320 THE TSCHERTSCHEN DESERT.
side of the dunes on the east, and on this there was not a particle of sand. The tongue of sand, which we crossed at its extreme low end or tip, thus tailed away towards the east. It was quite evident, that this eastern tip, lying as it did under the shelter of a particularly big accumulation of dunes, had been checked in its advance, and was unable to keep up with the regular forward march of the westward dunes. Fig. 263 gives a section through the big compact threshold which parted bajir No. 8 from No. 9, and fig. 264 a similar section of its successor between bajirs No. 9 and No. i o. The latter fig. shows the stage out of which the former fig. has developed. Here too the tongue of sand becomes gradually covered, and so tends slowly to assume the form of relief shown in fig. 263. The intervening stages of the development can be conceived without any difficulty. The rate of movement of the eastern dune-mass becomes a trifle accelerated when it begins to climb up the tongue of sand, for the line connecting its summit with its base is then gradually shortened, and the space beyond is filled more quickly by the down-pouring sand, while that space itself grows less and less in proportion as the sand advances westwards up the tongue.
Sr; NW
Fig. 264. VERTICAL SECTION OF AN UNCOMPLETED THRESHOLD, THE DOTTED LINES SHOWING THE PROGRESS OF THE LEE SIDE OF A DUNE-ACCUMULATION TRAVELLING UP OVER THE WINDWARD SIDE OF THE NEXT ACCUMULATION TO THE WEST.
I have already said, that the chains of dunes do not run in straight lines, but in festoons or a succession of long shallow bays, the cusps or horns of which are the parts most advanced towards the west, while the hollows of the festoons »hang back» as it were towards the east. As a rule these cusps or horns lie over against the tongues of sand. This disposition will explain also, why the thresholds between the bajirs are notched each by a gap or hollow in their middle, and are highest on the east, immediately at the foot of the leeward flank of the great accumulations of sand. This particular relief has an effect upon the direction and energy of the winds coming from the north-north-east and the south-south-west. The former sweep along the steep leeward flank (see fig. 265) until their progress is checked by the transverse threshold at the end of the bajir, and there a local »wind-screen» is formed by the cusp of the festoon. If however the wind is sufficiently powerful, it lifts the sand off the leeward flank of the great accumulation, sweeps it along its face, and drops it partly upon the threshold, partly on the sheltered side of the cusp. The same activity explains why the thresholds are narrowest in the middle and widen out towards the east, where they merge into the steep, leeward dune-flank, forming an inextricable chaos of dunes heaped in chaotic confusion one upon another. These, in consequence of the scores of different angles at which their constituent parts are built up and shaped, have never possessed any distinctive individuality, and consequently lack all those noticeable characteristics which recur with such amazing regu-
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