A mesa is an isolated, flat-topped elevation, ridge or hill, which is bounded from all sides by steep escarpments and stands distinctly above a surrounding plain. Mesas characteristically consist of flat-lying soft sedimentary rocks capped by a more resistant layer or layers of harder rock, e.g. shales overlain by sandstones. The resistant layer acts as a caprock that forms the flat summit of a mesa. The caprock can consist of either sedimentary rocks such as sandstone and limestone; dissected lava flows; or a deeply eroded duricrust. Unlike plateau, whose usage does not imply horizontal layers of bedrock, e.g. Tibetan Plateau, the term mesa applies exclusively to the landforms built of flat-lying strata. Instead, flat-topped plateaus are specifically known as tablelands.
Wanyuan City, which belongs to Dazhou City, Sichuan Province, gets its name because it is located in Wanqingchi and the source of Zhujiang River. it is located in the northeast of Sichuan Province, the hinterland of Dabashan Mountain, the junction of Sichuan, Shaanxi and Chongqing, and the confluence of seven counties and cities. it is the dividing line of climate between the north and south of China and the watershed of Jialing River and Han River. By the end of 2018, Wanyuan City had jurisdiction over 52 townships, covering an area of 4065 square kilometers, with a total population of 573600. Wanyuan City was established in July 1993 and has obvious geographical advantages. it is the main passage of land traffic out of Sichuan Province. It is known as the "lock key of Qinchuan". It is crossed by trunk lines such as Xiang-Yu Railway, Baomao Expressway, National Highway 210 and 347. Wanyuan City is known as "the Source of Wanbao". It is one of the three selenium-rich belts in China and the only natural selenium-rich area in Sichuan Province.