Latitude of Sichuan basin

Zishayan Basin in the east of Sichuan Province, China. Surrounded by mountains, there are Longmen Mountain and Qionglai Mountain in the west, Daba Mountain in the north, Wushan Mountain in the east and Dalou Mountain in the south, covering an area of 6.5438+0.8 million square kilometers. The elevation of the basin is mostly below 500 meters, and it drops to about 230 meters along the Yangtze River to the south. The western part of the basin is Chengdu Plain, which is low and flat, and is formed by alluvial sediments carried by Minjiang River and Tuojiang River. The soil is fertile, and Dujiangyan Water Conservancy Project has been built in guanxian in the northwest, which is an important granary in the agricultural developed Sichuan Basin. In the east of the basin, there are many low mountains and hills that strike northeast and southwest. The highest elevation of Huaying Mountain is 1 0,800 meters, which is the highest point in the basin. Fangshan Hill in the middle of the basin, accounting for 62% of the total area, is mainly composed of purple sandstone and shale. The lithology is loose, its weathered matter is rich in phosphorus and potassium, and its natural fertility is high, which is suitable for planting a variety of crops and economic trees.

Sichuan Basin is one of the four great basin in China, and most of the mountains around the basin are between 1000 m and 3000 m above sea level. The basin bottom is relatively low, with an altitude of 200-7.50 meters, which is a relatively depressed part on the second step of China's topographic division. Because the red rock series from Jurassic to Cretaceous are widely exposed on the surface, it is also called red basin. The western part is a plateau mountain with large uplift and vast territory, with an altitude of more than 4,000m, which belongs to the second level of topographic division in China.

Sichuan Basin is located in the subtropical zone, with Qinling Mountains and Daba Mountain blocking the cold wave in the north. Winter is mild, and there is almost no ice and snow. The average temperature in Leng Yue is 2? Higher than the middle and lower reaches of the Yangtze River at the same latitude. 6? At 54℃, spring comes one month in advance, which is beneficial to the growth of wintering crops such as winter wheat, rape and broad bean and subtropical perennial plants. Summer lasts about five months, and the annual precipitation is about 1 1,000 mm [mm], which is beneficial to rice growth. The temperature conditions in the south of the Yangtze River basin are good, with a frost-free period of 320? 6? 5365 days is beneficial to the cultivation of sweet orange, sugarcane and double-cropping rice. Luzhou and Hejiang regions produce more south subtropical fruits, such as longan and litchi. The basin has always been famous for its rich agricultural products and is known as the "land of abundance". The basin also contains coal, oil, natural gas, salt, apatite, sulfur and other minerals, and has developed into an important industrial base in southwest China. In the old society, the external traffic was inconvenient, and there was a saying that "it was difficult to get through the Shu Road and get to the sky". Nowadays, Chengdu-Chongqing, Baocheng, Cheng Kun, Sichuan-Guizhou and Xiangyu railways have been built, the Yangtze River waterway has been rectified, and air routes from Chengdu and Chongqing to major cities in China have been opened, and the traffic situation has been fundamentally changed. Chengdu and Chongqing are important economic and transportation centers.

Sichuan Basin is located in the east of Sichuan Province, with Qinghai-Tibet Plateau in the west, Yunnan-Guizhou Plateau in the south, Wushan Mountain in the east and Daba Mountain in the north, with an area of 1.8 million square kilometers. It is a typical basin in China. There are many hills and low mountains in the basin, with an altitude of about 500 meters. Minjiang River, Tuojiang River and Jialing River are tributaries in the north and Wujiang River in the south. During the formation of Sichuan Basin, fine sand and soil in the surrounding mountains and plateaus were alluvial to the bottom of the basin by running water, and substances containing iron, phosphorus and potassium were oxidized to purple, so Sichuan Basin is also called "purple basin". The soil in Sichuan basin is mostly purple soil, which is rich in nutrients such as phosphorus and potassium. The warm climate here is conducive to the growth of subtropical crops. It is an important agricultural area in ancient China, with the reputation of "Land of Abundance".

scenic spot

There are Chengdu, the provincial capital, Chongqing, the largest industrial town in southwest China, Panzhihua, a new steel city, and other regional central cities: Zigong, Neijiang and Luzhou. There are Dujiangyan in Chengdu, Qingcheng Mountain in Taoism, Emei Mountain and Three Gorges in Buddhism, Jianmen Shudao in northern Sichuan, Dazu Stone Carving, Leshan, Jinfo Mountain and so on. [Edit this paragraph] Topographic overview Sichuan Basin is located in the east of Sichuan. Zisha shale is widely distributed, which is called "Red Basin" and "Purple Basin" and is the lowest basin in China.

Sichuan Basin is surrounded by connected mountains, covering an area of about 6.5438+0.6 million square kilometers. Sichuan Basin is the home of most people in Sichuan and Chongqing. It is one of the most densely populated areas in China and even the world, and it is also the cradle of Bashu culture. Known as the "land of abundance". Zhuge Liang, the prime minister of Shu, once praised it as "fertile soil for thousands of miles". Li Bai once wrote a poem to prove that "until two rulers were in this area and squeezed in in the foggy age" and Bashu area was also a part of China civilization.

China is a famous red bed basin with the most typical shape, the southernmost latitude and the lowest elevation in the great basin of China. Located in the east of Sichuan Province, on the upper reaches of the Yangtze River, about 500 meters above sea level, the Yangtze River connects it with the East China Sea, making it the largest outflow basin in China. It covers an area of more than 260,000 square kilometers, accounting for 46% of the area of Sichuan Province. Sichuan Basin borders Qinghai-Tibet Plateau and Hengduan Mountains in the west, Qinling Mountains in the north, Loess Plateau in the east and Yunnan-Guizhou Plateau in the south. The rocks here are mainly composed of purple sandstone and shale. These two kinds of rocks are easily weathered and develop into purple soil. Purple soil is rich in nutrients such as calcium, phosphorus and potassium, and it is the most fertile natural soil in China. Sichuan Basin is the place with the most concentrated purple soil in China, and has the reputation of "Purple Basin".

Bottom of Sichuan basin

The bottom area of Sichuan Basin is about 6.5438+0.6 million square kilometers. According to its geographical differences, it can be divided into three parts: the plains in the west of the basin, the hills in the middle of the basin and the ridges and valleys in the east of the basin.

(1) The western basin plain between Longquan Mountain, Longmen Mountain and Qionglai Mountain is the largest plain in southwest China, covering an area of about 8,000 square kilometers. Because Chengdu is located in the plain, it is called Chengdu Plain. Chengdu plain is 450-750 meters above sea level, and the terrain inclines from northwest to southeast. The surface is flat, and the relative height difference is generally not more than 20-50 meters. It is composed of eight rivers, such as Mengjiang, Tuojiang, Fujiang and Qingyijiang, with fertile soil and dense canals. Known as "Tianfu".

(2) The basin hills between Longquan Mountain and Huaying Mountain are relatively low in topography, ranging from 300m to 500m above sea level, with a relative elevation difference of 50m to150m. The terrain is inclined from north to south, and the rock stratum is nearly horizontal. Under the long-term erosion and cutting of running water, stepped Fangshan hills, shallow hills in the south and deep hills in the north are formed. Purple sand shale with different hardness on the hilly surface is easy to weather into purple soil, rich in phosphorus and potassium, with high natural fertility and wide adaptability. It is the main producing area of grain and cash crops in this province.

(3) To the east of Huaying Mountain, there is a ridge valley in the east of the basin, which is composed of several belt-shaped anticline mountains and syncline wide valleys running near northeast and southwest. The mountain is steep and narrow, with an altitude of 700m- 1000m, of which Huaying Mountain is the highest peak at the bottom of the basin. After limestone on the top of the mountain is dissolved by rain, it often forms grooves, so most mountains have the characteristics of "one mountain with two ridges and one groove" or "one mountain with three ridges and two grooves". The valley between mountains is wide and gentle, with an altitude of 300-500 meters, in which hills and flat dams crisscross, which is the main area of industrial and agricultural production in parallel ridge and valley areas.

Marginal mountainous area of Sichuan basin

The marginal mountainous area of Sichuan Basin belongs to a strong uplift and fold belt. The landform is characterized by high altitude and obvious transition, surrounded by a series of Zhongshan and low mountains. Micang Mountain and Daba Mountain in the northern margin of the basin are nearly east-west, which are the southern wings of the famous Qinba Mountain area. Generally, the altitude is between 1, 500 m-2,200m, the mountain is steep and the valley is deep, and the relative height difference can reach 500- 1, 000m. The Dalou Mountain on the southern edge is a part of the majestic Yunnan-Guizhou Plateau. There are Longmen Mountain, Qionglai Mountain and Emei Mountain in the west. The elevations of the ridges are all between 65,438+0,500 meters and ~ 3,000 meters, with a relative height difference of 65,438+0,000 meters. The peak of Mount Emei is 3099 meters high, with a relative height difference of 2650 meters from the nearby plain. It is one of the four famous Buddhist mountains in China. [Edit this paragraph] Topographic features Sichuan Basin is located in the east of Sichuan Province in the upper reaches of the Yangtze River, surrounded by mountains and plateaus with an altitude of 2,000-3,000 meters, with Daba Mountain, Micang Mountain and Longmen Mountain in the north, Qionglai Mountain and Daliangshan Mountain on the edge of the Qinghai-Tibet Plateau in the west, Dalou Mountain in the south and Wushan Mountain in the east. Sichuan basin is a typical basin in structure and human landform. It has been a large depression area with relatively stable crust since Sinian. During the Indosinian Movement of Late Triassic, it became an inland lake basin, but its scope was much larger than that of today's Sichuan basin. Purple sandstone and shale with a thickness of 3000 ~ 4000 meters were accumulated in Mesozoic, so people also called it "red basin" or "purple basin". At the end of Mesozoic, the Sichuan movement made the periphery of the basin fold into mountains and the middle was relatively depressed, and the outline of the Sichuan basin was basically formed, which also caused large-scale deformation of the strata inside the basin. A group of NE-trending folds appeared in the east, which is called the eastern basin fold belt. The dome structure is formed in the middle, which is called the dome belt in the basin; The western part is characterized by subsidence, which has become the subsidence zone of the basin, laying the foundation for the three major geomorphic areas of the basin today. The Cenozoic Himalayan movement caused the surrounding mountains to rise again, the basin to sink again, and Chengdu to sink deeper. At the same time, the Yangtze River flows through Wushan and flows eastward, completing the unified Yangtze River water system and transforming the internal flow basin into the external flow basin.

Sichuan Basin is the largest outflow basin in China, covering an area of over 654.38+10,000 square kilometers. The hills and plains in the basin are interlaced, and the terrain is high in the north and low in the south. Because of the different surface morphology, Huaying Mountain and Longquan Mountain can be divided into three parts: ① East Sichuan Ridge Valley, which is roughly parallel to Huaying Mountain, is composed of several belt-shaped mountains running from northeast to southwest, with 700-800 meters above sea level, and many low hills and flat dams in the valley, with an altitude of 200-500 meters, which is the place where agriculture and population in East Sichuan are concentrated. (2) Fangshan lies between Huaying Mountain and Longquan Mountain. Due to the gentle dip angle of purple sand shale in the area, a large square hill is formed after cutting. The altitude is 350 ~ 450 meters, and the relative height is tens of meters. The local working people used the deep soil layer of Fangshan Mountain to build terraces all the way to the top of the mountain. ③ The plain is located to the west of Longquan Mountain. Known as West Sichuan Plain or Chengdu Plain, it covers an area of more than 6,000 square kilometers and is the largest plain in Sichuan Basin and Southwest China, with an altitude of about 600 meters. The famous Dujiangyan water conservancy project enables the plain to be irrigated by itself, with warm climate, fertile soil and early development, making it a "land of abundance" thousands of miles away. Sichuan basin belongs to Sichuan depression of Yangtze paraplatform. Paleozoic was relatively uplifted, lacking Devonian and Carboniferous. Indosinian movement transformed into a large depression, and fold uplift occurred in the late Yanshan period, especially after Himalayan movement. The basin pattern is mainly controlled by NE-SW and NW structural lines, forming a typical rhombic basin. Guangyuan, Ya 'an, Xuyong and Yunyang are the four vertices of the rhombus, with the east and west sides slightly longer, 380-430 km, and the north and south sides slightly shorter, 365,438+00-330 km. The connecting line of the four vertices of the diamond is roughly equivalent to the contour line of 650 ~ 750 meters in the basin, and the bottom of the basin and the marginal mountains are also divided by this line. Sichuan Basin was an inland lake basin10.40 billion years ago. Until 66 million years ago, the mountainous areas on the edge of the basin rose rapidly, and the water systems in the middle and upper reaches of the Yangtze River began to communicate. The eastward flow of lake water in the basin laid the present landform.

There are many low mountains and Zhongshan mountains on the edge of the basin, and the mountains are steep. Most of the rivers in the mountains on the edge of the basin are V-shaped valleys with a height difference of more than 500 ~ 1 000 meters, and the surface is rugged, so there is a saying in history that it is difficult to get through the Shu Road and get into the sky. Ridges are mostly at an altitude of 2,000-3,000 meters, and can exceed 3,000-4,000 meters in the northwest and west, such as 4,984 meters in Longmen Mountain, 3,099 meters in Emei Mountain and 479 1 meter in Xiaoxiangling Mountain. Limestone before Paleozoic is widely exposed on the surface, followed by slate, schist, crystalline limestone, quartzite, sandstone mudstone and conglomerate, and granite and basalt in some places. Karst landforms such as stone forest, karst cave, underground river and valley can be seen in limestone distribution area. Xingwen County in the southern margin of the basin is called "Shilin Cave Township" (see Xingwen Stone Forest). Famous mountains such as Twelve Peaks in Wushan and Jinfo Mountain are also mainly developed from limestone. Emei Mountain, which is composed of limestone, basalt and granite, and Qingcheng Mountain, which is composed of sandstone, mudstone and conglomerate, is known as "Emei is beautiful on earth" and "Qingcheng is quiet on earth", and is a famous tourist attraction in China.

Most of the bottom of the basin is 250 ~ 700 meters above sea level, and the terrain is inclined to the southeast. All rivers in the basin converge from the marginal mountains to the main stream of the Yangtze River at the bottom of the basin, forming a centripetal water system. The surface is covered by a large area of Mesozoic purplish red sandstone and mudstone, so it is called "red bed basin", which is the most concentrated area of Mesozoic continental red beds in China. Sichuan basin is a hilly basin with hills at the bottom, followed by low mountains and plains. [Edit this paragraph] Climate type The terrain of Sichuan Basin is blocked, and the temperature is higher than other areas at the same latitude. The average temperature in Leng Yue is 5 ~ 8℃, which is 2 ~ 4℃ higher than that in Shanghai, Wuhan in the same latitude and Guiyang in the south latitude. Extreme minimum temperature -6 ~-2℃. Frost and snow are scarce, and the frost-free period is 280 ~ 350 days throughout the year. Changning, located in the Yangtze River valley, is frost-free all year round.

Summer begins at the end of May and lasts for more than 4 ~ 5 months. The temperature in the hottest month is as high as 25 ~ 29℃, the Yangtze River basin is nearly 30℃, and the extreme maximum temperature in the southeast of the basin often exceeds 40℃. Chongqing and Pengshui used to have 44℃, so Chongqing is also one of the three "stoves" in the Yangtze River Basin. The sunny, hot and high temperature weather in midsummer caused a severe summer drought in the southeast of the basin.

The annual average temperature is 16 ~ 18℃. The active accumulated temperature above 10℃ is 4500 ~ 6000℃ and lasts for 8 ~ 9 months, belonging to the middle subtropical zone. The Yangtze River basin in the southeast is over 6000℃, such as Qijiang River is over 6 100℃, which is equivalent to the tropical climate of South Asia south of Nanling in China. The temperature in the basin is high in the east and low in the west, high in the south and low in the north, high in the bottom and low in the edge, and the isotherm distribution is concentric. The temperature in the mountainous area on the edge of the basin has vertical distribution characteristics. The elevation of Emei Mountain and Jinfo Mountain rises by 100 m, and the temperature drops by 0.55℃ and 0.6 1℃ respectively. The average annual temperature at the top of Mount Emei is only 3℃, and the active accumulated temperature above 10℃ is 586℃, which is equivalent to cold temperate zone and sub-cold zone in climate.

The annual precipitation in Sichuan Basin is 65,438+0,000 ~ 65,438+0,300 mm, and the precipitation in the mountainous areas around the basin is very rich. For example, the annual precipitation in the western mountainous area between Leshan and Ya 'an is 1.500 ~ 1.800mm, which is a prominent rainy area in China and is called "West China Rain Curtain". However, winter drought, spring drought, summer waterlogging and autumn Mao Mao rain are unevenly distributed in the year, and 70 ~ 75% of the rainfall is concentrated in 6 ~ 10. The maximum daily precipitation can reach 300 ~ 500mm. "Night rain" has been famous since ancient times, and it accounts for more than 60 ~ 70% of the total rainfall. The fog in the basin is big and wet, and the clouds are low and cloudy. Emei Mountain and Jinfo Mountain are the foggy areas in China, and the annual relative humidity is also the highest in China. The annual sunshine in the basin is only 900 ~ 1300 hours, and the annual solar radiation is 370 ~ 420 KJ/cm2, both of which are the lowest in the country, so it is called "the tree spring barks at the sun". [Edit this paragraph] Biological resources There are nearly 10,000 species of plants in Sichuan Basin, among which there are many ancient endemic species that cannot be compared with other areas in China. Metasequoia glyptostroboides, Liriodendron chinense, Zelkova schneideriana, Davidia involucrata, Celastrus mandshurica, Ailanthus altissima, Chimonanthus praecox, Eucommia ulmoides, Taxus chinensis, Cinnamomum camphora, Fokienia hodginsii, Taxus chinensis, Platycladus orientalis and so on. It can still be seen in the mountains on the edge of the basin and the valleys in the east of the basin. In the hot and humid valley, you can see ancient tropical relict plant, such as spinulosa spinulosa, Dryopteris pubescens, Osmunda japonica and Li Bai. Jinfo Mountain and Jinyun Mountain have established nature reserves respectively. Youyang, on the southeast edge of the basin, also has the highest white paulownia in the world, up to 44 meters.

The zonal vegetation in Sichuan Basin is subtropical evergreen broad-leaved forest, and the representative tree species are Castanopsis fargesii, Castanopsis emeiensis, Castanopsis hystrix, Cyclobalanopsis glauca, Cyclobalanopsis glauca, Quercus Baoshan, Schima superba, Schima superba, Phoebe bournei, Machilus Runnan and so on. The altitude is generally 1600 ~ 65438+. Followed by subtropical coniferous forest and bamboo forest composed of Pinus massoniana, Cunninghamia lanceolata and cypress. The marginal mountain areas are evergreen broad-leaved forest, evergreen broad-leaved and deciduous broad-leaved mixed forest, cold temperate mountain coniferous forest, and some areas are subalpine shrub meadow.

Sichuan Basin, a national treasure, is one of the most complete areas in China. According to statistics, except fish, there are 4 17 species of animals at the bottom of the basin, 487 species, 3 17 species and 288 species in the western, northern and southern mountainous areas of the basin, among which economic animals account for more than half. The mountainous area on the western edge of the basin is the best preserved and most concentrated area of endemic and ancient animals in China, including giant panda, golden monkey, pronghorn, grey golden monkey and white-lipped deer. There are also rare and unique animals such as red pandas, snow leopards, gazelles, macaques, crested deer, otters and mandarin ducks, blood pheasants, golden pheasants with red bellies, green-tailed pheasants, golden pheasants with white bellies and golden pheasants with red bellies.

Pingwu, Qingchuan, Beichuan, Baoxing, Tianquan, Hongya and Mabian in the western margin of the basin are the main distribution areas of giant pandas in China. Seven national nature reserves for giant pandas and golden monkeys have been established, including Tangjiahe National Nature Reserve, Wang Lang National Nature Reserve, Wolong National Nature Reserve, Fengtongzhai National Nature Reserve and Horn River National Nature Reserve.

The giant salamander in the gully on the edge of Youyang, Mabian, Pingwu and Qingchuan basins, the Chinese sturgeon and the white sturgeon in the Yangtze River and Jinsha River are also endemic to Sichuan, and all belong to the national protected animals. [Edit this paragraph] Economic overview There are coal, iron, natural gas, petroleum, salt, mirabilite, gypsum, phosphorus, aluminum, sulfur, copper, manganese, gold, graphite, mercury and other minerals in the basin, among which natural gas and mirabilite rank first in China, and there are also important strontium minerals in China. The reserves of salt rocks in central Sichuan reach 200-300 million tons. The reserve of hydraulic resources in the basin is nearly 50 million kilowatts. Industrial concentration, the main industrial cities are Chengdu, Chongqing, Mianyang, Zigong, Yibin, Neijiang, Nanchong, Luzhou and other cities.

This basin is a famous agricultural area in China. The purple soil distributed in the river basin covers an area of 6.5438+0.4 million square kilometers, which is neutral or neutral and alkaline, rich in mineral nutrients such as phosphorus and potassium, moderate in texture and good in permeability. It was cultivated and planted as early as the Western Han Dynasty.

The land utilization rate in the basin area is as high as 30 ~ 40%, and it is the largest rice and rape producing area in China. The output of sericulture, citrus, tung tree, Fraxinus mandshurica, gallnut, tremella fuciformis and coptis chinensis all ranks first in China. [Edit this paragraph] Geographical division Sichuan Basin can be clearly divided into two parts: marginal mountain and basin bottom, with areas of 654.38+10,000 and1.6,000 square kilometers respectively. There are many middle and low mountains in the marginal mountainous areas. The elements of the landscape are obviously transitional, such as the composition of animals and plants permeating the components of Central China, Southwest China, Qinghai-Tibet Plateau and North China respectively. Generally, there are 2 ~ 5 vertical natural zones from bottom to top in marginal mountainous areas. The marginal mountainous areas are the bases of various economic trees and timber forests in Sichuan. There is serious soil erosion in agriculture.

There are many hills, low mountains and plains at the bottom of the basin. The surface composition is new and single, sandy mudstone and Quaternary sediments. The climate belongs to the middle subtropical zone, and the heat is much higher than that in the marginal mountainous areas, but the precipitation is not as good as that in the marginal mountainous areas. Vegetation is replaced by artificial secondary forest trees, with low coverage and contiguous cultivated land. Drought and flood in summer are the biggest disasters in this area. The bottom of the basin is bounded by Longquan Mountain and Huaying Mountain, which can be divided into three parts: Chengdu Plain, hills in the middle of Sichuan and parallel ridges and valleys in the east of Sichuan. [Edit this paragraph] Formation of the Basin The Sichuan Basin is a part of the Yangtze Platform, which is a relatively stable area, but it has experienced two large-scale sea immersion. The first time started from the Cambrian more than 500 million years ago, and continued to the Silurian more than 370 million years ago, and it continued to sink into an ocean basin. The Silurian Caledonian movement took place, and the rest areas rose to land except the Longmenshan geosyncline in the west. At the end of Carboniferous period, 270 million years ago, due to the second seawater immersion, the basin was once again occupied by the ocean. The Permian sea and land alternated, forming Nankong, Songzao, Tianfu and other coal mines near Chongqing. At the end of Permian, magma erupted in the west of the basin, and basalts formed in Xiaojinding and Qingyinge areas of Emei Mountain at this time.

Triassic, 654.38+0.9 billion years ago, the Indosinian movement gradually raised the basin edge into a mountain, and the area submerged by seawater gradually rose to land, from a sea basin to a lake basin. At that time, the lake occupied almost all the territory of the present Sichuan Basin and was called "Bashu Lake", ending the history of seawater immersion. During the long period of 6,543.8 billion years in Mesozoic, the climate in the basin was warm and humid, and ferns, cycads and gymnosperms grew everywhere, which was another coal-forming period. Yongrong Coal Mine was formed in Triassic and Jurassic. From Changshou and Dianjiang in the east, Jiangyou and Qionglai in the west, at the foot of Daba Mountain in the north and Chishui in Guizhou in the south. This is also a region rich in natural gas. During this period, reptiles and dinosaurs ruled for a period of time. Mamenxilong in Hezhou discovered in 1957 is 22 meters long and 3.5 meters high. It is the largest and most complete dinosaur fossil in Asia.

At the end of Cretaceous 70 million years ago, another strong crustal movement "Yanshan Movement" took place. The mountains around the basin are constantly uplifting, and at the same time, many large faults are produced, such as Longmenshan fault in the west and Huayingshan fault in the east, which divides the basin into three parts. Bashu Lake has shrunk to only 20,000 square kilometers. The closed basin topography and sharply reduced water surface make the climate gradually dry and hot, and the sediments change from marine phase and land-sea alternating phase to continental phase. A large number of weathered, eroded and denuded materials have accumulated thousands of meters thick in the basin, forming purple sand, mud and shale. Gymnosperms decreased and dinosaurs became extinct. Under drought conditions, the inland lake was strongly evaporated, the concentration increased, and the salt accumulated continuously, forming a salt lake. Later, the sediments were buried and preserved in the stratum, and after a long geological process, the strata were formed. Zigong area is a famous well salt producing area.

The Neogene was influenced by Himalayan orogeny more than 20 million years ago. Two or three million years ago, in the Quaternary, the earth's crust underwent another tectonic movement. The water system on both sides of Wushan was eroded from the source, and * * * crossed Wushan, forming the world-famous Three Gorges of the Yangtze River, and the water from the basin was incorporated into the Yangtze River water system. As a result, Sichuan basin has experienced the changes of sea basin, lake basin and land basin from internal flow basin to external flow basin, from closed internal flow area to external flow area, and from accumulation to erosion.

Quaternary is an era when glaciers are widely distributed, and a large number of glaciers are developed in the northwest mountainous area of the basin. After the melting of glaciers, a large number of sediments were carried by Minjiang River and Tuojiang River and accumulated in Qianshu Lake in the west, eventually forming Chengdu Plain.

Sichuan Basin is rich in plains 7%, hills 52% and low mountains 4 1%. [Edit this paragraph] Sichuan Basin, a land of abundance, has fertile land, mild climate and abundant rainfall. Especially after the construction of Dujiangyan Water Conservancy Project in Qin Dynasty, Chengdu Plain became a very developed area in the history of China, and became the main grain supply base and the main source of tax revenue of the Central Dynasty, so it was called "Tianfu". Sichuan basin is surrounded by mountains and the traffic is blocked. In ancient times, it was called "the country with four blockades". In the era of cold weapons, it has a special strategic position that it is easy to defend but difficult to attack, thus avoiding the destruction of many wars in history and obtaining a relatively stable social environment, which is conducive to her social and economic development. Many far-sighted strategists in history, such as Sean and Zhuge Liang, regard Sichuan as the foundation of their country. In the middle and late Tang Dynasty, the war broke out in Guanzhong, and both Xuanzong and Tang Xizong fled to Chengdu, Sichuan to escape the war. In their eyes, Chengdu at that time was a place of singing and dancing, and it was called "the land of abundance" at that time. So, how did the name "Land of Abundance" come from? According to textual research, the word "Tianfu" first appeared in Zhou Li. Originally an official name, it is an official who specializes in keeping national treasures. Later generations used this as a metaphor for places with superior natural conditions, dangerous situation and rich specialties. The earliest name for Sichuan in history is "Tianfu", which comes from Zhuge Liang's "Longzhong Dui": "Yizhou is in danger, fertile land is thousands of miles away, and the land of abundance is due to it, so it will become an emperor's business." Yizhou in Han Dynasty includes Sichuan Basin and Hanzhong Basin. The famous historian of the Jin Dynasty, Chang Qu, said in the Records of Huayang Country: "Shu has thousands of miles of fertile land, which is called' land and sea'. Drought causes water seepage and rain blocks the water gate. Therefore, it is recorded that floods and droughts follow people, and hunger does not know. When there is no famine, the world is called abundance. " "Land of Abundance" has gradually become synonymous with Sichuan Basin in the writings of scholars in past dynasties.