Popular Science Monthly/Volume 10/February 1877/Ups and Downs of the Long Island Coast
By E. LEWIS, Jr.
"Daily it is forced home on the mind of the geologist that nothing, not even the wind that blows, is so unstable as the crust of this earth."—Darwin, in 1835.
OBSERVATIONS made around the shores of Long Island justify the conclusion that they have undergone important changes in time geologically recent. These changes appear to have arisen from a series of vertical movements, by which the coast has been alternately elevated and depressed.
In consequence of these movements the shore-line of the island has advanced and again receded, perhaps, in repeated instances: being at one time, upon the ocean-side, from fifty to seventy miles southward of where the waves now break; while at another period the highest hills of the island were largely if not wholly submerged. The persistence and extent of these movements are interesting and important questions in geology. We do not know at present how great the oscillations of the coast may have been, but enough is obvious, in the records they have left in the contour and structure of the island, to show that they have been much greater than is indicated on the adjacent mainland of Southern New England. We shall endeavor to follow these records, obscure and perplexing though they sometimes are, back to the period in which Long Island may be said to have had its origin—a period which witnessed the approach and presence of a great ice-sheet upon this coast.
It is not questioned, we believe, that Long Island is a terminal glacial moraine, and that the material of which it is composed is the débris of regions over which the ice moved in its progress toward the sea. Its underlying portions are beds of laminated sands and clays which have been referred to periods antecedent to the advent of the ice, and which constitute in one sense a part of the island. Its great mass, however, overlies these beds, and presents two general forms of structure. One is known as the "unmodified bowlder-drift," in which there are no stratified beds; the other is the "modified drift," or that in which the material has been distributed in layers chiefly by the action of waves. Much of the hill-region of the island presents the peculiar pell-mell structure of the one—the stratified gravels and sands of Southern Long Island are typical of the other. These differences in structure, and other facts to be mentioned, imply great changes in the relative level of land and sea upon the coast.
In considering these movements of oscillation it will be convenient to notice the latest first, and others in their order. A persistent invasion of the ocean upon the shores of the island has taken place 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 William Nicol, Esq., remains of a forest, portions of which are still flourishing on the adjacent uplands. These, Mr. Nicol writes, "are of oak, and are from twelve to twenty-four inches in diameter." Similar ones, he is informed, occur on the north side of the beach close to the ocean, which are covered by three feet of water at low tide. Eastward from this point the bay is broad and shallow for upward of twenty miles. The depth of water in it is from three to eight feet, with from two to three feet more in some parts of the channel. A tradition of the early settlers, which appears to have been received from the aborigines, is, that the whole area was once a fresh-water swamp, portions of which were so nearly dry at certain seasons of the year, that the Indians passed over it dry-footed to the beach.
A hundred and fifty years ago the bottom of this bay was covered in many places with remains of swamp vegetation, and stumps of trees, to the "great annoyance and astonishment of fishermen."
It is probable that this section of the bay was at one time a swamp or series of swamps like many now found on the contiguous uplands, and sufficiently above the level of the sea to admit of their free drainage into it, for it is certain that they were supplied by the same copious streams from the island which now empty into the bay.
The character of these swamps changed when the tides overflowed them. That the bay is comparatively modern is suggested by the fact that no great mounds of shells occur near it, such as were left by the aborigines along other parts of the coast. Yet it is certain that the country was thickly settled by Indians. Mr. Nicol writes, "There are fields known as old Indian fields which abound in shells, but they nowhere take the form of mounds."
A few miles eastward is the beautiful but shallow sheet of water known as Tiana Bay. It fills a depression in the almost level sands along this part of the coast, and is upon the site of a pine-forest. W. S. Pelletreau, Esq., of Southampton, informed us that he saw in it about three hundred stumps covered at low tide. They are of the same species of pine which now grows on the adjacent uplands.
In Peconic Bay, which divides the eastern part of Long Island into two very long necks of land, the submergence' of the shores has been extensive. Mr. E. F. Squires, of Riverhead, noticed not only areas of swamp, but of former cedar-forests, now permanently overflowed by the tides. One point is known as "Stump Landing."
On the north shore of the island are several tracts of "sunken meadow," over which the water at low tide is from ten to fifteen feet deep. These dead and submerged meadows are but little decayed, and are usually continuous with those now growing upon the shores.
On the flat shores of the south side of the island the encroachment of meadow upon the uplands is attended with interesting results. It forms first in depressions where the tides overflow. In this way knolls of upland, cultivated or perhaps covered with trees, become islands, which are in turn overflowed and covered by meadow, but some of the more elevated ones remain almost at the verge of the ocean. Barnum's, formerly Hog Island, in East Rockaway Bay, now being converted into an asylum for the paupers of Queen's County, is one of these.
It is well known that the beach on which the ocean breaks is gradually thrown inland upon the meadows. By this means old meadows are sometimes laid bare. It is stated, in Furman's "Antiquities of Long Island," that when Jones's Inlet was opened through the beach during a storm, it was found that the "bottom, laid bare, was solid meadow, in which were tracks of cattle, or of cloven-footed beasts."
The old meadow-bottoms, and sometimes masses of the tangled roots of upland vegetation, are torn up by waves during storms and thrown upon the beach. We have seen this turfy matter lying like windrows along the surf. Mr. Pelletreau informs us that just opposite the east end of Shinecock Bay "there was washed out by waves a large quantity of what is called meadow-bottom, partially decomposed vegetable matter, remains of fresh-water plants. . . . A few years since a violent storm washed away the sea-beach near Southampton, exposing at low tide, nearly at the brink of the ocean, a row of fence-posts that were put down by the first settlers." From these and other facts this careful observer concludes that the ocean, in that vicinity, has encroached upon the land about half a mile in two hundred years.
At Montauk Point, north of the lighthouse, is a low, swampy place, over which the tides sometimes rise. We are informed by Mr. J. F. Gould, who was for many years keeper of the lighthouse, that stumps are laid bare in front of this swamp, at the sea-margin, when the tide is extremely low. A similar phenomenon occurs at the extreme westerly end of the island. A few rods south of Fort Hamilton, at the entrance of New York Harbor, are the well-known Dyker Meadows (Fig. 1). They occupy the site of a swamp which is filled with the remains of upland and fresh-water vegetation.
The swamp was originally about a mile long, and was in one of the valley-shaped depressions common on the surface of the Long Island drift. It lost its character as a swamp by encroachment of the tides upon it, and was finally converted into a salt-marsh. This marsh abounds in stumps; a great number of large size, some of them three feet in diameter, have been seen by the writer at the verge of low water, and we have found them many rods from the shore where the water was ten feet deep. This area was certainly a portion of the original swamp when the land was sufficiently elevated to lift it above the level of the sea.
It is not necessary to further illustrate the present subsidence of this coast, but evidence of the extent of the movement will be of interest.
Fig. 1.—Section through the Dyker Meadows.
Horizontal scale, four inches to the mile; vertical scale, twenty feet to the inch.
In constructing the Erie Basin at Brooklyn in New York Harbor, Mr. George B. Brainerd, engineer, found the following series of deposits, the water being ten feet deep at low tide: Three and a half feet of mud, sand, etc.; ten feet compact peaty meadow. This gives twenty-three and a half feet of depression since the bottom of that meadow was the surface, and covered with vegetation at the level of the sea.
In 1867 John Nadir, Esq., United States Engineer at Fort Hamilton, carefully examined, by boring, the underlying formation around Fort Lafayette. The earth was penetrated to a depth of 53 feet at points between 800 and 1,000 feet from the shore, where the water at low tide was ten feet deep. The deposits passed through were as follows: twenty feet coarse sand and gravel, with few broken shells; three feet decayed meadow, with shells and, Diatomaceæ; seventeen feet gravel and sand, with broken shells; thirteen feet mud, quite compact, which appears to have been a marsh with scanty vegetation, and shells. This indicates a subsidence of the coast of at least sixty-three feet, or, in other words, the land of the coast was that number of feet higher than it now is, when the subsidence began. But there is reason to conclude that the elevation was much greater than sixty-three feet. If we take a step backward in the order of events, we find that, immediately previous to the elevation mentioned, there occurred a great depression of the coast. Possibly the highest hills of the island were carried under water. The evidences of this depression are found in the numerous beds of stratified sand and gravel—elevated beaches and other shore-formations—which lie along the central ridge of hills, and fronting the ocean from 100 to 260 or more feet above the level of the sea. At whatever heights these deposits occur, they suggest, if they do not prove, submergence of the coast to that extent.
From observations, made by the writer and others, it is ascertained that the summit of Hempstead Harbor Hill, which is 384 feet above tide, and the highest land upon Long Island, is a mass of stratified sand and gravel. The same is true of Janes Hill, in the West Hill group, said to be 383 feet high, and of Osborn's Hill, southwest of Riverhead, the height of which, according the United States Coast Survey, is 293 feet. In these instances, and in others similar, the layers are distinct and well defined. The stratification of this material was evidently the work of waves, but, whether of the ocean, or of a glacial lake or sea, admits of doubt. At present we cannot determine what the extent, contour, or elevation of the surface around these dome-like hills may have been; nor can we tell the original extent of the beds of assorted material, remains of which now cap the hills. That the denudation has been immense, is everywhere evident.
From the summits of the hills mentioned one overlooks southward a vast plain which extends to the ocean, ten miles distant; and the conclusion seems irresistible that every rood of that distance has been the shore-line of first an invading, afterward of a receding ocean, and the scene of those great coast-changes which waves produce. We may restore, in imagination, the hills of glacial rubbish crumbling before the stroke of waves, as during an immense period of time the subsidence of the land went on. So complete was the work of disintegration, that scarcely a bowlder remains in the low tracts fronting the ocean, but are numerous along the margin of the hills, and abound in the undisturbed drift which constitutes much of the hill-region.
The period of subsidence we are considering is referred to the "Champlain" of the geologists, so called by Prof. C. H. Hitchcock from the abundance of its peculiar deposits near that lake. It is a marked one in geological history. During its progress the deposits which the glaciers had left were altered in their contour, and redistributed by waters which encroached upon and finally covered them. The material thus redistributed would be left in layers, chiefly of gravels, sands, and clayey sands, if upon the ocean-margin, as is the case in Long Island.
In sinking wells into the beds formed during this period of subsidence, wood and shells have been found in a great number of instances. The wood sometimes occurs in logs of large size; oak and pine have been identified. We have a record of sixteen instances where wood has recently been found, and many others are mentioned by Thompson in his history, and by Mather in his excellent report on the geology of the island. These facts seem to imply that forests were upon the adjacent lands, which was not the case during the presence of glaciers upon the coast.
The shells found are at various depths—in one case at Gravesend 100 feet below the surface—and occur in all parts of the island where the stratified drift abounds. They have been found at Flatbush, Prospect Park, Bath, East New York, Farmingdale, Amagansett, and elsewhere.
The beds in which they occur are not of the low plains only, but many feet above tide. At Manhassett, as we are informed by John M. Clark, Esq., of that place, a well was dug, and at several feet below the surface-rubbish a layer of what appeared to be "creek-mud" was found, in which were a great number of shells of the oyster, clam, and scallop, many of them unbroken. The layer was about five feet thick, and throughout contained not only shells, but leaves, pinecones, also wood of pine and other species. This interesting deposit is about 200 feet above tide; but the contour of the present surface indicates plainly enough that an arm of a bay (Little Neck Bay) contiguous extended over this area when the land was sufficiently submerged to admit of it.
It is obvious that only the portion of Long Island which is more than 200 feet above tide was at that time dry land.
But the subsidence was greater than is indicated by this elevated deposit. The peculiar beds of stratified sands and gravels on the low plains already referred to, and which prove the former presence of the sea, are found at elevations of from 200 to 260 feet along the margins of the hills, and against or upon the unmodified bowlder-drift (Fig. 2). Beach-sands occur at 230 feet elevation, having the well-known structure of such beds. We have, too, the further fact, already noticed, that the tops of some of the highest hills of our island are composed of stratified gravel and sand.
Without insisting further on, this fact, however, we think a movement of subsidence is shown thus far of at least 260 feet; but facts of a most interesting and important character now being brought to light show that this is but part of the great movement of depression of the period we are considering. An artesian well is being bored on Barnum's Island, already mentioned, within two miles of the ocean. The island is but a knoll of tillable upland, surrounded by meadows and the waters of the bay.
The boring has reached a depth of 368 feet, or about 358 feet below
Fig. 2.—Section from Hills at Centre of the Island, near Wheatly, to the Ocean, showing Stratified Beds overlying the Unmodified Drift.
Vertical section, one inch to five hundred feet. Horizontal section, one inch to three miles.
tide-level, and is still in progress. We present below a statement of the series of deposits penetrated. Our table is prepared from the record of Theodore F. Carman, Esq., of Hempstead, Long Island, engineer of the work, and from eighty specimens of the layers passed through, furnished by that gentleman:
Our grouping of the deposits may suggest transitions more sharp than the specimens warrant. About 70 feet of the surface is of yellow or orange colored sand and gravel. Then occurs a bed of clay 56 feet thick, on the surface of which was found decayed vegetable matter having a strong odor of carbolic acid. Wood and lignite occurred in this bed. Beneath was found a layer of coarse gravel and sand.
None of the layers penetrated are of the unmodified drift. No shells have been found, nor other remains of animal life, excepting a very small fragment of a crinoidal stem which occurred in a thick bed of coarse silicious sand 160 feet below the surface. This specimen, which is much water-worn, Prof. Dana suggests may be of cretaceous species, but it affords little or no evidence that the deposit in which it was found is of Cretaceous age. The Silurian and Devonian fossils, which occur frequently in the drift of Long Island, cannot be considered as proof that the deposits are Silurian or Devonian.
With the specimens before us, we think these surface-beds, to a depth of about 180 feet, are post-glacial, and are formed from glacial drift.. Below this depth the beds are of dark clay and clayey silt-like sand, alternating with deposits of sand similar to that of the beaches upon the coast. Lignite occurs throughout, and a layer of it at 353 feet was penetrated with difficulty by the implements employed in boring the well.
The lignite found throws little light on the age of the beds. It is brought to the surface in small pieces, and that from the surface of the clay bed at 353 feet was formed from small stems of exogenous structure. The same is true of that found on the bed at 70 feet. This deposit of clay, 56 feet in thickness, seems closely analogous to many clays now upon, and at various depths beneath, the surface of the island. It is evidently a local deposit, such as might occur in a depression of the surface. Two tube-wells have been driven at no great distance from Barnum's Island, one 97, the other 194 feet, in which no similar layer of clay was detected. No green sand or marl deposits have been found. It seems probable that the beds below a depth of 180 feet were formed in tranquil waters of no great depth, possibly in an estuary sheltered by a beach of sand from the waves, into which streams discharged the waste of what was evidently a forested region. That they are preglacial, is, we think, quite certain. The period of transition between the Tertiary and the Drift, when the distant but advancing ice-sheet sent on its swollen streams, best answers the conditions. The existence of beds of stratified gravel and sand at 260 feet above the level of the ocean, and of similar beds at 180 feet below it, which we refer to the period of the Champlain subsidence, proves that a depression took place of at least 440 feet, and further that the coast was at least 180 feet higher than now when the depression began. That the elevation was much greater than that will be obvious as we proceed.
The glacial drift of Long Island, of which the Champlain deposits were formed, is without fossils, excepting such as occur in bowlders from older beds. But underneath it is a deep deposit of sands, gravels, and clayey sands, in which fossils have been found. Shells of the clam and oyster were taken from sands beneath the bowlder-drift at Lakeville, at a depth of 140 feet, by Henry Onderdonk, Jr., Esq., and a mass of shells, chiefly oyster, were found in sinking the well of the Nassau Gas-Light Company in Brooklyn, 127 feet below the surface, beneath a layer of unmodified drift, 70 feet thick. In this drift were many bowlders. A section of this well is shown in Fig. 3.
In digging wells these sandy and gravelly layers are generally found beneath the bowlder-drift; and the shells, wood, and lignite, evidently represent a period of milder climate than prevailed during the Ice period. Possibly they were deposited on the coast by the floods and swollen streams of the approaching glacier, and the coarsness of the upper portions may suggest a shoaling of the coast from an upward movement of the land as the Ice period came on.
That an elevation occurred during the progress of the Ice period is evident from the contour, as well as structure, of the drift of Long Island. On the north side of the island are numerous fiord valleys, constituting a series of harbors of unsurpassed beauty. They extend
Fig. 3.—Section of the Nassau Gas-Light Company's Well in Brooklyn.
into the island from two to six miles, having depths of water from tan to thirty, and in some instances fifty feet. Beneath the water is a deep deposit of ooze or sediment, known to be in one case forty feet thick. The banks on either side are from a few feet to 200 feet high.
It is apparent that these enormous valleys were not wholly cut into the drift after it was deposited, but rather were maintained while the deposit of drift was in progress, as valleys or water-courses, through which glacial streams may have been discharged into the ocean. These became filled, however, by an excessive accumulation of drift, as from rapid wasting of the ice, causing the outflowing streams to be arrested, and the waters to be discharged eastward or westward from Long Island Sound. But whether they were formed as we suggest, or were cut into the drift after it was deposited, it is quite certain that the coast was sufficiently elevated to permit the glacial floods to sweep the bottom of those valleys.
The 70 feet of drift of the Nassau Gas-Light Company's well is wholly below tide, and its unmodified structure shows that it was deposited above the sea-level, or out of the reach of waves, and further confirms the elevation of the coast in the glacial period.
But evidence of the elevation of the coast during the progress of the Ice age is shown in the interesting fact revealed by the United States Coast Survey, and noticed by Prof. Dana, that the old valley of the Hudson River exists a well-defined depression in the bed of the ocean through a distance of 89 miles southeastward from Sandy Hook. It is termed on the Coast Survey chart "a remarkable gorge." The soundings show that it comprises a continuous series of deep depressions in the ocean's bottom. Some of these are eight miles long and from one and a half to two and a half miles wide. The map (Fig. 4), kindly furnished by the publishers of Prof. Dana's "Manual of Geology," shows how the dotted lines of equal depths bend toward Sandy Hook, indicating the line of deepest water, or the old river-valley in question.
Fig. 4.—Map of the Submerged Border of the Continent off Long Island and New Jersey, with Lines of Equal Soundings in Fathoms. (From Dana's "Manual of Geology," p. 422.)
The Coast Survey chart shows us that at 28 miles from Sandy Hook the depression is 90 feet below the banks or ocean-bottom on either side. At 39 miles and at 51 miles the depression is 60 feet, and at 74 miles it is 72 feet, and at 89 miles it is 492 feet.
The average depth of the ocean over this depression is about one-third greater than on either side.
Through this valley, probably much deeper than now, during the period of elevation the Hudson flowed on its way to the ocean. Had it been, as it now is, deeply submerged, no river could have flowed in it, nor could it have been maintained as a valley while the deposit of drift was going on. The facts imply elevation of the region to an extent of from 300 to 400 feet above the present level of the sea. This would change in a marked manner the aspect of the coast. The site of the city of New York would be inland and greatly elevated, while the gorges of the Hudson and East Rivers would be deepened and widened by glacial torrents and ice. The ocean-border would be from 70 to 80 miles southward from the present shore of Long Island, and the deepest point attained in the artesian well on Barnum's Island would be above the level of the sea.
There is reason to conclude that the entire subsidence of our coast, from its greatest elevation in the glacial age to the greatest depression of the Champlain period which followed it, was from 600 to 700 feet, possibly much more than that. The elevation which followed carried its stratified deposits not only to their present height above tide, which, as we have seen, is about 260 feet, but at least 63 feet more than that when the buried marsh at Fort Lafayette was formed at the surface. By the present subsidence it is submerged or buried to that depth.
Here we pause. Further observations may confirm or correct our conclusions. Geology has not a more tangled skein than is presented in the structure of Long Island. There is evidence of minor oscillations, and pauses of movement, during which great clay deposits formed in depressions upon the surface, now deeply covered with drift or stratified sands, affording also some evidence that interglacial periods, perhaps of mild climate, occurred, but more observations and more facts are needed to justify a definite judgment on the subject.
Underneath the glacial drift, and underneath the sands which we refer to the advent of the ice, are beds of clay and colored sands which appear to be independent of the drift, and are referred to Tertiary or Cretaceous periods. They appear at the surface along the north side of the island, and are found buried by both unmodified drift and by coarse glacial rubble.
We present in tabular form the series of deposits which seem well defined on Long Island, and which represent the probable order of events, but they are fragmental, and perhaps do not occur anywhere in a continuous vertical series:
1. Shore and other surface formations.
2. Stratified gravels and sands of the Champlain subsidence with fossil shells of clam, oyster, and scallop; also wood and lignite.
5. Stratified gravels and sands with fossil shells of the oyster, also wood and lignite underneath the bowlder-drift.
6. Laminated sands and clays with decayed vegetation and lignite have been found; also one shark's tooth (C. angustidens).
The period we have considered is one of immense duration, but throughout there is no evidence of sudden or violent changes. No catastrophe has "set Long Island off from the mainland." In its wonderfully complex structure it is a monument of a state of things which has passed away, but also of a series of movements of oscillation which has continued to the present time.
But it is only after long periods of time that these become obvious, and we realize how completely old landmarks have disappeared.
The tourist in Italy lingers with astonishment before the erect columns of the temple at Pozzuoli in the bay of Baiæ, and sees, at a height of twenty feet above their base, proof of their long submergence in the waters. Moore said of them:
"These lonely columns stand sublime,
Flinging their shadows from on high,
But, on our own shores, beneath the clear waters, and on the hills we cultivate, are records of similar movements, vastly greater in extent, and running with marvelous continuity through periods so vast that all the centuries which have passed since the Pozzuoli marbles were erected seem but as yesterday.