Tarim from Ajagh-arghan downwards likewise followed a pre-existent chain of bajir depressions, running almost exactly parallel to the chain of the Ettek-tarim.
The differences that obtain between the Ettek-tarim and the Tarim have nothing whatever to do with the existence of the series of original bajir depressions. The former stream below Basch-arghan is generally much more plentifully equipped with forest than the latter is below Ajagh-arghan. Whereas the latter below Schirge-tschapghan is entirely destitute of forest, we already found forest beside the former at the point where we approached it, along the line of the route running to the village of Lop.
Again, the Ettek-tarim is more deeply embedded in sand than the Tarim is; indeed at this point there exists on the right bank of the latter a pretty broad zone of bare steppe, and its eastern or left bank is likewise pretty free from sand, or at all events it has far less than its neighbour has. It is precisely this circumstance that constitutes the greatest difference between the two rivers. The left or eastern bank of the Ettek-tarim is accompanied by a considerable ridge of dunes, which culminates in the conspicuous sandy eminence of Tagh-kum. In some places this ridge of dunes has begun to encroach upon the river-bed since the stream dried up, and thus the encroaching sand, shaping itself into thresholds, tends to restore the bajir depressions to their original appearance of a chain of detached hollows. The reason there exists no corresponding ridge of dunes on the east bank of the Tarim is that the Ilek, below the Arka-köl, served for a prolonged period as the sole channel for the entire volume of the Tarim system, which then proceeded, via Schirge-tschapghan and through the bed of the Tokus-tarim, to the problematical Utschu-köl, and that the whole of the eastern waterway fenced off the drift-sand sufficiently long for the region between the Ilek-Tokus-tarim and the lower Tarim to be swept practically free from sand. On the other hand there are dunes of not inconsiderable magnitude gathered on the eastern bank of the Ilek and on the northern bank of the Tokustarim, because in the desert regions to the north-east there exists no hydrographical hindrance to the free advance of the drift-sand.
The existing disturbance in the arrangement of the sand is therefore due to the three parallel rivers, the Ilek, Ettek-tarim, and Tschong-tarim, and that in the order named. Previously this region formed a compact sandy desert with an extraordinarily regular architecture and parallel chains of bajirs extending from north to south. The accumulations of sand grew at the same time continuously higher and more massive from east to west, the section being that shown in A on fig. 162. For some time after the Tarim broke away to the south and formed the Ilek arm along one of the chains of bajirs, no other change was observed in the disposition of the sand except that a continually broadening belt on the right (west) bank of the Ilek became free from sand (B). Later the water made a fresh path for itself through a more westerly chain of bajirs, the Ettek-tarim, and this stream was hemmed in by even higher accumulations of sand than the Ilek. The section is shown in C; whereas D shows the arrangement at the present time, the region between the Ilek and the Tarim being practically free from sand, while between the Tarim and the Ettektarim considerable dunes still persist. If the river maintains its present position for a sufficiently long period of time, these dunes too will continue their journey westwards, and overwhelm not only the bed, but also the forest, of the Ettik-tarim.
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