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.
Yima City, under the jurisdiction of Sanmenxia county-level city, is located in the west of Henan Province, located in the ancient road, north Yang Shaofeng, south Luoyi. It is 183 kilometers away from Zhengzhou, the provincial capital, between Luoyang, the ancient capital, and Sanmenxia, a new industrial city. With a total area of 112 square kilometers and a total population of 170900 (2014), there are 14 ethnic groups, including Han, Manchu, Mongolia, Uygur, Buyi, Zhuang and Yi. Its non-agricultural population is 161900, and the agricultural population is 9000. Yima City was established with the approval of the State Council in April 1981, under the jurisdiction of Luoyang area. Luoyang area was abolished in 1986 and placed under the jurisdiction of Sanmenxia City. Now it has jurisdiction over five offices: Qianqiu, Changcun, Qianqiu Road, Taishan Road, Chaoyang Road, Changcun Road and Xinyi Street. Yima City is located in the connection between the eastern developed areas and the western resource areas.