The formation of Yadan
The appearance of the earth is the result of the simultaneous action of internal and external forces, which are commonly referred to as internal and external factors. The internal force of the landform is the external reaction of changes in the earth's internal structure, while there are many types of external force, the most important of which are flowing water and wind.
For many years, in geography readings and textbooks, Yadan has been interpreted as a wind-eroded landform. It is believed that the formation of Yadan is the result of wind erosion, which is different from another type of wind-eroded landform-desert landform. resemblance. Actually this is not accurate. Wind is by no means the entire external force that forms Yadan.
In the north, east and west of Luobu Wasteland, Yadan covers an area of ??3,000 square kilometers. It is the second largest distribution area of ??Yadan landform in China.
There is such a large area of ??Yadan distribution in the Luobu Depression because there is the geological basis for the development of this landform, which is the so-called internal force. During the ancient Lop Nur period, a large area and deep lake sediments were left here. This mainly soil-based sedimentary structure is complex, containing a large amount of silt and fine sand, as well as hard mudstone layers and gypsum-cemented sand layers.
Although the wind and water in nature are fierce, they are also bullying the weak and afraid of the hard. Wind and water erosion only transport the loose sand layers in the Quaternary sediments layer by layer, and do not affect the hard mudstone or gypsum. The cement layer is helpless to a certain extent. As a result, pieces of mounds of soil were left in the wasteland, forming a unique uneven appearance, which we call Yadan.
There are various types of Yadan with different shapes, but the formation process is roughly similar.
Initially, it was weathering damage to the surface. Luobu Depression was once a large lake, and the lacustrine sediments left behind were formed over the geological years. Repeated water inflows and retreats formed a layer of mud, a layer of sand, and another layer of mud, Another layer of sand interlaced into a layered structure. The mudstone layer has a tight and hard structure and is generally not susceptible to erosion by wind and water. However, it cannot withstand temperature differences. When traveling in the Luobu Wilderness, one often hears sudden "pipai" sounds, sometimes like firecrackers, sometimes like howling wolves. No wonder the monk Faxian who passed by this place was so hairy that he said, "There are many evil ghosts and hot winds in the sand river." Dogen also said in "Shui Jing Zhu" that "there are few birds and many ghosts" here. In fact, these "evil ghosts" and "ghosts" are caused by Yadan. The Lop Nur area is in an extremely arid area, and the temperature difference between day and night changes drastically, often reaching over 30°C to 40°C. The effect of thermal expansion and contraction causes the exposed rocks to crack and make sounds. Even granite, which is known as "stubborn", can only remain stubborn in this climate environment and gradually break into pieces, not to mention mudstone! However, mudstone will not break into blocks like granite, because its structure is lamellar, and the breakup will also peel off layer by layer, forming many horizontal or vertical appearances, making the sand layers sandwiched between the mudstone layers Gradually exposed to the surface, creating conditions for the second stage of Yadan's formation.
After the surface is destroyed by weathering, wind and water have a target to wreak havoc. Due to wind erosion or water erosion, the loose sand layers accumulated between the mudstone layers on the surface were gradually transported to distant places. The originally flat surface became undulating and uneven, and the prototype of the Yadan landform was born.
The prototype of Yadan is more conducive to weathering and erosion. After the sand layer is exposed, external forces such as wind and water continue to exert their influence, further deepening and expanding the low-lying parts; the parts protruding from the surface are relatively stable due to the protection of the mudstone layer, but the exposed loose sand layer is eroded, thus Create all kinds of strange shapes. At this point, the Yadan landform was finally formed.
Naturally, after Yadan is formed, it is impossible to maintain its original appearance once and for all, because the influence of external forces, including wind and water, will never end, causing Yadan's appearance to change constantly. Always new. As erosion continues, the depressions will become larger and larger, while the raised mounds will shrink and become isolated, eventually collapsing and disappearing. This situation is common in the Aqike Valley on the east coast of Lop Nur, indicating that the Yadan landform has passed its heyday here and has begun to disappear.
When Chinese scholar Chen Zongqi and others inspected Lop Nur in the 1930s, they divided the Yadan landform into two major types and named them respectively: one is no more than 1 meter high and formed at a younger age, which is called Yadan"; another one 10 to 30 meters high, ancient, called "Maisai". In fact, these two types are nothing more than the primary and advanced stages of Yadan.
An investigation of the Yadan landform in Luobu Wasteland has proven that the external force that forms Yadan is not only wind, but also water, and there are three types: one is mainly formed by wind erosion Yadan, one type is mainly formed by water erosion, and the other type is formed by the combined action of wind and water. In this way, the original conclusion that "Yadan is a wind-eroded landform" is basically negated.
Yadang, which is mainly formed by wind erosion, is located in plains far away from the mountainous areas. Floods caused by precipitation in mountainous areas are generally inaccessible, and only wind exerts its power here. This type of Yadan is concentrated in the area from the south of Kongque River to the Loulan ruins. Yadans are generally 4 to 7 meters high. The depressions between Yadans run from northeast to southwest, which is consistent with the prevailing local wind direction, indicating the relationship between Yadans and the wind. According to the survey, the average annual wind erosion depth here is between 2.4 and 4.7 millimeters. According to this wind erosion speed, this Yadan was formed only a thousand years ago, after Loulan was abandoned. At that time, this area should have been a flat and fertile field.
Yadang, which is dominated by water erosion, is mainly distributed in areas adjacent to mountains. Sanlongsha Yadan in the eastern section of the Aqik Valley is a typical representative of this type of Yadan. Although the Lop Nur area is extremely dry, with an annual rainfall of only about 10 millimeters, the nearby mountains receive relatively large amounts of precipitation, sometimes up to 50 millimeters at a time. Moreover, in arid areas, precipitation is characterized by convective showers, which are strong in duration and short in duration. Once it rains, it rains in heavy pouring, and there is no vegetation on the surface to intercept it. It is easy to form flood flows, which will have a strong scouring effect on the loose surface. In the Xingdi ditch north of Lop Nur, the traces of past floods are as deep as 1.5 meters, which shows the magnitude of the flood and the strength of the erosion. The direction of Sanlongsha Yadan is 40° south to east, which is exactly perpendicular to the prevailing local wind direction and consistent with the flow direction of mountain floods. This shows that the Yadan here is proud of the strong wind, but bows to the water flow, indicating that floods are formed in this Yadan. leading role in it. On the surface of the protruding mounds and cliffs, there are clear traces of flood erosion, which are fundamentally different from the obvious layers formed by wind erosion. What is particularly interesting is that the Yadans here are neatly arranged in rows, which not only shows the power of the flood, but also looks like a huge fleet anchored at sea and ready to go. Some rows of yadan are shaped like steamed buns. It can be imagined that the long-term cleaning by water has shaped their current appearance.
Yadan, which is influenced by wind and water, is between the two types of Yadan mentioned above, with the famous Bailongdui Yadan and Longcheng Yadan as typical representatives. Although these Yadan are no longer related to water erosion from the appearance, they still left obvious traces of water erosion in their initial stage. The action of flowing water first washed the flat surface into countless ravines, exposing the loose sand layer to the surface, and then was eroded by the wind to form the current appearance. The effect of wind and water is actually water first and wind second. The direction of this piece of Yadan is consistent with the direction of the flood ditch and the prevailing local wind direction, indicating the influence of the two on it. The reason for the formation of this type of elegant dan has long been noticed by Li Daoyuan, a scholar in the Northern Wei Dynasty of my country, and gave a scientific explanation in his "Shui Jing Zhu". He believed that the formation of "Dragon City" was first caused by water. It was patted on the shore and then eroded by the wind, forming a dragon-like shape, so it was called "Dragon City".
The various Yadans are highly ornamental and are a special landscape that attracts tourists to Lop Nur. However, it is extremely difficult for you to visit Yadan in person. For example, in Longcheng Yadan, due to water erosion in the past, ravines and valleys filled the fields, becoming a natural obstacle preventing people from going to Loulan. At that time, "Hanshu" recorded that due to the dangerous terrain around Bailongdui, baggage vehicles could not pass. In order to "avoid the disaster of Bailong", a new north road was opened later, passing through Yiwu and Cheshi going west, bypassing Bailongdui. Then the silk route was changed from two to three. Even in the Shayadan area of ??Sanlong, which has a wide and flat corridor, vehicles can only take circuitous routes due to the obstruction of rows of yadan.
It has been observed that in a two-hour journey of a modern vehicle, the odometer only shows a distance of 11 kilometers, which is not much different from the walking speed of a person. However, during this 11-kilometer journey, there are 186 sharp turns. On average, It has to make three turns every two minutes, which shows how complex the terrain is.