The Mystery of Demon Towns

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Yardang landform is a typical wind-erosion landform, which is also known as wind -erosion groove or wind-erosion ridge. “Yardang”is a Uygur word, meaning steep earth mound. In the extremely arid regions, the dried lakebeds often crack because of drought; wind blows along these cracks, making them larger and larger, so many irregular dorsal carp-shaped ridges and wide and shallow grooves have been developed on the original flat ground, and such kind of fragmented ground is Yardang landform. Some landforms look like ancient castles, so they are commonly known as the “Demon Towns”.

In the early 20th century, many Chinese and foreign scholars who went to Lop Nor to inspect found the landform with a large area of uplifted mounds in the wilderness, so they asked the local guide what the name of the place was. The guide mistakenly thought that they were asking about the name of the landform, so he answered”Yardang”with the meaning of “steep earth mound”; since then, the word”Yardang” began to be spread in the field of geosciences in abroad, and “Yardang” has become the synonym for such type of landform.

The most typical Yardang landform is located near the ancient city of Loulan in northwest of Lop Nor. Many grooves in the region of Lop Nor in Tarim Basin can be more than ten meters deep and tens of meters to hundreds of meters long; the trend of the grooves is consistent with the prevailing wind direction, and sand will accumulate in the grooves. There are often white salt encrustations on the tops of the ridges, so they are also known as the “White Dragon Mounds”.

There are two key factors for the formation of Yardang landform: one is the geological foundation, namely the lacustrine sedimentary strata; and the other one is the erosion from external force, namely the deflation of the strong directional wind in the desert and the erosion of water. According to the investigation of Yardang landform in Lop Nor wilderness it has been proved that water is also an external force for the formation of Yardang landform in addition to wind, and there are three types of the landform: first, the Yardang landform formed mainly by wind erosion; second, the Yardang landform formed mainly by water erosion; and third, the landform formed by the combined action of wind and water.

The Yardang landform formed mainly by wind erosion is located in the plains far away from the mountains, for the flood formed by precipitation in the mountains generally cannot reach here, and only the wind-force takes effect. Such type of Yardang landform concentrates in the area from the south of the Peacock River to the ruins of Loulan. The landform is generally 4-7m high, the trend of the depressions of the landform is from northeast to southwest, which is consistent with the direction of the local prevailing wind, showing the relationship between the landform and the wind. According to the survey, the average annual wind erosion depth in this area is between 2.4-4.7mm; according to such a speed, the landform here was formed only a thousand years ago, and it was formed after the abandon of Loulan, so this region should be a flat and fertile field in those years.

During the history of the formation of lakes in arid areas, there would be repeating advances and retreats of water, forming the superimposed mudstone layers and sand layers. Wind and water can take away the loose sand layers, but not the hard mudstone layers and gypsum cementation layers. However, the compact mudstone layers are not indestructible. The expansion and contraction effect owing to the large temperature difference in the desert region finally leads to the cracks of the mudstone layers, the exposed sand layer will be taken away by wind and water, and then the mudstone layers will evolve to the groove-shaped layers; the parts covered by the mudstone layers are relatively stable, forming the elongated mounds, and then the Yardang landform gradually comes into shape.

The Sanlongsha Yardang at the eastern section of the Aqik Valley is in the trend of south by east, which is perpendicular with the prevailing northwest wind and in line with the direction of mountain flood, so it indicates that the flood has played a leading role in the formation of the landform here.

After the discovery of Yardang landform in Lop Nor wilderness, a number of similar landforms have also been found in other arid areas of the world, which are collectively referred to as Yardang landform. Even in China, the largest distribution area is not in the Lop Nor region where it was first found, but in the northwest of Qaidam Basin in Qinghai Province. The landform can be found everywhere in Xinjiang. The famous ones are as follows: Urho Demon Town in the northeast of Karamay City, Multicolored Bay in Beishawo of Jimsar County, and Wind Castles in the desert southwest of Qitai County, and so on.