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.
Tongchuan District, which belongs to Dazhou City, Sichuan Province, is located in the northeast of Sichuan Province and in the middle of Dazhou City, adjacent to Xuanhan County in the northeast, Dachuan District in the southwest and Pingchang County in the northwest. As of October 2018, Tongchuan District has jurisdiction over 19 townships and 3 subdistrict offices, covering an area of 900square kilometers and a resident population of 782100. It is a national advanced area of scientific and technological progress, a national demonstration area of harmonious community construction, a national demonstration base of agricultural products processing industry, an advanced area of safe construction, an advanced area of hilly areas, and a demonstration area of rural tourism. Tongchuan District, which was established in 1999 as the seat of the state and government, is the political, economic and cultural center of Dazhou City. The four leading groups of the Municipal Party Committee, the Municipal Government, the Municipal people's Congress, the Municipal people's political Consultative Conference and the major administrative departments are all located in Tongchuan, which is known as "the heart of Dazhou".