402 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
knolls or pinnacles, or for sharp crests of rapidly varying height. 
Except where Woodworth’s principle does apply, or masses of ice 
have been buried by the growing esker, or it has suffered erosion, 
the subglacial hypothesis calls for substantial uniformity of height, 
or at least for an even and continuous crest line, as well as for a 
close approximation of the crest to the level of the terminal plain, 
with a gradual rise above that level to the northward if the stream 
was sufficiently long-lived to permit the complete aggrading of its 
bed. The superglacial hypothesis, on the other hand, explains 
coincidence in height with the terminal plain, but requires it only 
where aggrading has continued during the entire period occupied in 
letting the esker down upon the ground. When aggradation has 
ceased earlier than this, the final adaptation of the esker to the 
uneven surface of the ground may give rise to pretty much all the 
observed irregularities of elevation. The high points or knolls are, 
however, as previously noted, best explained as due to late deposits 
contributed by hanging lateral valleys, and this harmonizes well with 
the fact that they occur usually at a bend in the glacial river. The 
subglacial hypothesis, on the other hand, appears to leave the excep¬ 
tionally high points unexplained. Height of the esker above its 
base and elevation above its terminal plain are not crucial tests ; but 
the superglacial stream appears, on the whole, more competent than 
the subglacial stream to account for the observed facts. 
Branching of eshers. — The subglacial streams of a stagnant ice 
sheet should branch in essentially the same fashion as the existing 
or postglacial streams of the same region, and the branches should, 
in general, be aggraded almost equally with the main stream. But 
this is clearly not the fashion of eskers, for they are little given to 
true, river-like branching, especially in their lower or more southern 
courses; and branches making large angles with the main esker are 
almost unknown. In fact, there are practically no branches, in the 
sense of minor tributaries to a main line, but we observe instead 
an occasional confluence of eskers of approximately equal size and 
length at very oblique angles, suggesting the primary rather than 
the secondary or lateral branching of rivers. This, as previously 
noted, is all precisely what we should expect in the case of super¬ 
glacial streams on the comparatively high gradient of the marginal 
slope of the ice sheet, with so great a thickness of easily eroded ice 
above the base level that the main stream had no difficulty, with its 
