258 THE BASIN OF EASTERN PERSIA AND SISTAN.
diverts a part of the river. According to Mr. Nikrashevich, superintendent of the dam, the inner terrace at the bottom of the old channel had a height of 2 feet before conditions were changed by the building of the darn, while the inner channel, where the stream to-day runs, has a depth of about i6 feet. That is, the bottom of the river to-day is 14 feet lower than it was at the time of the abandonment of the old channel a hundred years ago. Part of this difference, however, may be accounted for by filling of the latter subsequent to its abandonment.
Since the building of the dam the river has so filled up its channel above the artificial obstruction that the lower terrace has entirely disappeared and the stream has no proper channel, hut wanders this way and that over its own deposits. This wandering is causing the widening of the flood-plain, and there is great danger that ill time a sudden change in the course of the main stream will cause it to cut into the banks close to the dam and finally to break a way around the end of the latter. Such a catastrophe took place at Sultan Bend, a few miles up the river, where a dam was built about 1890 and was abandoned a few years later. Retaining walls were built in all directions, but nothing could prevent the river from cutting laterally when it was prevented from accomplishing its normal work of vertical erosion.
The material which is now being deposited by the Murg-ab seems to be the same as that which is exposed in the bluffs of the terraces. It consists of a very fine clayey sand well stratified and with a consistency like loess. It stands for years in nearly perpendicular bluffs, and preserves the marks of the pick indefinitely. It is said that as far as Tash Kupri, nearly a hundred miles upstream, the same fine sandy deposit continues, and only at that place does it become gravelly.
The terraces also continue far upstream. At Tash Kupri there are said to be two, one of them close to the river and the other 7o feet above it. At Sultan Bend, 15 miles above the dam at Hindu Kush, there are three terraces. At the top lies the great sand-covered alluvial plain, 7o feet above the river ; then comes a broad terrace covered with tamarisk and other bushes, and lying about half as high ; and lastly there is a small young terrace only io feet above the water. Here, again, as in so many other cases, there is no positive indication as to whether the terracing is due to climatic or tectonic causes. There are archeological indications that the flow of the Murg-ab one or two thousand years ago was more abundant than at present, and it is not impossible that the decrease in the size of the stream is connected with the building of the lower terraces.
THE HERI RUD.
The Heri Rud, or Tejen River, as it is called after it enters Russian territory, when taken by itself is no more conclusive as to the cause of the terraces than are the other rivers. To be sure, it flows directly across the northern mountain rim of the basin of Iran, and thereby differs from the other streams which we have considered. If the terraces are due to an extensive uplift of Kopet Dagli and the Paropamisus, the main axis of that uplift must have passed directly athwart the Heri Rud not far from what is now the Afghan border, accelerating the lower or northern portion of the stream and retarding the upper portion. The process of terrace-making under such circumstances would differ materially from that in cases