Wright.] 
140 
[January 19, 
general deposit; it is also largely composed of fresher looking and 
softer pebbles, showing that it has been subject to much less 
abrasion than the other, and that it is of more recent age ; it is 
also limited to the river valley, and finally is not overlaid by the 
Philadelphia brick clay which, so far as it extends, rests uncon- 
formably upon the general deposit of gravel. The general deposit 
of gravel in this region is composed almost exclusively of small, 
well rounded pebbles of quartz and of hard limestone which “ are 
not fresh looking, but are eaten and weather-worn by age.” 
The elevation of this implement-bearing gravel at Trenton is 
not far from forty feet above the present high water limit ; and 
Trenton is now at the head of tide-water. These gravels are 
continuous as a terrace all along up the river. As one ascends the 
river, however, their height (at least below the Water Gap) is 
reduced to fifteen or twenty feet above the present flood plain. 
But most significant of all the facts indicated are the character 
and position of the Philadelphia brick clay. This also is confined 
to the river valley and its tributaries, and rests unconformably 
upon the older gravel formations, rising to a height of one hun- 
dred and fifty feet above the river, and there ceasing. This 
elevation relative to the river is maintained with tolerable con- 
stancy as far up as Easton, where the bed of the river itself 
is one hundred and fifty-seven feet above tide level. Finally, 
this Philadelphia brick clay contains numerous boulders of 
considerable size, derived from the ledges of Medina sandstone 
and other rocks' above. This marks it as a deposit of the glacial 
flood sometime during the declining centuries of the great ice 
period. 
The succession of events would seem to be as follows : During 
the early part of the glacial period the ice accumulated in the 
upper portion of the valley of the Delaware to a depth of many 
hundred feet. Two and one half miles north of the Delaware at 
Martin’s Creek, Professor Lewis and myself saw, in going south, the 
last distinct evidences of direct glacial action at a height of six 
hundred and forty feet above the river and eight hundred and 
forty feet above the sea. Penobscot Knob, on the water-shed 
between the Susquehanna and the Lehigh east of Wilkesbarre, 
and only a few miles north of the southern limit of glaciation, 
