1878.] 
Glacial Drift in North America . 
63 
in his study cf the deposits left hy the ancient glaciers of 
the Sierra Nevada,* has arrived at a similar conclusion. 
Prof. Hall, in his “Geology of New York,” has given a 
large picture of a natural section on the shore of Lake Erie, 
in Chautauque county, where we may see as it were the till 
in course of formation. In some parts the top strata are 
only a little separated, with clay forced in between them ; in 
others they stand on edge, or are broken up and the frag- 
ments scattered throughout the till.t I have seen similar 
instances on the shores of Lake Ontario. In our own 
country there are also many examples of the same action ; 
one of the finest, that I have seen, being in the beautiful 
section of the glacial beds exhibited in the cliffs at the 
mouth of the Tyne, in Northumberland. In this way the 
lower portion of the ice appears to have become charged 
with stones and finer materials which were left on the sur- 
face when the great glacier melted back. The till is there- 
fore restridted to the area that the land-ice covered, and is 
as much a memorial of its former presence as the scratched 
and rounded rocks on which it generally lies. 
By the glaciated rock-surfaces up to the line I have 
already mentioned and their absence beyond, by the out- 
spread of the till limited by the same boundary, and by the 
disappearance of the decomposed surface-rocks up to but 
not beyond that margin, we know that the land-ice did not 
reach, in the valley of the Delaware, farther than the 
neighbourhood of Belvidere, which is 50 miles to the north 
of Trenton. What relations, then, do the beds of drift that 
I have described at Trenton bear to the ice-sheet ? To 
answer this question we must take into consideration what 
we know is taking place at the terminations of existing 
glaciers. Great streams of water run from underneath 
them, bearing along fragments of rocks that have been 
melted out of the glacier or fallen through crevices to the 
subglacial rivers. These are rounded by attrition, and 
spread out in sheets of pebbles often extending for miles 
below the terminations of the glaciers. From the enormous 
glacier that once filled the Delaware valley as far as Belvi- 
dere, and which was itself only a southern prolongation of 
the still greater northern ice-sheet, there must have been 
shed a vast amount of similar material. The large rounded 
drift that lies at the base of the quaternary beds at Trenton 
(No. 3 in Figs. 1, 2, 3) is almost undoubtedly a similar 
* Amer. Journ. Sci., 1875, p. 126. 
f Op. cit., Plate VIII. 
