Historical evolution of the Loess Plateau

About eight million years ago, the Loess Plateau was still a lake in Wang Yang. It starts from Sun Moon Mountain in Qinghai in the west, Luoyang in Henan in the east, Qinling in Shaanxi in the south and the Great Wall in northern Shaanxi in the north. The lake is very big, the water is sunny, and the area is equivalent to six Bohai Seas today.

It can be called the original lake of loess. On the west bank of this huge lake, there is a vast desert, and the south and east are barren hills. At that time, the weather on the earth was dry and cold, and there was a strong wind everywhere, and the wind blew up the dust on the ground.

Dust flying all over the sky was blown to the original loess lake, making the lake very turbid. Slowly, the dust sank to the bottom of the lake, accumulating bit by bit, layer by layer. Over time, after tens of millions of years of accumulation, the dust at the bottom of the lake is getting richer and richer.

Accumulated hundreds of meters to thousands of meters. Because this geological period is very cold, the dry and cold northwest wind blows on the mainland, so the dust powder mostly runs from northwest to southeast. In the process of dust powder operation, large particles always fall first.

Small and medium-sized ones fall into the vast loess lake in turn, thus forming a pattern of coarse to fine soil layer at the bottom of the lake from west to east. Near the northwest, the mud particles are coarse and close to the southeast.

Soil particles become finer in turn. After thousands of years of wind blowing, the soil at the bottom of the lake is getting thicker and thicker, and the sediment particles are arranged more and more closely under the induced force and gravity of the water surface swell, which lays the foundation for the hardness of loess in the future.

During the rainstorm season, strong mountain torrents will rush to the lake area with rocks, pebbles and coarse sand, and spread these stones on the clay layer accumulated by the lake. After heavy rain, sandstorms and dust particles will fall into the lake for a long time.

Cover those stones with a thick Sha Meng, which is often repeated in the lakeside area, so that several layers of debris structures can be formed. This movement has been going on for tens of millions of years.

Slowly, the climate on the earth is getting warmer, the storms are getting less severe, and the silt blown into the lake is getting less and less. As the weather gets warmer, grass vegetation gradually grows on the lake, and small creatures such as shellfish gradually appear in the lake.

About 20 million years ago, there were a lot of creatures here, including tropical animals such as elephants and rhinoceroses. Animals often play and eat by the lake, and occasionally some dead animals are buried in the mud by the lake and become fossils.

As time goes on, the earth is changing. About 6.5438+0.5 million years ago, the landform here changed a lot. The earth plate pushed from the South Indian Ocean collided with the Eurasian plate, and the whole lake area was slowly pushed up.

The bottom of the lake has been raised, and the lake will descend, rushing to the diwa area in the east and flowing to the East China Sea. Huge floods accumulated soil on the North China Plain, which laid the most important foundation for the formation of the North China Plain in the future.

With the passage of time, the loess lake area has been lifted higher and higher, while the lake is getting shallower and shallower. Finally, in about 8 million years, the lake finally dried up, and the loess was lifted out of the ground and gradually became a plateau. It has become the landform of today's loess high slope.

With the drying up of lakes and the formation of plateaus, the climate gradually became cold, and tropical animals such as elephants and rhinoceroses gradually became extinct, forming today's geographical environment and ecological environment. This is the formation process of the Loess Plateau.

Extended data:

The Loess Plateau is located between100 24 ′ ~14 and 34 ~ 40 20 ′ north latitude, which is the second step of the national landscape pattern. It spans China temperate zone and warm temperate zone from north to south, and includes semi-humid zone, semi-arid zone and arid zone from southeast to northwest.

Its eastern boundary is the west side of Taihang Mountain, its western boundary is Wushaoling, Sun Moon Mountain and Laji Mountain, its southern boundary is Qinling-Xionger Mountain and the northern foot of Songshan Mountain, and its northern boundary is the Great Wall. It spans seven provinces (regions) including Qinghai, Gansu, Ningxia, Mongolia, Shaanxi, Shanxi and Henan. The total area is 640,000 square kilometers.

The Loess Plateau is located in the middle and upper reaches of the Yellow River and Haihe River. The Yellow River Basin has been the birthplace of human civilization since ancient times. However, due to the inherent fragile natural environment and long-term unreasonable land use, vegetation has been destroyed.

Soil erosion is extremely serious, which has caused profound changes in the ecological environment of the Loess Plateau for two or three thousand years. The problem of soil erosion in the Loess Plateau has become an important restricting factor for its economic development.

The Loess Plateau is a unique geographical area in China, which has the following remarkable characteristics:

1, the ditch is steep and the terrain is undulating and broken.

The loess on the Loess Plateau is deep, porous and rich in calcium carbonate. Affected by long-term internal and external forces, the surface erosion and cutting are serious, fragmented and ravines are vertical and horizontal.

The climate of the Loess Plateau is dry, although the precipitation is small, it is concentrated and mostly heavy rain, which makes the Loess Plateau develop into a landscape with many ditches, steep slopes and broken terrain.

2. The climate is completely different.

The geographical distribution of temperature and precipitation in the Loess Plateau decreases from southeast to northwest. As far as the temperature is concerned, the Loess Plateau has the continental climate characteristics of Leng Xia fever in winter, with drastic changes in cold and summer.

The Loess Plateau is deep inland, with a strong continental climate and a large daily temperature difference of 10℃ ~ 25℃. As far as precipitation is concerned, the annual precipitation in the Loess Plateau is basically between 400 and 600 mm. ..

Moreover, precipitation is concentrated in July, August and September, accounting for 60% of the annual precipitation, and often occurs in the form of heavy rain. In other seasons, precipitation is less and evaporation is stronger, and drought is more common.

3. Water resources are scarce, and the contradiction between supply and demand is prominent.

In the arid and semi-arid Loess Plateau, precipitation is scarce and evaporation is vigorous. The evaporation on the water surface is more than twice that of precipitation, and precipitation is concentrated. The characteristics of the Loess Plateau determine that soil erosion is very serious, and the contradiction between supply and demand of water resources is increasingly prominent.

4. Mountains and hills cover a large area.

The loess plateau is dominated by hills and mountains, accounting for about 80% of the total area, while the valleys and plains only account for 20% of the total land area. This undulating landform structure is not suitable for farming.

In addition, poverty and backwardness, rapid population growth, single agricultural management, poor harvest of extensive planting, extensive farming and so on. At the same time, the phenomenon of indiscriminate reclamation is very serious, forming a vicious circle of "more reclamation and poorer, more reclamation and poorer".

5. Low vegetation coverage.

On the loess plateau, about 30% ~ 40% of the places are barren hills and bare ridges, leaving a scene of devastation. There are few artificial grasslands, and what's more, in some places, due to over-exploitation of hillsides, the topsoil loss is serious, the bedrock is exposed, and the vegetation coverage is further reduced, which intensifies the soil erosion in the Loess Plateau.

Baidu Encyclopedia-Loess Plateau