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.
Changning County, which belongs to Yibin City, Sichuan Province, is located in the southern edge of Sichuan Basin, in the heart of Yibin City, between longitude 10444 °22 "~ 105 °03 °30" east longitude 28 °15 °18 "~ 28 °470048", about 60 kilometers from north to south, 30 kilometers from east to west, an area of 1000.2 square kilometers, forest coverage of 62.8%, and the content of negative oxygen ions in the air can reach up to 47000 per cubic centimeter (Qidonggou). It is a famous hometown of longevity. As of August 24, 2019, Changning County has jurisdiction over 13 towns with a total population of 460400 (2018). There are 9 ethnic groups, including Han, Miao and Yi, among which 28 are over 100 years old, the oldest is 112 years old, and the number of people over 90 years old has reached 1075.