What is Ya Dan landform?

The origin of Ya Dan

Ya Dan, a term, has gradually become familiar to people since it was included in various textbooks and geography reading materials as a proper term of geomorphology in this century, although most people have never witnessed it. Why this landform is called "Ya Dan" is rarely discussed, and it is considered as a foreign language just like "karst landform".

In fact, the word "Ya Dan" is a native product of Xinjiang.

At the beginning of this century, some Chinese and foreign scholars who have been to Lop Nur found a large area of raised mounds in the wasteland of Lop Nur, so they asked the locals for guidance on place names. However, the tour guide mistakenly thought that he was asking the local name of this landform, so he answered "Yardang", which means "steep mound" in Uighur. The discoverer introduced the name, which was later translated from English, and "Jardan" became "Ya Dan". Since then, "Ya Dan" has become synonymous with this landform.

After the discovery of Ya Dan landform in Rob wasteland, many similar landforms have been found in many arid areas of the world, which are collectively called Ya Dan landforms. Even in China, the largest distribution area of Ya Dan is not in Lop Nur, which was first discovered, but in the northwest of Qaidam Basin in Qinghai. In Xinjiang, the distribution of Ya Dan is also ubiquitous, including the famous ghost town of Wuerhe in the northeast of Karamay City, Cai Wu Bay in Beishawo of Jimsar County, the windy city in the southwest desert of Qitai County and so on.

The arid desert environment has created a special landscape in Ya Dan. The arid desert environment has sealed Ya Dan in it, making it difficult for people to see its face and adding a lot of mystery to Ya Dan.

The formation of Ya Dan

The appearance of the earth is the result of internal and external forces, that is, internal and external factors. The internal force of geomorphology is the external reflection of the change of the internal structure of the earth, while there are many external forces, the most important of which are running water and wind.

For many years, Ya Dan has been interpreted as a wind erosion landform in geography books and textbooks, while Ya Dan is regarded as the result of wind erosion, which is similar to another wind erosion landform-desert landform. In fact, this is not accurate. Wind is by no means the whole external force that forms Ya Dan.

Ya Dan covers an area of 3,000 square kilometers in the north, east and west of Lop Nur wasteland, which is the second largest Ya Dan landform distribution area in China.

The reason why there is such a large area of Ya Dan distribution in Lop Nur Depression is that there is a geological basis for the development of this landform, that is, the so-called internal force. During the ancient Lop Nur period, a large area and deep lake deposits were left here. This soil-based sedimentary structure is complex, containing a lot of silt and fine sand, and at the same time sandwiched with hard mudstone layer and gypsum bonded sand layer.

Although the wind and water in nature are threatening, they are also bullying the weak and afraid of the hard. Wind and water erosion take away the loose sand layers in Quaternary sediments, and they are helpless to the hard mudstone or gypsum cementation layer. As a result, pieces of mounds were left on the wasteland, forming a strange uneven appearance, which is what we call Ya Dan.

Ya Dan has various types and shapes, but its forming process is similar.

At first, it was the weathering damage of the surface. Luobu sag used to be a big lake, but the lacustrine deposits left behind were formed in geological age. There have been repeated water inflow and retreat, which made the lake bottom form a layered structure of mud, sand, mud and sand. Mudstone layer is dense and hard, which is generally not easy to be eroded by feng shui, but it can't resist the effect of temperature difference. When traveling in Rob wasteland, you often hear sudden "knocking" sounds, sometimes like firecrackers and sometimes like wolves howling. No wonder the Fa passing by here is creepy, saying that "there are many evil spirits in Shahe", and Li Daoyuan also called it "fewer birds and more ghosts" in Zhu. In fact, this "evil spirit" and "ghost" are all caused by Ya Dan. Lop Nur is located in an extremely arid area, and the temperature difference between day and night changes dramatically, often reaching above 30℃~ 40℃ ... The effect of thermal expansion and contraction makes the exposed rock crack and make a sound. Even the so-called "stubborn" granite can only be stubborn and broken into pieces in this climatic environment, not to mention mudstone! Mudstone doesn't crack like granite, because its structure is layered, and the cracking is also peeled off layer by layer, forming many horizontal or vertical appearances, which gradually exposes the sand layer sandwiched between mudstone layers, creating conditions for the formation of the second phase of Ya Dan Formation.

After the surface is weathered and destroyed, it will be ravaged by wind and water. Under the action of wind erosion or water erosion, the loose sand layers accumulated between the surface mudstones are gradually transported far away, and the originally flat surface becomes uneven, and the embryonic form of Ya Dan landform is announced.

The rudiment of Ya Dan is more conducive to weathering and erosion. After the sand layer is exposed, external forces such as wind and water continue to act, which further deepens and expands the low-lying part; The part protruding from the surface is relatively stable because of the protection of mudstone, but the exposed loose sand layer is eroded and forms a strange shape. At this point, the landform of Ya Dan was finally formed.

Naturally, after the formation of Ya Dan, it is impossible to maintain its original appearance once and for all, because the external forces, including wind and water, will never end, making the appearance of Ya Dan change frequently. As the erosion continues, the depressions will become larger and larger, while the convex hills will shrink and become isolated, and eventually they will inevitably collapse and disappear. This situation can be seen everywhere in the Acic Valley on the east bank of Lop Nur, indicating that the Ya Dan landform has passed its heyday here and began to die out.

In 1930s, when China scholar Chen Zongqi visited Lop Nur, he divided the landforms of Ya Dan into two types and named them respectively: one with a height of no more than 1 m and a relatively young one named Ya Dan; The other is 10 ~ 30 meters high, which is very old and is called "Maisai". In fact, these two types are the primary stage and advanced stage of Ya Dan.

Through the investigation of Ya Dan landform in Lop Nur wasteland, it is proved that there are not only wind but also water, and there are three types of external forces that form Ya Dan: one is Ya Dan formed mainly by wind erosion, the other is Ya Dan formed mainly by water erosion, and the other is Ya Dan formed by feng shui interaction. This basically negates the original conclusion that Ya Dan is a wind erosion landform.

Ya Dan is mainly formed by wind erosion, which is distributed on the plains far away from the mountains. Floods caused by precipitation in mountainous areas generally can't reach, and only the wind exerts its force here. This kind of Ya Dan is mainly concentrated in the area from the south of Peacock River to Loulan Site. Ya Dan is generally 4 ~ 7 meters high, and the low pressure between Ya Dan is northeast-southwest, which is consistent with the prevailing local wind direction, indicating the relationship between Ya Dan and the wind. According to the survey, the average annual wind erosion depth here is between 2.4 and 4.7 mm. According to this wind erosion rate, this Ya Dan was formed a thousand years ago, and after Loulan was abandoned, it should be a flat land.

Ya Dan, which is dominated by flowing water erosion, is mainly distributed in the neighboring mountainous areas, and Sanlongshayadan, located in the east of Achik Valley, is a typical representative of this type of Ya Dan. Although the Lop Nur area is extremely dry, the annual rainfall is only 10 mm, but the precipitation in the nearby mountainous areas is relatively high, sometimes reaching 50 mm at a time. In addition, in arid areas, precipitation is characterized by convective showers, which are sudden and short-lived. Once precipitation and rain pour down, and there is no vegetation interception on the surface, it is easy to form flood flow, which will have a strong scouring effect on the loose surface. In the apricot ditch north of Lop Nur, the previous flood trace was as deep as 1.5 meters, which shows that the flood is great and the erosion is strong. The direction of Sanlongshayadan is 40 east-south, which is just perpendicular to the prevailing local wind direction, but consistent with the flood flow direction in mountainous areas, indicating that Ya Dan here is proud of the strong wind, but bowed to the current, indicating the leading role of flood in the formation of Ya Dan in this area. On the surface of protruding mounds and steep cliffs, there are also obvious traces of flood erosion, which is essentially different from the obvious levels formed by wind erosion. What is particularly interesting is that the Ya Dan here is neatly arranged in a row, which not only shows the power of the flood in that year, but also shows the majesty of a huge fleet moored in the sea, ready to fire at any time. Some Ya Dan chops are shaped like steamed bread. It is conceivable that it is the long-term cleaning of the water that has shaped today's appearance.

Ya Dan, which is influenced by geomantic omen, is between the above two kinds of Ya Dan, with the famous Bailongdui Ya Dan and Longcheng Ya Dan as typical representatives. Although these Ya Dan have now escaped from water erosion in appearance, they left obvious traces of running water in the initial stage. The action of flowing water first washed the flat surface into countless valleys, exposing the loose sand layer, and then eroded by the wind, forming what it is today. The function of wind and water is actually that there is water first, and then there is wind. The trend of Ya Dan is consistent with the trend of Hongshuigou and the prevailing local wind direction, which shows their influence on it. The reason for the formation of this Ya Dan was noticed by Li Daoyuan, a scholar of the Northern Wei Dynasty in China, who gave a scientific explanation in the Notes on Water Classics. He believes that the formation of "Dragon City" is called "Dragon City" because it was washed by water first and then eroded by the wind.

Ya Dan has high ornamental value and is a unique landscape that attracts tourists to Lop Nur. However, it is difficult for you to visit Ya Dan in person. For example, in Ya Dan, Longcheng, because the surface is eroded by water, there are valleys everywhere, which has become a natural obstacle to prevent people from going to Loulan. At that time, Hanshu recorded that due to the terrain and obstacles around Bailongdui, heavy vehicles could not pass, which was "to avoid Bailongdui". Later, a new northbound road was opened, which bypassed Bailongdui via Yiwu and the westbound automobile market, thus changing the Silk Road from two lanes to three lanes. Even in the Sanlongshayadan area where the corridor is wide and flat, vehicles can only make a detour because of the barriers of rows of Ya Dan. It is observed that during the two-hour journey, the odometer of modern vehicles only shows the distance of 1 1 km, which is not much different from the walking speed of people. However, this distance of 1 1 km has made 186 sharp turns, and it takes three turns every two minutes on average, so the terrain is complicated.

related data

Ya Dan is a geographical term, which refers to various Xiangtu-like deposits in ancient rivers and lakes eroded by wind in arid areas. At the beginning of the 20th century, when western explorers visited Lop Nur, Xinjiang, they saw many boat-shaped mounds distributed around the ancient lake in groups, generally about 6 meters high. They were written as Yardang according to the names of local Uighurs, and the word yardang has been circulated in foreign academic circles since then.

China Ya Dan has a landform area of more than 20,000 square kilometers, mainly distributed in the northwest of Qaidam Basin in Qinghai, the middle and lower reaches of Shule River in Xinjiang and around Lop Nur. The landform of Ya Dan in Xinjiang is only 3000-4000 square kilometers, which is very small. A typical Ya Dan is 4-5 meters high and 10-20 meters high, which is also called a countertop, that is, a square table. Ya Dan Group, located in Tebes wasteland in Chad Basin, Africa, has the largest range, about 260,000 square kilometers. The highest and largest Ya Dan is located in the southeast of the Iranian Root Desert, covering an area of about 20,000 square kilometers. Ya Dan is 200 meters high and the wind erosion valley is 500 meters wide. Ya Dan stretches like a ridge, several kilometers to more than ten kilometers long. Ya Dan, an ancient sea in Dunhuang, is 20- 100 meters above sea level. It belongs to the medium and large Ya Dan Group, with narrow wind-eroded valley, rich and colorful Ya Dan shapes and rare high-density types in the world. It is close to Dunhuang, with convenient transportation and excellent location advantages. It is a treasure house of tourism and scientific research.

From Anxi county, 265 kilometers northeast to Jiayuguan city, the Gobi scenery along the way is unique. Among them, Bulongji Township, located 45 kilometers east of the county seat, is a typical landform wonder of Ya Dan, with various and jagged wind erosion beaches on both sides of the road. Ya Dan landform is named after the most typical wind erosion landform in Ya Dan near Lop Nur. Due to thousands of years of wind and sun exposure, the sandstone layer with flat surface has formed many landforms such as wind erosion walls, wind erosion mushrooms, wind erosion columns, wind erosion ridges and gullies, wind erosion depressions, residual hills and castles. The scenery is spectacular and amazing. Anxi County, known as the "World Wind Bank", is located in a long and narrow corridor between two mountains, with an altitude of only 1 170 meters and low terrain. When the air flows in, the thin tube plays a role in accelerating the airflow movement, so strong winds often form. Sandstorms have already endangered the lives of Anxi residents and 3000 mu of cultivated land. There are 6 desert strongholds in the county, covering an area of more than 300,000 mu. This windy weather often makes the landforms of Ya Dan change greatly and have different shapes.

The Origin and Distribution of "Ya Dan Landform"

In the inland desert of China, there is a peculiar geographical landscape, which is composed of a series of long earth mounds extending intermittently and grooves distributed at intervals, and is called Ya Dan landform. In the last century, when junior high school and foreign scholars made a joint investigation in Lop Nur, they found this peculiar landform near Guloulan in the northwest of Lop Nur, and named it after the Uygur, and later translated it into Chinese as "Ya Dan landform".

Ya Dan landforms are found in many arid areas in the world, not only in Xinjiang, China. The road from You Xiang, Qinghai Province to southern Xinjiang is very desolate. You can see the "Ya Dan Landform" between Nanbaxian and Yiliping Highway, which is the largest distribution area of "Ya Dan Landform" in the northwest inland. However, Ya Dan landform is the most widely distributed in Xinjiang. In addition to the Ya Dan landforms of Lop Nur and Guloulan, the ghost town of Karamay and the windy city of Qitai are also typical landforms of Ya Dan.

"Ya Dan" is a Uyghur language, which originally means "hill with steep walls", and later refers to the landform combination of wind erosion ridges, hills, wind erosion troughs and depressions. Lake plains and alluvial plains in arid areas often crack due to shrinkage. Under the long-term erosion of directional wind, cracks gradually expand into grooves, and ridges as high as 5- 10 meters often appear between grooves. This landform is the most typical in Lop Nur, Tarim Basin, China.

Formation mechanism of Ya Dan landform

There are two key factors in the formation of Ya Dan landform. First, the geological basis for the development of this landform, that is, lacustrine sedimentary strata; The second is external force erosion, that is, strong directional wind erosion and flowing water erosion in the desert.

Lakes in arid areas often include repeated water inflow and water withdrawal in their formation history, thus developing overlying mudstone layers and sandy soil layers. Wind and running water can take away loose sandy soil layer, but it has limited effect on hard mudstone layer and gypsum cementation layer. However, the dense mudstone layer is not indestructible, and the expansion and contraction effect caused by severe temperature difference in desert area will eventually lead to the disintegration of mudstone layer, and the exposed sandy soil layer will be carried away by wind and running water and evolved into a groove shape; The part still covered by mudstone is relatively stable, forming large or small strip mounds, and the landform of Ya Dan gradually appears.

The external factors that form the landform of Ya Dan are generally considered to be the dominant factor, but this is not a single dominant factor. For example, in the east of Acic Valley, the direction of Sanlongshayadan is southeast, perpendicular to the prevailing northwest wind direction and consistent with the flow direction of mountain torrents, indicating that floods play a leading role in this Ya Dan landform; In addition, some Ya Dan are formed by the interaction of wind and flowing water, such as Ya Dan in Longcheng.

On the cold lake land of 654.38+700,000 square kilometers, the landform area of Ya Dan accounts for more than 94%. The Ya Dan landform in Qaidam is a lake deposit of Late Tertiary and Early Quaternary 75 million years ago, which was separated from the water body due to the uplift of geological movement, during which the crust of salt sand condensate was carved by the west wind. They are widely distributed in the northwest of Qaidam, which is one of the largest and most typical Ya Dan landscapes in the world, especially Nanbaxian and Yiliping, with a distribution area of 1000 square kilometers. Because of its strange and grotesque landforms, erratic winds, mysterious and horrible winds generated by strange landforms, and the local rocks are rich in iron and strong in geomagnetism, the compass often fails, which leads to the inability to tell the direction and get lost. It is regarded as a ghost town, maze and unique by the world. The total area of Yadanlin here is about 2 1 1,000 square kilometers, with an average elevation of 3,260 meters. It is the largest wind erosion soil forest group discovered in China so far.

The height of Yadanlin ranges from about four or five meters to twenty or thirty meters, and its length and width range from tens to hundreds of meters. On the whole, some are similar to ancient castles, temples, emperors' tombs and accounts of thousands of troops; Some are similar to "whales frolicking in the sand", "millions of sea lions facing the sun" and "Wanlong formation", which are very spectacular. From a close distance, they are also full of weather: some mounds are like warships full of sails and about to sail; Some are like manly cocks, crowing with their necks stretched out; Some are like pavilions in a small bridge, and some are like lovely animals such as horses, camels, elephants, lions, tigers, whales, dragons and birds. Everything is mysterious and wonderful.

In 1950s, eight southern female geologists came here to look for oil resources, and died because they got lost, so the place of "Southern Eight Immortals" came into being. Wind erosion landform is a natural wonder in Qinghai.

/70 cffyinkgqfm 2 e 88 ium _ a/baike/pic/item/99636 c 0 EFE 67 eed 97 bcbe 18d . jpg

/view/34548.html？ wtp=tt