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.
Nanxian County, which belongs to Yiyang City, Hunan Province, is located in the border of Hunan and Hubei provinces and the hinterland of Dongting Lake area. Nanxian County is connected with Shishou, Public Security and Songzi in Hubei Province in the north, Anxiang and Hanshou counties in Changde City in the west, Huarong County in Yueyang City in the east, Yuanjiang City in Yiyang City in the south, and Datong Lake, Beizhouzi, Jinpan, Nanwan Lake and Qianshanhong in the southeast, which is one of the 36 border counties in Hunan Province. In 1988, Nanxian County was approved as a "revolutionary base county" by the State Council. The land area is 1321 square kilometers, with a total population of 794100 (2009). South County under the jurisdiction of 13 towns and 2 townships, there are Luowen Huahai graffiti Village, Dechang Park, Maocaojie Bridge, Nanzhou Square, Nanzhou National Wetland Park and other scenic spots. In 2016, the GDP of the whole county was realized.