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.
Ansai District, which belongs to Yan'an City, Shaanxi Province, is located in the hinterland of the inland Loess Plateau, the edge of Ordos Basin, in the north of Shaanxi Province, due north of Yan'an City, west of Zhidan County, Jingbian County of Yulin City in the north, Zichang County in the east, and Ganquan County and Baota District in the south. Between longitude 108 °5 "44"-109 °26 "18" east and latitude 36 °30 °45 "- 37 °19" 3 "north. The distance between the north and south lines is 92 kilometers, and the east-west distance is 36 kilometers, with a total area of 2950 square kilometers, accounting for 8.04 percent of the total area of Yan'an. Ansai has been known as the "upper county throat" since ancient times. It is located in the cultural fusion zone in which the national culture of the Central Plains represented by Yangshao culture develops northward and the northern national culture represented by Yinshan rock painting culture develops to the south.