Accaiii'ihifto)) of Dnnnl hix. — rj>]i(iin. 35T 
•century or more. Avitli pr()l):il)ly much addition from the thick, 
inner part of the ice-sheet, containino-. from whichever source, 
little or no drift, passed and moulded these hills in their smoothly 
•oval, or round, or elongated forms. 
It is thus readily seen why the amount of finally englacial drift 
upon the surface of drumlius is usually less than on intervening 
tracts of low ground and on those parts of the drift-bearing area 
from which the ice-shi'et was more rapidly melted away. 
We can also understand why these accunudations are so fre- 
quently found capping the top of low hills of the Ijed-rocks. 
since these projected through the ice that Iny l)eneath the super- 
glacial, and afterward again englacial drift stratum, and so were 
obstacles to favor an aggregation of that drift, either as a com- 
plete drumlin resting on the hill of rock. or as a lenticular slope of 
till, of which al)uiidant examples are found in New Hampshire, 
collected on the stoss or thc' lee side of the rock hill and occa- 
sionally in slopes of this form, covering l)oth these most exposed 
and most sheltered sides of the hill thickly and its intervening 
tianks thinly, with visible outcrop of the rock only at its summit. 
Powderhorn hill, in Chelsea. Mass., one of the largest druni- 
lins near Boston, rising about 200 feet above its base, wliich is 
near the sea level, and having an exceptionally elongated form, 
with the length of three-({uarters of a mile and one-fourth as 
great width, atl'ords evidence that a slight thickness of ice re- 
mained beneath it when it was accumulated. Extensive excava- 
tions. 20 to M) feet deep, in each end of this drumlin consist 
wholly of till, with no trace of any bed or seam of stratified drift. 
In one of these sections, about 30 feet high, on the north side of 
its southeastern end, I observed a nearly vertical, irregular course 
of fracture, from one to six inches wide, filled with sand and fine 
gravel l»rought by percolating water, where this long hill had suf- 
fered a slight dislocation in sinking as the underlying ice melted. 
8uch fractures, extending deeply into the hill, were also found in 
the construction of the reservoir on its top, which gave nnicii 
trouble l)y leaking, until the bottom was made impervious with 
cement. The till where not so fractured is water-tight, and 
numerous reservoirs on other drumlins near Boston have ])een 
free from this ditticulty. The narrowness of Powderhorn hill, in 
proportion to its length. [)robably caused it to sink more une(|nally 
than most of the drumlins in this district. 
