Читать книгу AMERICA - Joel Cook - Страница 63

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.