Popular Science Monthly/Volume 33/August 1888/Drift-Sands and their Formations
|DRIFT-SANDS AND THEIR FORMATIONS.|
FIVE large sand-tracts may be designated in Europe—the German lowland, extending from Holland through Germany to Russia (about 340,000 English square miles in area); the Dano-Germanic island plains (20,000 square miles), including Schleswig-Holstein, Denmark, and Jutland; the Austro-Hungarian Danubian plain (about 42,000 square miles); the Landes of France (about 5,400 square miles); and the sea-coast sands of Russia, Germany, Belgium, Holland, and France. These extensive regions have, for the most part, either been made amenable to cultivation, or at least protected from the assaults of the winds by preservative plantations. Tracts of this sort are often much more fruitful than we are accustomed to suppose them to be. But there are also in Europe large fields of sand which are hardly if at all covered with plants. They are the dunes and sands on the coasts of Prussia, Pomerania, Jutland, many of the Danish and Frisian islands, Hanover, Oldenburg, Holland, Belgium, and France. Large systems of dunes extend in France from Brittany to near the Pyrenees. But there are also exposed sand-tracts in the interior. The most extensive of them are in Hungary, of which the most important is probably the sand-barren of the Banat. It forms an oval about thirty-five kilometres long by eleven kilometres broad, and has an area of nearly four hundred square kilometres. It presents the appearance of a rolling region with elongated hills. often resembling mountains, which for the most part follow the trend of the steppe, as well as the direction of the prevailing wind, which is here from southeast to northwest. The hills rise, counting from the nearest valleys, to a height of fifty-five metres, inclining gently on the windward side, but presenting a steep slope on the lee. This steppe has indeed always in historical times included bare spaces, varying in number and extent according to the existing state of cultivation. Till the beginning of the present century the bare drift-sand had so gained the upper hand that there was in the middle of the tract a continuous extent of one hundred and fifty square kilometres in which grass and shrub land could be found only in small spots and streaks. Only the northeastern and southeastern part of the steppe consisted of grass-land, in which bare spaces were likewise not wanting. Many other such sand-districts might be named. For example, there were in the County Pesth, in 1809, nearly 1,600 square kilometres of more or less bare, but not continuous sands.
Let us inquire into the origin of these sand-masses. The open sea is usually bordered by deposits of sand and gravel. Where the shore rises in steep rocks, these deposits are concealed under the water; but where the coast is alluvial, as is generally the case in the smaller seas, the whole shore consists of sand and shingle; and the sea-drift extends up upon the dry land. The waves are constantly bringing up new material to the beach, chiefly consisting of fine sand. A storm stirs the water to a considerable depth, pushes the sand forward, and bears it with the waves high upon the shore. On the retreat of the waves, a part of the sand remains, because it is specifically lighter than water. When the storm has subsided, and the sea has withdrawn to its proper limits, the sand becomes dried, and falls under the power of the strong, restless wind, which takes up all the fine particles, and when it rises to be a storm wind some of the coarser ones too, and carries them away into the country. Here it meets impediments of various forms and efficiency; then it drops its load irregularly, and gradually piles up along the shore those hills which are generally known as sea-coast dunes.
This edging of dunes which borders the flat sea-shore is of various breadths in different places. In some places it becomes so piled up as to cause a retreat of the waves. New dunes are constantly formed farther toward the sea, and the dune-zone is widened. Sometimes the widening is effected by the emigration of the bare dunes toward the interior. In other places the dunes are broken into by the sea-water, and suffer loss of breadth.
When there is no overflowing of the shore or breaking up of the dunes, the system usually consists of three more or less connected rows of hills. First, the fore-dunes, which first receive the material added by the waves; then the high dunes back of them, which, receiving the drift-sand, gradually rise in height and become better fitted to protect the mainland against wind and sand; third, the inner dunes, a series of low hills lying back of the high dunes, which are formed of the sand that is blown over the high dunes. [This division is, however, theoretical, and not always recognizable in the fact.]
The two sides of the dune slope at different angles. On the windward side the rise is gradual, at an angle seldom of more than from 5° to 10°. But the other side is much steeper, and usually offers an angle of 30°. The variations that appear are explained by differences in the cohesion of the sand; coarse sand forms a steeper slope than fine sand. The dunes have an average height of some forty-five or fifty feet; but there are some of greater altitude. In Jutland, for example, and on the Frisian and Courland low ground, and in the Landes, they rise to from one hundred and fifty to two hundred feet. In breadth, they vary from three hundred to three thousand feet; but they reach an extraordinary development in the Landes, where breadths of thirty thousand feet are not rare.
The large sand stretches in the interior were formerly sea-bottoms. Another origin has been sought for the Banat sands of the Hungarian steppe, in the supposition that they are a product of the Danube, which, tremendously disturbed by southeastern storms in the vicinity of Palanka, has thrown its sand-weighted spray into the air to be carried far inward, dropping its heavier constituents along the way. This hypothesis needs to be mentioned only to be contradicted.
It can hardly be supposed that an inland drift-sand district has lain bare during historical time, or since that long-past epoch when the retreating waters left it dry. On the contrary, sand constantly acquires an increasing verdancy through the unceasing efforts of Nature, and becomes at last covered with a thick carpet of the lower plants, or with wood, and thereby fully protected against the wind. This gradual process of binding goes on the more speedily when it is not interrupted by man. The presence of a former coating of humus is easily proved by digging into the sand; for, everywhere that the sand-hills are removed, dark-colored strata appear, which were certainly once surface-soil covered with plants. The wind-blowings occasionally bring to light carbonized relics and reed-shaped holes which are unmistakably derived from tree-roots. Cylindrical massive or hollow tufa formations are also observed, the shapes, direction, and ramifications of which likewise point to roots; and they have probably been formed from roots through the infiltration of calciferous water.
The origin of the bare sand-spots may be traced to the agency of man. The immigrating nomadic populations required for their herds not wood, but pasture and tillable land, and mercilessly cleared away the forest. The land thereby became arid, and wherever a pasture or meadow was not established, the sand, deprived of its covering, became a prey to the winds. Even if this view be regarded as a hypothesis that can not be proved, it is at least illustrated and made comprehensible by events which are historically authenticated or are still taking place. When the Turks were driven out of Hungary, the sand-tracts, for the most part, lay waste. The Italian Griselini, who traveled through the Banat under a commission of the Empress Maria Theresa, wrote: "For nearly eight German miles in length and from nine to ten thousand fathoms in breadth, the sand, when it is not moist, is so fugitive that it is taken up by the wind and deposited in little hills of various heights."
The once well-clothed level sand region of Tidsvild in Zealand, where a religious house was built in the twelfth century, was, at a later date, through carelessness and the destruction of the woods during the Swedish invasion of 1658-'60, given up to the ravages of the winds. Wide tracts and even valleys, like Tomb, and, in 1730, Tibirke, were overwhelmed with sand. The Government was aroused by these disasters, and earnestly undertook the work of irrigating the sand. The enterprise was successful, a fact of which a memorial stone erected in the territory bears witness, in an inscription in Danish, German, and Latin, relating, "The drift sand was watered at the command of Kings Frederick and Christian, by the faithful industry of Warden Frederick von Granu and Roehl's skilled hand." A similar instance of the letting loose of the drift-sand through the careless destruction of the woods is recorded in East Prussia.
There are cases even now where, through the greed or ignorance of man, bare sand-tracts are allowed to be formed in the midst of cultivated lands. This takes place, for example, where grass-land is pastured to excess, or the turf is trodden out by the too frequent passage of large herds over the same spot. Exposure to the direction of the prevailing winds, subjecting broken spots to frequent sweepings and promoting the washing out of ruts by rains, poor farming, and careless burning of the shrubbery are also dangerous, and in Hungary have led to the enactment of laws regulating the treatment of sand-lands.
As has already been mentioned, the noxious quality of sand consists not so much in its own infertility as in its being subject to transportation by strong winds and deposition upon fertile spots, where it buries and destroys the lower vegetation. When it is thus driven or flies away from its original place it receives the name of drift or flying sand. While single grains have but little cohesive force, sand behaves toward the wind like water; and the method of the formation of dunes is undoubtedly very similar to that of the formation of waves. If the sand was quite even and horizontal and the wind blew regularly in the same direction, it would not get at the sand. But the surface of a sand bed is not even; it consists of the roundish heads of the sand grains that form the upper layer. The wind blowing over them moves them out of their place, and, the individual grains being roundish, they roll. The continuous pressure of the wind extends their movement, and these grains striking upon the projecting grains that are still at rest, disturb them, and the movement spreads more and more. The smaller grains at last no longer touch the ground, and only the heavy ones retain the springing character of their motion, till the wind is restrained and weakened by some fixed objects—plants or buildings—and is compelled to let part or all of its load fall. These objects are thus exposed to be submerged in sand; and hence it is that we so often see fields of low plants and even villages overwhelmed. It is interesting to observe the different ways in which different objects receive the wind. A tight wall does not catch the sand immediately in front of itself. A furrow is formed just before it, through the generation of side-currents, which receive the sand from actual contact with the wall. There is then formed a sand-ridge parallel to the wall, but at first separated from it by a hollow; but afterward, when the ridge has become high enough to shield the wall from the wind, the side-currents are extinguished, and the sand advances to it. The sand driven over the wall by the wind falls at a considerable distance behind it. A striking illustration of this process was formerly to be seen at the church of Altpillau, on the Baltic. The village, which had previously surrounded the church, was removed farther to the east on account of the presence of the sand, but the church had to be left where it was. A sand-ridge some twelve or twenty feet high was formed around it, but nowhere reached the walls of the building; and while the congregation were obliged to climb over the ridge, they never found the church-doors buried. A broken wall, an open fence, or a quick-set hedge behaves quite differently toward the advancing sand. No furrow is formed in front of it. The air-current forces a large part of its load through the openings, but is so weakened by the obstruction that it drops it before and behind the fence. A little wall is formed around a tree-trunk, which is not, however, of great extent behind it. In isolated bushes and tufts of herbage, the intervals between the single stalks are filled up with sand, and a little mound is gradually formed.
Like the inland drift-sands, the dunes of the coast also migrate. Fortunately, it is only the dry sand that is destitute of cohesion. Were this not the case, a whole dune might be taken up and removed to another spot during a very heavy storm. Sand possesses considerable capillarity, by virtue of which the ground-water at its bottom rises through its substance; so that an apparently dry dune is so moist only a few feet below its surface, as to form a compact mass. Rain, however, only temporarily lends it a certain degree of fixedness. The air present between the grains of sand permits to rain-water a slow percolation, so that it has been observed, particularly with fine sand, that water ascends in it from below more rapidly than it descends from above. Rain-water, not being all sucked up by the sand, has to run down the slope, and, therefore, not rarely washes deep furrows in the mass. Sand that has been moistened by rain is more tractable after drying than before it was wet, because the interposition of the water has separated the grains, and they are more easily moved by the wind. The wind can only wear away the surface of a dune. It therefore takes the direction of the ascent of the dune, and carries the sand with it. A space free from wind is formed upon the top of the dune, where the larger grains fall upon the ground and run down on its other side, forming a nook in which they are enabled by their cohesion to remain, while the finer grains are carried farther by the wind. The dunes thus maintain their general forms while slowly advancing. The progress of the dunes has been frequently observed, and attempts have been made to measure it. A series of observations for twenty-three years in the barrens of the Banat gives an average of two metres a year. It is estimated, according to Count Adelbert Baudissen, on the island of Sylt, that the dunes are moving from west to east at the rate of four metres a year. Hagen names a rate of five and a half meters a year for two dunes on the Friesian lowlands, and Krause, four metres a year for another one. Elie de Beaumont describes dunes in Brittany that have moved since 1666 at the rate of seven metres a year; and Behrendt gives the average annual progress as from five to six metres.
A traveling dune is stopped by no obstacles. With the irresistibility of an element only slower than water or fire, it presses forward, burying field and wood, and even whole villages. The spires of church-towers may still be seen projecting out of the sandy sea of Brittany, testifying to the presence of former dwelling-places, there. The whole northern point of Jutland has been • given up by man to the advancing sand.
In the seventeenth century an old churchyard was found, over which a dune had taken its course, on the Courland lowlands north of Kranz. A sand hill that separated the hamlet Sarkan from the parish-village Rositte, and which had to be crossed by the pastor, has since then disappeared in the gulf. Great dunes now cover the village of Lattenwalde, which was so laid waste during the Seven Years' War by plundering, quartering of the Russians, infectious diseases, and fire, that the sand had only a heap of ruins to cover up. The village of Kunzen, with its church and seventeen homesteads, was ruined in the same way in the course of the last and the beginning of the present century; and now the dune, continuing its journey, has permitted the skulls and skeletons of the former churchyard on the west side to be again exposed.
The village of Pillkoppen has had a remarkable fate. The inhabitants left the place about the middle of the last century, and founded New Pillkoppen, at about a mile away. Then the dune went on in an unanticipated course, and old Pillkoppen has risen anew since the third decade of the present century; but the sand is already again a foot high in the potato-garden of the new school-house.
A fine wood near Schwarzort has been almost systematically destroyed by a dune advancing toward the southeast. It was composed of primitive oaks, lindens, and firs, and was in the year 1800 about five kilometres long, while now the dune has hardly left a kilometre and a half of it. Schumann says of this wood that "in about ten years after the tree has gone into the southern side of the moving dune, it emerges again from the north side. But the boughs which have been dried out and withered up during the interval are broken, ground up, and reduced to atoms as soon as the sand has left them. The same occurs later to the rotted stems. Few of these trees show more than an inch over the surface of the sand; and it is only the thicker and hardier trunks that can maintain themselves so as to project from two to five metres over the diminished dune. With most of them the sap-wood disappears down to the surface, the bark with all, which, however, is still present beneath. Frequently, the bark alone is left, while the wood has rotted away. Such trees are marked only by a hardly perceptible bark-ring, and the careless traveler is in danger of falling into the holes they have left." The time may be fixed with an approach to accuracy when the whole wood shall have been destroyed, and Schwarzort itself will be threatened. Schumann estimates the yearly progress of the dune at twelve metres, and gives the last trees still eighty years before they shall be overwhelmed.
In the small islands west of Jutland, the progress of the dunes is illustrated by a diminution of the islands themselves. For the sea eats the shore away year by year as it is left bare, and when the dunes have marched over the whole islands, and precipitated themselves on the eastern side of them into the sea, the islands will have disappeared, and ships will find free sailing-ground where men are now living and cattle and sheep are pasturing.
Of course, man struggles to defend himself against this enemy. The only way of counteracting its movements is to cover the sand with vegetation and make it inaccessible to the attacks of the wind, and this is not very easily accomplished. The sand consists chiefly—seventy-five to ninety-eight per cent—of uncultivable quartz sand, in which only easily satisfied plants can be made to grow. The wind is, besides, sometimes so strong on the sea-coast as to permit lowly plants to grow only with difficulty, and trees not at all. Sand is, moreover, so fugitive a substance that plants are liable to be torn from it before they have taken firm root. But these hindrances can be overcome, though with difficulty. One of the first instances in which a sand mass was thus tamed was in Denmark in 1738. The sands of the Landes in France have been bound with entire success. Measures of precaution were undertaken in the neighborhood of Dantzic about the middle of the last century. As everywhere else on the Baltic, the dunes had been covered by Nature, except on the side toward the sea, with firs and bushes of all sorts, and thereby protected against the wind. But the ignorant greed of men had removed the wood, grubbed up the stumps, allowed cattle to tread the heaths at will, and treated the dunes so recklessly that their protective covering disappeared, and their sand masses were exposed to the winds. Consequently, at the beginning of the eighteenth century the villages of Kleinvogler and Schmergrube were wholly and Polski partly overwhelmed. It was not till about the middle of the century, when the dunes nearer to Dantzic began to encroach upon the fir-wood appertaining to the city, that measures of protection were thought of. The first measure suggested was the planting of fences of fir-boughs on the comb of the dune, to intercept the sand brought by the wind. This scheme failed. The deposits of sand in front and rear of the hedges made the constant planting of new barriers over the old ones necessary; and the dune increased in height at an alarming rate, involving a great danger of the sudden breaking down of the ridge, when the destruction effected by the sand would be worse than if it had been let alone.
In this dilemma, the Natural History Society of Dantzic, in 1768, offered a prize for the best answer to the question, "What are the most effective and cheapest means of preventing the overflow of the lowlands with sand, and of stopping the further growth of the dunes?" Titius, Professor of Natural History in Wittenberg, gained the prize, by an essay in which he indicated the restoration of the coast-woods as the only permanent remedy, and the planting of a sand-grass (Arundo arenaria) as the measure with which the immediate emergency might be met. His suggestion was not carried out. But when Dantzic became part of Prussia, in 1793, the Government took the matter energetically in hand. Burgher Sören-Biörn, who was of Danish birth, recommended the application of Titius's plan, and was intrusted with the redemption of one of the most threatening dunes. Having accomplished this work to the general satisfaction, he was made inspector of plantations, and in this capacity superintended the work till his death in 1819.
Since then, the work of fixing the drift-sands has been begun in several places. As a generally approved preliminary measure, a fore-dune is first formed. This is done by planting between the dune and the sea two parallel fences, about six feet apart, and rising some eight feet above the level of the sea. These structures weaken the force of the sea-winds so much as to cause a considerable proportion of the sand to fall between them, or on either side of them. Thus a dune is formed. It is secured by planting several rows of Arundo arenaria—a grass that can not be buried by sand, because the more it is covered the better it flourishes, throwing out strong roots from below, and even growing in length above. The fore-dune protects the real dune against waves during storms, against a part of the wind, and against a continued overflow of new sand. Time is also gained for providing a covering of plants. First the sand-grasses are planted, then herbage-plants, then heaths and willows, and finally, in the least protected places, trees, of which firs are preferred.
The cultivation of the inland drift-sand is somewhat easier; but in either case it requires continuous, active work. Seeds of suitable sand-plants are sown and covered with limbs, straw, etc., till the resultant growth has become strong enough to stand against the wind. The ground is gradually improved by means of these plants, till at a later period useful plants can be cultivated, and the former desert can be turned into tillable land or wood. The conversion is, indeed, a tedious process, but the result is profitable.—Translated for the Popular Science Monthly from Das Ausland.