Origin of Eskcrs. — Crosby. 25 
usually lie in the lee of prominent topographic reliefs — ledges 
or hills, which might, conceivabl}', have grooved the bottom 
of the ice as it flowed past them. Xor is it easy to see how 
tunnels due in any direct way to ice-flow and pressure could 
have been so indififerent to the topography as eskers are often 
observed to be, forsaking broad valleys hundreds of feet in 
depth to continue, perhaps in a very oblique or closely par- 
allel course, across an uneven upland. As previously noted, 
there must be, as long as the ice is in motion, a tendency to 
equalize the bSisal pressures and prevent the development of 
open spaces by flow of the ice and also by flow, in obedience to 
the ice pressure, of the underlying drift or ground moraine ; 
and, furthermore, the effects of unec^ual pressure would be, 
at least to some degree, neutralized by the differential 
or localized melting induced by the pressure through 
the lowering of the melting point. Again, in their 
longer reaches as well as in their minor meanders, 
€skers are often obliquely or directly transverse to 
the last recorded ice movement ; and to that extent the move- 
ment of the ice must have tended strongly to obliterate or 
sweep away the deposits of the subglacial streams as fast as 
they were formed. Stone says,* "The longer meanderings 
transverse to the direction of ice flow certainly add some diffi- 
culties to the hypothesis of subglacial streams." This diffi- 
culty, together with the fact that the formation and mainten- 
ance of tunnels transverse to the ice movement is well-nigh 
inconceivable, and the further fact that eskers and the plains 
to which they are tributary were clearly formed at a time when 
the ice was so far wasted as to have a very irregular and frag- 
mentary margin, have led Davis and other adherents of the 
subglacial theory to hold with the superglacialists that eskers 
were formed mainly, at least, after the ice became stagnant. 
But it is obvious that the control of the subglacial drainage 
demanded by the subglacial theory must originate in the mo- 
tion of the ice ; although, it may, conceivably, survive the ces- 
sation of that motion. In other words, the subglacial theory 
of eskers requires us to suppose that the su])glacial streams 
were established before the ice ceased to move; but that, as a 
rule, eskers were formed by these streams after the ice l^ecame 
* U. S. Geol. Surv., Mon. 34, p. 426. 
