380 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
That observations on the existing ice of the northern hemisphere 
have made no important positive contributions to the theory of eskers 
is, perhaps, not an overstatement. We may fairly conclude, how¬ 
ever, 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 unfor¬ 
tunately no observations bearing upon the origin of eskers have 
been made here; and perhaps none are possible, since the margin 
of this greatest of living ice sheets is almost everywhere deeply 
submerged in the Antarctic 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 observed, 
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 hypotheses as have been or may be 
suggested by the facts. Fortunately the principal facts are now 
well determined; but, although a good general agreement exists 
among glacialists as to what constitutes anesker, it appears advisable 
to enumerate briefly the main features before proceeding to a critical 
comparison of the rival hypotheses. 
Chakactekistics of Eskers. 
Form. — The typical esker is a steep sided, narrow crested and 
more or less winding ridge, varying in height above the surrounding 
country up to 100 and even 150 feet. The lateral slopes usually 
approximate the maximum angle of repose for gravel. Although 
the crest is often of even height for considerable distances, it is more 
commonly diversified by cols and knolls, and not infrequently it 
widens into level topped plateaus. Woodworth (’94, p. 197) has 
shown that the variations in both height and breadth often find a 
reasonable explanation in the ratio of depth to breadth of the orig¬ 
inal deposit, before the disappearance of the retaining walls of ice. 
The eskers rather rarely receive distinct tributaries, but they are 
often composite, splitting up into two or several ridges, which wind 
and anastomose, enclosing kettles and even large, irregular basins 
