238 THE DESERT OF LOP.
On the i oth March, the first day we were at work, we covered a distance of 9140 m. In that space the desert underwent a great and noticeable change. In the neighbourhood of the ruins of Lôu-lan the surface was relatively broken, the vertical relief being greater there than it was farther south. Nor was this entirely due to the erosive action of the wind; it was due rather to the fact that this strip of country was formerly dry shore with forest and steppe vegetation, whereas the surface farther south was formerly covered by the lake, and consequently was level and free from forest. The houses of Lôu-lan, as well as its toghrak groves, its tamarisks, and its kamisch, all stood upon small elevations of the usual description, that is to say typical jardangs, and in this way were protected against the wind-erosion. Shortly after leaving our starting-point the kötäk diminished in quantity, and sometimes there was none at all to be seen. The tamarisk-mounds were very few and the kamisch-stubble came to an end. The mollusc-shells too, which were so numerous along the old lake-shore, now grew fewer. Everything pointed to the inference, that we were turning our backs upon what was formerly a well planted lake-shore, and were advancing across the bottom of an ancient lake. And this conclusion derives additional confirmation from the presence of the ruins and the traces
of the old road, which unquestionably ran along the northern shore of the lake. Owing to the varying consistency of the surface formations, the face of the desert changes a good ..deal. The accompanying photographs (pl. 24-27) will give' a better idea of the jardangs on the northern shore of the lake than any amount of description in words. Even during the first day out they grew smaller and farther apart. Fig. 147 shows an ordinary vertical section through the jardangs in the northern part of the desert; they are about a meter high. It is very curious to see the way in which they have been »undermined» or undercut by the wind. The portion which projects like the edge of a table is composed of harder clay, possessed of a greater power of resistance. The underlying stratum consists of softer material, and into this the wind cuts like a knife. Nor can there exist the slightest doubt but that this insidious undercutting does accelerate the work of destruction. In some cases the »stalk» which supports the table-like top of the jardang is so thin that it evidently cannot be long before it is broken right through. In many places breaches of this description in the clay elevations seem to have taken place quite recently. The blocks of clay thus thrown down lie in the gullies the wind has excavated, and consequently become in an especial manner exposed to its annihilating energy. In this way layer after layer is planed away off the face of the desert, while fresh gullies are excavated and fresh jardangs formed, the general effect being a gradual lowering of the surface as a whole.
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