Elisee Reclus, in describing the coast dunes of Gascony, observes that when, as sometimes happens, the sands are not heaped in a continuous, irregular bulwark, but deposited in isolated hillocks, they have a tendency to assume a crescent shape, the convexity being turned seawards, or towards the direction from which the prevailing winds proceed. This fact, the geological bearing of which is obvious, is not noticed by previous French writers or even by Andresen, though a semi-lunar outline has been long generally ascribed to inland dunes. It is, however evident that such a form would naturally be produced by the action of a wind blowing long in a given direction upon a mass of loose sand with a fixed centre—such as is constituted by the shrub or stone around which the sand is first deposited—and free extremities. On a receding coast, dunes will not attain so great a height as on more secure shores, because they are undermined and carried off before they have time to reach their greatest dimensions. Hence, while at sheltered points in South-western France, there are dunes three hundred feet or more in height, those on the Frisic Islands and the exposed parts of the coast of Schleswig-Holstein range only from twenty to one hundred feet. On the western shores of Africa, it is said that they sometimes attain an elevation of six hundred feet. This is one of the very few points known to geographers where desert sands are advancing seawards, [Footnote: "On the west coast of Africa the dunes are drifting seawards, and always receiving new accessions from the Sahara. They are constantly advancing out into the sea."—Naumann, Geognosie, ii., p.1172.] and here they rise to the greatest altitude to which sand-grains can be carried by the wind. The hillocks, once deposited, are held together and kept in shape, partly by mere gravity, and partly by the slight cohesion of the lime, clay, and organic matter mixed with the sand; and it is observed that, from capillary attraction, evaporation from lower strata, and retention of rain-water, they are always moist a little below the surface. [Footnote: "Dunes are always full of water, from the action of capillary attraction. Upon the summits, one seldom needs to dig more than a foot to find the sand moist, and in the depressions, fresh water is met with near the surface."—Forchhammer, in Leonhard and Bronx, for 1841, p.5, note. On the other hand, Andresen, who has very carefully investigated this as well as all other dune phenomena, maintains that the humidity of the sand ridges cannot be derived from capillary attraction. He found by experiment that a heap of drift-sand was not moistened to a greater height than eight and a half inches, after standing with its base a whole night in water. He states the minimum of water contained by the sand of the dunes, one foot below the surface, after a long drought, at two per cent, the maximum, after a rainy month, at four per cent. At greater depths the quantity is larger. The hygroscopicity of the sand of the coast of Jutland he found to be thirty-three per cent, by measure, or 21.5 by weight. The annual precipitation on that coast is twenty-seven inches, and as the evaporation is about the same, he argues that rain-water does not penetrate far beneath the surface of the dunes, and concludes that their humidity can be explained only by evaporation from below.—Om Klitformationen, pp. 106-110. In the dunes of Algeria, water in so abundant that wells are constantly dug in them at high points on their surface. They are sunk to the depth of three or four inches only, and the water rises to the height of a metre in them.—Laurent, Memoire sur le Sahara, pp. 11, 12, 13. The same writer observes (p. 14) that the 'hollows in the dunes are planted with palms which find moisture enough a little below the surface. It would hence seem that proposal to fix the dunes which are supposed to threaten the Suez Canal, by planting the maratime pine and other trees upon them, is not altogether so absurd as it has been thought to be by some of those disinterested philanthropists of other nations who were distressed with fears that French capitalists would lose the money they had invested in that great undertaking. Ponds of water are often found in the depression between the sand-hills of the dune chains in the North American desert.]
By successive accumulations, they gradually rise to the height of thirty, fifty, sixty, or a hundred feet, and sometimes even much higher. Strong winds, instead of adding to their elevation, sweep off loose particles from their surface, and these, with others blown over or between them, build up a second row of dunes, and so on according to the character of the wind, the supply and consistence of the sand, and the face of the country. In this way is formed a belt of sand-dunes, irregularly dispersed and varying much in height and dimensions, and sometimes many miles in breadth. On the Island of Sylt, in the German Sea, where there are several rows, the width of the belt is from half a mile to a mile. There are similar ranges on the coast of Holland, exceeding two miles in breadth, while at the mouths of the Nile they form a zone not less than ten miles wide. The base of some of the dunes in the Delta of the Nile is reached by the river during the annual inundation, and the infiltration of the water, which contains lime, has converted the lower strata into a silicious limestone, or rather a calcarous sandstone, and thus afforded an opportunity of studying the structure of that rock in a locality where its origin and mode of aggregation and solidification are known.
The tide, though a usual, is by no means a necessary condition for the accumulations of sand out of which dunes are formed. The Baltic and the Mediterranean are almost tideless seas, but there are vast ranges of dunes on the Russian and Prussian coasts of the Baltic and at the mouths of the Nile and many other points on the shores of the Mediterranean. The vast shoals in the latter sea, known to the ancients as the Greater and Lesser Syrtis, are of marine origin. They are still filling up with sand, washed up from greater depths, or sometimes drifted from the coast in small quantities, and will probably be converted, at some future period, into dry land covered with sand-hills. There are also extensive ranges of dunes upon the eastern shores of the Caspian, and at the southern, or rather south-eastern, extremity of Lake Michigan. [Footnote: The careful observations of Colonel J. D. Graham, of the United States Army, show a tide of about three inches in Lake Michigan. See "A Lunar Tidal Wave in the North American Lakes," demonstrated by Lieut.-Colonel J. D. Graham, in the fourteenth volume of the Proceedings of the American Association for the Advancement of Science.] There is no doubt that this latter lake formerly extended much farther in that direction, but its southern portion has gradually shoaled and at last been converted into solid land, in consequence of the prevalence of the north-west winds. These blow over the lake a large part of the year, and create a southwardly set of the currents, which wash up sand from the bed of the lake and throw it on shore. Sand is taken up from the beach at Michigan City by every wind from that quarter, and, after a heavy blow of some hours' duration, sand ridges may be observed on the north side of the fences, like the snow wreaths deposited by a drifting wind in winter. Some of the particles are carried back by contrary winds, but most of them lodge on or behind the dunes, or in the moist soil near the lake, or are entangled by vegetables, and tend permanently to elevate the level. Like effects are produced by constant sea-winds, and dunes will generally be formed on all low coasts where such prevail, whether in tideless or in tidal waters.
Jobard thus describes the modus operandi, under ordinary circumstances, at the mouths of the Nile, where a tide can scarcely be detected: "When a wave breaks, it deposits an almost imperceptible line of fine sand. The next wave brings also its contribution, and shoves the preceding line a little higher. As soon as the particles are fairly out of the reach of the water they are dried by the heat of the burning sun, and immediately seized by the wind and rolled or borne farther inland. The gravel is not thrown out by the waves, but rolls backwards and forwards until it is worn down to the state of fine sand, when it, in its turn, is cast upon the land and taken up by the wind." [Footnote: Staring, De Bodun van Nederland, i., p. 327, note.] This description applies only to the common every-day action of wind and water; but just in proportion to the increasing force of the wind and the waves, there is an increase in the quantity of sand, and in the magnitude of the particles carried off from the beach by it, and, of course, every storm in a landward direction adds sensibly to the accumulation upon the shore.
Sand Banks.
Although dunes, properly so called, are found only on dry land and above ordinary high-water mark, and owe their elevation and structure to the action of the wind, yet, upon many shelving coasts, accumulations of sand much resembling dunes are formed under water at some distance from the shore by the oscillations of the waves, and are well known by the name of sand banks. They are usually rather ridges than banks, of moderate inclination, and with the steepest slope seawards, [Footnote: Kohl, Inseln und Marschen Schleswig Holsteins, ii., p. 33. From a drawing in Andresen, Om Klitformationen, p. 24, it would appear that on the Schleswig coast the surf-formed banks have the steepest slope landwards, those farther from the shore, as stated in the text.] and their form differs little from that of dunes except in this last particular and in being lower and more continuous. Upon the western coast of the island of Amrum, for example, there are three rows of such banks, the summits of which are at a distance of perhaps a couple of miles from each other; so that, including the width of the banks themselves, the spaces between them, and the breadth of the zone of dunes upon the land, the belt of moving sands on that coast is probably not less than eight miles wide.
Under ordinary circumstances, sand banks are always rolling, landwards, and they compose the magazine from which the material for the dunes is derived. [Footnote: Sand banks sometimes connect themselves with the coast at both ends, and thus cut off a portion of the sea. In this case, as well as when salt water is enclosed by sea-dikes, the water thus separated from the ocean gradually becomes fresh, or at least brackish. The Haffs, or large expanses of fresh water in Eastern Prussia—which are divided from the Baltic by narrow sand banks called Nehrungen, or, at sheltered points of the coast, by fluviatile deposits called Werders—all have one or more open passages, through which the water of the rivers that supply them at last finds its way to the sea.] The dunes, in fact, are but aquatic sand banks transferred to dry land. The laws of their formation are closely analogous, because the action of the two fluids, by which they are respectively accumulated and built up, is very similar when brought to bear upon loose particles of solid matter. It would, indeed, seem that the slow and comparatively regular movements of the heavy, unelastic water ought to affect such particles very differently from the sudden and fitful impulses of the light and elastic air. But the velocity of the wind currents gives them a mechanical force approximating to that of the slower waves, and, however difficult it may be to explain all the phenomena that characterize the structure of the dunes, observation has proved that it is nearly identical with that of submerged sand banks. [Footnote: Forchhammer ascribes the resemblance between the furrowing of the dune sands and the beach ripples, not to the similarity of the effect of wind and water upon sand, but wholly to the action of the wind; in the first instance, directly, in the latter, through the water. "The wind-ripples on the surface of the dunes precisely resemble the water-ripples of sand flats occasionally overflowed by the sea; and with the closest scrutiny, I have never been able to detect the slightest difference between them. This is easily explained by the fact, that the water-ripples are produced by the action of light wind on the water which only transmits the air-waves to the sand."—Leonhard und Bronn, 1841, pp. 7, 8.] The differences of form are generally ascribable to the greater number and variety of surface accidents of the ground on which the sand hills of the land are built up, and to the more frequent changes, and wider variety of direction, in the courses of the wind.
CHARACTER OF DUNE SAND.
"Dune sand," says Staring, "consists of well-rounded grains of quartz, more or less colored by iron, and often mingled with fragments of shells, small indeed, but still visible to the naked eye. [Footnote: According to the French authorities, the dunes of France are not always composed of quartzose sand. "The dune sands" of different characters, says Bremontier, "partake of the nature of the different materials which compose them. At certain points on the coast of Normandy they are found to be purely calcareous; they are of mixed composition on the shores of Brittany and Saintonge, and generally quartzose between the mouth of the Gironde and that of the Adour."—Memoire sur les Dunes, Annales des Ponts et Chaussees, t. vii., 1833, 1er semestre, p. 146.
In the dunes of Long Island and of Jutland, there are considerable veins composed almost wholly of garnet. For a very full examination of the mechanical and chemical composition of the dune sands of Jutland, see Andresen, Om Klitformationen, p. 110. Fraas informs us, Aus dem Orient, pp. 176, 177, that the dune sands of the Egyptian coast arise from the disintegration of the calcareous sandstone of the same region. This sandstone, composed in a large proportion of detritus of both land and sea shells mingled with quartz sand, appears to have been consolidated under water during an ancient period of subsidence. A later upheaval brought it to or near the surface, when it was more or less disintegrated by the action of the waves and by meteoric influences—a process still going on—and it is now again subsiding with the coast it rests on.
The calcareous sand arising from the comminution of corals forms dunes on some of the West India Islands.—Agassiz, Bulletin of the Museum of Comparative Zoology, vol. i.] These fragments are not constant constituents of dune sand. They are sometimes found at the very summits of the hillocks, as at Overveen; in the King's Dune, near Egmond, they form a coarse, calcareous gravel very largely distributed through the sand, while the interior dunes between Haarlem and Warmond exhibit no trace of them. It is yet undecided whether the presence or absence of these fragments is determined by the period of the formation of the dunes, or whether it depends on a difference in the process by which different dunes have been accumulated. Land shells, such as snails, for example, are found on the surface of the dunes in abundance, and many of the shelly fragments in the interior of the hillocks may be derived from the same source." [Footnote: De Bodem van Nederland, i., p. 323.]
Sand concretions form within the dunes and especially in the depressions between them. These are sometimes so extensive and impervious as to retain a sufficient supply of water to feed perennial springs, and to form small permanent ponds, and they are a great impediment to the penetration of roots, and consequently to the growth of trees planted, or germinating from self-sown seeds, upon the dunes. [Footnote: Staring, De Bodem van Nederland, i., p.317. See also Bergsoe, Reventlov's Virksomhed, ii., p. 11.
"In the sand-hill ponds mentioned in the text, there is a vigourous growth of bog plants accompanied with the formation of peat, which goes on regularly as long as the dune sand does not drift. But if the surface of the dunes is broken, the sand blows into the ponds, covers the peat, and puts and end to its formation. When, in the course of time, marine currents cut away the coast, the dunes move landwards and fill up the ponds and thus are formed the remarkable strata of fossile peat called Martorv, which appears to be unknown to the geologists of other parts of Europe." — Forchhammer, in Leonhard und Bronn, 1841, p. 18. Martorv has a specific gravity thrice as great as that of ordinary peat in consequence of the pressure of the sand.—Asbjornsen, Torv og Torvdrift, p.26.]
Interior Structure of Dunes.
The interior structure of the dunes, the arrangement of their particles, is not, as might be expected, that of an unorganized, confused heap, but they show a strong tendency to stratification. This is a point of much geological interest, because it indicates that sandstone may owe its stratified character to the action of other forces as well as of water. The origin and peculiar character of these layers are due to a variety of causes.
For example, a south-west wind and current may deposit upon a dune a stratum of a given color and mineral composition, and this may be succeeded by a north-west wind and current, bringing with them particles of a different hue, constitution, and origin. Again, if we suppose a violent tempest to strew the beach with sand-grains very different in magnitude and specific gravity, and, after the sand is dry, to be succeeded by a gentle breeze, it is evident that only the lighter particles will be taken up and carried to the dunes. If, after some time, the wind freshens, heavier grains will be transported and deposited on the former, and a still stronger succeeding gale will roll up yet larger kernels. Each of these deposits will form a stratum. If we suppose the tempest to be followed, after the sand is dry, not by a gentle breeze, but by a wind powerful enough to lift at the same time particles of very various magnitudes and weights, the heaviest will often lodge on the dune while the lighter will be carried farther. This would produce a stratum of coarse sand, and the same effect might result from the blowing away of light particles out of a mixed layer, while the heavier remained undisturbed. [Footnote: The lower strata must be older than the superficial layers, and the particles which compose them may in time become more disintegrated, and therefore finer than those deposited later and above them.
Hull ingeniously suggests that, besides other changes, fine sand intermixed with or deposited above a coarser stratum, as well as the minute particles resulting from the disintegration of the grains of the latter, may be carried by rain in the case of dunes, or by the ordinary action or sea-water in that of sand-banks, down through the interstices in the coarser layer, and thus the relative position of sand and gravel may be changed.—Oorsprong der Hollandsche Duinen, p. 103.]
Still another cause of apparent stratification may be found in the occasional interposition of a thin layer of leaves or other vegetable remains between successive deposits, and this I imagine to be more frequent than has been generally supposed. Some geologists have thought that the sand strata of dunes are of annual formation; [Footnote: Schomann, Geologische Wanderungen durch die Preussischen Ost-See Provinzen, 1869, p. 81.] but the autumnal deposit of foliage from neighboring trees and shrubs furnishes a more probable explanation of the division of the sand-heaps into regular layers.
A late distinguished American admiral communicated to me an interesting observation made by him at San Francisco, which has an important bearing on the arrangement of the particles of sand in dunes and other irregular accumulations of that substance. In laying out a navy-yard at that port, a large quantity of earthy material was removed from the dunes and other hillocks and carted to a low piece of ground which required filling up. Sand of various characters, fine and coarse gravel, and common earth were dropped promiscuously by the carts as accident or convenience dictated, and of course they were all confusedly intermixed. Some time after, when the new ground was consolidated, various excavations were made in it, and the different materials of which the filling was composed were found to be stratified with considerable regularity, according to their specific gravity.
Two explanations of this remarkable fact suggest themselves to me, which, however, do not perhaps exclude others. San Francisco is subject to earthquakes, and though violent or even sensible shocks are not very frequent, it is highly probable that, as is shown to be the case in many other countries, by late seismological observations, there are, in the course of the year, a great number of slight shocks which escape unscientific observation. A frequent repetition of slight tremblings of the earth would, like any other moderate mechanical agitation, probably produce the separation of a miscellaneous mass, like that described, into distinct layers. Again, the Pacific coast, like all others upon an open sea, is exposed to incessant concussion from the shock of the waves, which is repeated many thousand times a day. This concussion is often sensibly felt by the observer, and it seems not in the least improbable that the agitation may have tended to produce a stratified arrangement in the case at San Francisco, as well as in all coast dunes and other accumulations of loose mineral material in similar situations. Kohl observes that the shore on the landward side of the files of dunes often trembles from the shock of the waves on the beach, [Footnote: Inseln und Marschen, etc., ii., p. 34.] and Villeneuve established by careful experiment that at Dunkerque the ground is sensibly agitated by the same cause, in stormy weather, to a distance of more than a mile from the sea.
The eddies of strong winds between the hillocks must also occasion disturbances and re-arrangements of the sand layers, and it seems possible that the irregular thickness and the strange contortions of the strata of the sandstone at Petra may be due to some such cause. A curious observation of Professor Forchhammer suggests an explanation of another peculiarity in the structure of the sandstone of Mount Seir. He describes dunes in Jutland, composed of yellow quartzose sand intermixed with black titanian iron. When the wind blows over the surface of the dunes, it furrows the sand with alternate ridges and depressions, ripples, in short, like those of water. The swells, the dividing ridges of the system of sand ripples, are composed of the light grains of quartz, while the heavier iron rolls into the depressions between, and thus the whole surface of the dune appears as if covered with a fine black network.
The sea side of dunes, being more exposed to the caprices of the wind, is more irregular in form than the lee or land side, where the arrangement of the particles is affected by fewer disturbing and conflicting influences. Hence, the stratification of the windward slope is somewhat confused, while the sand on the lee side is found to be disposed in more regular beds, inclining landwards, and with the largest particles lowest, where their greater weight would naturally carry them. The lee side of the dunes, being thus formed of sand deposited according to the laws of gravity, is very uniform in its slope, which, according to Forchhammer, varies little from an angle of 30 degrees with the horizon, while the more exposed and irregular weather side lies at an inclination of from 5 degrees to 10 degrees. When, however, the outer tier of dunes is formed so near the waterline as to be exposed to the immediate action of the waves, it is undermined, and the face of the hill is very steep and sometimes nearly perpendicular.
Geological Importance of Dunes.
These observations, and other facts which a more attentive study on the spot would detect, might furnish the means of determining interesting and important questions concerning geological formations in localities very unlike those where dunes are now thrown up. For example, Studer supposes that the drifting sand-hills of the African desert were originally coast dunes, and that they have been transported to their present position far in the interior, by the rolling and shifting leeward movement to which all dunes not covered with vegetation are subject. The present general drift of the sands of that desert appears to be to the south-west and west, the prevailing winds blowing from the north-east and east; but it has been doubted whether the shoals of the western coast of Northern Africa, and the sands upon that shore, are derived from the bottom of the Atlantic, in the usual manner, or, by an inverse process, from those of the Sahara. The latter, as has been before remarked, is probably the truth, though observations are wanting to decide the question. [Footnote: "The North African desert falls into two divisions: the Sahel, or western, and the Sahar, or eastern. The sands of the Sahar were, at a remote period, drifted to the west. In the Sahel, the prevailing east winds drive the sand-ocean with a progressive westward motion. The eastern half of the desert is swept clean."—Naumann, Geognosie, ii., p. 1173.] There would be nothing violently improbable in the a priori supposition that they may have been in part first thrown up by the Mediterranean on its Libyan coast, and thence blown south and west over the vast space they now cover. But inasmuch as it is now geologically certain that the Sahara is an uplifted bed of an ancient sea, we may suppose that, while submerged, it was, like other sea-bottoms, strewn with sand, and that its present supply of that material was, in great proportion, brought up with it. Laurent observed, some years ago, that marine shells of still extinct species were found in the Sahara, far from the sea, and even at considerable depths below the surface. [Footnote: Memoires sur le Sahara Oriental p. 62] These observations have been confirmed past all question by Desor, Martins, and others, and the facts and the obvious conclusion they suggest are at present not disputed. But whatever has been the source and movement of these sands, they can hardly fail to have left on their route some sandstone monuments to mark their progress, such, for example, as we have seen are formed from the dune sand at the mouth of the Nile; and it is conceivable that the character of the drifting sands themselves, and of the conglomerates and sandstones to whose formation they have contributed, might furnish satisfactory evidence as to their origin, their starting-point, and the course by which they have wandered so far from the sea. [Footnote: Forchhammer, after pointing out the coincidence between the inclined stratification of dunes and the structure of ancient tilted rocks, says: "But I am not able to point out a sandstone formation corresponding to the dunes. Probably most ancient dunes have been destroyed by submersion before the loose sand became cemented to solid stone, but we may suppose that circumstances have existed somewhere which have preserved the characteristics of this formation."—Leonhard und Bronn, 1841, p. 8, 9. Such formations, however, certainly exist. Laurent (Memoire sur le Sahara, etc., p. 12) tells us that in the Algerian desert there are "sandstone formation" not only "corresponding to the dunes," but, actually consolidated within them. "A place called El-Mouia-Tadjer presents a repetition of what we saw at El-Baya; one of the funnels formed in the middle of the dunes contains wells from two metres to two and a half in depth, dug in a sand which pressure, and probably the presence of certain salts, have cemented so as to form true sandstone, soft indeed, but which does not yield except to the pickaxe. These sandstones exhibit an inclination which seems to be the effect of wind; for they conform to the direction of the sands which roll down a scarp occasioned by the primitive obstacle."
"At New Quay the dune sands are converted to stone by an oxide of iron held in solution by the water which pervades them. This stone, which is formed, so to speak, under our eye, has been found solid enough to be employed for building."-Esquiros, L'Angleterre, etc., in Revue des Deux Mondes, 1864, pp. 44, 45.
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 shells, 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 coarseness, 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-shore winds is unfavorable to the formation of dunes, and, though marine currents lodge vast quantities of sand, in the form of banks, on that coast, its shores 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 shores, 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 quantity of dune land upon the Atlantic and Baltic shores 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 incumbrance, 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 accumulated 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 grasses 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 accumulation 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 accumulated, 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, assigns 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 assumed 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 grass; 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 hoary 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 seashore, 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, grasses, shrubs, and trees, which nature has assigned 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 trespassers 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 quantities of sand on flat lee-shores, there are many cases where it continually encroaches on those same shores 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 accumulate upon the coast of Schleswig-Holstein and Jutland, and were there no conflicting influences, the shore 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 shore 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 quantity of fine shore-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 shore 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 retard. On the whole, the dunes on the coast of the German Sea, notwithstanding the great quantity 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 shores 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 circumstances, that no general rate can be assigned 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 shore 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 shore, partly dead and partly dying, and were carted off by the people. A few only survived, and still frequent the shores at the mouth of the brooks. The eel, however, has gradually accommodated itself to the change of circumstances, 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 shells 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 shell 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 shell 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 passed 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 shore. [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 morass; [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 quantity, 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.
Especially destructive are they when, by any accident, a cavity is opened into them to a considerable depth, thereby giving the wind access to the interior, where the sand is thus first dried, and then scooped out and scattered far over the neighboring soil. The dune is now a magazine of sand, no longer a rampart against it, and mischief from this source seems more difficult to resist than from almost any other drift, because the supply of material at the command of the wind is more abundant and more concentrated than in its original thin and widespread deposits on the beach. The burrowing of conies in the dunes is, in this way, not unfreqnently a cause of their destruction and of great injury to the fields behind them. Drifts, and even inland sand-hills, sometimes result from breaking the surface of more level sand deposits, far within the range of the coast dunes. Thus we learn from Staring, that one of the highest inland dunes in Friesland owes its origin to the opening of the drift sand by the uprooting of a large Oak. [Footnote: De Bodem van Nederland, i. p. 425.]
Great as are the ravages produced by the encroachment of the sea upon the western shores of continental Europe, they have been in some degree compensated by spontaneous marine deposits at other points of the coast, and we have seen in a former chapter that the industry of man has reclaimed a large territory from the bosom of the ocean. These latter triumphs are not of recent origin, and the incipient victories which paved the way for them date back perhaps as far as ten centuries. In the meantime, the dunes had been left to the operation of the laws of nature, or rather freed, by human imprudence, from the fetters with which nature had bound them, and it is scarcely three generations since man first attempted to check their destructive movements. As they advanced, he unresistingly yielded and retreated before them, and they have buried under their sandy billows many hundreds of square miles of luxuriant cornfields and vineyards and forests.
On the west coast of France a belt of dunes, varying in width from a quarter of a mile to five miles, extends from the Adour to the estuary of the Gironde, and covers an area of nine hundred and seventy square kilometres, or two hundred and forty thousand acres. When not fixed by vegetable growths, these dunes advance eastwards at a mean rate of about one rod, or sixteen and a half feet, a year. Wo do not know historically when they began to drift, but if we suppose their motion to have been always the same as at present, they would have passed over the space between the sea coast and their present eastern border, and covered the large area above mentioned, in fourteen hundred years. We know, from written records, that they have buried extensive fields and forests and thriving villages, and changed the courses of rivers, and that the lighter particles carried from them by the winds, even where not transported in sufficient quantities to form sand-hills, have rendered sterile much land formerly fertile. [Footnote: The movement of the dunes has been hardly less destructive on the north side of the Gironde. See the valuable articles of Elisee Reclus in the Revue des Deux Mondes for December 1862, and several later numbers, entitled "Le Littoral de la France."] They have also injuriously obstructed the natural drainage of the maritime districts by choking up the beds of the streams, and forming lakes and pestilential swamps of no inconsiderable extent. In fact, so completely do they embank the coast, that between the Gironde and the village of Mimizan, a distance of one hundred miles, there are but two outlets for the discharge of all the waters which flow from the land to the sea; and the eastern front of the dunes is bordered by a succession of stagnant pools, some of which are more than six miles in length and breadth. [Footnote: Laval, Memoire sur les Dunes du Golfe de Gascongne, Annales des Ponte et Chaussees, 1847, p. 223. The author adds, as a curious and unexplained fact, that some of these pools, though evidently not original formations but mere accumulations of water dammed up by the dunes, have, along their western shore, near the base of the sand-hills, a depth of more than one hundred and thirty feet, and hence their bottoms are not less than eighty feet below the level of the lowest tides. Their western banks descend steeply, conforming nearly to the slope of the dunes, while on the north-east and south the inclination of their beds is very gradual. The greatest depth of these pools corresponds to that of the sea ten miles from the shore. Is it possible that the weight of the sands has pressed together the soil on which they rest, and thus occasioned a subsidence of the surface extending beyond their base?
A more probable explanation of the fact stated in the note is suggested by Elisee Reclus, in an article entitled Le Littoral de la France, in the Revue des Deux Mondes for September 1, 1864, pp. 193, 194. This able writer believes such pools to be the remains of ancient maritime bays, which have been cut off from the ocean by gradually accumulated sand banks raised by the waves and winds to the character of dunes.] A range of dunes extends along the whole western coast of Jutland and Schleswig-Holstein, and the movement of these sand-hills was formerly, and at some points still is, very destructive. The rate of eastward movement of the drifting dunes varies from three to twenty-four feet per annum. If we adopt the mean of thirteen feet and a half for the annual motion, these dunes have traversed the widest part of the belt in about twenty-five hundred years. Historical data are wanting as to the period of the formation of these dunes and of the commencement of their drifting; but there is recorded evidence that they have buried a vast extent of valuable land within three or four centuries, and further proof is found in the fact that the movement of the sands is constantly uncovering ruins of ancient buildings, and other evidences of human occupation, at points far within the present limits of the uninhabitable desert. Andresen estimates the average depth of the sand deposited over this area at thirty feet, which would give a cubic mile and a half for the total quantity. [Footnote: Andresen, On Klitformationen, pp. 56, 79, 82]
The drifting of the dunes on the coast of Prussia commenced not much more than a hundred years ago. The Frische Nehrung is separated from the mainland by the Frische Haff, and there is but a narrow strip of arable land along its eastern borders. Hence its rolling sands have covered a comparatively small extent of dry land, but fields and villages have been buried and valuable forests laid waste by them. The loose coast-row has drifted over the inland ranges, which, as was noticed in the description of these dunes on a former page, were protected by a surface of different composition, and the sand has thus been raised to a height which it could not have reached upon level ground. This elevation has enabled it to advance upon and overwhelm woods, which, upon a plain, would have checked its progress, and, in one instance, a forest of many hundred acres of tall pines was destroyed by the drifts between 1804 and 1827.
Control of Dunes by Man.
There are three principal modes in which the industry of man is brought to bear upon the dunes. First, the creation of them, at points where, from changes in the currents or other causes, new encroachments of the sea are threatened; second, the maintenance and protection of them where they have been naturally formed; and third, the removal of the inner rows where the belt is so broad that no danger is to be apprehended from the loss of them.
In describing the natural formation of dunes, it was said that they began with an accumulation of sand around some vegetable or other accidental obstruction to the drifting of the particles. A high, perpendicular cliff, which deadens the wind altogether, prevents all accumulation of sand; but, up to a certain point, the higher and broader the obstruction, the more sand will heap up in front of it, and the more will that which falls behind it be protected from drifting further. This familiar observation has taught the inhabitants of the coast that an artificial wall or dike will, in many situations, give rise to a broad belt of dunes. Thus a sand dike or wall, of three or four miles in length, thrown in 1610 across the Koegras, a tide-washed flat between the Zuiderzee and the North Sea, has occasioned the formation of rows of dunes a mile in breadth, and thus excluded the sea altogether from the Koegras. A similar dike, called the Zijperzeedijk, has produced another scarcely less extensive belt in the course of two centuries. A few years since, the sea was threatening to cut through the island of Ameland, and, by encroachment on the southern side and the blowing off of the sand from a low flat which connected the two higher parts of the island, it had made such progress, that in heavy storms the waves sometimes rolled quite across the isthmus. The construction of a breakwater and a sand dike have already checked the advance of the sea, and a large number of sand-hills has been formed, the rapid growth of which promises complete future security against both wind and wave. Similar effects have been produced by the erection of plank fences, and even of simple screens of wattling and reeds. [Footnote: Staring, De Bodem van Nederland, i., pp. 329-331. Id., Voormaals en Thans, p. 163. Andresen, Om Klitformationen, pp. 280, 295.
The creation of new dunes, by the processes mentioned in the text, seems to be much older in Europe than the adoption of measures for securing them by planting. Dr. Dwight mentions a case in Massachusetts, where a beach was restored, and new dunes formed, by planting beach grass. "Within the memory of my informant, the sea broke over the beach which connects Truro with Province Town, and swept the body of it away for some distance. The beach grass was immediately planted on the spot; in consequence of which the beach was again raised to a sufficient height, and in various places into hills."—Travels, iii., p. 93.]
The dunes of Holland are sometimes protected from the dashing of the waves by a revetement of stone, or by piles; and the lateral high-water currents, which wash away their base, are occasionally checked by transverse walls running from the foot of the dunes to low-water mark; but the great expense of such constructions has prevented their adoption on a large scale. [Footnote: Staring, i., pp. 310, 332.] The principal means relied on for the protection of the sand-hills are the planting of their surfaces and the exclusion of burrowing and grazing animals. There are grasses, creeping plants, and shrubs of spontaneous growth, which flourish in loose sand, and, if protected, spread over considerable tracts, and finally convert their face into a soil capable of cultivation, or, at least, of producing forest trees. Krause enumerates one hundred and seventy-one plants as native to the coast sands of Prussia, and the observations of Andresen in Jutland carry the number of these vegetables up to two hundred and thirty-four.
Some of these plants, especially the Arundo arenaria or arenosa, or Psamma or Psammophila arenaria—Klittetag, or Hjelme in Danish, helm in Dutch, Dunenhalm, Sandschilf, or Hugelrohr in German, gourbet in French, and marram in English—are exclusively confined to sandy soils, and thrive well only in a saline atmosphere. [Footnote: There is some confusion in the popular use of these names, and in the scientific designations of sand-plants, and they are possibly applied to different plants in different places. Some writers style the gourbet Calamagrostis arenaria, and distinguish it from the Danish Klittetag or Hjelme.] The arundo grows to the height of about twenty-four inches, but sends its strong roots with their many rootlets to a distance of forty or fifty feet. It has the peculiar property of flourishing best in the loosest soil, and a sand-shower seems to refresh it as the rain revives the thirsty plants of the common earth. Its roots bind together the dunes, and its leaves protect their surface. When the sand ceases to drift, the arundo dies, its decaying roots fertilizing the sand, and the decomposition of its leaves forming a layer of vegetable earth over it. Then follows a succession of other plants which gradually fit the sand-hills by growth and decay, for forest planting, for pasturage, and sometimes for ordinary agricultural use.
But the protection and gradual transformation of the dunes is not the only service rendered by this valuable plant. Its leaves are nutritious food for sheep and cattle, its seeds for poultry; [Footnote: Bread, not indeed very palatable, has been made of the seeds of the arundo, but the quantity which can be gathered is not sufficient to form an important economical resource.—Andresen, Om Klitformationen, p. 160.] cordage and netting twine are manufactured from its fibres, it makes a good material for thatching, and its dried roots furnish excellent fuel. These useful qualities, unfortunately, are too often prejudicial to its growth. The peasants feed it down with their cattle, cut it for rope-making, or dig if up for fuel, and it has been found necessary to resort to severe legislation to prevent them from bringing ruin upon themselves by thus improvidently sacrificing their most effectual safeguard against the drifting of the sands. [Footnote: Bergsoe, Reventlovs Virksomhed, ii., p. 4.]
In 1539 a decree of Christian III., king of Denmark, imposed a fine upon persons convicted of destroying certain species of sand-plants upon the west coast of Jutland. This ordinance was renewed and made more comprehensive in 1558, and in 1569 the inhabitants of several districts were required, by royal rescript, to do their best to check the sand-drifts, though the specific measures to be adopted for that purpose are not indicated. Various laws against stripping the dunes of their vegetation were enacted in the following century, but no active measures were taken for the subjugation of the sand-drifts until 1779, when a preliminary system of operation for that purpose was adopted. This consisted in little more than the planting of the Arundo arenaria, and other sand-plants, and the exclusion of animals destructive to those vegetables. [Footnote: Measures were taken for the protection of the dunes of Cape Cod, in Massachusetts, during the colonial period, though I believe they are now substantially abandoned. A hundred years ago, before the valley of the Mississippi, or even the rich plains of Central and Western New York, were opened to the white settler, the value of land was relatively much greater in New England than it is at present, and consequently some rural improvements were then worth making, which would not now yield sufficient returns to tempt the investment of capital. The money and the time required to subdue and render productive twenty acres of sea-sand on Cape Cod, would buy a "section" and rear a family in Illinois. The son of the Pilgrim, therefore, abandons the sea-hills, and seeks a better fortune on the fertile prairies of the West. See Dwight, Travels, i., pp. 92, 93.] Ten years later, plantations of forest trees, which have since proved so valuable a means of fixing the dunes and rendering them productive, were commenced, and have been continued ever since. [Footnote: Andresen, Om Klitformationen, pp. 237, 240.] During this latter period, Bremontier, without any knowledge of what was doing in Denmark, experimented upon the cultivation of forest trees on the dunes of Gascony, and perfected a system, which, with some improvements in matters of detail, is still largely pursued on those shores.
The example of Denmark was soon followed in the neighboring kingdom of Prussia, and in the Netherlands; and, as we shall see hereafter, these improvements have been everywhere crowned with most flattering success.
Under the administration of Reventlov, a little before the close of the last century, the Danish Government organized a regular system of improvement in the economy of the dunes. They were planted with the arundo and other vegetables of similar habits, protected against trespassers, and at last partly covered with forest trees. By these means much waste soil has been converted into arable ground, a large growth of valuable timber obtained, and the further spread of the drifts, which threatened to lay waste the whole peninsula of Jutland, to a considerable extent arrested.
In France, the operations for fixing and reclaiming the dunes—which began under the direction of Bremontier about the same time as in Denmark, and which are, in principle and in many of their details, similar to those employed in the latter kingdom—have been conducted on a far larger scale, and with greater success, than in any other country. This is partly owing to a climate more favorable to the growth of suitable forest trees than that of Northern Europe, and partly to the liberality of the Government, which, having more important landed interests to protect, has put larger means at the disposal of the engineers than Denmark and Prussia have found it convenient to appropriate to that purpose. The area of the dunes already secured from drifting, and planted by the processes invented by Bremontier and perfected by his successors, is about 100,000 acres. [Footnote: "These plantations, perseveringly continued from the time of Bremontier, now cover more than 40,000 hectares, and compose forests which are not only the salvation of the department, but constitute its wealth." —Clave, Etudes Forestieres, p. 254. Other authors have stated the plantations of the French dunes to be much more extensive.] This amount of productive soil, then, has been added to the resources of France, and a still greater quantity of valuable land has been thereby rescued from the otherwise certain destruction with which it was threatened by the advance of the rolling sand-hills.
The improvements of the dunes on the coast of West Prussia began in 1795, under Soren Bjorn, a native of Denmark, and, with the exception of the ten years between 1807 and 1817, they have been prosecuted ever since. The methods do not differ essentially from those employed in Denmark and France, though they are modified by local circumstances, and, with respect to the trees selected for planting, by climate. In 1850, between the mouth of the Vistula and Kahlberg, 6,300 acres, including about 1,900 acres planted with pines and birches, had been secured from drifting; between Kahlberg and the eastern boundary of West Prussia, 8,000 acres; and important preliminary operations had been carried on for subduing the dunes on the west coast. [Footnote: Kruse, Dunenbau, pp. 34, 38, 40.]