CROSBY : ORIGIN OF ESKERS. 
381 
' with floors of till and holding water; and occasionally an esker is 
double, consisting of two distinct but contiguous parallel ridges. 
The esker ridge may be uninterrupted for long distances, but is 
usually more or less discontinuous; and an esker system (to which 
Stone has proposed to restrict the name osar), embracing all the 
esker ridges referable to one glacial river or drainage system, may 
be of any length up to 100 and even 150 miles. 
Compositio7i and structure. — Eskers consist chiefly of coarse, and 
often of very coarse, gravel, mingled or interstratified with which 
is usually much coarse sand, although the sand may be at times 
insuflicient to till the interstices between the pebbles and cobbles. 
The proportion of sand and the fineness of all the material are 
greatest in the wide, flat topped portions of an esker, corresponding 
to lake-like expansions of the glacial stream ; but even here fine sand 
and clay are of rare occurrence. The gravel is more or less rounded 
and water-worn, and includes a larger proportion of far-traveled 
material than do the adjacent masses of till or ground moraine. 
This is a necessary deduction from the fact that the esker drift has 
been transported by water as well as by ice, and it has been fully 
confirmed by observation. These materials are rudely, irregularly, 
and very often indistinctly stratified, and a sort of anticlinal structure, 
due to lateral sliding and settling as the retaining walls of ice melted 
away, is a characteristic, though by no means a constant, feature. 
Bowlders, sometimes of considerable size, are rather rarely found 
resting on the slopes of eskers, and more commonly partially or 
wholly imbedded. Also, eskers may be more or less completely 
buried by delta and over-wash plains, and valley terraces and flood 
plains, but never by till or ground moraine. 
Topographic and geologic relations. — Eskers and esker systems 
or osars, unlike the terminal moraines of the great ice sheet, exhibit 
a tendency to conform in trend with the movement of the ice as 
recorded in striae, the major axes of drumlins, and bowlder trains ; 
and this conformity is often surprisingly close. This means that 
while occurring chiefly, with other forms of modified drift, in valleys, 
eskers are to a good degree independent of the topography and 
often do not hesitate to forsake, or to cross at all angles, large and 
well-accentuated vallevs, in order to adhere to their normal courses. 
They may thus rise to levels far above and cross tracts quite free 
from all other types of modified drift. But there is a limit to their 
