Origin of Eskcrs. — Crosby. 35 
Relations of Eskers to Frontal and Delta Plains. — The 
main facts under this head are well determined and there is 
little question or controversy concerning them. My purpose 
is simply to call attention to the very numerous plains of mod- 
ified drift, including many true delta plains, which have no 
tributary eskers and, apparently, never have had. They have, 
however, undoubtedly been formed through the agency of 
glacial streams. If these were superglacial streams, the ex- 
planation of the absence of associated esker^, is simply, as 
previously noted, that the plain was formed, and the stream 
diverted before its channel was sufficiently base-levelled to 
permit any notable aggrading. But if, instead, they were 
subglacial streams, the aggrading of their beds must have been 
in progress during their entire existence, or at least during 
all the time required for the formation of the terminal plain ; 
and the aggrading must, approximately, have kept pace with 
the upward growth of the plain, else we should not have the 
good general agreement in bight between eskers and plains 
so commonly observed. That a large majority of superglacial 
streams should disappear and leave no record, save in the 
terminal plains, is not surprising; but it is difficult to under- 
stand how subglacial streams can do the same. They should 
certainly be marked, either by aggraded or by eroded chan- 
nels ; for we cannot suppose that eskers were formed and 
subsequently swept away by currents which left the ice-contact 
slopes of the .sand-plains intact. 
Composition and Structure of Eskers. — Little need be 
added here to what has been noted under the characteristics 
of eskers. The true significance and evidential value of the 
main facts have been well expressed by Stone, where he says,* 
"My conclusion is that where the whole of a ridge of till, from 
which the finer detritus has plainly been washed by water, has 
lost all signs of stratification and has a pell-mell structure, the 
best interpretation is that it was deposited upon the ice in a 
superficial or englacial channel, and that when the ice under- 
neath the sediment melted, the gravel -slid down irregularly 
and the original stratification was lost. * * * * j,-, general 
we remark : A stratified internal structure is consistent with 
either subglacial or superglacial streams. Pell-mell structure 
of a large mass of glacial gravel strongly favors the hypoth- 
* U. S. G. S., Mon. 34., p. 424. 
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