26 The .Imcrican Geologist. "^"'y- ^^^-• 
stationary. This nice adjustment of conditions made the 
streams relatively long-lived ; and makes it appear the more 
improbable that they promptly abandoned their courses when 
the eskers were finished. The meanders of eskers, which are 
cetainly among the most constant and striking features, rather 
seldom exhibit any definite or causal relation to the topography 
of the bedrock and till ; and, since they cannot be correlated 
directly with the movement of the ice, the subglacialist can only 
suggest that they may have been determined by one or moie 
systems of crevasses, of the existence of whichthere is little or 
no independent evidence. In view of all these considerations, 
it is, perhaps, not too much to say that the superglacial theory 
afifords at once the simplest and most natural explanation alike 
of the general trend of eskers, the major deviations from this 
trend, and the minor deviations or meanders. 
Length of Eskers and Esker Systems. — In the absence of 
crevasses, superglacial streams are limited in length only by 
the breadth of the zone of ablation ; and superglacial eskers 
only by the breadth of the zone of englacial drift which has 
become superglacial by ablation ; and the last, in turn, depends 
upon the hight to which the englacial drift has risen in the 
ice. We have seen: that observations on existing glaciers 
are practically valueless as evidence of crevassing in the Pleis- 
tocene ice-sheet; that the Greenland ice-cap is free from 
crevasses except near the margins, where it breaks over the 
restraining mountains ; that, if the Pleistocene ice-sheet had 
been eftectively crevassed (that is, from top to bottom)' in 
its passage over the vast peneplain tracts of the glaciated areas, 
the crevasses would have been gradually obliterated during 
the extremely slow cessation of the movement, anterior to the 
period of final ablation of the now stagnant ice-sheet, when 
eskers were formed ; that even on the moraine and forest-cov- 
ered outer zone of the Malaspina glacier, crevasses are prac- 
tically w^anting, the streams originating within this zone 
through surface melting being superglacial and so continu- 
ing as they flow down the terminal ice slope ; and that glacial 
potholes are by no means conclusive proof of the existence of 
niouUns or crevasses in the ice sheet at these points. Add to 
all these considerations the probability that such crevasses as 
might possibly appear in the ice-sheet and survive the cessa- 
