Sand dune

“ Wherever the sand-dune tracts are located, and that is right along the coast, will be found an irregular succession of hummocks and hillocks, drifted by the wind, which are very characteristic. On the summits of these hillocks an Elymus has taken root in times past, and, as the sand drifts up, it keeps growing on and up too, so that a quaint spectacle is presented of large stretches to the view wherein sand-dunes, entirely bare of all vegetation at their base and on their sides, are crowned with a living cap of the brightest green—a tuft of long, waving grass blades which will not down. ”

“ When the night shadows come into the dune country, the spell of mystery and poetry comes with them. The sorcery of the dark places leads us into a land of dreams and unreality.
Out on the tremulous surface of the lake, we may fancy the lifting of silvery paddles in the path of the moon’s reflections, and the furtive movement across the bar of light, of mystic shapes in phantom canoes.
Mingled with the lispings of the little waves, we may hear ghostly prows touch the sand, and see spectral figures file into the hills. The faint echoes of strokes upon flint come out of the shadows. ”

“ The inland sand plains of Europe are either derived from the drifting of dunes or other beach sands, or consist of diluvial deposits, or are ancient sea-beds uplifted by geological upheaval. As we have seen, when once the interior of a dune is laid open to the wind, its contents ars soon scattered far and wide over the adjacent country, and the beach sands, no longer checked by the rampart which nature had constrained them to build against their own encroachments, are also carried to considerable distances from the coast. ”

“ The big ranges of sand dunes that skirt the southern and eastern shores of Lake Michigan, and the strip of sparsely settled broken country back of them, contain a rich fund of material for the artist, poet, and nature lover, as well as for those who would seek out the oddities of human kind in by-paths remote from much travelled highways. ”

“ The laws which govern the formation of dunes are substantially these. We have seen that, under certain conditions, sand is accumulated above high-water mark on low sea and lake shores. 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 accumulation is formed which constitutes the foundation of a dune. ”

“ A long sand dune, some fifteen feet high, runs parallel with the sea, and in the face of this dune strong walls, doorways, and ruins of all sorts are embedded. The sand in many places has been removed sufficiently to uncover entire rooms and passages, and the whole beach below is literally covered with broken tiles, apparently Roman, which presumably formed the roofs of the ruined dwellings. ”

“ It is well known that the front of a sand dune generally consists of sand deposited on a slope inclined at the angle of repose, say between 30° and 35°, and rolled into place up the long back slope of the dune by the wind. ”

“ Maples and Oaks, relics of the time when the dune area was fertile, may still be found dying and being buried over by drifting sand. Now it is a different community of plants and animals living here. The continually shifting sand and the scarcity of water limit the variety of life found, but each dune dweller is specially adapted to this homesite, and no matter how lush, green, and more attractive a neighboring meadow may look to us, many of these specialized organisms could not survive there at all. ”

“ As the tide falls, the sand laid bare is rapidly dried by the wind and carried above high-water mark. Then, safe beyond the reach of the waves, the minute particles are borne still farther from the water, and striking against some piece of drift-wood, bush, or tuft of grass, quickly build a hillock, which grows larger and larger as more sand falls upon it, and a dune is formed many feet in height. ”

“ This distinguishes small ripples from dunes, where the coarsest materials are generally in the troughs.
Wind-blown sand moves up the gentle upwind side of the dune by saltation or creep. Sand accumulates at the brink, the top of the slipface. When the buildup of sand at the brink exceeds the angle of repose, a small avalanche of grains slides down the slipface. Grain by grain, the dune moves downwind. ”

“ In most cases these wind-borne materials take on the form of dunes, or heaps of sand, varying from a few feet to several hundred feet in height. It is characteristic of these hills of blown sand that they move across the face of the country. Under favourable conditions they may journey scores of miles from the shore. The marching of a dune is effected through the rolling up of the sand on the windward side of the elevation, when it is impelled by the current of air to the crest where it falls into the lee or shelter which the hill makes to the wind. ”

“ 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. ”

“ This shrinking of summer’s vegetation, this uncovering of the body of the dune, has permitted the winter gales to reach the sand, and all up and down the great wall, on the tops of the dunes, the surface sand is moving. ”

“ Often near the shore the sandy barrier has been broken down by the wind or by the Page 178 waves, and a long gulley formed, which cuts deep into the dunes, and through which the sand drifts inland till it reaches a steep bank clothed with rushes, against which it heaps itself, and so, rising higher with the storms of each winter, forms another dune. This process has been going on for ages. The sands are for ever shifting, but moss begins to grow in sheltered spots; such wild flowers as can flourish there bloom and decay ”

“ Dunes in Death Valley consist of sand from local materials found in nearby mountains. The primary source of this sand is believed to be the Cottonwood Mountains to the west and northwest. Although the shapes and patterns of individual dunes may vary daily or seasonally, the dune field itself is relatively stable. ”

“ These islands are made wholly of limestone, the product of reef-building corals, and of lime from the sea water. The limestone sand of the beaches has been blown up into great dunes, some more than two hundred feet in height. Much of the loose dune sand has been changed to firm rock by percolating waters, which have dissolved some of the limestone and deposited it again as a cement between the grains.
Dunes occur not only in arid regions, but also wherever loose sand lies unprotected by vegetation from the wind. ”

“ THE DUNE COUNTRY WHILE there are immense stretches of sand dunes in other parts of the world, it is of a particular dune country, to which many journeys have been made, and in which many days have been spent, that this story will be told. ”

“ As individuals they go to pieces as soon as they touch these irregular formations, and become merely confused heaps of sand. Assuming the average journey, which they travel, to be twenty-five miles, since the mean yearly motion is about sixty-one feet, the life of a sand-dune may be estimated at more than 2,000 years. Since the desert is somewhat broken in places by ravines-and low hills, it is probable that but few of them make the full journey without at some time losing their identity. ”

“ On the coast of France they attain a height of 225 feet, and on the Atlantic border of the Sahara Desert are more than twice that elevation. But the desert sands are exceedingly fine and dust-like from attrition, and move in greater volume than is possible for the coarser sands of our coasts. They are whirled and tossed in the gale like dense smoke, but nowhere do they roll on as do waves of the ocean, as is sometimes stated. The transition of a sand-dune is by transfer and deposition of the individual particles of which it is composed. ”

“ The dunes are built up by slow accretions, and at the top the sand-grains are smaller than at the bottom. The process by which they are formed is a continual rolling of sand-grains up-hill by wind-force, and it is obvious that the lightest ones will attain the greatest elevation. These, too, are the ones that, on reaching the top of the hillock, roll-over on the protected side of the dune, and there form a mass of fine sand. ”

“ Formation of dunes.—In contrast with dust deposited from the atmosphere, wind-blown sand is commonly aggregated into mounds and ridges in the process of lodgment. These mounds and ridges are dunes. Once a dune is started, it occasions the further lodgment of sand, and is a cause of its own growth. Dunes sometimes reach heights of 200 or 300 feet, but they are much more commonly no more than 10 or 20 feet in height. On plane surfaces, there is a limit in height above which they do not rise, though the limit is different under different conditions. ”

“ Along coasts, dune ridges are often transverse to the wind, and groups of dune hillocks are frequently elongate in the same direction. Here the source of supply of the sand is itself an elongate belt, often transverse to the dominant wind, and the resulting dunes often have great length transverse to the wind. Where the wind has strong mastery over the sand, the longitudinal tendency is seen, even along coasts. ”

“ It is sometimes practicable to cross the dune-tract in a light vehicle, if the weather happens to be cool and one’s horses are in good condition. But crossing them is, however, never safe, for there is no water to be had within their repulsive bounds. The bones of many a lost wanderer lie there, covered by the sand streaming over the flat dune-top, under the lea of which he may have crept in the vain hope of getting shelter from the flame-hot wind from the north. ”

“ For the bones had evidently been washed out of the sandy dune running along the beach, which had been deposited at a time when the present coast lay ten to twenty metres below the surface of the sea, thus hundreds or thousands of years ago, undoubtedly before the time when the north coast of Asia was first inhabited by man. The dune sand is, as recently exposed profiles show, quite free from other kitchen-midden remains than those which occur upon its surface. ”

“ The dunes sweep for many miles along the Lake Michigan coasts. They are post-glacial, and are undergoing slow continual changes, both in form and place,—the loose sand responding lightly to the action of varying winds.{16}
The “fixed dunes” retain general forms, more or less stable, owing to the scraggly and irregular vegetation that has obtained a foothold upon them, but the “wandering dunes” move constantly. The fine sand is wafted in shimmering veils across the smooth expanses, over the ridges to the lee slopes. ”

“ You may find an occasional Apple tree growing out of the sand, rooted in a more fertile soil below, a reminder of the time when that bit of the Neck was a rich farmland. The roots of the Beach Plum also reach down to the water table, and it is thus able to grow out of the sand, although its seedlings cannot take root in the sand. Most of all, you will have an opportunity to note many special animal and plant peculiarities the dune dwellers have developed to suit their particular environment. ”

“ The velocity of wind at the bottom of the air is not so great as that higher up, and as a dune is built up, a level is presently reached where the stronger upper winds sweep away as much sand as is brought to the top. The very even crests of many dune ridges are probably to be accounted for in this way. Wind-blown or eolian sand, not piled up in heaps or ridges, is somewhat widespread, but does not constitute dunes. ”

“ On the surface of these sands beach-grass of several kinds, and young pine-trees (Pinus rigida) maintain a doubtful struggle for life. This dune does not materially differ from a very large number which cover the banks on the north shore of Loner Island. Their source is the débris of the banks reduced to sand by the action of the waves. The lighter portions of this sand are carried up the slope during fierce winds, and the process is now in operation during every gale. The present forests may delay, but cannot arrest, the final inundation of the land where the sand-hills crown the coast. ”

“ The long, undulating lines of ridged sand inspire thoughts of hopeless melancholy. The sparse vegetation, which in its struggle for life pathetically seizes and holds the partially fertile spots among these ever-shifting masses, has the appearance of broken submission. The wildly tangled {20}roots—derelicts of the sands—which have been deserted and left to bleach in the sun by the slow movement of the great hills, emphasize the feeling of isolation. ”

“ The dunes yonder must be gigantic. But of course on a world the size of ancient Earth, and which was waterless save for snow-patches at its poles, the size to which sand dunes could grow had no limit. ”

“ Another range of sand dunes, at least twenty miles long, and forming hills some two or three hundred feet high, is on the eastern end of Alkali Lake in the same State. Dunes of equal height have been formed on the eastern shore of Lake Michigan, and at Grand Haven and Sleeping Bear have drifted over the woodlands, so as to leave only the dead tops of trees exposed. The erosive power of these drifting sands is often an important agent in wearing away the rocks upon which they strike. ”