The dunes near the mouth of the Nile, the lower sands of which have been cemented together by the infiltration of Nile water, would probably show a similar stratification in the sandstone which now forms their base.
Dana describes a laminated rook often formed by the infiltration of water into the sand dunes on the Hawaian islands.--Corals and Coral Islands, 1872, p.155.]
If the sand of coast dunes is, as Staring describes it, composed chiefly of well-rounded, quartzose grains, fragments of sh.e.l.ls, and other constant ingredients, it would often be recognizable as coast sand, in its agglutinate state of sandstone. The texture of this rock varies from an almost imperceptible fineness of grain to great coa.r.s.eness, and affords good facilities for microscopic observation of its structure.
There are sandstones, such, for example, as are used for grindstones, where the grit, as it is called, is of exceeding sharpness; others where the angles of the grains are so obtuse that they scarcely act at all on hard metals. The former may be composed of grains of rock, disintegrated indeed, and re-cemented together, but not, in the meanwhile, much rolled; the latter, of sands long washed by the sea, and drifted by land-winds. There is, indeed, so much resemblance between the effects of driving winds and of rolling water upon light bodies, that there might be difficulty in distinguishing them; but after all, it is not probable that sandstone, composed of grains thrown up from the salt sea, and long tossed by the winds, would be identical in its structure with that formed from fragments of rock crushed by mechanical force, or disintegrated by heat, and again agglutinated without much exposure to the action of moving water.
Dunes of American Coasts.
Upon the Atlantic coast of the United States, the prevalence of western or off-sh.o.r.e winds is unfavorable to the formation of dunes, and, though marine currents lodge vast quant.i.ties of sand, in the form of banks, on that coast, its sh.o.r.es are proportionally more free from sand-hills than some others of lesser extent. There are, however, very important exceptions. The action of the tide throws much sand upon some points of the New England coast, as well as upon the Beaches of Long Island and other more southern sh.o.r.es, and here dunes resembling those of Europe are formed. There are also extensive ranges of dunes on the Pacific coast of the United States, and at San Francisco they border some of the streets of the city.
The dunes of America are far older than her civilization, and the soil they threaten or protect possesses, in general, too little value to justify any great expenditure in measures for arresting their progress or preventing their destruction. Hence, great as is their extent and their geographical importance, they have, at present, no such intimate relations to human life as to render them objects of special interest in the point of view I am taking, and I do not know that the laws of their formation and motion have been made a subject of original investigation by any American observer.
Dunes of Western Europe.
Upon the western coast of Europe, on the contrary, the ravages occasioned by the movement of sand dunes, and the serious consequences often resulting from the destruction of them, have long engaged the earnest attention of Governments and of scientific men, and for nearly a century persevering and systematic effort has been made to bring them under human control. The subject has been carefully studied in Denmark and the adjacent duchies, in Western Prussia, in the Netherlands, and in France; and the experiments in the way of arresting the drifting of the dunes, and of securing them, and the lands they shelter, from the encroachments of the sea, have resulted in the adoption of a system of coast improvement substantially the same in all these countries. The sands, like the forests, have now their special literature, and the volumes and memoirs, which describe them and the processes employed to subdue them, are full of scientific interest and of practical instruction.
Dunes of Gascony.
In the small kingdom of Denmark, inclusive of the duchies of Schleswig and Holstein, the dunes cover an area of more than two hundred and sixty square miles. The breadth of the chain is very various, and in some places it consists only of a single row of sand-hills, while in others, it is more than six miles wide. [Footnote: Andersen, Om Klitformationen, pp. 78, 202, 275.] The dunes of the Prussian coast are vaguely estimated to cover from eighty-five to one hundred and ten thousand acres; those of Holland one hundred and forty thousand acres; and those of Gascony more than two hundred thousand acres. I do not find any estimate of their extent in other provinces of France, or in the Baltic provinces of Russia, but it is probable that the entire quant.i.ty of dune land upon the Atlantic and Baltic sh.o.r.es of Europe does not fall much short of a million of acres. [Footnote: In an article on the dunes of Europe, in vol. 29 (1864) of Aus der Natur, p. 590, the dunes are estimated to cover, on the islands and coasts of Schleswig Holstein, in North-west Germany, Denmark, Holland, and France, one hundred and eighty-one German, or nearly four thousand English square miles; in Scotland, about ten German, or two hundred and ten English miles; in Ireland, twenty German, or four hundred and twenty English miles; and in England, one hundred and twenty German, or more than twenty-five hundred English miles. Pannewitz (Anleitung zum Anbau der Sandfluchen), as cited by Andresen (Om Klitformationen, p. 45), states that the drifting sands of Europe, including, of course, sand plains as well as dunes, cover an extent of 21,000 square miles. This is, perhaps, an exaggeration, though there is, undoubtedly, much more desert-land of this description on the European continent than has been generally supposed. There is no question that most of this waste is capable of reclamation by simple planting, and no mode of physical improvement is better worth the attention of civilized Governments than this.
There are often serious objections to extensive forest planting on soils capable of being otherwise made productive, but they do not apply to sand wastes, which, until covered by woods, are not only a useless inc.u.mbrance, but a source of serious danger to all human improvements in the neighborhood of them.] This vast deposit of sea-sand extends along the coasts for a distance of several hundred miles, and from the time of the destruction of the forests which covered it, to the year 1789, the whole line was rolling inwards and burying the soil beneath it, or rendering the fields unproductive by the sand which drifted from it. At the same time, as the sand-hills moved landwards, the ocean was closely following their retreat and swallowing up the ground they had covered, as fast as their movement left it bare.
Age, Character, and Permanence of Dunes.
The origin of most great lines of dunes goes back past all history.
There are on many coasts several distinct ranges of sand-hills which seem to be of very different ages, and to have been formed under different relative conditions of land and water. [Footnote: Krause, speaking of the dunes on the coast of Prussia, says: "Their origin belongs to three different periods, in which important changes in the relative level of sea and land have unquestionably taken place....
Except in the deep depressions between them, the dunes are everywhere sprinkled, to a considerable height, with brown oxydulated iron, which has penetrated into the sand to the depth of from three to eighteen inches, and colored it red. ... Above the iron is a stratum of sand differing in composition from ordinary sea-sand, and on this, growing woods are always found.... The gradually acc.u.mulated forest soil occurs in beds of from one to three feet thick, and changes, proceeding upward, from gray sand to black humus."
Even on the third or seaward range, the sand gra.s.ses appear and thrive luxuriantly, at least on the west coast, though Krause doubts whether the dunes of the east coast were ever thus protected.--Der Dunenbau, pp.
8, 11.] In some cases there has been an upheaval of the coast line since the formation of the oldest hillocks, and these have become inland dunes, while younger rows have been thrown up on the new beach laid bare by elevation of the sea-bed. Our knowledge of the mode of their first acc.u.mulation is derived from observation of the action of wind and water in the few instances where, with or without the aid of man, new coast dunes have been acc.u.mulated, and of the influence of wind alone in elevating new sand-heaps inland of the coast tier, when the outer rows are destroyed by the sea, as also when the sodded surface of ancient sands has been broken, and the subjacent strata laid open to the air.
It is a question of much interest, in what degree the naked condition of most dunes is to be ascribed to the improvidence and indiscretion of man. There are, in Western France, extensive ranges of dunes covered with ancient and dense forests, while the recently formed sand-hills between them and the sea are bare of vegetation, and in some cases are rapidly advancing upon the wooded dunes, which they threaten to bury beneath their drifts. Between the old dunes and the new there is no discoverable difference in material or in structure; but the modern sand-hills are naked and shifting, the ancient, clothed with vegetation and fixed. It has been conjectured that artificial methods of confinement and plantation were employed by the primitive inhabitants of Gaul; and Laval, basing his calculations on the rate of annual movement of the shifting dunes, a.s.signs the fifth century of the Christian era as the period when those processes wore abandoned. [Footnote: Laval, Memoire sur les Dunes de Gascogne, Annales des Ponts et Chaussees, 1847, 2me semestre, p. 231. The same opinion had been expressed by Bremontier, Annales des Ponts et Chaussees, 1833, 1er semestre, p. 185.]
There is no historical evidence that the Gauls were acquainted with artificial methods of fixing the sands of the coast, and we have little reason to suppose that they were advanced enough in civilization to be likely to resort to such processes, especially at a period when land could have had but a moderate value.
In other countries, dunes have spontaneously clothed themselves with forests, and the rapidity with which their surface is covered by various species of sand-plants, and finally by trees, where man and cattle and burrowing animals are excluded from them, renders it highly probable that they would, as a general rule, protect themselves, if left to the undisturbed action of natural causes. The sand-hills of the Frische Nehrung, on the coast of Prussia, were formerly wooded down to the water"s edge, and it was only in the last century that, in consequence of the destruction of their forests, they became moving sands.
[Footnote: "In the Middle Ages," says Willibald Alexis, as quoted by Muller, Das Buch der Pflanzenwelt, i., p. 16, "the Nebrung was extending itself further, and the narrow opening near Lochstadt had filled itself up with sand. A great pine forest bound with its roots the dune sand and the heath uninterruptedly from Danzig to Pillau. King Frederick William I. was once in want of money. A certain Herr von Korff promised to procure it for him, without loan or taxes, if he could be allowed to remove something quite useless. He thinned out the forests of Prussia, which then, indeed, possessed little pecuniary value; but he felled the entire woods of the Frische Nebrung, so far as they lay within the Prussian territory. The financial operation was a success. The king had money, but in the material effects which resulted from it, the state received irreparable injury. The sea-winds rush over the bared hills; the Frische Haff is half-choked with sand; the channel between Elbing, the sea, and Konigsberg is endangered, and the fisheries in the Haff injured. The operation of Herr von Korff brought the king 200,000 thalers. The state would now willingly expend millions to restore the forests again."] There is every reason to believe that the dunes of the Netherlands were clothed with trees until after the Roman invasion. The old geographers, in describing these countries, speak of vast forests extending to the very brink of the sea; but drifting coast dunes are first mentioned by the chroniclers of the Middle Ages, and so far as we know they have a.s.sumed a destructive character in consequence of the improvidence of man. [Footnote: Staring, Voormaals en Thans, p. 231.
Had the dunes of the Netherlandish and French coasts, at the period of the Roman invasion, resembled the moving sand-hills of the present day, it is inconceivable that they could have escaped the notice of so acute a physical geographer as Strabo; and the absolute silence of Caesar, Ptolemy, and the encyclopaedic Pliny, respecting them, would be not less inexplicable.] The history of the dunes of Michigan, so far as I have been able to learn from my own observation, or that of others, is the same. Thirty years ago, when that region was scarcely inhabited, they were generally covered with a thick growth of trees, chiefly pines, and underwood, and there was little appearance of undermining and wash on the lake side, or of shifting of the sands, except where the trees had been cut or turned up by the roots. [Footnote: The sands of Cape Cod were partially, if not completely, covered with vegetation by nature. Dr.
Dwight, describing the dunes as they were in 1800, says: "Some of them are covered with beach gra.s.s; some fringed with whortleberry bushes; and some tufted with a small and singular growth of oaks. ... The parts of this barrier which are covered with whortleberry bushes and with oaks, have been either not at all or very little blown. The oaks, particularly, appear to be the continuation of the forests originally formed on this spot. ... They wore all the marks of extreme age; were, in some instances, already decayed, and in others decaying; were h.o.a.ry with moss and were deformed by branches, broken and wasted, not by violence, but by time."--Travels, iii., p. 91]
Nature, as she builds up dunes for the protection of the seash.o.r.e, provides, with similar conservatism, for the preservation of the dunes themselves; so that, without the interference of man, these hillocks would be, not perhaps absolutely perpetual, but very lasting in duration, and very slowly altered in form or position. When once covered with the trees, shrubs, and herbaceous growths adapted to such localities, dunes undergo no apparent change, except the slow occasional undermining of the outer tier, and accidental destruction by the exposure of the interior, from the burrowing of animals, or the upturning of trees with their roots, and all these causes of displacement are very much less destructive when a vegetable covering exists in the immediate neighborhood of the breach.
Protection of Dunes.
Before the occupation of the coasts by man, dunes, at all points where they have been observed, seem to have been protected in their rear by forests, which served to break the force of the winds in both directions, [Footnote: Bergsoe (Reventlovs Virksomhed, ii., 3) states that the dunes on the west coast of Jutland were stationary before the destruction of the forests to the east of them. The felling of the tall trees removed the resistance to the lower currents of the westerly winds, and the sands have since buried a great extent of fertile soil.
See also same work, ii., p. 124.] and to have spontaneously clothed themselves with a dense growth of the various plants, gra.s.ses, shrubs, and trees, which nature has a.s.signed to such soils. It is observed in Europe that dunes, though now without the shelter of a forest country behind them, begin to protect themselves as soon as human trespa.s.sers are excluded, and grazing animals denied access to them. Herbaceous and arborescent plants spring up almost at once, first in the depressions, and then upon the surface of the sand-hills. Every seed that sprouts, binds together a certain amount of sand by its roots, shades a little ground with its leaves, and furnishes food and shelter for still younger or smaller growths. A succession of a very few favorable seasons suffices to bind the whole surface together with a vegetable network, and the power of resistance possessed by the dunes themselves, and the protection they afford to the fields behind them, are just in proportion to the abundance and density of the plants they support.
The growth of the vegetable covering can, of course, be much accelerated by judicious planting and watchful care, and this species of improvement is now carried on upon a vast scale on the sandy coasts of Western Europe, wherever the value of land is considerable and the population dense.
Use of Dunes as a Barrier against the Sea.
Although the sea throws up large quant.i.ties of sand on flat lee-sh.o.r.es, there are many cases where it continually encroaches on those same sh.o.r.es and washes them away. At all points of the shallow North Sea where the agitation of the waves extends to the bottom, banks are forming and rolling eastwards. Hence the sea-sand tends to acc.u.mulate upon the coast of Schleswig-Holstein and Jutland, and were there no conflicting influences, the sh.o.r.e would rapidly extend itself westwards.
But the same waves which wash the sand to the coast undermine the beach they cover, and still more rapidly degrade the sh.o.r.e at points where it is too high to receive partial protection by the formation of dunes upon it. The earth of the coast is generally composed of particles finer, lighter, and more transportable by water than the sea-sand. While, therefore, the billows raised by a heavy west wind may roll up and deposit along the beach thousands of tons of sand, the same waves may swallow up even a larger quant.i.ty of fine sh.o.r.e-earth. This earth, with a portion of the sand, is swept off by northwardly and southwardly currents, and let fall at other points of the coast, or carried off, altogether, out of the reach of causes which might bring it back to its former position.
Although, then, the eastern sh.o.r.e of the German Ocean here and there advances into the sea, it in general retreats before it, and but for the protection afforded it by natural arrangements seconded by the art and industry of man, whole provinces would soon be engulfed by the waters.
This protection consists in an almost unbroken chain of sand banks and dunes, extending from the northernmost point of Jutland to the Elbe, a distance of not much less than three hundred miles, and from the Elbe again, though with more frequent and wider interruptions, to the Atlantic borders of France and Spain. So long as the dunes are maintained by nature or by human art, they serve, like any other embankment or dike, as a partial or a complete protection against the encroachments of the sea; and on the other hand, when their drifts are not checked by natural processes, or by the industry of man, they become a cause of as certain, if not of as sudden, destruction as the ocean itself whose advance they r.e.t.a.r.d. On the whole, the dunes on the coast of the German Sea, notwithstanding the great quant.i.ty of often fertile land they cover, and the evils which result from their movement, are a protective and beneficial agent, and their maintenance is an object of solicitude with the Governments and people of the sh.o.r.es they defend.
[Footnote: "We must, therefore, not be surprised to see the people here deal as gingerly with their dunes as if treading among eggs. He who is lucky enough to own a molehill of dune pets it affectionately, and spends his substance in cherishing and fattening it. That fair, fertile, rich province, the peninsula of Eiderstadt in the south of Friesland, has, on the point towards the sea, only a tiny row of dunes, some six miles long or so; but the people talk of their fringe of sand hills, as if it were a border set with pearls. They look upon it as their best defence against Neptune. They have connected it with their system of dikes, and for years have kept sentries posted to protect it against wanton injury."--J. G. Kohl, Die Inseln u. Marschen Schleswig-Holsteins, ii., p. 115.]
The eastward progress of the sea on the Danish, Netherlandish, and French coasts depends so much on local geological structure, on the force and direction of tidal and other marine currents, on the volume and rapidity of coast rivers, on the contingencies of the weather and on other varying circ.u.mstances, that no general rate can be a.s.signed to it.
At Agger, near the western end of the Liimfjord, in Jutland, the coast was washed away, between the years 1815 and 1839, at the rate of more than eighteen feet a year. The advance of the sea appears to have been something less rapid for a century before; but from 1840 to 1857, it gained upon the land no less than thirty feet a year. At other points of the sh.o.r.e of Jutland the loss is smaller, but the sea is encroaching generally upon the whole line of the coast. [Footnote: Andersen, "Om Klitformationen," pp. 68-72.]
The Liimfjord.
The irruption of the sea into the fresh-water lagoon of Liimfjord in Jutland, in 1825--one of the most remarkable encroachments of the ocean in modern times--is expressly ascribed to "mismanagement of the dunes"
on the narrow neck of land which separated the fjord from the North Sea.
At earlier periods the sea had swept across the isthmus, and even burst through it, but the channel had been filled up again, sometimes by artificial means, sometimes by the operation of natural causes, and on all these occasions effects were produced very similar to those resulting from the formation of the new channel in 1825, which still remains open. [Footnote: Id., pp. 231, 232. Andresen"s work, though printed in 1861, was finished in 1859. Lyell (Antiquity of Man, 1863, p.
14) says: "Even in the course of the present century, the salt-waters have made one eruption into the Baltic by the Liimfjord, although they have been now again excluded."] Within comparatively recent historical ages, the Liimfjord has thus been several times alternately filled with fresh and with salt water, and man has produced, by neglecting the dunes, or at least might have prevented by maintaining them, changes identical with those which are usually ascribed to the action of great geological causes, and sometimes supposed to have required vast periods of time for their accomplishment.
"This breach," says Forchhammer, "which converted the Liimfjord into a sound, and the northern part of Jutland into an island, occasioned remarkable changes. The first and most striking phenomenon was the sudden destruction of almost all the fresh-water fish previously inhabiting this lagoon, which was famous for its abundant fisheries.
Millions of fresh-water fish were thrown on sh.o.r.e, partly dead and partly dying, and were carted off by the people. A few only survived, and still frequent the sh.o.r.es at the mouth of the brooks. The eel, however, has gradually accommodated itself to the change of circ.u.mstances, and is found in all parts of the fjord, while to all other fresh-water fish, the salt-water of the ocean seems to have been fatal. It is more than probable that the sand washed in by the irruption covers, in many places, a layer of dead fish, and has thus prepared the way for a petrified stratum similar to those observed in so many older formations.
"As it seems to be a law of nature that animals whose life is suddenly extinguished while yet in full vigor, are the most likely to be preserved by petrification, we find here one of the conditions favorable to the formation of such a petrified stratum. The bottom of the Liimfjord was covered with a vigorous growth of aquatic plants, belonging both to fresh and to salt water, especially Zostera marina.
This vegetation totally disappeared after the irruption, and, in some instances, was buried by the sand; and here again we have a familiar phenomenon often observed in ancient strata--the indication of a given formation by a particular vegetable species--and when the strata deposited at the time of the breach shall be accessible by upheaval, the period of eruption will be marked by a stratum of Zostera, and probably by impressions of fresh-water fishes.
"It is very remarkable that the Zostera marina, a sea-plant, was destroyed even where no sand was deposited. This was probably in consequence of the sudden change from brackish to salt water ... It is well established that the Liimfjord communicated with the German Ocean at some former period.
To that era belong the deep beds of oyster sh.e.l.ls and Cardium edule, which are still found at the bottom of the fjord. And now, after an interval of centuries, during which the lagoon contained no salt-water sh.e.l.l fish, it again produces great numbers of Mytilus edulis. Could we obtain a deep section of the bottom, we should find beds of Ostrea edulis and Cardium edule, then a layer of Zostera marina with fresh-water fish, and then a bed of Mytilus edulis. If, in course of time, the new channel should be closed, the brooks would fill the lagoon again with fresh water; fresh-water fish and sh.e.l.l fish would reappear, and thus we should have a repeated alternation of organic inhabitants of the sea and of the waters of the land.
"These events have been accompanied with but a comparatively insignificant change of land surface, while the formations in the bed of this inland sea have been totally revolutionized in character."
[Footnote: Forchhammer, Geognostiche Studien am Meeres-Ufer, Leonhard und Bronn, Jahrbuch, 1841, pp. 11, 13.] Coasts of Schleswig-Holstein, Holland, and France. On the islands on the coast of Schleswig-Holstein, the advance of the sea has been more unequivocal and more rapid. Near the beginning of the last century, the dunes which had protected the western coast of the island of Sylt began to roll to the east, and the sea followed closely as they retired. In 1757, the church of Rantum, a village upon that island, was obliged to be taken down in consequence of the advance of the sand-hills; in 1791, these hills had pa.s.sed beyond its site, the waves had swallowed up its foundations, and the sea gained so rapidly, that, fifty years later, the spot where they lay was seven hundred feet from the sh.o.r.e. [Footnote: Andresen, Om Klitformationen, pp.
68, 72.] The most prominent geological landmark on the coast of Holland is the Huis te Britten, Arx Britannica, a fortress built by the Romans, in the time of Caligula, on the main land near the mouth of the Rhine.
At the close of the seventeenth century, the sea had advanced sixteen hundred paces beyond it. The older Dutch annalists record, with much parade of numerical accuracy, frequent encroachments of the sea upon many parts of the Netherlandish coast. But though the general fact of an advance of the ocean upon the land is established beyond dispute, the precision of the measurements which have been given is open to question.
Staring, however, who thinks the erosion of the coast much exaggerated by popular geographers, admits a loss of more than a million and a half acres, chiefly worthless mora.s.s; [Footnote: Voormaals en Thans, pp. 126, 170.] and it is certain that but for the resistance of man, but for his erection of dikes and protection of dunes, there would now be left of Holland little but the name. It is, as has been already seen, still a debated question among geologists whether the coast of Holland now is, and for centuries has been, subsiding. I believe most investigators maintain the affirmative; and if the fact is so, the advance of the sea upon the land is, in part, due to this cause. But the rate of subsidence is at all events very small, and therefore the encroachments of the ocean upon the coast are mainly to be ascribed to the erosion and transportation of the soil by marine waves and currents.
The sea is fast advancing at several points of the western coast of France, and unknown causes have given a new impulse to its ravages since the commencement of the present century. Between 1830 and 1842, the Point de Grave, on the north side of the Girondo, retreated one hundred and eighty metres, or fifty feet per year; from the latter year to 1846, the rate was increased to more than three times that quant.i.ty, and the loss in those four years was about six hundred feet. All the buildings at the extremity of the peninsula have been taken down and rebuilt farther landwards, and the lighthouse of the Grave now occupies its third position. The sea attacked the base of the peninsula also, and the Point de Grave and the adjacent coasts have been for thirty years the scene of one of the most obstinately contested struggles between man and the ocean recorded in the annals of modern engineering.
Movement of Dunes.
Besides their importance as a barrier against the inroads of the ocean, dunes are useful by sheltering the cultivated ground behind them from the violence of the sea-wind, from salt spray, and from the drifts of beach sand which would otherwise overwhelm them. But the dunes themselves, unless their surface sands are kept moist, and confined by the growth of plants, or at least by a crust of vegetable earth, are constantly rolling inwards, and thus, while, on one side, they lay bare the traces of ancient human habitations or other evidences of the social life of primitive man, they are, on the other, burying fields, houses, churches, and converting populous districts into barren and deserted wastes.