotheses. 
Although recognizing, as previously stated, that other ex- 
planations, such as the filling of abandoned channels by surface 
slide and wash, are entitled to some consideration, and that 
they are all, probably, essential to the complete theory of es- 
kers, it is proposed in what follows to take account only of the 
two main hypotheses ; viz., that eskers have been formed b}' the 
active agency of subglacial streams, or of superglacial streams. 
And here, again, as already noted, it is merely a question of 
relative importance, since I hold with Davis and other advo- 
cates of the subglacial hypothesis that neither can wisely be 
discarded in toto. Subglacial tunnels are a realitv at the pres- 
ent time and were, doubtless, to some extent a feature of the 
Pleistocene ice-sheet ; and it would certainly be hazardous to 
deny that deposits formed in them have never escaped obliter- 
ation on the disapperance of the ice. 
Superglacial Hypothesis. 
This explanation of eskers assumes a stagnant, margmai 
zone of the ice-sheet at least one hundred miles in maximum 
width, practically free from crevasses, sufficiently wasted by 
ablation to be more or less abundantly covered by engla- 
cial drift which has become superglacial, with a general 
southward slope, and, toward the southern border at least,, 
thin enough to reflect in its surface contours in some de- 
gree, the underlying topograjjhy, and even to ])ermit the 
more prominent land forms to rise as nunataks above 
its surface. At the southern margin of the ice the 
elevation or grade of the superglacial stream finds a limit 
or control in a barrier of rock or till against which the ice may 
temporarily terminate, or in a body of standing water (a 
glacial lake) held against the ice by such a barrier in north- 
