Yardang landform

       1 The origin of the word Yardang

Yardang is the transliteration of the Uygur language, which originally refers to the hills with steep wall, a kind of wind erosion landforms in dry areas. During the year of 1899 to 1903, the Swedish geographer and explorer Sven heding started an expedition to Xinjiang, China. He found the extensively distributed hill landform around the Lop Nur. He asked the Uighur guide where they were in, and got the answer “Ya’erdan”. Sven heding spelt it as "Yardang" according to the pronunciation of the guide, and wrote it in his book, the central Asia and Tibet. Then this book was widely circulated, and Chinese scholars retranslate the “yardang” into Chinese Yardang. From then on, Yardang became the name of such kind of landform, and was accepted by geographers and geologists.

Yardang landform is a kind of typical wind erosion landform, distributed in the extreme arid or semi-arid areas. It is based on the incompletely consolidated Neogene and Quaternary fluvial and lacustrine deposits formed since twenty million years ago. Constructed by the structural uplift and other geological agents such as wind, water and gravity, it is changed into the landform with the shape of ridges of hills or streamlining and pillars. Yardang landform is widely distributed in Lop Nur, which is typical and of abundant type. There are various types of Yardang body in the park, all of which are well arranged. The products of each stage of development are retained, serving as the most typical Yardang landform in China and even in the world.

2 The classification of Yardang landform

According to different classification principles, Yardang landform has different classification systems. There are two kinds of classification systems at home. Zongqi Chen (1936) divided Yardang into mesas and yardangs according to the size and formed era, the former of which is 10 to 30 meters high while the latter of which is less than 1 meters. Meanwhile, Xunjie Xia divided Yardang in the Lop Nur into three types according to the external agents, which are wind erosion dominated, water erosion dominated and water erosion integrated with wind erosion. There are also two classification systems abroad. Cooke (1993) divided it into three categories according to the size. Based on the analysis of the shape and developing period and integrating the qualitative and quantitative methods, Halinov et al. (1989) divided the Yardang in the Qaidam basin into several types, the shapes of which are like mesa, fangs, cone, pyramid, long ridge, arch back, whales back and low flow whales back.

There are about more than five thousand large and small Yardang body in the park, which significantly vary in length, height, strike and shape. Based on the qualitative description and quantitative measurement of Yardang body, it can be divided into five categories according to the shape, which are like the ridge hillock, wall, tower, columnar, and monadnock.

The ridge-shape Yardang landform

The external shape of ridge-shape Yardang landform is thin. New tectonic activities elevates the previously deposited diluvium and lake sediments, and thus generates structural joints and fractures. These fractures are eroded by the water into gullies, and then the erosion by wind gradually shapes it into the ridge-shape Yardang landform. The ridge-shape Yardang landform is the primary stage of the development of Yardang landform, whose form is the most complete. Its monomer can be hundred meters to kilometers long, with its width 50-400 meters and its height 25-60 meters. 

Figure 4-1 the ridge-shape Yardang landform

2. The wall-shape Yardang

The ridge-shape Yardang continues to be eroded, with its width narrowing, the middle part breaking up and the height decreasing. Finally, the external shape is like a wall, which is largely independent and forms wall-shape Yardang (Figure 4-2). The wall-shape Yardang with long and straight top is long beach like. Then the top is eroded to form the bottom connected but top jagged Yardang dody, which is like fangs. It can also form the top of the streamlined ups and downs, similar to the whale back, which is known as the whale back shape Yardang. Although it is somewhat wrack, it is overall like a wall. So it fall into the category of the wall-shape Yardang landform.

Figure 4-2 the wall-shape Yardang

   3. The tower-shape Yardang

The wall-shape Yardang continues to be eroded, gradually forming the tower-shape Yardang, whose aspect ratio is the closest (Figure 4-3). Usually the bottom part is 4 to 30 meters in diameter, and 5 to 25 meters in height. Generally, the diameter of the bottom part is larger than that of the upper part. If the upper part is round as the Ger, it is called dome-shape Yardang. When the wind erodes the tower-shape Yardang from different directions, the bottom part can be shaped as irregular polygon while the side is shaped as triangular pyramid, known as the pyramid-shape Yardang. The pyramidal Yardang belongs to this type. If some soft layer in the central part of the tower-shape Yardang is blown to be eroded, it is shaped as that the diameter of the upper part is small while that of the bottom part is large, called seal Yardang. Shapes such as "Buddha" and "great men statue" all belong to this type. 

Figure 4-3 tower-shape Yardang

        4. The columnar Yardang

Tower-shape Yardang evolves into columnar one, which bears obvious height characteristics and thus is called columnar Yardang (Figure 4-4). It is generally 2 to15 meters in diameter and 5 to 25 meters high. Because composition of Yardang body is different, the weathering resistance is not the same. The columnar Yardang is easy to form an up-coarsening shape, called mushroom-shape Yardang. However, Tower-shape Yardang body collapses and develops into almost cylindrical form, known as cylindrical Yardang. 

Figure 4-4 columnar Yardang

        5. Yardang monadnock

Finally, Yardang body evolves to die out. It will collapse along the joints, becoming disorderly massive deposited Yardang monadnock (Figure 4-5). The original occurrence thus can’t be told, and the Yardang landform disappears. In the right environment, a new round of Yardang landform develops. 

Figure 4-5 Yardang monadnock

In general, the transition from the ridge-shape Yardang to Yardang monadnock reflects the process of the development of Yardang landform. From infancy to old age, it is obvious that the erosion degree is heavier, that the gulch is wider, that the top is sharper, and that the size becomes smaller, the specific morphological characteristics of which are shown in Table 4-1.