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 writer[4] 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, Connecticut, and along the Hudson River. And those of the Island, in their variations, correspond so accurately with the rocks of the localities mentioned that it seems probable that they came from those localities. For example, the boulders on the east end are like the granite, gneiss, mica slate, green-stone, and sienite of Rhode Island and the east part of Connecticut; opposite New London and the mouth of the Connecticut River are boulders like the granites, gneiss, and hornblende rock of those localities; opposite New Haven, are found the red sandstone and conglomerate, fissile and micaceous red sandstone, trap conglomerate, compact trap, amygdaloid and verd antique; opposite Black Rock are the granites, gneiss, hornblende, quartz, and white lime-stone, like those in Fairfield County; and from Huntington to Brooklyn, hornblende, crystalline lime-stone, trap, red sandstone, gneiss, and granite, are the same in appearance as those found in the vicinity of the Hudson River."
The earliest historical description of Long Island, in Daniel Denton's "A Brief Description of New York, formerly called New Amsterdam," published in London in 1670, remarks that "the greatest part of the Island is very full of timber, as Oaks, white and red, Walnut-trees, Chestnut-trees, which yield stores of Mast, etc." The same record says:
"For wild beast there is Deer, Bear, Wolves, Foxes, Raccoons, Otters, Musquashes, and Skunks. Wild fowl there is a great store of, as Turkeys, Heath-hens, Quails, Partridges, Pigeons, Cranes, Geese of several sorts, Brants, Widgeons, Teal, and divers others. Upon the south side of Long Island in the winter lie store of Whales and Grampusses, which the inhabitants begin with small boats to make a trade, catching to their no small benefit. Also, an innumerable multitude of seals, which make an excellent oyle; they lie all the winter upon some broken Marshes and Beaches or bars of sand before mentioned, and might be easily got were there some skilful men would undertake it."