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.
Pu'an County, which belongs to the Buyi and Miao Autonomous Prefecture in southwestern Guizhou Province, is located in the southwest of Guizhou Province, adjacent to Qinglong County to the east, Xingren County and Xingyi City to the south, Panzhou City in Liupanshui City to the west, and Liuzhi Special Zone in Liupanshui City to the north. In 2015, the county has a total area of 1454 square kilometers, with jurisdiction over 2 streets, 8 towns and 2 townships, and Panshui Street where the county seat is located; it has an area of 254000 mu of arable land and a total population of 331900, inhabited by more than ten ethnic groups such as Han, Miao and Buyi. In 2014, Pu'an County completed a GDP of 5.015 billion yuan. There are 46 rivers in Pu'an County, the theoretical reserve of water energy is 270000 kilowatts, and there are 28 kinds of mineral deposits such as coal, gold, iron, silicon, lead and zinc. Puan County has the oldest in the world.