292 The American Geologist. May, 1885 
calcareous nodules, containing shell fragments, which they re- 
gard as evidence of some glacial recession and reiidvance."^' It 
seems to me possible, however, in view of the absence of other 
indications of glacial oscillation there, that the formation of 
the nodules took place in superglacial drift exposed by abla- 
tion on the ice surface and afterward covered by increasing 
snow and ice accumulation and ontlow, but requiring no rc- 
advance of the glacial boundary. f Another and more proba- 
ble explanation, as I think, is that these rare nodules, like the 
abundant fragments of marine shells in the same till, are ref- 
erable to the early Pleistocene time when the area of Massa- 
chusetts bay is known by these shells to have had nearly its 
present relations to the sea level, and to the succeeding time 
of great epeirogenic uplift of this region and of all the north- 
ern half of this continent, to which the Pludson and Califor- 
nian submarine fjords and the subaerial erosion contour of 
the now submerged Fishing Banks between Cape Cod and 
Newfoundland bear witness.;]; During this uplift, before it 
had culminated in the Glacial period, leaching waters perco- 
lating through the shell beds may have formed hard calcare- 
ous layers which were eroded and broken up by the ice-sheet 
to yield the glaciated nodules of the till. 
All the fifty-five species whose shell fragments are identi- 
fied in the drumlins of Boston and its vicinity are still living. 
They include no exclusively northern species, but indicate 
that the sea had even a somewhat warmer temperature than 
now in Massachusetts hny. If this reference of their origin 
to beds antedating an epeirogenic movement which caused ice 
accumulation in that district is true, they give very impressive 
testimony of the geologic brevity of the Glacial period. The 
same is also to be said of the similarly large representation of 
the early Pleistocene nuvrine fauna which is preserved in sec- 
tions underl3'ing the outer morainic margin of the drift in 
*Am. Jour. Sci., Ill, vol. xLvni, pp. 486-496, Doc, 1894. 
fCompare my paptT 1)11 "Conditions of Accumulation of Drumlins," 
x\m. Geologist, vol. x, pp. :?y9-362, Dec, 1892. 
XAu. Geologist, vol. vi, pp. 327-339, Dec, 1890, Am. Jour. Sci., Ill, 
vol. XLVT, ])p. 114-121, Aug-., 1893. Proc ]k)St()n Society of Natural His- 
tory, vol. XXVI, 1893, pj). 42-48 (also in Am. Jour. Sci., 111. vol. xlvii, \)\) 
123-129, Feb., 1894). 
