Man and Nature Part 19

_Formation of Dunes._

The laws which govern the formation of dunes are substantially these. We have seen that, under certain conditions, sand is acc.u.mulated above high-water mark on low sea and lake sh.o.r.es. So long as the sand is kept wet by the spray or by capillary attraction, it is not disturbed by air currents, but as soon as the waves retire sufficiently to allow it to dry, it becomes the sport of the wind, and is driven up the gently sloping beach until it is arrested by stones, vegetables, or other obstructions, and thus an acc.u.mulation is formed which const.i.tutes the foundation of a dune. However slight the elevation thus created, it serves to stop or r.e.t.a.r.d the progress of the sand grains which are driven against its sh.o.r.eward face, and to protect from the further influence of the wind the particles which are borne beyond it, or rolled over its crest, and fall down behind it. If the sh.o.r.e above the beach line were perfectly level and straight, the gra.s.s or bushes upon it of equal height, the sand thrown up by the waves uniform in size and weight of particles as well as in distribution, and if the action of the wind were steady and regular, a continuous bank would be formed, everywhere alike in height and cross section. But no such constant conditions anywhere exist. The banks are curved, broken, unequal in elevation; they are sometimes bare, sometimes clothed with vegetables of different structure and dimensions; the sand thrown up is variable in quant.i.ty and character; and the winds are s.h.i.+fting, gusty, vortical, and often blowing in very narrow currents. From all these causes, instead of uniform hills, there rise irregular rows of sand heaps, and these, as would naturally be expected, are of a pyramidal, or rather conical shape, and connected at bottom by more or less continuous ridges of the same material.

On a receding coast, dunes will not attain so great a height as on more secure sh.o.r.es, because they are undermined and carried off before they have time to reach their greatest dimensions. Hence, while at sheltered points in Southwestern 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 sh.o.r.es 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 seaward, and here they rise to the greatest alt.i.tude 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.[426] By successive acc.u.mulations, 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 some times 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 calcareous 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.

_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 sh.e.l.ls, small indeed, but still visible to the naked eye.[427] These fragments are not constant const.i.tuents 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 coa.r.s.e 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 acc.u.mulated. Land sh.e.l.ls, such as snails, for example, are found on the surface of the dunes in abundance, and many of the sh.e.l.ly fragments in the interior of the hillocks may be derived from the same source."[428]

J. G. Kohl has some poetical thoughts upon the origin and character of the dune sands, which are worth quoting:

"The sand was composed of pure transparent quartz. I could not observe this sand without the greatest admiration. If it is the product of the waves, breaking and crus.h.i.+ng flints and fragments of quartz against each other, it is a result which could be brought about only in the course of countless ages. We need not lift ourselves to the stars, to their incalculable magnitudes and distances and numbers, in order to feel the giddiness of astonishment. Here, upon earth, in the simple sand, we find miracle enough. Think of the number of sand grains contained in a single dune, then of all the dunes upon this widely extended coast--not to speak of the innumerable grains in the Arabian, African, and Prussian deserts--this, of itself, is sufficient to overwhelm a thoughtful fancy.

How long, how many times must the waves have risen and sunk in order to reduce these vast heaps to powder!

"During the whole time I spent on this coast, I had always some sand in my fingers, was rubbing and rolling it about, examining it on all sides, holding a little s.h.i.+ning grain on the tip of my finger, and thinking to myself how, in its corners, its angles, its whole configuration, it might very probably have a history longer than that of the old German nation--possibly longer than that of the human race. Where was the original quartz crystal, of which this is a fragment, first formed? To what was it once fixed? What power broke it loose? How was it beaten smaller and ever smaller by the waves? They tossed it, for aeons, to and fro upon the beach, rolled it up and down, forced it to make thousands and thousands of daily voyages for millions and millions of days. Then the wind bore it away, and used it in building up a dune; there it lay for centuries, packed in with its fellows, protecting the marshes and cherished by the inhabitants, till, seized again by the pursuing sea, it fell once more into the water, there to begin the endless dance anew--and again to be swept away by the wind--and again to find rest in the dunes, a protection and a blessing to the coast. There is something mysterious about such a grain of sand, and at last I went so far as to fancy a little immortal spark linked with each one, presiding over its destiny, and sharing its vicissitudes. Could we arm our eyes with a microscope, and then dive, like a sparling, into one of these dunes, the pile, which is in fact only a heap of countless little crystal blocks, would strike us as the most marvellous building upon earth. The sunbeams would pa.s.s, with illuminating power, through all these little crystalline bodies. We should see how every sand grain is formed, by what multifarious little facets it is bounded, we should even discover that it is itself composed of many distinct particles."[429]

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.[430]

_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 wind as well as of water. The origin and peculiar character of these layers are due to a variety of causes. A southwest wind and current may deposit upon a dune a stratum of a given color and mineral composition, and this may be succeeded by a northwest wind and current, bringing with them particles of a different hue, const.i.tution, 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 coa.r.s.e sand, and the same effect might result from the blowing away of light particles out of a mixed layer, while the heavier remained undisturbed.[431] Still another cause of 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.

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 t.i.tanian 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.

_Form of Dunes._

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 landward, 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 with the horizon, while the more exposed and irregular weather side lies at an inclination of from 5 to 10. 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 s.h.i.+fting 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 southwest and west, the prevailing winds blowing from the northeast and east; but it has been doubted whether the shoals of the western coast of Northern Africa, and the sands upon that sh.o.r.e, 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.[432] There is nothing violently improbable in the supposition that they may have been first thrown up by the Mediterranean on its Libyan coast, and thence blown south and west over the vast s.p.a.ce they now cover. But whatever has been their source and movement, 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.[433]

If the sand of coast dunes is, as Staring describes it, composed chiefly of well-rounded quartzose grains, fragments of sh.e.l.ls, and other constant ingredients, it would often be recognizable as coast sand, in its agglutinate state of sandstone. The texture of this rock varies from an almost imperceptible fineness of grain to great coa.r.s.eness, and affords good facilities for microscopic observation of its structure.

There are sandstones, such, for example, as are used for grindstones, where the grit, as it is called, is of exceeding sharpness; others where the angles of the grains are so obtuse that they scarcely act at all on hard metals. The former may be composed of grains of rock, disintegrated indeed, and recemented 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 would be difficulty in distinguis.h.i.+ng them;[434] 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.[435]

_Inland Dunes._

I have met with some observations indicating a structural difference between interior and coast dunes, which might perhaps be recognized in the sandstones formed from these two species of sand hills respectively.

In the great American desert between the Andes and the Pacific, Meyen found sand heaps of a perfect falciform shape.[436] They were from seven to fifteen feet high, the chord of their arc measuring from twenty to seventy paces. The slope of the convex face is described as very small, that of the concave as high as 70 or 80, and their surfaces were rippled. No smaller dunes were observed, nor any in the process of formation. The concave side uniformly faced the northwest, except toward the centre of the desert, where, for a distance of one or two hundred paces, they gradually opened to the west, and then again gradually resumed the former position.

Poppig ascribes a falciform shape to the movable, a conical to the fixed dunes, or _medanos_, of the same desert. "The medanos," he observes, "are hillock-like elevations of sand, some having a firm, others a loose base. The former [latter], which are always crescent shaped, are from ten to twenty feet high, and have an acute crest. The inner side is perpendicular, and the outer or bow side forms an angle with a steep inclination downward. When driven by violent winds, the medanos pa.s.s rapidly over the plains. The smaller and lighter ones move quickly forward, before the larger; but the latter soon overtake and crush them, whilst they are themselves s.h.i.+vered by the collision. These medanos a.s.sume all sorts of extraordinary figures, and sometimes move along the plain in rows forming most intricate labyrinths. * * A plain often appears to be covered with a row of medanos, and some days afterward it is again restored to its level and uniform aspect. * * *

"The medanos with immovable bases are formed on the blocks of rocks which are scattered about the plain. The sand is driven against them by the wind, and as soon as it reaches the top point, it descends on the other side until that is likewise covered; thus gradually arises a conical-formed hill. Entire hillock chains with acute crests are formed in a similar manner. * * * On their southern declivities are found vast ma.s.ses of sand, drifted thither by the mid-day gales. The northern declivity, though not steeper than the southern, is only sparingly covered with sand. If a hillock chain somewhat distant from the sea extends in a line parallel with the Andes, namely, from S. S. E. to N.

N. W., the western declivity is almost entirely free of sand, as it is driven to the plain below by the southeast wind, which constantly alternates with the wind from the south."[437]

It is difficult to reconcile this description with that of Meyen, but if confidence is to be reposed in the accuracy of either observer, the formation of the sand hills in question must be governed by very different laws from those which determine the structure of coast dunes.

Captain Gilliss, of the American navy, found the sand hills of the Peruvian desert to be in general crescent shaped, as described by Meyen, and a similar structure is said to characterize the inland dunes of the Llano Estacado and other plateaus of the North American desert, though these latter are of greater height and other dimensions than those described by Meyen. There is no very obvious explanation of this difference in form between maritime and inland sand hills, and the subject merits investigation.[438]

_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.[439] In some cases, there has been an upheaval of the coast line since the formation of the oldest hillocks, and these have become inland dunes, while younger rows have been thrown up on the new beach laid bare by elevation of the sea bed. Our knowledge of the mode of their first acc.u.mulation is derived from observation of the action of wind and water in the few instances where, with or without the aid of man, new coast dunes have been acc.u.mulated, and of the influence of wind alone in elevating new sand heaps inland of the coast tier, when the outer rows are destroyed by the sea, as also when the sodded surface of ancient sands has been broken, and the subjacent strata laid open to the air.

It is a question of much interest, in what degree the naked condition of most dunes is to be ascribed to the improvidence and indiscretion of man. There are, in Western France, extensive ranges of dunes covered with ancient and dense forests, while the recently formed sand hills between them and the sea are bare of vegetation, and 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 s.h.i.+fting, 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 s.h.i.+fting dunes, a.s.signs the fifth century of the Christian era as the period when these processes were abandoned.[440]

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.[441]

There is every reason to believe that the dunes of the Netherlands were clothed with trees until after the Roman invasion. The old geographers, in describing these countries, speak of vast forests extending to the very brink of the sea; but drifting coast dunes are first mentioned by the chroniclers of the Middle Ages, and so far as we know they have a.s.sumed a destructive character in consequence of the improvidence of man.[442] 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 s.h.i.+fting of the sands, except where the trees had been cut or turned up by the roots.[443]

Nature, as she builds up dunes for the protection of the sea sh.o.r.e, provides, with similar conservatism, for the preservation of the dunes themselves; so that, without the interference of man, these hillocks would be, not perhaps absolutely perpetual, but very lasting in duration, and very slowly altered in form or position. When once covered with the trees, shrubs, and herbaceous growths adapted to such localities, dunes undergo no apparent change, except the slow occasional undermining of the outer tier, and accidental destruction by the exposure of the interior, from the burrowing of animals, or the upturning of trees with their roots, and all these causes of displacement are very much less destructive when a vegetable covering exists in the immediate neighborhood of the breach.

Before the occupation of the coasts by civilized and therefore destructive 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,[444] and to have spontaneously clothed themselves with a dense growth of the various plants, gra.s.ses, shrubs, and trees, which nature has a.s.signed to such soils. It is observed in Europe that dunes, though now without the shelter of a forest country behind them, begin to protect themselves as soon as human trespa.s.sers are excluded, and grazing animals denied access to them. Herbaceous and arborescent plants spring up almost at once, first in the depressions, and then upon the surface of the sand hills. Every seed that sprouts, binds together a certain amount of sand by its roots, shades a little ground with its leaves, and furnishes food and shelter for still younger or smaller growths. A succession of a very few favorable seasons suffices to bind the whole surface together with a vegetable network, and the power of resistance possessed by the dunes themselves, and the protection they afford to the fields behind them, are just in proportion to the abundance and density of the plants they support.

The growth of the vegetable covering can, of course, be much accelerated by judicious planting and watchful care, and this species of improvement is now carried on upon a vast scale, wherever the value of land is considerable and the population dense. In the main, the dunes on the coast of the German Sea, notwithstanding the great quant.i.ty of often fertile land they cover, and the evils which result from their movement, are, upon the whole, a protective and beneficial agent, and their maintenance is an object of solicitude with the governments and people of the sh.o.r.es they protect.[445]

_Use of Dunes as a Barrier against the Sea._

Although the sea throws up large quant.i.ties of sand on flat lee-sh.o.r.es, there are, as we have seen, many cases where it continually encroaches on those same sh.o.r.es and washes them away. At all points of the shallow North Sea where the agitation of the waves extends to the bottom, banks are forming and rolling eastward. Hence the sea sand tends to acc.u.mulate upon the coast of Schleswig-Holstein and Jutland, and were there no conflicting influences, the sh.o.r.e would rapidly extend itself westward.

But the same waves which wash the sand to the coast undermine the beach they cover, and still more rapidly degrade the sh.o.r.e at points where it is too high to receive partial protection by the formation of dunes upon it. The earth of the coast is generally composed of particles finer, lighter, and more transportable by water than the sea sand. While, therefore, the billows raised by a heavy west wind may roll up and deposit along the beach thousands of tons of sand, the same waves may swallow up even a larger quant.i.ty of fine sh.o.r.e earth. This earth, with a portion of the sand, is swept off by northwardly and southwardly currents, and let fall at other points of the coast, or carried off, altogether, out of the reach of causes which might bring it back to its former position.

Although, then, the eastern sh.o.r.e of the German Ocean here and there advances into the sea, it in general retreats before it, and but for the protection afforded it by natural arrangements seconded by the art and industry of man, whole provinces would soon be engulfed by the waters.

This protection consists in an almost unbroken chain of sand banks and dunes, extending from the northernmost point of Jutland to the Elbe, a distance of not much less than three hundred miles, and from the Elbe again, though with more frequent and wider interruptions, to the Atlantic borders of France and Spain.[446] So long as the dunes are maintained by nature or by human art, they serve, like any other embankment or dike, as a partial or a complete protection against the encroachments of the sea; and on the other hand, when their drifts are not checked by natural processes, or by the industry of man, they become a cause of as certain, if not of as sudden, destruction as the ocean itself whose advance they r.e.t.a.r.d.

_Encroachments of the Sea._

The eastward progress of the sea on the Danish and Netherlandish coast, and on certain sh.o.r.es of the Atlantic, depends so much on local geological structure, on the force and direction of tidal and other marine currents, on the volume and rapidity of coast rivers, on the contingencies of the weather and on other varying circ.u.mstances, that no general rate can he a.s.signed to it.

At Agger, near the western end of the Liimfjord, in Jutland, the coast was washed away, between the years 1815 and 1839, at the rate of more than eighteen feet a year. The advance of the sea appears to have been something less rapid for a century before; but from 1840 to 1857, it gained upon the land no less than thirty feet a year. At other points of the sh.o.r.e of Jutland, the loss is smaller, but the sea is encroaching generally upon the whole line of the coast.[447]