Popular Science Monthly/Volume 8/January 1876/The Formation of Sand-Dunes
|THE FORMATION OF SAND-DUNES.|
ON the south shore of Long Island there intervenes between the uplands and the ocean a narrow beach on which the waves continually break. It is composed chiefly of clean, grayish-white, silicious sand. Other matters present are mica, garnet, and magnetic-iron sands, but, excepting a few localities, these are not in quantity sufficient to alter the general character of the beach. The sand-grains are small; some of them exceedingly minute. We found, in specimens of drifted sand, 1,920 particles in the weight of a troy grain. This will give, for a pound avoirdupois, more than 13,000,000, and about 1,450,000,000 in a cubic foot of sand. The comparison of a "great multitude" to the "sands of the sea-shore" is wonderfully vivid and impressive. Examined by the aid of a microscope, these delicate grains are seen to have lost the sharp, angular features of broken quartz, and closely resemble pebbles, irregular in form, but smooth and rounded. They are wave-worn bowlders on a small scale.
This beach, which is seldom more than one-third of a mile broad, constitutes the coast-line from Coney Island at the entrance to New York Harbor, to the Nepeague Hills, a distance of about one hundred miles, but broken by occasional inlets through which the tides ebb and flow. Throughout this distance, scarcely a pebble of any considerable size occurs. Mather, in the "Geological Survey of the State of New York," commenting on this magnificent beach-line, says, "In Europe, there is no deposit of a similar character to compare with it in extent."
Eastward from the Nepeague hills, which are of sand, along the ocean-side of Montauk Point, high bluffs of bowlder-drift reach the shore, strewing it with their falling débris. Here may be seen on a grand scale the process by which rocks are transformed into the fine sand of which the beach is composed. The waves throw their whole force upon the shore, carrying forward with tremendous roar tons of bowlders and pebbles which roll back as the waves recede. This process is repeated with every wave. The stones thus rolled and tossed lose something of their volume, and scarcely one can be found that does not show signs of disintegration and decay. All of them are penetrated by moisture, some are fractured by frost, and others, weakened by chemical changes, are dashed in pieces. The sand-beach represents the silicious matters of these comminuted rocks. Its position along the coast is determined by the set of the waters, but its contour of sand-hills is determined by winds. These, in their endless play, have carved it into every form possible to drifting sands. Mather observed that "where the beach is above the reach of the surf, it is covered by a labyrinth of hillocks of drifting sand, imitating almost all the varieties of form which snow-drifts present after a storm." These are sand-dunes, or dunes, as they are termed by Lyell, and their surprising mobility, in the ever-changing direction and force of the winds, is a subject of scientific and popular interest.
Everywhere on the beach, in a dry, windy day, the sand-grains on the surface are in motion. They are not carried through the air like dust, except to a limited extent, when the winds are violent, but roll or bound along the surface. Their motion, therefore, represents to the eye, although less perfectly than snow or dust, the motions of the invisible air.
The dunes are built up by slow accretions, and at the top the sand-grains are smaller than at the bottom. The process by which they are formed is a continual rolling of sand-grains up-hill by wind-force, and it is obvious that the lightest ones will attain the greatest elevation. These, too, are the ones that, on reaching the top of the hillock, roll-over on the protected side of the dune, and there form a mass of fine sand. But the winds are not uniform in force, and a consequence is, the dunes are laminated in their structure, coarse and fine layers alternating. The winds change in direction too, changing the position of the sands, and thus the dunes are not only laminated, but irregularly bedded in their structure, closely resembling in this respect that of beaches formed by the plunge and flow of waves. Both structures simply represent wave-motions, one of the water, the other of the air. Fig. 1 represents a section of a large sand-dune, and Fig. 2 a similar but coarser formation hardened into sandstone.
The exterior form of a dune undergoes continual change in dry weather from gravity. The grains of sand roll down its sides until the fine traces of wind-sculpture are obliterated, and a somewhat uniform outline is obtained. It is found that in case of dry sand the angle the side of the dune will finally assume is about 32°. But the winds rarely permit regularity in the form of dunes. A slight breeze becomes a strong one when it rises to the top of an obstacle, or is turned around it, and every dune, however small, becomes a means of so distributing the air-waves that their force and eroding power are increased. From these and similar causes, the contour of the mobile sand-hills is scarcely more permanent than that of the waves in whose spray they lie. But it is the dry sand only that is put in motion by the winds; only a few inches below the surface it is uniformly moist, and on that account somewhat adhesive. This moisture above where the sand is saturated is capillary water, that is, water held by the attraction of the sand-grains, and is about thirty per cent, of the mass by weight. It rises through the sand to the surface as evaporation goes on, and thus in this climate of rainfall the dunes are rendered more permanent than on rainless deserts.
The formation of a sand-dune seems a simple process, and it is surprising how small an object may be the nucleus of one, and indirectly of a series of them. A bush, or tuft of grass, or only a twig, as we have seen, raised above a level surface, breaks the force of the wind, and immediately the sand-grains, which are rolling along the surface, are arrested, and form a minute hillock on the windward side of the obstacle. This increases in size—the sand-grains, as before observed, are driven up its slope, and fall on the sheltered side. The mound thus formed produces currents and eddies in the moving air, and the form and position of other hillocks are determined by the new conditions. By the means indicated, dunes are formed on our narrow beaches thirty feet high; but there are dunes on our coast much higher than that, as will presently be noticed. Their size depends mainly on the abundance and condition of the material, and exposure to winds. On the coast of France they attain a height of 225 feet, and on the Atlantic border of the Sahara Desert are more than twice that elevation. But the desert sands are exceedingly fine and dust-like from attrition, and move in greater volume than is possible for the coarser sands of our coasts. They are whirled and tossed in the gale like dense smoke, but nowhere do they roll on as do waves of the ocean, as is sometimes stated. The transition of a sand-dune is by transfer and deposition of the individual particles of which it is composed.
A wonderfully vivid description of a sand-storm is given by Mr. Southworth, in his "Four Thousand Miles of African Travel:" "I was sitting at my table in the midst of the glorious sunshine of-Africa. Slowly the southern horizon began to grow obscure. A huge mountain of sand, growing grander and grander, advanced rapidly.... The doom-palms and date-trees, frosted with clouds of white birds, the spires and minarets slowly losing their outlines in the dense obscurity.... It came nearer and nearer. Its front was absolutely perpendicular. To breathe was difficult and oppressive, and it was darker than the darkest night I ever knew. Sand covered the ground to the thickness of an inch."
It is easy to see that this more accurately describes a dust-storm than it does the movement of sand on our beaches. It is the fine material only which is thus swept through the air. The coarser sands are driven along the surface, and constitute the hills of the desert, and they are built up as similar ones are of the still coarser sands of our coast. Grain by grain they rise at the touch of the invisible architect. This is true not only of the great dunes, but of the smaller ones, or ripple-marks, which cover the surface of the sands. These beautifully cut and wavy furrows represent the undulatory movement of the air. With a full breeze, they are all seen to be in motion. The grains hop and bound along as the air passes, and the form shown in Fig. 3 is the one which the sands continually assume. But, even while
we watch, each little ridge or mound has been transferred to the space which was a furrow only a few moments before.
These sand-ripples rise on the sandy floor, however level and smooth they may be, as the wind in passing strikes it, in a series of wave-like undulations.
Ripple-marks thus formed are, sometimes, as we have witnessed, covered by drifting sand, and are retained with wonderful distinctness, when the material is hardened into sandstone. All the vast beds of this material existing in the crust of the globe are but the compacted ruins of rock still older, and their furrowed tablets repeat to our eyes the rhythmic beat of winds and waters in ages long past. Fig. 4 represents a slab of sandstone covered with ripple-marks, evidently produced by water, but which differs in form only from those produced by wind.
Sand-dunes are not only blown away piecemeal, but the winds pour upon their flanks a ceaseless shower of sand, and, as the frail masonry gives way, the falling grains are caught and carried on by the gale. By this natural sand-blast rocks are sculptured on the highlands of the Rocky Mountains, and the glass of windows on exposed beaches is sometimes cut through.
On the north side of Long Island, upon the banks along the Sound, are a great number of sand-hills from twenty to eighty feet high. The banks are of glacial drift, with bowlders of immense size, and eastward of Port Jefferson Harbor, for upward of forty miles, are crowned in many places by these broken, desolate hills. In some places they advance slowly inland. A farm, near the village of Baiting Hollow, in Suffolk County, has lost from this cause thirty acres in half a century. Other farms have lost valuable land in a similar way, and we are informed that, during the time mentioned, 100 acres of arable and timber land have been inundated, and are now deeply covered with drifting sand in this immediate neighborhood. At this point is the great dune known on the Coast Survey charts as Friar's Head. Its top is 150 feet above tide, but it stands on the bank which is half that height, so that 75 feet of that elevation is drifting sand. It was originally formed many yards inland, as others are continually
forming, but, by the ceaseless wearing away of the bluffs, it is now upon their brink. It is evidently of considerable age, as its wind-ward slope is covered by a thick growth of beach-grass, bayberry and other bushes, with stunted trees of beach and cedar quite at its top.
From this point the weird architecture of the sand-hills is singularly impressive. There is formed, to the southeast of Friar's Head, a great semicircle of sand, between which and the dune is a floor of several acres in extent swept by the winds. This floor, the original surface of the drift now laid bare, is rich in the remains of an old Indian settlement. Hundreds of specimens—including arrow-heads of flint, jasper, and quartz, axes of various sizes, and other articles of utility—have been picked up.
The sand blown from this spot and from the flanks of the dune constitutes the semicircular wall spoken of. It is one-eighth of a mile inland, and lies directly against a forest of oak and pine, burying many of the trees to a height of thirty to forty feet, only their dead and barkless tops being visible. On the surface of these sands beach-grass of several kinds, and young pine-trees (Pinus rigida) maintain a doubtful struggle for life. This dune does not materially differ from a very large number which cover the banks on the north shore of Loner Island. Their source is the débris of the banks reduced to sand by the action of the waves. The lighter portions of this sand are carried up the slope during fierce winds, and the process is now in operation during every gale. The present forests may delay, but cannot arrest, the final inundation of the land where the sand-hills crown the coast. In Europe the maritime pine and other species of plants whose habitat is the silicious sand have not only arrested the movement of it, but have covered immense areas of waste land with valuable forest. Our native pitch-pine, the Pinus rigida above mentioned, also flourishes on the most sandy soils. There is proof that it formerly grew on portions of the south beach of Long Island, where its foliage was moistened by the spray of the ocean, nor does the occasional overflow of the tides soon destroy it. If these trees are planted abundantly over the surface of these broken hills of sand, their movement would be delayed if not permanently arrested. The sands lie motionless where the force of the wind is broken.