Chapter XIV: The Contest Between Man and Wind-Borne Sand

The Winds — Wind and Sand as Agents of Denudation — The Combat of the Inhabitants with Sand-laden Winds — The Dune-Belts — Origin of the Sand — Composition of Dune-Sand — The Forms of Dunes — Dimensions and Rate of Movement — Irresistibility of Blown Sand — Protection afforded by Topographical Features — Effect of Artificial Obstructions on the Formation of Dunes — Fixing of Dunes by Vegetation and Moisture — Storm-Walls and Fences — The Dunes of Gennâh — Wind-borne Sand beneficial in the South of the Oasis — Formation of Terraces of Wind-borne Materials — Encroachment of Dunes in the South of the Oasis.

No one who has sojourned in Kharga long enough to compare it with other parts of Egypt will feel inclined to dispute the statement that the depression is one of the most windy places in the country. Only one day in ten or eleven is calm, though if the wind has been blowing fairly steadily during the day it generally drops at night. Five times out of six its direction is from the north, and when unaccompanied by sand the wind acts as a welcome moderator of the temperature.

In a desert region, owing to the surface being unprotected by vegetation, there is nearly always an abundance of weathered material loosely exposed on the surface, the lighter portions of which are quickly swept up and carried along as soon as the wind attains even a moderate velocity. On the actual dunes even a light breeze is sufficient to set the surface-layers in motion, while on the open plain the wind becomes visibly charged with sand directly it attains the velocity of a moderate breeze. The frequency with which the wind blows has already been mentioned, and when we add that on one day in three it attains a velocity sufficient to enable it to easily carry sand in suspension, its power as a transporting agent can readily be conceived. Not only does the wind carry the sand along from one place to another, dumping it behind or in front of every obstacle it may meet, but the combination of sand and wind forms a denuding force of no mean power, capable of planing and grooving the exposed surfaces of the hardest of rocks.

Now, although opinions may differ as to the part played in the past by wind and sand in the shaping and formation of the oases-depressions of the Libyan Desert, there can be no question of the effects they are producing at the present day. Wind-borne sand is indeed the curse of the oasis, and although its evil effects may be mitigated or altogether staved off for a time, the longer this force of Nature is opposed by man the greater and more overwhelming is its final victory. If a full account of the human occupation of the oases could be written, it would be very largely the history of an incessant combat between man and Nature; and although man may for a time gain the upper hand, and even make the sand-laden winds to some extent serve his purposes, he is in the end generally forced to abandon those places in which he has been at such pains to establish himself.

In the Libyan Desert, both on the plateaux and in the depressions, blown sand has a marked tendency to collect into dunes occupying definite north and south belts of country, lying parallel to the direction of the prevailing winds. This peculiar and fortunate disposition depends on a number of circumstances, such as the mean direction of the wind, the points of greatest supply of the material, and on other causes which are not perhaps as yet thoroughly understood.

In Kharga Oasis the best marked and most important belt lies somewhat to the west of the central axis of the depression. At the north end, in the neighbourhood of Um el Dabâdib, it consists of a number of isolated but parallel lines of dunes, which southwards pass over and round the hill-massif of Jebel Tarif, eventually uniting and forming a more compact belt of sand, with an average width of 8 or 10 kilometres, which continues right through the depression into the desert to the south. This line of sand sets a limit to the extension of cultivation to the west. A second belt comes from the plateau to the north of Qasr Lebekha, passes immediately to the east of Jebel Têr and the village of Kharga, and continues southwards until it spreads out and is arrested, more or less by artificial means, in the vicinity of Gennâh. Other smaller and less defined belts exist to the south of Ain el Tawîl, to the east of Headquarters, and near the foot of, and parallel to, the eastern wall of the depression.

Although it has been stated by more than one writer that the great sand accumulations of the Libyan Desert originate in the denudation of the Nubian Sandstone which occupies large areas to the south of the oases, I have never myself met with any evidence in support of such a view. In my opinion the bulk of the sand must be regarded as originating in the arenaceous deposits of post-Middle Eocene age, which largely occupy and are exposed on the surface of the country between the Mediterranean Sea and latitude 29° N. We know from personal observation that some of the great sand-belts, which cross the intervening limestone plateau to the southern oases, commence in this region, and the supply of sand made available by the denudation of the beds in question is inexhaustible. During the passage of this sand across the great desert tableland the individual grains of silica doubtless become much rounded and reduced by attrition, though the total loss is probably more than counterbalanced by the very considerable additions received in the form of calcareous grains, derived from the limestones forming the surface of the high desert.

An examination of the dunes of Kharga Oasis at once shows that the sand is not by any means composed solely of siliceous grains, but that there is a considerable proportion of white granules of limestone. A number of samples collected at random from the dunes in the central part of the depression were all found to contain a visible proportion of calcareous grains, and one of these, collected from the big dune to the south-east of Headquarters, was qualitatively examined for me by Mr. Garsed, and shown to contain 7·7 per cent. of calcium carbonate. It would be difficult to account for the presence of these calcareous grains on the assumption that the sand is derived from the Nubian Sandstone, though the denudation of the latter in and to the south of the oases must give rise to a very large amount of siliceous sand, which goes to swell the bulk of the dunes which have invaded this country from beyond the limestone plateau to the north.

The dominant form of dune in the oasis is of crescentic or horse-shoe shape, a form specially typical of desert regions where there are prevailing winds in any one direction. These ‘barchans,’ as they are called, which are always disposed with the concave, steeply-inclined sides facing southwards, are found of every size, and exhibit many variations of the simple crescentic pattern. In many parts of the oasis the sand-belts are made up of a number of isolated and promiscuously disposed barchans; in others the dunes have joined together into a compact mass, in which the typical shape of the individual barchan is more or less obliterated.

Solitary barchans, being well-defined isolated masses, lend themselves to observation better than the large and continuous belts of sand. They occur of all sizes, from little baby crescents, a metre or two across, to enormous masses 30 or 40 metres high and 200 to 300 metres in breadth. In all cases, except when their steady march southwards has been temporarily interrupted by southerly winds, the southern face, flanked by the horns of the crescent, is a straight slope of from 30 to 33 degrees, its inclination is, in fact, the angle of rest of loose dry sand, and it is formed of the sand which is continually blown up the opposite slope and dropped over the crest.

In a moderate breeze the whole barchan progresses steadily in a southerly direction without loss or gain of sand; in stronger winds, while still continuing its course, the dune may at the same time be either losing or gaining in actual bulk. The rate at which the barchans travel varies with their size. The larger the dune the greater the amount of material to be moved, so that, as a general rule, small barchans progress much more rapidly than large ones. The actual rate may vary from 10 to 20 metres per annum, the average being about 17. For instance, the big isolated dune on the road between Headquarters and Kharga village moved 11 metres southwards between December 8, 1907, and December 8, 1908, while during the same period a small dune in the Bellaida district travelled 21 metres in the same direction. The dimensions of these two dunes is as follows:

+------------------------------------+-------------+----------+
| | Kharga Road | Bellaida |
| | Dune. | Dune. |
+------------------------------------+-------------+----------+
| | Metres. | Metres. |
| Vertical height of centre of crest | 17 | 10·5 |
| Maximum breadth (east and west) | 259 | 160 |
| Maximum length (north and south) | 230 | 70 |
+------------------------------------+-------------+----------+

These are merely instances from dunes which I have had under observation for a number of months, full details of which I hope shortly to publish.

Approached on a bright sunny day from the south side, the angle of slope within the crescent of a barchan is most deceptive. Nine out of ten persons will confidently state that the face of the dune cannot be less than 60 or 70 degrees from the horizontal, though, as a matter of fact, it never exceeds by more than two or three degrees a third of a right angle. Unless held together by vegetation or compacted by moisture, it cannot exceed the angle of rest of dry blown sand, which is about 32 degrees. In normal weather this face of a barchan is a straight, unrippled, even slope of loose sand from top to bottom; when a light or gentle breeze is blowing from the north, the dune can be seen ‘smoking’ at the top of the crest, the sand being blown up the much more gentle and markedly rippled northerly slope, and dropped over the crest on to the steep, smooth, southerly face. The bulk of this sand in a light breeze falls on the upper part of the slope, equilibrium being adjusted from time to time by a downward sliding of the superficial layers. A cross-section of the dune, from north to south, would indeed show it to be composed of well-stratified layers of sand, dipping steadily southwards at a little over 30 degrees.

In this district I have never observed southerly winds hold for a sufficient length of time to do more than slightly modify the general form of a barchan, though during their continuance the progress of the dune is arrested, the outline of the crest becomes considerably altered, ripple-marks appear on the steep southerly face, and an apron of sand of greater or lesser dimensions forms at its foot.

The irresistibility of drift sand is well seen where isolated hills lie in the path of the dunes. When such hills are narrow, and elongated in a north and south direction, they generally act as a wedge, divide the wind, and force any sand it may be carrying to pass on either side. When, however, a hill has northerly flanks of any considerable width, it will only form a temporary check to the southward passage of the sand. The progress of the latter may be arrested for a while, until hollows have been filled in and the general slopes reduced; but eventually the sand will pass up along the lines of least resistance to the summit. This is beautifully illustrated at Jebel Tarif, the great isolated, flat-topped, hill-massif lying between Um el Dabâdib and Kharga village, standing over 300 metres, or 1,000 feet, above the surrounding plain. Instead of being diverted by this obstacle, the sand, coming from the plateau to the north and north-west of Um el Dabâdib, surmounts the slopes, and continues in its normal direction across the flat-topped summit. The latter has been cut up into numberless parallel north- and south-disposed ridges, separated by deep troughs, which are now to a considerable extent filled with sand. Active erosion must, therefore, at the present day be confined to the upper portions of the slopes of the valleys and to the intervening ridges.

Naturally in such a region the inhabitants have been quick to take advantage of any protection afforded by topographical features, and it is not surprising to find that the most important of the ancient monuments, as well as the greatest area of modernly cultivated land, lie in a comparatively sheltered position to the south of Jebel Têr. Even there, however, trouble is constantly experienced on the east side, owing to the tendency of the belt of sand-dunes, which comes from the vicinity of Qasr Lebekha, to spread laterally. Along the east side of the village the dunes are close up against the outlying houses, and the sand continually surmounts and pours over the walls which from time to time are erected by the natives to act as barricades.

Sheltered places of the above description are rare, the majority of the smaller settlements being situated on the open plain, where the only precaution which can be taken is to select the sites for wells as far away as possible from the well-defined sand-belts. But in windy weather the sand-blast is not confined to the neighbourhood of the dunes; the wind rages across the open plains, carrying dense clouds of sand in suspension, and devastating the works of man wherever the latter offer the slightest obstacle to its progress.

In order to protect his crops from the fierce onslaught of wind-borne sand the native is compelled to erect fences and barricades along the northern boundaries of his fields. These serve his purpose for the time being, the sand which would otherwise have cut and beaten down the growing crops being deposited on either side of the fence, where the velocity of the wind is checked. The arrested sand accumulates with increasing rapidity as the size of the obstruction becomes greater, a veritable dune, constituting a really efficient protection to the cultivated lands, being formed over the site of the original small and insignificant hedge. Unfortunately, this sand continues to collect, and although the cultivator may have satisfactorily insured himself against the frequent damage caused to his crops by their exposure to wind-driven sand, he is now confronted with the still more serious prospect of having his lands overwhelmed by the ever-increasing dunes, which have grown to considerable proportions as the result of his efforts to protect himself.

Fortunately for the occupiers of these outlying settlements, the progress of such dunes, formed more or less artificially on the northerly sides of the cultivated tracts, is not at the same rate as that of isolated dunes moving across the open desert plain. Owing to the presence of a certain amount of moisture in the superficial layers of the ground in the neighbourhood of the wells and the cultivated lands, plants of various kinds are enabled to thrive, with the result that the dunes become to some extent ‘fixed’ by wild vegetation. The most valuable plant in this connection is the tamarisk, which by means of immensely long roots continues to thrive and keep its head above growing dunes with remarkable facility. To a certain extent, moreover, the moisture from the soil itself rises by capillarity into the base of the sand, and assists the vegetation in binding it into a compact mass, which is not liable to movement in the same degree as dry sand. The rate of progress of such dunes may therefore be reduced to a considerable extent, but the movement can never be altogether stopped. As we have seen in the case of the hamlet of Meheriq, the sand eventually gets the upper hand, blotting out the arable lands, burying the palm-groves, and forcing man to abandon his habitations.

In the neighbourhood of the Corporation’s Headquarters a considerable area of the floor of the depression has been brought under cultivation during the last three years. This tract is exposed on every side, there being no protection whatever in the shape of natural features. Although occasional sandstorms are experienced from the south, and even from the east and west sides, practically the whole of the trouble encountered comes from northerly sandstorms and sand-laden winds. It was found quite impossible to raise crops unless the fields were protected by storm-walls or fences, the young plants being cut down as if by a scythe. So long as the reclamation of land proceeded from south to north at a fairly rapid rate, the accumulation of sand against these fences was of little moment, being, indeed, rather welcome than otherwise, as the sand was found to be of considerable value when spread out and ploughed into the heavy clay soil with which this area is superficially covered. When, however, a storm-wall remains for several consecutive months exposed to the open desert to the north, the accumulation of sand becomes a more serious matter, and care has to be taken to avoid the formation of dunes, which it might be afterwards difficult to get rid of. A fairly satisfactory mode of procedure is the planting of narrow belts of ‘sesban,’ or other quickly-growing shrub, along the northern limits of the newly reclaimed lands; a hedge of this description forms an efficient shelter to the fields, while at the same time it allows a good deal of the sand to filter through and become absorbed by the soil.

Most of the dunes in the Gennâh district are more or less covered with tamarisk and other wild vegetation, but that their progression southwards has only been retarded, and not prevented, is evident from an examination of the country. Large areas of land have been abandoned owing to the invasion of the dunes, and the magnificent wells of Gennâh are threatened with destruction within a very short term of years, unless special measures are adopted to cope with the advance of the sand. The available land in the vicinity of the village has long been insufficient to utilize the water discharged by these wells, which has in consequence to be led by a channel of several kilometres in length to lower ground south-east of the Gorn el Gennâh. This channel is being constantly pushed to the south, and as the ground rises in that direction, it may eventually become impossible to maintain the conduit at a sufficiently low level to carry the water of the wells to this outlying district.

The southern portion of the oasis is the only area where sand and wind can be said to be of any actual benefit to the inhabitants, but in this area, owing to the broad plains of alluvial clay, the suspended material consists to a very great extent of fine argillaceous particles. At many of the wells in the Dakhakhin and Beris districts the cultivators encourage the gradual deposition on their fields of the sand and clay-dust borne by the northerly winds. The mixture forms an excellent loam, and an annual dressing of the material is regarded as desirable, and even necessary, on account of its value as a fertilizer. The result of this gradual deposition of wind-borne material has been that the cultivated terraces have in many cases attained to such elevations that the discharges of the wells have suffered very considerable diminution, owing to the increased height of the outlets.

To the west of the villages of Maks and Beris the great belt of sand has already enveloped a number of wells, and appears to be still encroaching—as the individual dunes follow their normal course in a direction slightly east of south—on the area occupied by the existing cultivated lands. Fortunately there are still extensive tracts of cultivable land comparatively free from accumulations of drift sand, and doubtless the inhabitants, when driven by the irresistible forces of Nature from the localities they now occupy, will again, as they have so frequently been compelled to do in the past, migrate to pastures new.