Page:Popular Science Monthly Volume 10.djvu/450

434

BSERVATIONS made around the shores of Long Island justify the conclusion that they have undergone important changes in time geologically recent. These changes appear to have arisen from a series of vertical movements, by which the coast has been alternately elevated and depressed.

In consequence of these movements the shore-line of the island has advanced and again receded, perhaps, in repeated instances: being at one time, upon the ocean-side, from fifty to seventy miles southward of where the waves now break; while at another period the highest hills of the island were largely if not wholly submerged. The persistence and extent of these movements are interesting and important questions in geology. We do not know at present how great the oscillations of the coast may have been, but enough is obvious, in the records they have left in the contour and structure of the island, to show that they have been much greater than is indicated on the adjacent mainland of Southern New England. We shall endeavor to follow these records, obscure and perplexing though they sometimes are, back to the period in which Long Island may be said to have had its origin—a period which witnessed the approach and presence of a great ice-sheet upon this coast.

It is not questioned, we believe, that Long Island is a terminal glacial moraine, and that the material of which it is composed is the débris of regions over which the ice moved in its progress toward the sea. Its underlying portions are beds of laminated sands and clays which have been referred to periods antecedent to the advent of the ice, and which constitute in one sense a part of the island. Its great mass, however, overlies these beds, and presents two general forms of structure. One is known as the "unmodified bowlder-drift," in which there are no stratified beds; the other is the "modified drift," or that in which the material has been distributed in layers chiefly by the action of waves. Much of the hill-region of the island presents the peculiar pell-mell structure of the one—the stratified gravels and sands of Southern Long Island are typical of the other. These differences in structure, and other facts to be mentioned, imply great changes in the relative level of land and sea upon the coast.

In considering these movements of oscillation it will be convenient to notice the latest first, and others in their order. A persistent invasion of the ocean upon the shores of the island has taken place