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.
Tacheng: the county-level city under the jurisdiction of Tacheng area of Yili Prefecture, Xinjiang, is located in the northwest of Xinjiang Uygur Autonomous region and the Tacheng basin on the northwest edge of Junggar Basin, with a total area of 4356.6 square kilometers. It is bordered by the Republic of Kazakhstan in the northwest, Emin County in the east and Yumin County in the south. It is 530 kilometers to the east from Urumqi, the capital of the autonomous region, and only 12 kilometers to the west from the first-class national port, Baktu Port. It is one of the closest open cities in China to the border. Tower is referred to as "Tarbatai" (Mongolian marmot). Tacheng is the seat of the Tacheng Prefectural Committee of the Communist Party of China and the administrative office of Tacheng area. In 1984, the county was removed and the city was established. In 1992, it was listed as a further open city along the border by the State Council. In June 1992, with the approval of the State Council, the national border was established.