Origin of Eskers. — Crosby. . 7 
fairly conclude, however, that the Antarctic ice-cap alone, of 
all existing ice masses, realizes at all closely, the conditions of 
the Pleistocene ice-sheets of northeastern North America and 
northwestern Europe ; but unfortunately, no observations bear- 
ing upon the origin of eskers have been made here; and per- 
haps none are possible, since the margin of this greatest of liv- 
ing ice-sheets is, at most points, deeply submerged in the Ant- 
arctic ocean. Thus we are baffled at every point, and can only 
say that the testimony of existing ice is almost wholly negative, 
indicating only where and how eskers have not been formed. 
Since, then, the formation of eskers has nowhere been ob- 
served, our only resource in seeking an explanation of this 
highly specialized type of drift is in a close study of existing 
examples, followed by a rigid testing of such working hypoth- 
eses as have been or may be suggested by the facts. Fortun- 
ately the principal facts are* now well determined; but, although 
a good general agreement exists among glacialists as to what 
constitutes an esker, it appears advisable to briefly enumerate 
the main features before proceedinsf to a critical comparison of 
the rival hypotheses. 
CHARACTERISTICS OF ESKERS. 
Form. — The typical esker is a steep-sided, narrow-crested 
and more or less winding ridge, varying in hight above the 
surrounding country up to 100 and even 150 feet. The later- 
al slopes usually approximate the maximum angle of repose 
for gravel. Although the crest is often of even hight for con- 
siderable distances, it is more commonly diversified by cols and 
knolls ; and not infrequently it widens into level-topped pla- 
teaus. Woodworth has shown that the variations in both hight 
and breadth often find a reasonable explanation in the ratio of 
depth to breadth of the original deposit, before the disappear- 
ance of the retaining walls of ice.* Eskers rather rarely 
receive distinct tributaries ; ])ut they are often composite, split- 
ting up into two or several ridges, which wund and anastomose, 
enclosing kettles and even large, irregular basins with floors of 
till and holding water ; and occasionally an esker is double, con- 
sisting of two distinct but contiguous parallel ridges. The 
esker ridge may be uninterrupted for long distances, but is 
• Proc. Bost. Sue. .Vat. Hist., xxvi, p. 197. 
