According to the reporter's observation last summer (19 19) and the reference of eastern and western books, it can be concluded that the West Lake is a lagoon. The topography of the West Lake is surrounded by mountains in the south, west and north, but there is an alluvial plain in the east, and the cities of Zhejiang Province are on this alluvial plain. All the soil is deposited by Qiantang River. When the sediment carried by the river reaches the estuary, some of it will sink. First, the speed of the river will be reduced because of the resistance of the sea. Secondly, because seawater contains salt, salt will reduce the cohesion of river molecules. For the above two reasons, all the great rivers in the Yangtze River, such as the Nile in Egypt, the Ganges in India, the Yellow River and the Yangtze River in China, have become deltas when they enter the sea. In this way, the alluvial plain near Hangzhou is only a delta formed by Qiantang River. If we go to the next floor to investigate the geology near the West Lake, we will know that there are not only alluvial soil in the east of the West Lake, but also alluvial soil in the west. If we can recall the situation when the Qiantang River was just formed, before all the alluvial soil sank, Hangzhou was still a Wang Yang, and the West Lake was just a small bay near the mouth of the Qiantang River. Later, the Qiantang River sediment slowly blocked the mouth of the bay and became a lagoon. When it was first formed, the area of Lihu Lake was larger than the outer lake now. Later, due to the intersection of the north and south peaks of Yuquan, Liangfengjian and Longjing streams, the soil brought by the streams flowed into the lake, and the speed suddenly decreased and deposited. Lihu Lake silted up quickly because it was on the side of the mountain. Such as Gengjiabao, Jinsha Port, Maojiabao, etc., were all formed by alluvial soil under the stream. If it hadn't been for dredging and dredging in the Song, Yuan, Ming and Qing Dynasties, not only Lihu Lake would have been destroyed, but the outer lake would have been silted up. In other words, if there is no artificial excavation, the West Lake will be eliminated naturally. Now we can still walk by the lake and enjoy the beautiful scenery, which is also a proof that human beings can conquer nature. In summer, the water level in the outer lake is only four feet deep. Because the inner lake is close to the mountain side and receives more sediment than the outer lake, the water is shallower than the outer lake. However, under Su Causeway Sixth Bridge, Daiyu Bridge and Xiling Bridge, the water level is slightly deeper, and the deepest place is about six feet. This is because the water level of the lake is like a mirror, the flow rate is extremely slow, and the smallest mud particles contained in the water also sink. The bridge on the single dike is the traffic throat of the lake and the only way to flow from Lihu Lake into the outer lake. The lake is rapidly popular, and the fine soil in the water cannot be precipitated. Try to see all the places in the West Lake. The ointment is piled deep, alone on Xiqiao Bridge. Looking down, the water is very clear, and rocks can be seen, just because the water is so fast that it is impossible to leave enough fragrant plaster. To sum up, the current situation of the West Lake is due to soil deposition under the Qiantang River and the blockage of the original bay mouth. As for the generation age of the West Lake, how far is it from now? This problem is not easy to solve. From a historical point of view, the West Lake has a long history. Before the Tang Dynasty, although unknown, since Li Houzhu, Bai Juyi and Su Dongpo successively worked in Wulin, the name of West Lake has become famous all over the country. But from the geological point of view, the formation of the West Lake is a recent event, in the latest era of geology. This era is an alluvial era. The human beings in the world came into being at the end of the diluvial period and the beginning of the alluvial period, so the West Lake came into being, of course, after the world had human beings. The rocks in western, southern and northern Hunan are very old. The northwest, such as Geling and Baoshishan, is composed of trachyte. Trachyte is a kind of igneous rock, which is widely distributed in the south coast of China. According to Richthofen, a famous German geologist, there are many porphyries from Ningbo to the South China Sea (trachyte and quartz porphyry are the same kind of rocks), ranking first in the world. This kind of quartz porphyry and trachyte was formed by volcanic eruption. According to American geologist Willis, the eruption period was probably between Triassic and Jurassic. The south of the West Lake is the same as the west, such as Jiuyao Mountain, Fifteen Ridges, South Peak and Lingyin. This series consists of sandstone and limestone, among which limestone is the most widely distributed. The most thrilling purpose of the limestone mountain peak is Feilai Peak in front of Yunlin Temple, which is green and graceful, with many Buddha statues carved on it, just like a natural rockery. There are many caves near the West Lake, such as Yuru Cave and Shiwu Cave, which are also made of limestone. The limestone and sandstone strata are even older than the trachyte in the north, about the Carboniferous and Permian in Paleozoic (almost 300 million years ago). Although the rocks in western Hunan, western Hunan and northern Hunan are very old, the soil on the east bank of the West Lake is very new, which was formed during the alluvial period. We need to know the age of the West Lake. As long as we know the sediment discharged from Qiantang River, it is time to deposit Hangzhou on the mainland. Since the formation of the West Lake, the Qiantang River Delta has gradually spread in the sea, and now it has reached the mouth of Hangzhou Bay, about 120 miles from the provincial capital Hangzhou. If we speculate on the annual expansion speed of Qiantang River Delta in the sea, it is not difficult to know the formation age of West Lake. The length of Qiantang River and the size of its surface are comparable to those of Longhe River and Bohe River in Europe, as can be seen from the following: Longhe River is 5 10 Li long; Po 4 18 mile; Qiantang River is 400 Li. The Longhe River basin covers an area of 94,800 square miles; Qiantang River 24000. The height of the source of the Longhe River is 1200- 1500 feet; Po River 12608 ft; Qiantang river is 5900 feet. Rivers on the same slope of the Longhe River enter the Mediterranean Sea, and the seabed depth of the estuary is the same as that of the Qiantang River estuary. The headwaters of the Dupo River and the Longhe River are in the Alps, which is more than twice as high as the Qiantang River starting from Huangshan Mountain in Anhui Province. Therefore, the growth rate of Qiantang River Delta should not be as fast as that of Longhe River Delta and Tongpo River Delta. The Longhe Delta grew by 15 miles during 1500 years, with an average growth of one mile every 100 years. The Po River Delta grew by 20 miles during 1800, and almost one mile every 100. If the Qiantang River Delta grows as fast as the Pohe River and the Longhe River and grows by one mile every hundred years, then 120 Li of sediment will be accumulated in 2000. According to these calculations, the West Lake was formed at least 12,000 years ago.