depression like promontories of a much-indented coast-line. Rising with very steep faces to heights of 120 to 200 feet above the intervening depressions and commanding extensive views, these ridges furnished excellent natural bases for the towers, and the Chinese engineers did not fail to make the most of them. On this account it was always easy for us to sight these towers from afar. It was curious to note, as the survey progressed to the south-west, how much care they had taken to place their signalling towers for a direct distance of more than twenty-four miles in an almost straight line, as if they had fixed their positions by sighting with a diopter ! But it would need a large-scale map to discuss such details.
But, quite apart from the skill of this early military engineering, there was enough of interest in certain physical features of this forlorn region to reward attention. As I made my way slowly from tower to tower I found myself skirting the coast-line of an ancient lake basin now partially dried up. While crossing in succession its bays and inlets, all occupied by abundant Toghrak groves and reed-beds, and then again the boldly sculptured clay ridges which formed the headlands between (Figs. 18o, 181), I could not help noticing that the latter almost all ran in the same direction from south-east to north-west. Where the bays were wider there could be seen within them strings of isolated clay terraces exactly parallel to the ridges. Elsewhere the latter had a continuation formed by similar clay terraces projecting farther into the marshy basin and still maintaining the same bearing.
The bays and intervening ridges clearly owed their direction to the carving done by running water, which had once descended from the foot of the distant mountains and across that gravel glacis now so terribly dry. This became quite certain when, just below one of the ridges crowned on its top, some 200 feet higher, by a conspicuous watch-tower, I came upon a deep-cut dry river bed. It came from the south-east, and showed unmistakable signs of having been washed by occasional floods at a relatively recent period. The banks were so steep that the camels could not be got across without difficulty. Elsewhere,
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