Page:Popular Science Monthly Volume 10.djvu/455

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439
THE LONG ISLAND COAST.

island were carried under water. The evidences of this depression are found in the numerous beds of stratified sand and gravel—elevated beaches and other shore-formations—which lie along the central ridge of hills,[1] and fronting the ocean from 100 to 260 or more feet above the level of the sea. At whatever heights these deposits occur, they suggest, if they do not prove, submergence of the coast to that extent.

From observations, made by the writer and others, it is ascertained that the summit of Hempstead Harbor Hill, which is 384 feet above tide, and the highest land upon Long Island, is a mass of stratified sand and gravel. The same is true of Janes Hill, in the West Hill group, said to be 383 feet high, and of Osborn's Hill, southwest of Riverhead, the height of which, according the United States Coast Survey, is 293 feet. In these instances, and in others similar, the layers are distinct and well defined. The stratification of this material was evidently the work of waves, but, whether of the ocean, or of a glacial lake or sea, admits of doubt. At present we cannot determine what the extent, contour, or elevation of the surface around these dome-like hills may have been; nor can we tell the original extent of the beds of assorted material, remains of which now cap the hills. That the denudation has been immense, is everywhere evident.

From the summits of the hills mentioned one overlooks southward a vast plain which extends to the ocean, ten miles distant; and the conclusion seems irresistible that every rood of that distance has been the shore-line of first an invading, afterward of a receding ocean, and the scene of those great coast-changes which waves produce. We may restore, in imagination, the hills of glacial rubbish crumbling before the stroke of waves, as during an immense period of time the subsidence of the land went on. So complete was the work of disintegration, that scarcely a bowlder remains in the low tracts fronting the ocean, but are numerous along the margin of the hills, and abound in the undisturbed drift[2] which constitutes much of the hill-region.

The period of subsidence we are considering is referred to the "Champlain" of the geologists, so called by Prof. C. H. Hitchcock from the abundance of its peculiar deposits near that lake. It is a marked one in geological history. During its progress the deposits

  1. A ridge of hills, varying in height from one hundred and fifty to three hundred and eighty-four feet above tide, extends, with some interruptions, through Central Long Island. They are drift with bowlders; but nowhere show rock in place, as some have supposed.
  2. Many bowlders on Long Island are of immense size; one, at Manhassett, contains upward of 20,000 cubic feet. Two others now or recently in the same valley are, in circuit, 108 and 126 feet respectively. One, on Strong's Neck, in Suffolk county, has a content of 14,000 cubic feet. Bowlders are found on the tops of the highest hills, and form an enormous rip-raps in some places where they have fallen as the banks were undermined by waves.