NORTHWEST SIDE OF THE LAKE.
In Sistan lacustrine and fluviatile terraces merge into one another, but cause no
confusion, as they both tell the same story. Scattered among them and often parallel to them are numerous structural terraces, due to the hardness of the gravel among the upper and of the green clay among the lower strata. Most careful work is needed to avoid confusing them with their neighbors. A fourth class of terraces is due to warping. None of these has been detected, but the older lacustrine terrace has been warped, and the corresponding stream terraces must have been similarly affected.
The lacustrine terraces agree with those of fluviatile origin ; two are well developed, and a third is now in process of formation. They indicate that since the last heavy gravels were deposited on the smooth plain which caps the upper terrace (fig. 169) the lake has stood at three different levels, at two of which it has cut high bluffs in the soft clays or formed large beaches where the shore is less steep. The bluffs are best developed on the northwest side of the lake. At Bereng, where there has been no warping, the two upper levels are about 15 and 25 feet above the level occupied by the lake surface of January, 1904, which was perhaps 5 feet lower than the extreme high level of floods. Just north of Bereng all the space between the I5-foot beach and the present beach is covered with beach material, and the same is true wherever the shores of the lake are fairly steep. Where the shores are flat, on the contrary, the two beaches are sharply separated. The 15-foot beach appears to be parallel to the present shoreline. The 25-foot beach, on the contrary, has been warped up to a height of 30o feet. At no time, apparently, did the lake level reach any great height, for even a rise to the 15-foot level would mean that the lake of Sistan must permanently overflow to that of Zirrah, and a rise to 25 feet would cause the two lakes to coalesce into one and would involve an immense increase in area.
The recent history of the lake resolves itself into a few simple changes. At the time of the volcanic eruptions of Chaku the lacustrine deposits at the northwest corner of the Hamun-i-Sistan and along the western side had been uplifted and covered with layers of gravel and silt. The gravel layers appear to have numbered two before the eruptions began and one during their progress or after their completion. When the deposition of gravel carne to an end a vast and very smooth gravel plain (I, fig. 169) surrounded the Hamun and concealed a series of lacustrine deposits, very complete at the north, but worn away by the Bendan stream at the south. At Chaku the plain stood several hundred feet higher than the lake, although not so high as at present ; at Bereng it stood at the level of to-day. When matters were in this state the lake stood at the 25-foot level (A, fig. 169), and remained there long enough to form bluffs on all sides of the lake. Near Chaku these are 500 to 600 feet high and are capped with lava ; toward the south they gradually descend till at Bereng they are but 20 or 3o feet high. At Daghaz the bluffs still border the lake and are being actively undercut. They present an almost perpendicular cliff loo feet high. From here northward they gradually retreat, and at the same time grow higher, till at Chaku they are 3 miles from the water. Where