CKOSBY : OEIGIN OF ESKEKS. ' 
407 
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 height 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 understand how sub¬ 
glacial streams can do the same. They should certainly be marked 
either by aggraded or by eroded channels, 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 (’99, p. 424), My conclu¬ 
sion 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 underneath the sediment melted, the 
gravel slid down irregularly and the original stratification was lost. 
In 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 hypothesis that 
it was deposited on the ice, not beneath it.” Add to this confession 
of a subglacialist the undoubted fact that the structure of the 
typical esker is essentially pell-mell, or at least chaotic to such a 
degree that Davis (’92, p. 489) has hesitated to describe the very 
normal eskers of the Boston Basin as stratified in any true or 
ordinary sense, and that the occasional appearance of anticlinal 
stratification must be chiefly, at least, the result of sliding as the 
gravel gradually adjusts itself to the slowly vanishing walls of ice, 
and it becomes apparent that no argument fatal or even inimical to 
the superglacial origin of eskers lurks in the coarse, rude, chaotic or 
anticlinal structure of the latter. Even the open-work gravel, on 
which Davis so confidently relies, appeals to me as 'finding its readi¬ 
est explanation, as previously noted, in the loosening up and differ¬ 
ential settling of the coarse and irregular detritus during the melting 
and vertical recession of its icy floor, and I make bold to claim it as 
a specially cogent argument for the superglacial origin of the eskers 
in which it occurs. 
