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 Dune. | Bellaida 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.
SAND EROSION ON SUMMIT OF JEBEL TARIF.