[415] Few seas have thrown up so much sand as the shallow German Ocean; but there is some reason to think that the amount of this material now cast upon its northern sh.o.r.es is less than at some former periods, though no extensive series of observations on this subject has been recorded. On the Spit of Agger, at the present outlet of the Liimfjord, Andresen found the quant.i.ty during ten years, on a beach about five hundred and seventy feet broad, equal to an annual deposit of an inch and a half over the whole surface.--_Om Klitformationen_, p. 56.
This gives seventy-one and a quarter cubic feet to the running foot--a quant.i.ty certainly much smaller than that cast up by the same sea on the sh.o.r.es of the Dano-German duchies and of Holland, and, as we have seen, scarcely one fourth of that deposited by the Atlantic on the coast of Gascony. See _ante_, p. 453, note.
[416] Sand heaps, three and even six hundred feet high, are indeed formed by the wind, but this is effected by driving the particles up an inclined plane, not by lifting them. Bremontier, speaking of the sand hills on the western coast of France, says: "The particles of sand composing them are not large enough to resist wind of a certain force, nor small enough to be taken up by it, like dust; they only roll along the surface from which they are detached, and, though moving with great velocity, they rarely rise to a greater height than three or four inches."--_Memoire sur les Dunes, Annales des Ponts et Chaussees_, 1833, 1er semestre, p. 148.
Andresen says that a wind, having a velocity of forty feet per second, is strong enough to raise particles of sand as high as the face and eyes of a man, but that, in general, it rolls along the ground, and is scarcely ever thrown more than to the height of a couple of yards from the surface. Even in these cases, it is carried forward by a hopping, not a continuous, motion; for a very narrow sheet or channel of water stops the drift entirely, all the sand dropping into it until it is filled up.
The character of the motion of sand drifts is well ill.u.s.trated by an interesting fact not much noticed hitherto by travellers in the East. In situations where the sand is driven through depressions in rock beds, or over deposits of silicious pebbles, the surface of the stone is worn and smoothed much more effectually than it could be by running water, and you may pick up, in such localities, rounded, irregularly broken fragments of agate, which have received from the attrition of the sand as fine a polish as could be given them by the wheel of the lapidary.
Very interesting observations on the polishing of hard stones by drifting sand will be found in the Geological Report of William P.
Blake: _Pacific Railroad Report_, vol. v, pp. 92, 230, 231. The same geologist observes, p. 242, that the sand of the Colorado desert does not rise high in the air, but bounds along on the surface or only a few inches above it.
[417] Wilkinson says that, in much experience in the most sandy parts of the Libyan desert, and much inquiry of the best native sources, he never saw or heard of any instance of danger to man or beast from the mere acc.u.mulation of sand transported by the wind. Chesney"s observations in Arabia, and the testimony of the Bedouins he consulted, are to the same purpose. The dangers of the simoom are of a different character, though they are certainly aggravated by the blinding effects of the light particles of dust and sand borne along by it, and by that of the inhalation of them upon the respiration.
[418] In the narrow valley of the Nile, bounded as it is, above the Delta, by high cliffs, all air currents from the northern quarter become north winds, though, of course varying in partial direction, in conformity with the sinuosities of the valley. Upon the desert plateau they incline westward, and have already borne into the valley the sands of the eastern banks, and driven those of the western quite out of the Egyptian portion of the Nile basin.
[419] "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.
[420] In parts of the Algerian desert, some efforts are made to r.e.t.a.r.d the advance of sand dunes which threaten to overwhelm villages. "At Debila," says Laurent, "the lower parts of the lofty dunes are planted with palms, * * * but they are constantly menaced with burial by the sands. The only remedy employed by the natives consists in little dry walls of crystallized gypsum, built on the crests of the dunes, together with hedges of dead palm leaves. These defensive measures are aided by incessant labor; for every day the people take up in baskets the sand blown over to them the night before and carry it back to the other side of the dune."--_Memoires sur le Sahara_, p. 14.
[421] Organic const.i.tuents, such as comminuted sh.e.l.ls, and silicious and calcareous exuviae of infusorial animals and plants, are sometimes found mingled in considerable quant.i.ties with mineral sands. These are usually the remains of aquatic vegetables or animals, but not uniformly so, for the microscopic organisms, whose flinty cases enter so largely into the sandbeds of the Mark of Brandenburg, are still living and prolific in the dry earth. See WITTWER, _Physikalische Geographie_, p. 142.
The desert on both sides of the Nile is inhabited by a land snail, and thousands of its sh.e.l.ls are swept along and finally buried in the drifts by every wind. Every handful of the sand contains fragments of them.
FORCHHAMMER, in LEONHARD Und BRONN"s _Jahrbuch_, 1841, p. 8, says of the sand hills of the Danish coast: "It is not rare to find, high in the knolls, marine sh.e.l.ls, and especially those of the oyster. They are due to the oyster eater [_Haemalopus ostralegus_], which carries his prey to the top of the dunes to devour it." See also STARING, _De Bodem van_, N.
I. p. 321.
[422] There are various reasons why the formation of dunes is confined to low sh.o.r.es, and this law is so universal, that when bluffs are surmounted by them, there is always cause to suspect upheaval, or the removal of a sloping beach in front of the bluff, after the dunes were formed. Bold sh.o.r.es are usually without a sufficient beach for the acc.u.mulation of large deposits; they are commonly washed by a sea too deep to bring up sand from its bottom; their abrupt elevation, even if moderate in amount, would still be too great to allow ordinary winds to lift the sand above them; and their influence in deadening the wind which blows toward them would even more effectually prevent the raising of sand from the beach at their foot.
Forchhammer, describing the coast of Jutland, says that, in high winds, "one can hardly stand upon the dunes, except when they are near the water line and have been cut down perpendicularly by the waves. Then the wind is little or not at all felt--a fact of experience very common on our coasts, observed on all the steep sh.o.r.e bluffs of two hundred feet in height, and, in the Faroe Islands, on precipices two thousand feet high. In heavy gales in those islands, the cattle fly to the very edge of the cliffs for shelter, and frequently fall over. The wind, impinging against the vertical wall, creates an ascending current which shoots somewhat past the crest of the rock, and thus the observer or the animal is protected against the tempest by a barrier of air."--LEONHARD und BRONN, _Jahrbuch_, 1841, p. 3.
The calming, or rather diversion, of the wind by cliffs extends to a considerable distance in front of them, and no wind would have sufficient force to raise the sand vertically, parallel to the face of a bluff, even to the height of twenty feet.
It is very commonly believed that it is impossible to grow forest trees on sea-sh.o.r.e bluffs, or points much exposed to strong winds. The observations just cited tend to show that it would not be difficult to protect trees from the mechanical effect of the wind, by screens much lower than the height to which they are expected to grow. Recent experiments confirm this, and it is found that, though the outer row or rows may suffer from the wind, every tree shelters a taller one behind it. Extensive groves have thus been formed in situations where an isolated tree would not grow at all.
Piper, in his _Trees of America_, p. 19, gives an interesting account of Mr. Tudor"s success in planting trees on the bleak and barren sh.o.r.e of Nahant. "Mr. Tudor," observes he, "has planted more than ten thousand trees at Nahant, and, by the results of his experiments, has fully demonstrated that trees, properly cared for in the beginning, may be made to grow up to the very bounds of the ocean, exposed to the biting of the wind and the spray of the sea. The only shelter they require is, at first, some interruption to break the current of the wind, such as fences, houses, or other trees."
[423] 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 a.s.sociation for the Advancement of Science_.
[424] STARING, _De Bodem van Nederland_, i, p. 327, note.
[425] The princ.i.p.al special works and essays on this subject known to me are:
BReMONTIER, _Memoire sur les Dunes, etc._, 1790, reprinted in _Annales des Ponts et Chaussees_, 1833, 1er semestre, pp. 145-186.
_Rapport sur les differents Memoires de M. Bremontier_, par LAUMONT et autres, 1806, same volume, pp. 192, 224.
LEFORT, _Notice sur les Travaux de Fixation des Dunes, Annales des Ponts et Chaussees_, 1831, 2me semestre, pp. 320-332.
FORCHHAMMER, _Geognostische Studien am Meeres Ufer_, in LEONHARD und BRONN, _Jahrbuch, etc._, 1841, pp. 1, 38.
J. G. KOHL, _Die Inseln und Marschen der Herzogthumer Schleswig und Holstein_, 1846, vol. ii, pp. 112-162, 193-204.
LAVAL, _Memoire sur les Dunes du Golfe de Gascogne, Annales des Ponts et Chaussees_, 1847, 2me semestre, pp. 218-268.
G. C. A. KRAUSE, _Der Dunenbau auf den Ostsee-Kusten West-Preussens_, 1850, 1 vol. 8vo.
W. C. H. STARING, _De Bodem van Nederland_, 1856, vol. i, pp. 310-341, and 424-431.
Same author, _Voormaals en Thans_, 1858, pages cited.
C. C. ANDRESEN, _Om Klitformationen og Klittens Behandling og Bestyrelse_, 1861, 1 vol. 8vo, x, 392 pp., much the most complete treatise on the subject.
ANDRESEN cites, upon the origin of the dunes: HULL, _Over den Oorsp.r.o.ng en de Geschiedenis der Hollandsche Duinen_, 1838, and GROSS"s _Veiledning ved Behandlingen af Sandflugtstraekningerne_, 1847; and upon the improvement of sand plains by planting, PANNEWITZ, _Anleitung zum Anbau der Sandflachen_, 1832. I am not acquainted with either of the latter two works but I have consulted with advantage, on this subject, DELAMARRE, _Historique de la Creation d"une Richesse millionaire par la culture des Pins_, 1827; BOITEL, _Mise en valeur des terres pauvres par le Pin maritime_, 1857; and BRINCKEN, _Ansichten uber die Bewaldung der Steppen des Europaischen Russlands_, 1854.
[426] "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 und BRONN, 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 drift sand was not moistened to a greater height than eight and a half inches, after standing 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 quant.i.ty 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 is 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 metres 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 the proposal to fix the dunes which are supposed to threaten the Suez Ca.n.a.l, by planting the maritime pine and other trees upon them, is not altogether so absurd as it is thought to be by some of those disinterested philanthropists of other nations who are distressed with fears that French capitalists will lose the money they have invested in that great undertaking.
Ponds of water are often found in the depressions between the sand hills of the dune chains in the North American desert.
[427] 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 sh.o.r.es 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.
[428] _De Bodem van Nederland_, i, p. 323.
[429] J. G. KOHL, _Die Inseln und Marschen der Herzogthumer Schleswig und Holstein_, ii, p. 200.
[430] STARING, _De Bodem van Nederland_, i, p. 317. See also, BERGSoE, _Reventov"s Virksomhed_, ii, p. 11.
"In the sand-hill ponds mentioned in the text, there is a vigorous 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 an end to its formation. When, in the course of time, marine currents cut away the coast, the dunes move landward 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. 13.
[431] 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.
[432] "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." See _ante_, p. 16, note.--NAUMANN, _Geognosie_, ii, p. 1172. See _Appendix_, No. 58.
[433] 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 circ.u.mstances have existed somewhere which have preserved the characteristics of this formation."--LEONHARD und BRONN, 1841, p. 8, 9.
Such formations, however, certainly exist. I find from Laurent (_Memoire sur le Sahara, etc._, p. 12), that in the Algerian desert there exist "sandstone formations" not only "corresponding to the dunes," but actually consolidated within them. "A place called El-Mouia-Tadjer presents a repet.i.tion 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." See _Appendix_, No. 59.
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.
[434] 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 former fluid; 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.