Origin of Eskers. — Crosby. 23 
together with that due to superficial melting and becoming sub- 
glacial through the medium of crevasses, is gathered into 
streams which, it is stated, hold to meridional courses or, more 
exactly, conform with the ger»2ral trend of the ice movement, 
in obedience to a direct control exerted by the ice. This con- 
trol being sufficient to force the streams to flov/ up hill and 
over elevations of 100 to 400 feet, we must, apparently, sup- 
pose that their courses were determined before the motion of 
the ice cea)sed and while it was still thick enough to hold to its 
normal trend regardless of even quite strongly accentuated 
topographic features. How the subglacial channel or tun- 
nel originated is not considered, beyond the suggestion that it 
may be due to the enlargement of longitudinal crevasses ; but 
the point is not vital, for it is obvious that the subglacial water 
must escape and that, no matter how closely pent, there must 
alwa3'is be lines of least resistance to its flow. 
At and beyond the margin of the ice, the phenomena must be 
essentially the same as for superglacial streams ; and a frontal 
barrier or sill of any kind causing the stream to rise, perhaps 
fountain-like, must favor the aggrading of the floor of the tun- 
nel by the coarser detritus and a corresponding elevation of the 
roof by corrasion and melting. If the stream is sufficiently long- 
lived, this process may continue until the deposit attains the 
level of its terminal alluvial cone or delta. The breadth of the 
deposit is sharply limited by the stability of the ice arch ; and if 
we should assume two hundred feet as the maximum breadth 
of ice tunnels, portions of most important eskers would demand 
some other explanation, and in many cases a breadth of 500 
or 1000 feet would prove inadequate. In fact, the subglacialist 
finds it convenient in such cases to suppo'se : either that the 
stream rose through crevasses, perhaps in consequence of the 
clogging of the tunnel, and became superglacial for a longer or 
shorter distance ; or that the tunnel became, by excessive ab- 
lation, locally roofless, or open tO' the sky, and the resulting 
canyon was widened by the recession or melting back of the 
walls. As in the case of superglacial streams, the water is 
supposed to be diverted after a time, and the deposit left in the 
abandoned tunnel gradually develops the steep lateral slopes 
and other formal features of eskers during the slow melting 
of the retaining walls and arch of ice. 
