The mystery of the colorful hills

　　On July 7, 2020, the 209th meeting of the UNESCO Executive Board was held in Paris, France. At this meeting, the two geoparks in Xiangxi, Hunan and Zhangye, Gansu recommended by my country were officially approved as “World Geoparks”. So far, the number of global geoparks owned by my country has reached 41, accounting for a quarter of 161 locations in the world, ranking first in the world.
　　 Speaking of the Zhangye Geopark in Gansu, many people first think of the Danxia landform, but what comes to mind is the red-orange-yellow-green, layered colorful hills. In fact, many people have not figured out the relationship between these hills and Danxia. Even people who have actually been to Zhangye may not really figure it out. Next, let us explore the secrets of the colorful hills together.
　　The hilly wonder “Rainbow Mountain”
　　 is located at the junction of Linze County and Sunan County in Zhangye City, Gansu Province. When we approached there, we saw that the rocks in front of us were colorful, like a magnificent picture against the blue sky and white clouds, as if someone had put a gorgeous colorful robe on the mountain. Geologists call this spectacle “color hills” (“color hills” for short), and others vividly call it “rainbow mountains”.
　　 When the beauty photos of the colorful hills in Zhangye became popular on the Internet, many people came here admiringly. But some people said that after they came to the scene in person, they didn’t feel as stunning as the professional photographers took. Could the photos become “photo tricks”? According to the local guide, it is not ruled out that there are excessively modified photos and grandstanding. In fact, the colorful hills are indeed very beautiful, but this kind of beauty has a lot to do with the viewing time, viewing angle and weather. Whenever it is sunny and sunny after rain, the colorful hills will show its most beautiful scenery.
　　 From a geological point of view, colored hills are actually hilly landforms with a relative height of less than 200 meters. They are low hills with rounded tops, gentle slopes and inconspicuous foot lines. The hills are the products of the quasi-plainization stage formed by various external dynamic geological processes such as erosion and denudation for a long time. They are the transition from mountains to plains. For example, the hills in southern Shandong and northern Jiangsu are the remaining bedrock hills on the quasi-plain. There are also some hilly landforms often connected with mountains. For example, the colorful hills in Zhangye are the piedmont hills located at the northern foot of the Qilian Mountains. They have survived the vicissitudes of life and have survived the vicissitudes of life. It is like an old man who is frail and dying, quietly telling us the story of the years.
　　 The colorful hills are not unique to Zhangye. Similar geological phenomena have also been found in the colorful beaches in Burqin County, Altay Region, the northern tip of Xinjiang, and Peru in South America. About 10 years ago, due to global warming, a large area of ​​ice and snow melted through the Andes Mountains in Peru, and the snow line continued to rise. At this time, someone discovered that there were beautiful stripes of purple, red, yellow and turquoise on a mountain with an altitude of more than 5,000 meters. The locals called it Vinicunca, which means “Rainbow Mountain” or “Colorful Mountain.” . The beauty there is hidden under the snow and ice of the inaccessible mountains. Only occasional herds of alpacas roam the slopes. Now it is famous and tourists are increasing. On average, as many as thousands of people set foot on the ridge of Rainbow Mountain every day. Faced with this situation, some scholars pointed out anxiously that the influx of tourists will accelerate the erosion and destruction of the natural landscape.
　　Color Hill in past lives
　　 if we look closely rock Zhangye colored hills, then you will find that they are typical of siltstone and mudstone, are the product of sedimentation. Geological dating results show that they were formed about 135 to 096 million years ago. Geologists have discovered that as early as the Cretaceous period of the Mesozoic, the earth’s climate became quite warm, and dinosaurs were ruling the world. At that time, the colored hills of Zhangye were a vast lake. After a long period of millions of years, layers of sediment were deposited on the bottom of the lake. Later, with climate and geological changes, the lake dries up, and the sediment at the bottom of the lake has been compacted, dehydrated, and consolidated into diagenesis, and slowly becomes sedimentary rock with clear layers.
　　 Nowadays, the reason why the rock layers of the colorful hills in Zhangye are slanted is that they were horizontal or nearly horizontal at the beginning of the deposition, and then they appeared to bend and twisted with the crustal movement, as if they were squeezed. In the book, some places are arched in the middle, and some places are sunken. Therefore, uneven hills are formed under the effects of external wind, sun, rain and other weathering and erosion, and the rock layers are perfectly displayed on its spine like a canvas.
　　 Why do the hills here show different colors? This is related to the mineral composition contained in the rock formations and also depends on the wet and dry climate conditions when the sedimentary rocks were formed. Chinese scholar Wang Jiemin and other studies have found that if the deposition process is in an arid environment, oxidation will be the main effect. The iron-bearing minerals are mainly red hematite (iron-containing Fe3+); if they are in a humid environment, they will The main effect is reduction, and the iron-bearing minerals are mainly goethite (containing ion Fe2+). Therefore, the different colors presented by the rocks precisely reflect the geological environment during the diagenesis period. The Fe3+ content in the rocks is high, so the color of the stratum will be mainly purple or brick red. With the gradual decrease of Fe3+ content and the gradual increase of Fe2+ content, the color of the formation will change from red to gray-green, yellow-gray, and blue-gray.
　　Difference “Cai Hill” and “Danxia” is
　　 now the most beautiful color Zhangye hills, why it is always said to Danxia it?
　　 One of the reasons is related to the long-standing disputes in the academic circles about the colorful hills and Danxia landform. “Danxia Landform” is a geomorphological term coined by Chinese geologists. It was first named after Danxia Mountain, which “looks like a danxia and shining brightly” in Renhua County, Shaoguan City, Guangdong. It refers to a variety of strangely shaped landforms, such as castles, formed under the combined effects of flowing water erosion, differential weathering, and gravity collapse due to the development of joints, uneven iron-calcium mixing and cementation of thick red glutenite with gentle occurrence. , Pagoda-shaped, needle-shaped, columnar, rod-shaped or peak forest-shaped, etc. However, the opinions of geologists on the definition of Danxia landform are not completely unified, and different documents give different definitions. Many laymen, with little knowledge, put the name “Danxia” on some similar landscapes, including the colorful hills, which was misleading.
　　 Another reason is that Zhangye does have Danxia landforms, such as Binggou Danxia and Dalibagou Danxia. Among them, Binggou Danxia is dominated by strange rocky landscapes such as Danya cliffs and fort-shaped lonely mountains, while Dalibagou Danxia is famous for its sandstone window lattice structure with its facade resembling window lattices and the overall structure like a palace. Experts call it “the most typical Danxia landform developed in the arid area of ​​northern China and the only Danxia landform and colorful hilly landscape complex in China”. However, after arriving in Zhangye, many tourists did not go to see the Danxia landform, but went directly to the colorful hills, but mistakenly thought that what they saw was the Danxia landform.
　　 The Nantai colorful hilly scenic area in Zhangye is only 10 kilometers away from the Binggou Danxia scenic area, and consists of the same set of strata (called “red layers” by geologists). Why do they form distinctly different landscapes? Peng Hua, a famous geographer in China, believes that the red beds developed in the river or lake environment of inland basins during the geological period have obvious laws in their rock composition: the edge of the basin is dominated by huge thick mixed mud and gravels of proluvial , Towards the center of the basin, it gradually changes into diluvial conglomerate, alluvial conglomerate, glutenite, sandstone and river-lacustrine fine sand, siltstone or argillaceous rock. Therefore, from the edge of the basin to the center, the hardness of the formed sedimentary rock will become smaller and smaller. In contrast, relatively hard rock formations are easy to develop into Danxia landforms, while relatively soft rock formations are easy to form red beds and hills. Because it does not have Chibi Danya, it is generally not considered Danxia landforms.
　　 Therefore, the biggest difference between the colorful hills and the Danxia landform is that the colorful hills are characterized by the bright colors of rocks; while the Danxia landform is characterized by the weird rock shape and bright red. For example, for the difference between the colorful hills of Nantai in Zhangye and the Danxia landform in Binggou, my country’s famous tourism geologist Professor Li Zhongdong made a vivid analogy: “If Nantai is famous for its colorfulness, it shows us the beauty of sandstone landforms. , Then Binggou is superimposed with the beauty of sandstone landforms on the basis of the beauty of colors.”
　　 In addition, it should be noted that some people may be foolish to distinguish Danxia and Yadan landforms, and then confuse them with colored hills. The Danxia landform is mainly formed by the erosion of surface water, and the color is red or purplish red; while the Yadan landform is mainly distributed in arid desert areas and is eroded by strong winds. There is no specific color, such as Yadan in Dunhuang, Gansu. Devil City. In contrast, colored hills may be the result of surface runoff, wind and glaciers and other external forces, and the colors are also much richer.
　　 As a new topography, the scientific research value and tourism value of colored hills are very high. The Zhangye Geopark has successfully entered the ranks of the world’s geoparks, which will better show my country’s unique colorful hills in front of the people of the world, allowing more people to understand, appreciate and protect it.