During the Palæozoic ages, the New York ridge seems to have been a land-surface; for the Silurian, Devonian, and Carboniferous rocks were deposited on both sides of it in New England, New York, and Pennsylvania, but no traces of them have been found upon it. In each of these ages the sea flowed in over some portion of the continent, and deposited on the inundated surfaces sediments containing more or less complete representatives of the prevalent forms of life; and these, now fossilized, afford means for identifying and classifying the strata.
In the Cambrian age the continent, composed of Laurentian and Huronian rocks, was broad and high, and the Cambrian strata (Acadian group) were deposited only along its margin.
At the beginning of the Silurian age the sea rose over its shores, covering most of the land-surface, but leaving the Canadian highlands, the Adirondacks, the Blue Ridge, with its New York spur, unsubmerged. Then during all the thousands of years in which the Trenton limestone group was accumulating by organic agencies, the slow growth and deposition after death of the hard parts of animals, and the other thousands of years in which the Hudson River and Utica shales were formed in a shallowing sea, this old land was exposed to wear from rain and wind, sun and frost.
In like manner when the Upper Silurian and Devonian seas in turn flooded more limited portions of the adjacent lands, covering them with new layers of sediment, the old ridges and highlands which have been enumerated, with large additions to their areas made in the Silurian age, were suffering constant abrasion and reduction of altitude.
In the Carboniferous age all the country for a great distance east, north, and west of New York, was above the sea, but along the coast in Rhode Island and Eastern Massachusetts were marshes where a luxuriant vegetation was forming peat-beds that were destined, in after-times, to become seams of coal; and in Pennsylvania, and thence westward in Ohio and Illinois, were vast tracts of swamp—half water, half land—which are now the most extensive coal-basins in the world. During all these ages the belt of highlands which separates the valley of the Hudson from that of the Connecticut was probably much higher than now, and stood as a witness of the varying phases of the unending war between land and sea, and saw the continent created and destroyed again and again; but in all these changes it took no part.
In the latter part of the Carboniferous age the Alleghanies proper were gradually elevated, the convex folds forming mountain-ridges, the depressed or synclinal arches becoming the slowly-deepening coal basins. In the end all the country between the Atlantic and the Mississippi stood as a broad and elevated continental area. Subsequently the sea rose and fell upon its margin, leaving there the record of its oscillations in the deposits of the recent geological ages, but no considerable portion of its surface has since been submerged.
The Triassic age was a stormy one in the region about New York.