In journeying up the Delaware and approaching Trenton, we have passed
through a region of most interesting geological development. All along
are evidences of the deposit of the drift from above, which is
popularly known as the "Trenton gravel." The Delaware flows southeast
from the Kittatinny Water Gap to Bordentown, and then, impinging
against the cretaceous stratified rocks of New Jersey, abruptly turns
around a right-angled bend and goes off southwestward towards
Philadelphia. The river has thus deposited the Trenton gravels,
composed of the drift of most of the geological formations in its
upper waters, throughout its course, on the Pennsylvania side from
Trenton down below Philadelphia. This deposit is fifty feet deep on
the river bank in Philadelphia, and underlies the river bed for nearly
a hundred feet in depth. At Bristol the deposit stretches two miles
back from the river, and at Trenton it is almost universal. The
material, which in the lower reaches is generally fine, grows coarser
as the river is ascended, until at Trenton immense boulders are often
found imbedded. We are told by geologists that at the time of the
great flood in the river which deposited the gravel, the lower part of
Philadelphia, the whole of Bristol and Penn's Neck and almost all
Trenton were under water. The gravel has disclosed bones of Arctic
animals--walrus, reindeer and mastodon--and also traces of ancient
mankind. The latter have been found at Trenton and on Neshaminy Creek,
indicating the presence of a race of men said to have lived about
seven thousand years ago. The river has also made immense clay
deposits all along, which was done at a time when the water flowed at
a level more than a hundred feet higher than now.
In the early geological history of the Delaware it is found that all
southern New Jersey lay deep beneath the Atlantic, whose waves broke
against the ranges of hills northwest and north of Philadelphia, and
an inlet from the sea extended into the great Chester limestone valley
behind them. This whole region, then probably five hundred feet lower
than now, was afterwards slowly upheaved, and the waters retreated.
Subsequently the climate grew colder, and the great glacial ice-cap
crept down from Greenland and Labrador, forming a huge sea of ice,
thousands of feet thick, which advanced on the Delaware to Belvidere,
sixty miles north of Philadelphia. Then there came another gradual
change; the land descended to nearly two hundred feet below the
present level, and again the waters overflowed almost the whole
region. This was ice-cold, fresh water, bearing huge icebergs and
floes, which stranded on the hills, forming a shore on the higher
lands northwest of Philadelphia. The river channel was then ten miles
wide and two hundred feet deep all the way down from Trenton, and a
roaring flood depositing the red gravel along its bed. As the torrent,
expending its force, though still filled with mud and sand from the
base of the glacial ice-cap, became more quiet, it laid down the
clays, the stranded icebergs dropping their far-carried boulders all
along the route. This era of cold water and enormous floods is
computed to have occupied a period of about two hundred and seventy
thousand years, and then the "Ice Age" finally terminated. The land
rose about to its present level, the waters retreated, and elevated
temperatures thawed more and more of the glaciers remaining in the
headwaters, so that there came down the last great floods which
deposited the "Trenton gravel." The river was still wide and deep, and
Arctic animals roamed the banks. Mankind then first appeared, living
in primitive ways in caves and holes, and hunting and fishing along
the swollen Delaware ten thousand years ago. Occasionally they dropped
in the waters their rude stone implements and weapons, which were
buried in the gravel, and, being recently found, are studied to tell
the story of their ancient owners. The river deposited its gravel and
the channel shrunk with dwindling current, moving gradually eastward
as it eat its way into the cretaceous measures. The primitive man
retired, making way for the red Indian, and the present era dawned,
with the more moderate climate, and with again a slow sinking of the
land, which the geologists say is now in progress.