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.
Xigu, Lanzhou area, located in the west gate of Lanzhou City, is the core industrial zone of Gansu Province and Lanzhou City, and the largest petrochemical base in western China. The region has a total area of 385 square kilometers and has jurisdiction over 40 administrative villages, 8 streets and 70 communities in 5 towns and 1 township. In 2015, the region's GDP reached 29.781 billion yuan. Xigu District has become one of the areas with the densest factors of production in the western region. It has won the titles of "National Spark Program Advanced County", "National Science and Technology Progress County", "National Science Popularization demonstration District", "National Cultural Advanced County", "National Family Planning Advanced County and District", Gansu Province double support Model City "six consecutive championships" and so on.