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.
Mianning County, which belongs to Liangshan Yi Autonomous Prefecture of Sichuan Province, is located in the southwest of Sichuan Province, in the north of Liangshan Yi Autonomous Prefecture, adjacent to Xiaoxiangling in the east, Xichang and Yanyuan in the south, Muli and Jiulong in the west, and Bodhisattva Gang and Shimian County in the north. The total area is 4420 square kilometers. Mianning is located in the Panxi Rift Valley, and the Anning River and Nanya River originate in the territory. The Yalong River winds through the southwest and is known as the rice granary on the banks of the Anning River. It is a "major county of national grain production" and "a big county for pig transfer". More than 30 kinds of minerals have been proved to be of mining value, among which the reserves of rare earths rank second in the country. Mianning is an old revolutionary base. In 24 (1935), after the long March of the Red Army passed through Mianning, the Red Army was established to enter Sichuan.