Page:Popular Science Monthly Volume 10.djvu/456

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which the glaciers had left were altered in their contour, and redistributed by waters which encroached upon and finally covered them. The material thus redistributed would be left in layers, chiefly of gravels, sands, and clayey sands, if upon the ocean-margin, as is the case in Long Island.

In sinking wells into the beds formed during this period of subsidence, wood and shells have been found in a great number of instances. The wood sometimes occurs in logs of large size; oak and pine have been identified. We have a record of sixteen instances where wood has recently been found, and many others are mentioned by Thompson in his history, and by Mather in his excellent report on the geology of the island. These facts seem to imply that forests were upon the adjacent lands, which was not the case during the presence of glaciers upon the coast.

The shells found are at various depths—in one case at Gravesend 100 feet below the surface—and occur in all parts of the island where the stratified drift abounds. They have been found at Flatbush, Prospect Park, Bath, East New York, Farmingdale, Amagansett, and elsewhere.

The beds in which they occur are not of the low plains only, but many feet above tide. At Manhassett, as we are informed by John M. Clark, Esq., of that place, a well was dug, and at several feet below the surface-rubbish a layer of what appeared to be "creek-mud" was found, in which were a great number of shells of the oyster, clam, and scallop, many of them unbroken. The layer was about five feet thick, and throughout contained not only shells, but leaves, pinecones, also wood of pine and other species. This interesting deposit is about 200 feet above tide; but the contour of the present surface indicates plainly enough that an arm of a bay (Little Neck Bay) contiguous extended over this area when the land was sufficiently submerged to admit of it.

It is obvious that only the portion of Long Island which is more than 200 feet above tide was at that time dry land.

But the subsidence was greater than is indicated by this elevated deposit. The peculiar beds of stratified sands and gravels on the low plains already referred to, and which prove the former presence of the sea, are found at elevations of from 200 to 260 feet along the margins of the hills, and against or upon the unmodified bowlder-drift (Fig. 2). Beach-sands occur at 230 feet elevation, having the well-known structure of such beds. We have, too, the further fact, already noticed, that the tops of some of the highest hills of our island are composed of stratified gravel and sand.

Without insisting further on, this fact, however, we think a movement of subsidence is shown thus far of at least 260 feet; but facts of a most interesting and important character now being brought to light show that this is but part of the great movement of depression