34 The American Geologist. •'"^^'' i^^i'. 
channel, perhaps scores of miles in length, before the esker is 
uncovered at any point south of which it rises to a higher ele- 
vation. How the subglacial stream is to be diverted while the 
ice is -still several hundred feet thick above it, and how it 
fails, in general at least, to build an esker in its new channel, 
are points which have not, perhaps, been duly considered. 
Several writers have emphasized, perhaps unduly in some 
cases, the brevity of the time required for the formation of an 
extensive sand-plain and its tributary esker, a few years, or 
even a isingle season, being considered sufficient in most cases. 
On the other hand, we are asked to suppose that the subgla- 
cial tunnel, often many miles in length, is formed at a time 
when the ice still has a definite motion, to the controlling in- 
fluence of which the tunnel is supposed to owe its general 
trend, that the meanders and transverse reaches of the tunnel 
are not obliterated by the motion of the ice, and that the tunnel 
as a whole survives the necessarily slow cessation of the ice 
movement. This insures to the timnel and the stream which 
formed and maintains it, a good degree of longevity and time 
for the slow building of the esker and terminal plain. But 
these deposits testify, in composition and structure, to rapid 
work by torrential streams ; and the stationary ice margin 
during their formation points unequivocally to the same con- 
clusion. It is an interesting question, therefore, as to what 
the subglacial stream was doing during the relatively long 
period of its existence anterior to this brief period of tre- 
mendous constructive activity ; and also as to why it should 
then abruptly abandon the channel to which it had adhered so 
long. In brief, the trend of the subglacial stream demands 
ice control and a long life ; its deposits demand a short and in- 
tensely active life followed by a sudden disappearance from 
the scene of its labors. But, whether it be long- or short-lived, 
its existence should be recorded in the ground moraine, in 
the form of interbedded gravels before the ice ceased to move, 
and of erosion channels after the ice became stationary. These 
phenomena, however, are of rare occurrence, and still more 
rarely can they be correlated with subglacial streams or eskers. 
This important problem will receive farther consideration in 
connection with the source of the material of eskers and sand- 
plains. 
