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.
Subei Mongolian Autonomous County, which is under the jurisdiction of Jiuquan City, is located in the northwest of Gansu Province, on the north and south sides of the western end of the Hexi Corridor. The county is divided into two disconnected areas, Nanshan and Beishan, with a total area of 66748 square kilometers. it is bordered by three counties and cities of one country and three provinces (regions). It has jurisdiction over 2 towns and 2 townships, with a total population of 11741 (2012), of which 4446 are Mongolian, accounting for 37.9%. Subei County had nomadic activities in the pre-Qin period; in the Western Han Dynasty, it was incorporated into the territory of the Central Plains Dynasty, belonging to Dunhuang County; after the continuous war, the ownership was changed repeatedly, the local regime and nomadic sphere of influence were crisscross, and the control of the central regime was weak. It belongs to Gansu Province since the Qing Dynasty, and Subei Bureau was set up in the Republic of China in twenty-five years (1937). Liberation in July 1950, 19