A History of the City of Brooklyn. Stephen M. Ostrander

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СКАЧАТЬ of Long Island, and how little it has excited the notice of travelers, it is not surprising that its geological character as well as other peculiarities should have remained so long unexplored. Until quite recently very few scientific men have even deigned to give it a passing notice, though the assertion may be safely hazarded that scarcely any other tract of land of equal extent on the American Continent furnishes more abundant room for the imagination of geologists to play upon, or that imposes a stronger necessity for conjecturing the operation of some tremendous agency, which in its freaks had invaded the domains of both the land and the ocean, and after completing its sport had silently retired without leaving a track to determine its origin or identify its form."

      The geologist of the present day does not seem to regard the field as one calling in the same degree for the exercise of the imagination, though the more definite knowledge acquired and made familiar since the time of the publication just quoted has in one sense vastly extended the opportunity for speculation. Certainly it no longer can be said that scientific men have neglected the investigation of the subject.

      Commenting on the investigations of Dr. Dwight, Prime says:—

      "From all these considerations, the inference has been regarded as legitimate that Long Island was once through its whole extent attached to the main; and some powerful agency, the form of which is now left entirely to conjecture, forced the separation which is now marked by the intervening Sound. One of the most plausible suppositions is that the separation has been effected by some resistless torrent of water, which, under peculiar circumstances that it is impossible now to determine, has swept out the intervening land, and left its channel to be occupied by the waters of the ocean."

      Thus vaguely were the early speculations set forth. With a well developed glacial theory to aid him the modern geologist is able to present a fairly circumstantial picture of probable conditions in the past. We now know with reasonable certainty that Brooklyn rests on soil that is a monument to a vast force quite different from any that were included in the hesitating speculations of the early writers.

      In an admirable review of the subject written by Charles M. Skinner we are presented with a picturesque outline of the glacial theory. We are reminded that Brooklyn stands on rubble that was rolled down from the New England mountains to the northward by a glacier larger than the combined areas of all the glaciers now existing on the earth. How many thousands of years ago this great glacier began its work we may only guess within somewhat liberal margins. This continent of ice covered the whole of the northern part of North America, burying mountains beneath its bulk and hollowing the beds of the great fresh-water seas that Chicago and its sister cities front upon to-day, burying, too, for aught we know, the remains of civilizations, though nothing at present has been taken out of the glacial drift, except rude stone implements, to show what the probable condition of man was at that time.

      This ice lay so deep that not even Mount Washington barred its advance, and to-day geologists find the summit of this mountain heaped with blocks of stone that were dragged from other points and left there when the ice melted; for glaciers are not stationary, like ice on ponds and marshes, but have an onward movement toward their point of melting that varies, with the slope of their beds, from six to thirty-six inches a day. In Greenland the whole interior is covered with ice thousands of feet thick, the movement of which is hindered by a wall of mountains that nearly surrounds that island, but wherever a valley opens a way for it the ice sends down a tongue to the sea, and from these tongues the ocean currents break off the icebergs that float down the Atlantic. In their descent these glaciers act as plows, wearing off so much earth and rock from the hills that the icebergs are freighted with them, and where they melt their stony burdens sink to the bottom of the sea, forming the Grand Banks of Newfoundland.

      The ice that buried upper North America acted in the same manner as the Greenland ice to-day: it eroded the mountains, it sent off bergs, and the rocks and gravel that it tore from the hills by a pressure of a thousand tons to the square yard were dropped at its foot, where they formed a moraine, as it is called. These moraines, which may be seen at the feet of the glaciers in Switzerland and British Columbia, and that sometimes make heaps and hills of rock, like rude forts, forty and fifty feet high, are trifling affairs to the shoals left by the great glacier of the ice age, for that can be traced from the Atlantic coast nearly to the Mississippi River. Long Island, measuring approximately 120 miles in length, is a small part of the dump of this glacier, and it is sometimes possible to tell where the stones came from that are found on the surface. For example, there are in Brooklyn anthophyllite from Westchester county, feldspar and green mica from Fort George, basalt from the Palisades of the Hudson, and a block of labradorite was found on Myrtle Avenue that had been carried down from the Adirondacks, three hundred miles.

      The members of the United States geological survey, supported by the New York and other state surveys, have studied into the course and volume of the glacier and mapped its moraine from Montauk Point westward nearly half across the continent. By this survey we learn that the gneiss that crosses under the East River and approaches the surface at Astoria, is the only bed rock to be found on Long Island, Brooklyn resting on a cushion of glacial drift that in some places is three hundred feet deep. Originally there were cliffs of gneiss edging the Atlantic, but the great glacier shaved these down to mere ledges. Central Park, New York, preserves a number of these ledges, rounded off into "sheep backs" and scratched by the pieces of stone that formed a grinding surface to the under side of the ice, while every now and then a boulder comes to the top of the ground in Brooklyn that is scored and almost polished by rubbing against those ledges. Pieces from that very outcrop in Hell Gate are found in Brooklyn streets.

      We are also reminded in Mr. Skinner's review that manufacturers of brick, tile, terra cotta, pottery, and porcelain in other states have to rely in part on the clay beds that environ Brooklyn for their material, and, in fact, that clay and sand are the only economic mineral products of Long Island. The explanation of this is that Brooklyn clays are rich in silica, which is apt to be deficient in the clays of New Jersey. Without silica the clays are weak, and bricks and utensils made from them readily crack and crumble; but by mixing properly the best results are obtained. Excellent sand for glass-making is also found in and near Brooklyn.

      There are many evidences in support of the theory that since the completion of the great glacier's work the surface of Long Island has subsided considerably. A recent writer4 on the geology of Long Island says:

      "The shore at the west end of the island has also undergone decided changes—even within the memory of persons now living. Personal witnesses have testified that about the first of this century Coney Island was composed of high and extensive sand hills, which have since been flattened down to a low beach, sometimes covered by the tides. About the same time salt meadow-grass was annually cut on a part of the beach now far out into the ocean. We are also informed that cedar-trees were cut for fence-posts, and other timber for firewood, about 150 years ago, on land which is now submerged by the ocean a mile and a half or two miles from the shore. There was also a house standing upon what was known as Pine Island, the site of which is now beneath the breakers, at a considerable distance from the present shore."

      Within the range of Kings County a stratum of salt meadow has been found at a depth of one hundred and twenty feet, and at other points within the county shells have been found fifty and sixty feet below the surface. What is generally called the "back-bone of Long Island" is a ridge of low hills beginning at the western end within the limits of Kings County and running almost the whole length of the Island. Of the boulders or erratic blocks found on the Island in this central range of hills and between them and the north shore, Mr. Bayles writes:—

      "The boulders or erratic blocks found upon the Island are mostly met with on the central range of hills and between them and the north shore. They are often contained in a stratum which is interstratified with deposits of sand, clay, and gravel, and is often exposed along the coast. Some of the blocks, when first disinterred, exhibit scratches upon one or more of their sides. Rocks of the same constituent formation are found in Rhode Island, СКАЧАТЬ