8 The American Geologist. 
Julv, 1902. 
usually more or less discontinuous; and an esker system (to 
which Stone has proposed to restrict the name osar), embrac- 
ing all the esker ridges referable to one glacial river or drain- 
age system, may be of any length up to lOO and even 150 miles. 
Composition and Structure. — Eskers consist chiefly of 
coarse, and often of very coarse, gravel, mingled or inter-strati- 
fied with which is usually much coarse sand, although the sand 
may be at times insufficient to fill 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 gla- 
cial stream; but even here fine sand and clay are of rare occur- 
rence. The gravel is more or less rounded and water-worn, 
and includes a larger proportion of far-travelled 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, irregu- 
larly, and very often indistinctly, stratified ; and a sort of anti- 
clinal structure, due to lateral sliding and settling as the retain- 
ing walls of ice melted aw^ay, is a characteristic, though by no 
means a constant, feature. Bowlders, sometimes of consider- 
able size, are rather rarely found resting on the slopes of esk- 
ers, and more commonly partially or wholly imbedded. Also, 
eskers may be more or less completely buried by delta and 
overwash 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 move- 
ment of the ice as recorded in striae, the major axes of drum- 
lins and bowlder-trains ; and this conformity is often surpris- 
ingly 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 
valleys, 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 
topographic indepen dence, as recently noted by Stone.* 
* U. S. G. S. Mon., xxxiv, p. 36. 
