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.
Yongshan County, which belongs to Zhaotong City, is located in the northeast of Yunnan Province. Xiluodu Hydropower Station is the second largest hydropower station in China and the third largest hydropower station in the world. In 1727, the governor of Yungui Ertai suppressed Pingmitie, and the county was named "Yongshan" by the imperial court, which meant to obey discipline forever. In 2014, Yongshan County has jurisdiction over 6 towns and 9 townships, with 4 neighborhood committees and 133 administrative villages. The county government is stationed in Xiluodu Town, 680 kilometers southwest from the provincial capital Kunming and 200 kilometers south from Zhaotong Municipal Government. Located at the northwest edge of Zhongshan Mountain in northeastern Yunnan, the plateau is broken because it is cut by the Jinsha River and its tributaries. The highest elevation is 3199.5 meters, and the lowest elevation is 340 meters. The main rivers are Jinsha River, Sabina River, Damotan River, Jingdi River, Longchong River and so on. It belongs to the humid climate of the subtropics.