Natural moat refers to a large ditch formed naturally to cut off traffic. Natural trenches can cut off traffic if it is dangerous. More refers to the Yangtze River. "Sui Shu Five Elements": "The Yangtze River is a natural barrier, which was thought to be separated by the north and the south in ancient times. How can the northern army fly today? " Li Bai's poem Jinling in Tang Dynasty: "The sky of Jinling is spectacular, and the natural moat is full of waves." Deng Jin Qianjiang's poem "Watching the Tide of the Bohai Sea and Keeping Lanzhou" says: "Yunlei Mountain is a dangerous place with golden soup, and famous officials have been high in the fence since ancient times." Mao Zedong's "Water Turning Tour" said: "A bridge flies north and south, and the natural barrier becomes a thoroughfare." ; Referring to other rivers, Song Liu Yong's poem "Looking at the Tide of the Sea" said: "Clouds and trees surround the embankment, and the waves roll with frost and snow, and the natural moat is endless." This refers to Qiantang River. Li Jing-fen's Poems on the Capital of Liang (III): "The natural moat waves set the sun, and the Taihang mountains shine on the Central Plains." This refers to the Yellow River.
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