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.
Yongchang County, which belongs to Jinchang City, Gansu Province, is located in the northwest of Gansu Province, in the east of Hexi Corridor, the northern foot of Qilian Mountain and the southern edge of Alashan platform. It is bordered by Wuwei to the east, Jinchuan to the north, Shandan to the west and Sunan to the south. The geographical coordinates are 101 °04 east longitude 102 °43 mi, 37 °47 mi 38 °39'N. The longest distance from east to west is 144.8 km, and the widest distance from north to south is 144.55 km, with a total area of 7439.27 square kilometers. As of 2014, Yongchang County has jurisdiction over 6 towns and 4 townships, with a total of 111administrative villages, 917 village groups and 10 communities, with a total population of 235300 (by the end of 2017), mainly Han nationality, and ethnic minorities such as Mongolian, Hui and Pan. In 2017, Yongchang County realized the regional gross domestic product.