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.
Ziyang District, which belongs to Yiyang City, Hunan Province, is located in the north of the central city of Yiyang City, facing Changsha-Zhuzhou-Xiangtan Urban agglomeration to the east and Changde Hanshou County to the west, with a population of 423000 and an area of 680sq km. It has jurisdiction over five towns, one township, two subdistrict offices and one provincial industrial park (Changchun Economic Development District). Ziyang is an important transportation hub and material distribution center in central and northern Hunan, and it is the "key place to undertake the gradient transfer of processing trade" approved by the Ministry of Commerce. The region has formed four major industrial clusters dominated by equipment manufacturing, electronic information, light industrial textiles and agricultural products (food) processing, communications, electric power and other infrastructure have developed rapidly, and land and water transportation has formed a dense network. Ziyang District is a national ecological construction demonstration area, the national Dongting Lake Wetland Ecological Reserve, and it is also one of the two provincial counties with excellent environmental quality in Hunan Province. Guan Yunchang