Page:Popular Science Monthly Volume 10.djvu/451

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435
THE LONG ISLAND COAST.

in recent time, in consequence of which its bluffs have been under-rained by waves, and the lowlands submerged. Immense bowlders lie upon some portions of the shores, which indicate the sites of recent banks and headlands.

The wearing away of an exposed coast, like that of the east end of Long Island, suggests a subsidence, by which it is continually being: brought under dominion of the waves. Did no such change occur, the abrasion would be retarded, or might finally cease, unless, indeed, the falling material be removed by a coastwise drift of the water. The tendency of wave-motion is to throw upon shore the waste of the cliffs, thus raising a breakwater on which the waves expend their force. But, where low grounds along the ocean-margin become permanently overflowed, the proof is conclusive that a change of relative level has taken place. There has been no abrasion by waves, but silently and imperceptibly the tides have advanced upon the uplands.

Around the shores of Long Island are large areas of recent forest, swamp, and meadow, with remains of their peculiar forms of vegetation in many cases undecayed, covered by water to depths of from one to sixteen or more feet. Some facts illustrating this were presented to the Natural History Section of the Long Island Historical Society, in May, 1868, a synopsis of which was published in the American Naturalist for August of that year. A few of these, with others of importance since discovered, are offered in this paper:

The movement under consideration is by no means a local one, but occurs along the Atlantic border from Labrador to the Capes of Delaware, and in a lesser degree to Florida. Prof. G. H. Cook, in his admirable "Report on the Geology of New Jersey," cites many instances along the coast of that State where swamps of cedar and other forms of vegetation are now submerged, or covered with salt meadow.

On the south side of Long Island are about 40,000 acres of salt marsh and meadow. Their vast stretches of level surface, fringed by the Great South Bay and the beach on the one side, and by uplands on the other, present a scene of rare and surpassing beauty. The meadow rests upon a floor of gravel and sand. It varies in thickness from a few inches at the uplands to eight or ten feet near the beach, and is filled with the roots of grasses throughout. It has been formed by growth and accumulation at the surface, for the meadow-grasses thrive only at or near high-tide level where rainfall and sunshine can reach them. The increase of the meadow in thickness has, therefore, just kept pace with the deepening of the water, or in other words, with the sinking of the coast.

Beneath the meadows remains of swamp and forest are found. These are fast rooted, and often six feet beneath the surface. At Islip a great number of stumps are found in the salt meadow of Wil-