Popular Science Monthly/Volume 13/October 1878/The Geological History of New York Island and Harbor
|THE GEOLOGICAL HISTORY OF NEW YORK ISLAND AND HARBOR|
I. NEW YORK in Ancient Geological Times.—The rocks which compose New York Island and underlie the adjacent country on the north and east are chiefly gneiss and mica-schist, with heavy intercalated beds of coarse-grained, dolomitic marble and thinner layers of serpentine. These are all distinctly stratified, and have once been sedimentary beds deposited horizontally—sandstones, shales, and limestones—but now, upheaved and set on edge, are by metamorphism converted into compact crystalline strata with the obliteration of all fossils—if fossils they contained. The age of these rocks has not yet been accurately determined, although they have usually been supposed to be Lower Silurian, and a continuation of those which contain the marble-beds of Western Massachusetts and Vermont. There are some reasons, however, why they should be regarded as still older. That they do not form the southern prolongation of the marble belt of Vermont is indicated by the facts that both the marble-beds and the rocks associated with them are so unlike in the two localities that they can hardly be parts of the same formation. In Vermont, the marbles occur in what is essentially a single belt, are fine-grained, usually banded and mottled, are nearly pure carbonates of lime, and the rocks immediately associated with them are gray siliceous limestones, quartzites, and slates. In Westchester County, and on New York Island, on the contrary, the marbles are very coarsely crystalline dolomites (double carbonates of lime and magnesia), which occur in a number of parallel belts, are generally of uniform white or whitish color, and have no rocks associated with them that can represent the quartzites and argillites of Vermont. On the whole, the group of strata which forms New York Island has so great a similarity to some portions of the Laurentian series in Canada that it is difficult to resist the conviction that they are of the same age.
The Canadian series is supposed to be not less than 50,000 feet in thickness, consisting of somewhat different elements in different parts, but mainly of gneiss and crystalline schists with numerous beds of dolomitic marble and serpentine, and containing, as most characteristic minerals, magnetic iron-ore and apatite (phosphate of lime). The beds stand at a high angle, and, although having once formed great folds and even mountains, by ages of surface-erosion they have been worn down to a merely undulating surface. On the east bank of the Hudson, at and above New York, we have almost precisely the same state of things, viz.: 1. A belt of crystalline rocks forming apparently a continuous series to and beyond the Connecticut line; 2. Strata set nearly vertical, once forming high hills or mountains, now worn down by long exposure to a mere rolling surface; 3. The series composed chiefly of gneiss and crystalline schists, with heavy beds of dolomitic marble and thinner bands of serpentine; and, 4. Containing in its western portion where it joins the New Jersey iron belt—with which it is inseparably connected—important beds of magnetic iron-ore, while apatite is one of the most common disseminated minerals. From these and other reasons which might be mentioned, the New York rocks are regarded by the writer as of Laurentian age. They seem to have formed a ridge which was a part of a range of highlands that ran down on the eastern side of our continent, having the same general direction with the Alleghanies, but being very much older than the more recent folds of that chain. Indeed, judging from the character of the rocks composing it, the immense amount of surface-erosion it has suffered, and the absence of overlying strata, we must regard it as one of the oldest portions of the continent.
Staten Island is in part a continuation of the New York belt of Laurentian rocks—the eastern side being composed of granite and serpentine, the western of trap and Triassic sandstone and owes its relief to that fact. South of this point the ridge sinks down and is covered
with more recent strata, but it apparently reappears at Trenton and Philadelphia. Thus it would seem to be a sort of spur of the Blue Ridge, the oldest chain of the Alleghany belt, diverging from it in Fulton County, New York, and following a nearly parallel course southwestward.
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. The trough between the New York axis and the Blue Ridge was occupied by water, and in this trough the Triassic shales and sandstones were deposited. A similar trough east of New York, where now is the valley of the Connecticut, was also a lagoon or estuary in which similar sediments accumulated, but not so quietly as the strata composing the older formations in the same region were laid down. It is evident that Nature's forces were in great activity during the period under consideration, for we find the greatest diversity in the product of these forces. The Triassic beds consist of shales, sandstones, and conglomerates. Of these the shales accumulated in comparatively clear and quiet water; and at various levels we find them filled with the remains of fishes that inhabited the lagoons where they were deposited. These fishes occur in thousands, confined to layers a few inches thick, mostly complete and mature individuals, showing that they were killed suddenly by some poisoning of the water in which they lived, its complete withdrawal, or a substitution of fresh for salt, or vice versa. These fish-bearing shales alternate with conglomerates that are sometimes beds of large bowlders—the result of violent water-action alone: a shore—or with strata of ripple-marked, sun-cracked sandstone, pitted with the impressions of rain-drops, and bearing the footprints of thousands of animals, great and small, which made these mud-banks their feeding-grounds. Here and there we find twigs of coniferous trees of the Araucarian family, or fragments of the fronds of cycads and ferns; much more frequently casts of the trunks and branches of trees mingled pell-mell, and evidently collections of drift-wood.
The footprints referred to above are generally three-toed, and resemble the tracks of birds. In dimension they vary from one to twenty inches long, and are supposed to have been made by a peculiar group of biped, birdlike reptiles, which possessed the world in Mesozoic times, and inhabited the shores of North America in great numbers during the Triassic age.
The alternations of coarse and fine strata, with their characteristic fishes and footprints, are repeated in the Trias on the west side of the Hudson until they form a series which has a thickness of several thousand feet. As the ripple-marks, sun-cracks, and other evidences of exposure to the air, occur at several levels, they prove the gradual subsidence of the trough where those sediments accumulated, with which the filling from the wash of the land kept pace, affording a succession of fresh surfaces where the winds and waves as well as living creatures left their autographs. Although as yet but partially examined and imperfectly read, these records, like the Assyrian tablets, have told us many interesting things, and they constitute a treasury of ancient lore which is destined for ages to supply new material for the geological history of this region.
From what we have already learned of the circumstances in which the Triassic rocks of our neighborhood were formed, we may conclude that a depressed area once existed between the New York ridge and the New Jersey highlands, and that this trough was an estuary swept by the tides, much like the Bay of Fundy. Here, as there, the shallows and mud-flats exposed by the ebb were places of resort for many of the animals inhabiting the district; but there is this difference, that in the lapse of time the fauna of the country has completely changed, and the fishes which inhabited the waters of the Triassic estuary, as well as the reptilian monsters that perambulated its shores, have now utterly disappeared from the face of the earth.
The fishes of the Trias, being found at various localities both in New Jersey and the Connecticut Valley, early attracted attention, and many of them were described by Mr. W. C. Redfield—for many years a leading scientist of New York. More recently large collections of them have been made by the writer, so that now they are pretty well known. They form some twenty species of four genera—all ganoids— related to the Lepidosteus and Amia of our interior lakes and rivers.
Of the molluscous life of the age in this region we, know almost nothing, since the marine deposits which contain its remains are not now above the ocean-level, and the fresh-water and estuary beds exposed to our observation have yet yielded none. Of the land-animals scarcely any traces have been found except their footprints. These prove that a motley crowd of reptiles and amphibians, some of huge size, and, according to our notions, of uncouth and hideous shapes, thronged the shores of our Triassic bay in such numbers, so swift and so well armed for attack and defense, that this must have been anything but a congenial place of residence for a peaceably-disposed citizen.
The hills which overlooked the Triassic lagoons—as they now do their exposed beds, the plains of New Jersey—were covered with forests of Araucarian pines, and the lowlands with thickets of sago palms and ferns, while gigantic scouring rushes lined the marshy shores. There were no oaks, maples, nor walnuts in the forests, and probably no flower-bearing shrubs or herbs in the undergrowth, for nearly all the fruits and flowers belong to the angiosperms and palms, neither of which had yet made their appearance on the earth's surface. Hence, the vegetation must have been sombre and uninteresting, compared with that of the present day, and, as there were no grasses in it, ill-adapted to the wants of man or the higher animals.
At the close of the Triassic age this region became the scene of great and destructive physical changes, which must have completely altered its aspect. Along the Triassic belt, both east and west, subsidences took place, or displacements by lateral pressure which tilted up all the strata until they stood at an angle with the horizon of 15°, where they still remain—those on the east dipping eastward, those of New Jersey toward the west. At the same time deep fractures reached the source of molten matter below, and this was forced up, either in dikes through vertical fissures, or in sheets between the beds of the stratified rocks. Cooling in place, these trap-sheets are now conformable to the associated strata, and seem at first sight to be normal portions of it, but the metamorphism which they have produced in the beds above and below them show that they are intrusive. The erosion which has since acted upon the surface of this region has cut away the softer sandstones and shales, leaving the outcropping edges of the trap-sheets in high-relief, and these are now known as the Palisade Range, First and Second Newark Mountains, etc.
It has been suggested that the New Jersey and Connecticut basins were once connected by strata which occupied all the interval between them, and that by the subsidence of the sides or the elevation of the central portion an arch was formed the crown of which has been removed by erosion. It seems, however, scarcely probable that some thousands of feet of Triassic rocks, including thick beds of hard and resistant trap, should have been so completely carried away from the interval of 100 miles now separating the Triassic basins, that not a trace of them should be anywhere left. There is apparently good evidence also that the trap-sheets of the Connecticut Valley issued from fissures there, and appertained to a distinct line of disturbance; and, further, that the materials composing the Triassic series in each belt were derived from the adjacent highlands, and were spread by currents which swept up and down two narrow troughs.
To some persons, the most interesting fact in regard to the Trias yet remains to be mentioned, and this is that from the quarries sunk in its sandstone-beds—of which the most important are at Bellville, New Jersey, and Portland, Connecticut—has been taken the brown-stone to which we owe the architectural beauty and monotony of the best portions of our city. Copper is also frequently found impregnating the Triassic rocks, but it has generally proved only a snare to those who have attempted to work it, the deposit being small and unreliable.
During the time in which the Jurassic rocks were deposited in other places, the Atlantic coast of North America seems to have been above the sea-level, for we find here no strata which are certainly of that age. Some writers have called Jura-Trias the beds described as Triassic on the preceding pages, but up to the present time no facts have been brought to light which justify this usage. Possibly the uppermost beds of the series may hereafter be found to contain Jurassic fossils, but none such have yet been discovered, while a number of well-known European Triassic species have been obtained from what are considered as the highest portions of the group in Virginia and North Carolina.
In the Cretaceous age the region about New York sunk below its Triassic level, and the sea came in over a belt of country which was before, and is now, dry land. The waves in their advance cut away much of the shore which opposed their progress, both rock and soil, and spread a sheet of sea-beach composed of gravel and sand as far as they reached inland. This old beach we now know as the Raritan sands, and they contain great quantities of leaves, branches, and trunks of trees, which had grown on the sinking coast. On examination, they prove to be entirely different from those contained in the Triassic rocks, consisting mainly of the remains of angiospermous plants—the highest botanical group, and such as form the prevailing vegetation of the present day. Among these Cretaceous plants we find the leaves of oaks, magnolias, and other genera now living in our forests. These prove that, in the long interval—the Jurassic age—which intervened between the Triassic and Cretaceous, the vegetation of the world had been completely revolutionized; at least, that most of the genera and species which prevailed during the Triassic age had passed away, and been superseded by such as had been before unknown.
When the water stood at a moderate depth over the sunken shore, the Amboy clays settled down upon the sandy bottom. These were apparently derived from the feldspar of the granites which compose the neighboring highlands; the quartz, unaffected by chemical action, and less finely comminuted, remaining as sand and gravel nearer its place of origin.
As the water deepened, true marine conditions supervened along the coast, and the first two of the New Jersey marl-beds were formed from the remains of animals which inhabited the sea, and such as were washed into it from the adjacent shore. The green-sand of these marl-beds is derived chiefly from the countless number of microscopic shells of the Foraminifera, which filled the waters of the Cretaceous sea here, as in many other places. Its green color is due to glauconite, a silicate of lime, potash, alumina, etc. White chalk is likewise composed chiefly of the shells of Foraminifera, but these lived in deeper water, and were of different kinds from those that produced the greensand. The fertilizing property of the marls is due to the potash and phosphorus they contain.
The marl-beds are also vast cemeteries, in which are stored the more or less perfect remains of the larger land and water animals of Cretaceous times. Among these we find the shells of Ammonites—the great coiled cephalopods—and a large number of other mollusks characteristic of the Cretaceous fauna. There have likewise been discovered in the marl-beds numerous remains of large reptiles, both herbivorous and carnivorous. Among these are Hadrosaurus and Lœlaps—the representatives of Iguanodon and Megalosaurus of the Old World—Mosasaurus, and many others. Hadrosaurus was herbivorous, while Lœlaps was a carnivore. Both were biped, terrestrial reptiles, thirty feet long, standing fifteen to twenty feet high, and of very peculiar and interesting structure. The former will be remembered as that of which the spirited restoration, made by Prof. Hawkins for the Central Park, was destroyed by the order of Judge Hilton. Mosasaurus was a snake-like, marine lizard, some sixty or seventy feet long, and of pronounced carnivorous habits. These, with their associates, probably densely populated the land and sea, while the air was the special domain of the huge flying dragons—the pterodactyls. With such a numerous and so enterprising a population, it is evident that life in this time and region was full of variety.
At the close of the Cretaceous age the animal life, both sea and land, was again revolutionized, but by causes which we cannot yet fully understand, as the physical conditions remain nearly the same, and the flora suffered little change. The facts, however, are unquestionable. All the great reptilian fauna disappeared as if by magic, and gave place to herds of mammals, numerous and large it is true, but far inferior in size and armament to their predecessors. In the sea, the whole Ammonite family disappeared at once, and other great changes took place, so that in the upper or Tertiary bed of green-sand, deposited in the same place and under nearly the same conditions as the lower and Cretaceous two, but we know not how many thousands of years after, not a single one of all the species of Cretaceous mollusks, radiates, or marine vertebrates, mingled its remains with those of the new-comers.
II. New York in the Ice Period.—The excavation of New York Harbor and the trough of the Hudson seems to have been effected in late Tertiary times. During the first portion of the Tertiary age—the Eocene—the coast from New York southward was low, and the sea washed the base of the Alleghany Mountains, covering the coast-plain and depositing upon it the uppermost and most recent of the marl-beds of New Jersey. But in the middle and later Tertiary epochs—the Miocene and Pliocene—all the northern portion of the continent stood higher above the sea than now, for we find there no marine deposits of that age; and the immense numbers of fiords, or submerged valleys which fringe the coast, are, as Dana long since pointed out, the results of subaërial erosion and proofs of elevation. A genial climate then prevailed to the Arctic Sea, and all the continent was covered with a more luxuriant flora, and inhabited by a more varied fauna, than can now be found anywhere on its surface.
This was, indeed, for America, the golden age of animals and plants, and in all respects but one—the absence of man—the country was more interesting and picturesque than now. We must imagine, therefore, that the hills and valleys about the present site of New York were covered with noble trees, and a dense undergrowth of species, for the most part different from those now living there; and that these were the homes and feeding-grounds of many kinds of quadrupeds and birds, which have long since become extinct. The broad plain which sloped gently seaward from the highlands must have been covered with a subtropical forest of giant trees and tangled vines teeming with animal life. This state of things doubtless continued through many thousands of years, but ultimately a change came over the fair face of Nature more complete and terrible than we have language to describe. From causes which are not yet fully understood, and into the discussion of which we cannot here enter, the climate of the northern hemisphere became gradually more severe, and that of Greenland, from being what it had been for ages, like that of our Southern States, became arctic as we now find it, and its luxuriant forests were replaced by fields of snow and ice. But the change did not stop here, for with increasing cold the ice-sheets spread southward and covered successively the mountains of Labrador, the Canadian highlands, and the hills of New England and New York. At the culmination of the Glacial period the ice reached as far south as Staten Island and Trenton, and all the country north of this line was buried under a great moving mass of ice, in places several thousand feet in thickness. At this time the present climate of Greenland had been transferred to New York; in the strongest possible contrast to that earlier time when the present climate of New York prevailed in Greenland. In the advent of the Ice period not only were all kinds of animals and plants exterminated or driven southward, and thus what had been a paradise was converted into a howling wilderness, but even the topography of the country was greatly modified. The ice-sheet moving from the north ground down or rounded over all projecting rock-masses, and filled up valleys with the débris, producing great abrasion in some places, and accumulations in others, until the whole face of the country was changed. In the vicinity of New York the ice moved from north-northwest to south-southeast, and was of such thickness that it crossed the trough of the Hudson diagonally, and probably, because this had been filled with transported material, was by it little deflected from its course. In other localities the old river-valleys were sometimes completely obliterated, and the drainage of the surface given new channels and even new direction. To this cause we may attribute the blocking up of the old line of drainage from the lake-basin through the Hudson, and its diversion to the present course of the St. Lawrence.
Now that the glaciers have left this region and have retreated again to the far north, we everywhere see evidence of the stupendous changes they wrought in the country over which they moved. North of the line which marks the margin of the glacier, we find the contour-lines rounded over and softened, ridges of granite converted into domes, and the hardest rocks grooved and striated, or ground smooth and even polished. The whole surface of New York Island, where the rock is exposed, shows marks of glacial action, the upturned edges of gneiss being ground off to form a nearly plane surface, or, where ridges of more massive rock had existed, these are rounded over to form roches moutonnées. Fine examples of the latter may be seen in Central Park and on the east side of the island near Harlem.
The material which occupies so much of the troughs of the Hudson and East River is mostly glacial drift, clay, gravel, sand, and bowlders, scraped from the highlands by the great ice-sheet into these preglacial gorges. It is probable they were once filled to the brim, and that they were subsequently reëxcavated in part by the floods of water which resulted from the melting ice. After these ceased, and they were occupied by water standing at its present level, and moved only by tidal action, they were more or less silted up by the deposit of fine mud brought down by the larger and smaller streams, here checked in their
flow and losing their transporting power. The southern and lower portion of New York Island, which was under the lee of the higher, was covered with deposits left by the retreating glacier, and these were never afterward entirely removed. Here are now beds of sand and gravel which have in places been penetrated to the depth of one hundred feet or more. On the higher parts of the island and the adjacent country, the rock is generally bare or covered with soil, but even here depressions are filled with bowlders, clay, or gravel, often to the depth of several feet, and large transported bowlders are everywhere scattered over the surface. These latter have sometimes been derived from the rocks of the island, but most of them seem to have come from distant points, and always from the north and west. Rounded masses of trap are very common among the bowlders, and these have been brought across the Hudson, for there is no trap in place on the east side of the river. The trap-ledge which forms the summit of the Palisades is everywhere worn and scratched by glacial action, and the markings which it bears are generally concordant in direction with those of the rocks of New York Island and Westchester County, viz., about north-northwest and south-southeast. Even on the river-face of the hills which form the east bank of the Hudson, the bearing of the glacial scratches is essentially the same, showing that the movement of the great ice-sheet was little affected by any such trifling irregularity of the surface beneath it as the Hudson Valley.
We have no measure of the amount of erosion which New York Island and the adjacent country suffered during the Ice period, but it is not improbable that a mass a hundred feet in thickness was taken from the surface of all the region occupied by the ice.
Most of the finer material ground up by the glaciers was washed out to sea and deposited as the "Champlain clays." Of these there is very little showing in the vicinity of New York, since none of the coast from this point southward has been raised to display them; but a great continental elevation has since taken place toward the north, bringing them at Croton Point 100 feet, at Albany 250, at Burlington 400, at Montreal 500, at Labrador 800, at Davis Straits 1,000, and at Polaris Bay 1,800 feet above the present sea-level.
The coarser portion of the grist ground by the glacier remains as beds of gravel and sand, or heaps of bowlders scattered over the surface of the country where they were left as local moraines, or as the gravel bars of streams flowing beneath the glacier. The greatest accumulation of material transported by the ice in all the country about New York is seen on Long Island, which is indeed a great terminal moraine heaped up along the margin of the continental glacier. As is generally known, Long Island is mostly composed of heaps of gravel and sand, which sometimes form hills from 200 to 300 feet in height, and in these no solid rock has been found in any exploration yet made. The formation of this huge gravel-bank seems to have been, in brief, as follows: The great ice-sheet, moving down from the north in Connecticut and Southern New York, passed over a region occupied mostly by hard, crystalline rocks. These were extensively worn away by it, and much of the material taken from the surface was pushed on as by a great scraper to its margin. When the ice-sheet reached the line of Long Island Sound, it passed from the area of upturned crystalline rocks on to the comparatively soft horizontal Tertiary and Cretaceous strata, which here formed a plain stretching seaward, from the highlands, just as they now do in New Jersey and more southern States. These were scooped out to form the basin of Long Island Sound, and the material excavated from it, as well as much brought from the country lying farther north, was banked up between this basin and the ocean. Thus it will be seen that, of the water-connections of New York Harbor, Long Island Sound is much the most modern; and yet, as a part of it occupies the site of the valley of a large stream—the Housatonic, with perhaps the Connecticut—which passed through the Hell Gate gorge, its formation must have been begun in preglacial times.
As has been said, the rock foundations of Long Island are almost entirely concealed, but a number of cases are reported of the penetration in wells of strata containing Cretaceous fossils, and there is little doubt that the Cretaceous series of New Jersey and Staten Island, represented by the Raritan sands, and the Amboy, Keyport, and Staten Island clays, once formed a continuous margin to the continent, all the way around to Nantucket. These strata still probably underlie a large part of Long Island where they have been protected from erosion by the heavy beds of drift that cover them, while the shore-waves have eaten away all exposed portions. Evidence strongly confirmatory of the view that Cretaceous rocks have been scooped out of the basin of Long Island Sound is afforded by the fact that the drift of Long Island contains in immense numbers imperfectly-rounded blocks of a reddish-brown sandstone, filled with the impressions of dicotyledonous leaves—a rock nowhere yet found in place, but one which is probably the representative of the leaf-bearing Cretaceous sandstone of the Raritan River.
Whether the overlying Tertiary beds will be found on Long Island is perhaps doubtful, since they are not conterminous with the Cretaceous; but, from the fact that an outlier of this formation exists at Gay Head, Martha's Vineyard, it is highly probable that it was once continuous from Southern New Jersey.
On the preceding pages the history of the vicinity of New York has been traced backward for some millions of years. This history has been read from rock-graven records, which, although meagre and mutilated, give the generalities of the narrative with a truth and fidelity which shame all human history. It would be a pleasant duty to predict the future of this region, even in the same degree of fullness; but the future is as unknown to the geologist as to others. He learns, however, from his studies, that what we call terra firma is a type of instability, and that there is nothing stable but the law of change; and he can prophesy with confidence that in the distant future the history of the distant past will be, in part at least, repeated. Even now changes are in progress which, if they should continue a few thousand years, would very profoundly affect not only the aspects of this region, but its adaptability to human occupation. A number of facts indicate that the coast of New Jersey and Long Island is gradually sinking. From the marshes of New Jersey are taken the trunks of trees which could not have grown there except when it was drier ground, and on the shore stumps are seen, now under water, of trees which must have grown on land. So, too, the sea throws up in storms portions of turfy soil, once covered only by the air, and similar soil has been reached below the sea-level in pits dug through drifted sand along its margin. It is also said that the land boundaries have been changed and farms diminished even where the wash of the shore-waves produced no effect. The rate of this subsidence is very slow—only a few inches in a century—and it may at any time be arrested or reversed; but, should it continue, as it may, for some thousands of years, it would result in a submergence of land now valued at hundreds of millions of dollars, and a complete change of position in the seats of commerce and industry, which must always centre about this harbor. This possible catastrophe is, however, so uncertain and remote that it seems hardly sufficient to disturb the equanimity of at least this generation of inhabitants.
III. Why New York is the Commercial Metropolis op the United States.—The great commercial advantages of the site of the city of New York attracted the attention of the first voyagers who came to these shores. When Hendrick Hudson, passing through the Narrows, found within a commodious, landlocked harbor, and a broad and beautiful river, which floated his ships in safety more than a hundred miles into the interior of the continent, he clearly foresaw, and predicted, that this would be the great entrepot of foreign trade for the New World. The subsequent history of New York has fully demonstrated the advantages of its position, since a population of more than 2,000,000 has gathered immediately around its harbor, and it has become not only the business metropolis of a great nation, but the second in importance of the markets of the world. Those who have witnessed and shared the progress and prosperity of the city have been generally well satisfied to enjoy these, without any special inquiry into the causes which have produced them; and, indeed, it is not unlikely that they have accepted them as simply the fruit of their own intelligence and energy. It is doubtful, however, whether the merchants of New York have been more shrewd and enterprising than those of the other ports on our coast. It is not flattering to the vanity of men to assert that they are what their surroundings make them, but it is nevertheless in a great measure true, and New-Yorkers are probably no exception to the rule. The real secret of the unparalleled growth of New York lies in the peculiar topography of its vicinity.
The city is set on an island, of which the shore on every side is swept by tide-water. On the west it is bounded by the Hudson—river we call it, but really an arm of the sea—in which the ebb and flow of the tide are perceptible as far as Troy, one hundred and fifty miles from its mouth. On the east the island is encircled by tideways called Spuyten Duyvel Creek, Harlem River, and East River, the latter a deep channel which connects New York Harbor with Long Island Sound, and thus affords an important artery of internal commerce, and another outlet to the ocean. These two great natural canals, the Hudson and East Rivers, embracing the long and narrow island between them, unite in New York Harbor, one of the most beautiful and commodious in the world. Seen from the city, it seems to be completely landlocked, but communicates with the ocean through the Narrows, with Newark Bay through the Kill van Kull, and thence by Arthur's Kill with Raritan Bay.
Thus it will be seen that New York Harbor is the centre of a series of navigable tideways which add greatly to its adaptation to the wants of commerce, and constitute the most peculiar physical features in its surroundings. The little map given on the next page will show the connection of this system of water-ways more distinctly than any verbal description can.
To those who have not made topography a study, the interest and mystery of the origin of the navigable channels leading into New York Harbor will not be apparent, and it may seem an easy explanation to assume that they have been formed by the ebb and flow of the tide which sweeps through them. The tides at New York, however, do not rise to a great height, and have very little eroding power. It should also be said that the channels are far too deep to have been cut by any agents now in operation. For instance, at Polhemus's Dock the depth of Hell
Gate channel is 170 feet, and there are many places in the East River where the depth is over 100 feet. The greatest depth of water in New York Harbor and the Hudson River is about sixty feet, but this does not represent the true depth of the channels, since they have been very much silted up, and their rock-bottoms are probably 200 or 300 feet below the water-surface. If they could be cleared of clay, sand, and gravel, they would be seen to be narrow gorges cut in solid rock as deep as that of Niagara, and resembling some of the canons of the Western rivers. It is therefore certain that they could not have been produced by tidal action. There are only two ways in which such chasms could be formed: first, by earthquakes, opening fissures in the rocks; and, second, by the erosion of flowing streams. That they are not earthquake fractures is certain, since no such fissures are found in the country about in the line of these channels, and their rocky walls show no sign of disturbance, being similar on opposite sides, and doubtless continuous below. They have, in fact, been formed by draining streams when this part of the continent stood much higher than now above the ocean-level. The evidence of this is cumulative and conclusive. The facts which prove it are, briefly, as follows:
1. The trough of the Hudson has been shown, by the soundings of the Coast Survey, to be distinctly marked upon the sea-bottom out to a point some eighty miles southeast of New York, and where the water is now over 500 feet deep. Here we reach the true margin of the continent, where the shore plunges rapidly down into the depths of the ocean; and here was for ages the mouth of the Hudson River; for the channel which leads to it could not possibly have been excavated except upon a land-surface.
2. Explorations made over a large part of the territory lying between the Atlantic and the Mississippi show that many of the draining streams are now flowing far above their ancient beds, and that these sometimes lie below the present ocean-level. For example, the Ohio flows in a valley the bottom of which is occupied by sand and gravel at least 100 feet thick. The rock-bottoms of the streams which empty into the Great Lakes are at their mouths sometimes 200 feet below the water-level. The Mohawk Valley is filled to a great depth with loose materials, the surface of which forms for long distances a nearly level plain, through which the present river meanders.
Innumerable instances of this kind could be cited, all of which go to prove that for ages the eastern half, at least, of this continent stood 500 to 600 feet higher above the ocean than now, and that during this time the draining streams with swiftly-flowing currents cut the surface into a network of deep channels not unlike the canons of some of the rivers of the far West.
There seems to be good reasons for believing also that in this period of elevation the stream which drains the basin of the Great Lakes, called in different parts of its course the St. Mary's, the Detroit, the Niagara, and the St. Lawrence, flowed not through the modern channel, which passes the Thousand Islands and the Lachine Rapids, but, leaving the basin of Lake Ontario at its southeastern corner, traversed the now deeply-buried channel of the Mohawk, and entered the present valley of the Hudson somewhere near Albany—precisely where has not yet been determined, as heavy beds of drift cover and conceal its course for many miles in that vicinity. From Albany this ancient Hudson River flowed through a deeper and wilder valley than the present one, which is half filled with water, passed what is now New York Island, far below the present water-surface, was joined at the Battery by a large tributary from the east, issued from the highlands by a picturesque gate at the Narrows, and, traversing a littoral plain, emptied into the ocean eighty miles southeast from New York.
The limits of this article will not permit the presentation of all the facts which sustain this view, but a few of them will suffice to show that it hardly admits of doubt. These are briefly as follows:
An ancient connected line of drainage passes through the basin of the Great Lakes at least 200 feet below the present water-surface, deepening eastward, and reaching a level much below that of the bed of the St. Lawrence. An old channel at least 200 feet deep connects Lake Huron and Lake Erie. Detroit is situated on the western side of it, and the rock lies there 130 feet below the surface.
Many of the streams now flowing into Lake Erie were once tributaries to the ancient river which traversed its valley and joined it far below the present water-level.
The old channel connecting Lake Erie and Lake Ontario apparently passed through Canada between Long Point and Hamilton. Heavy beds of drift, by which it is filled and concealed, here occupy the surface. The Niagara now flows over a rock-bed, for this is a comparatively modern river, which, following the line of lowest surface-levels, passed over a spur from the south shore of the lake-basin when the old channel was filled by glacial drift.
Some of the streams draining into the basin of Lake Ontario in former times cut their channels below the present ocean-level. All the salt-wells of Syracuse are sunk in one of these, which is filled with gravel and sand saturated with brine issuing from the Salina group that forms its walls. The rock-bottom of this old river-bed was reached in some of these wells at a depth of fifty feet below the present level of tide-water.
The valley of the Mohawk is a very deep channel of erosion, now half filled, which must have been traversed by a large stream flowing eastward at a level below that of the present ocean; and everything indicates that this was the ancient outlet of the basin of the Great Lakes.
The channel of the Hudson is apparently the only possible continuation of this long line of drainage. As has been remarked, it is of great and yet unknown depth. The clay by which it is partially filled has been penetrated to a depth of about 100 feet along its margins. How deep it is in the middle portion can only be conjectured; but Hell Gate channel, which has been kept comparatively free by the force of the tides, is in places known to be nearly 200 feet deep; and, since this is a channel of erosion formed by a stream draining into the Hudson, the ancient bed of the Hudson must be still lower.
From the depth and distinctness of the old river-course on the submerged plain outside of the Narrows, we may reasonably infer that the old channel at New York is not much less than 300 feet deep.
We are compelled to conclude from these and other facts of similar import: 1. That the topographical features of the vicinity of New York were for the most part fashioned by the erosion of a system of watercourses which, in preglacial times, when the continent was higher than now, cut their valleys much deeper than would now be possible.
2. That there was here a group of hills composed of crystalline rocks, a sort of spur from the Alleghany belt, and that this range of hills was then seventy or eighty miles inland from the ocean, separated from it by a plain similar in its topographical relations to that which lies between the highlands of our Southern States and the present shore of the Atlantic.
3. At the period under consideration a river draining the basin of the Great Lakes, and in size the second on the continent, followed the course of the Mohawk and Hudson, and, passing through the New York hills, there left the highlands and flowed quietly on to the ocean.
4. Where New York Harbor now is, this great river received two important tributaries—one from the east through Hell Gate channel, which joined it at the Battery, the other from the west through the gorge of the Kill van Kull. Of these, the first is now represented by the Housatonic, then a larger stream, with a longer course and more tributaries; the second was formed by the Passaic and Hackensack, which united at the head of what is now Newark Bay, arid emptied into the Hudson at the entrance to the Narrows. The junction of these two considerable branches so near each other seems to have produced the expansion of the valley which is now New York Harbor. This must then have been a very picturesque spot, as its outlet oceanward was a narrow pass bordered by the hills of Staten and Long Island, 500 feet in height. On the north, it was overlooked on one hand by the great wall of the Palisades, which rose 700 feet above the river; on the other, by a bold shoulder or headland, 400 feet in height, now New York Island, then a promontory, which separated the Housatonic and the Hudson to their junction at its southern extremity.
5. After the lapse of unnumbered ages, during which this nook among the hills was slowly prepared for the important part it was to play in the history of the yet unborn being—man—a quiet subsidence of the land or elevation of the water began in this region. Gradually the sea flowed in over its shores, crept up the valleys of the streams, checking their flow and converting them into tideways, until it washed the base of the highlands. Up to this time the surface of the littoral plain in its gradual submergence formed a broad expanse of shallow
water bounded by a monotonous line of beach, with no good harbors—a shifting, dangerous shore, such as is most dreaded by mariners. By further subsidence, however, the water flowed up into the valleys among the New York hills and into the deeper river-channels, making of the first safe, landlocked harbors, of the second navigable inlets or tide-ways. In this manner were produced the magnificent harbor and the system of natural canals connected with it, which determined the position and created the subsequent prosperity of the commercial emporium of the New World.
The subsidence which resulted in the formation of New York Harbor and its connections seems also to have affected all the coast, and the influx of the sea-water filled the valleys of the rivers which drained the Atlantic slope south of New York, and gave it the fringed and irregular outline which constitutes its most striking characteristic. James River, York River, the Potomac, the Susquehanna, and the Delaware, are, like the Hudson, half-drowned rivers—if we may use the expression—for all the lower portions of their valleys are estuaries, in which the tide sets up to the base of the highlands. But that portion of the littoral plain which separates these estuaries is too low, too much cut up with water-ways, and its harbors are too shallow and ill-defined, to afford proper sites for great shipping-ports. Hence the cities of this region—Richmond, Washington, Baltimore, and Philadelphia—are situated at the head of navigation, where the rivers come down from the highlands on to the plain; and they are located like Albany, remote from the seaboard, with which they are connected by long and somewhat tortuous channels of inland navigation. New York, on the contrary, is located directly on the coast, because here alone the highlands reach to the sea, and their submerged valleys and river-channels form commodious, rock-girt harbors, immediately accessible from the ocean. It will be seen at a glance that this fact gives it great commercial advantages, and has been the most potent influence in making this the chief port of entry for the country.
On the Southern coast there is no harbor at all comparable with that of New York except Norfolk. This is deep, roomy, and accessible to the sea—advantages which are destined to give it permanent and increasing importance. But it is less central to the population and business of the country; and, while its inland water connections through Chesapeake Bay and the tidal rivers which open into it are more extensive than those which the harbor of New York possesses, they are less favorably situated in their relations to the present and future internal commerce of the country.
The great advantage which New York enjoys for trade with the interior consists in its accessibility from the basin of the Great Lakes where the most rapid accumulation of population and wealth of the last half-century has taken place, and where the business of the country is destined to concentrate in the future. As has been stated in the preceding pages, the drainage of the lake-basin apparently flowed for ages by New York; and it is an interesting fact that the great tide of population and business which has set in from the Eastern States toward the interior has chiefly passed through the gap cut in the highlands by the old river whose course we have endeavored to trace. The topographical features of this pass led to the construction of the Erie Canal, and it was comparatively easy to reëstablish there the old line of water communication. In later years the same influences caused the construction through it of the most important railroad line of the world. The natural advantages of this route are such as to give New York and her connections with the interior a positive and inalienable superiority over all competing ports and lines of traffic—a superiority which, though it may be temporarily abrogated by municipal misgovernment, or be diverted from public to private profit by individual or corporate rapacity, will ultimately and always assert itself, and give to this city a continuance of the prosperity that has attended her past career.