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.
Yanchang County, which belongs to Yan'an City, Shaanxi Province, is located in the north of Shaanxi Province and east of Yan'an City. It is 73 kilometers from east to west and 55 kilometers from north to south, with a total area of 2368.7 square kilometers. The early human activities in Yanchang County can be traced back to the Neolithic Age. Western Jin Tai began two years (266 years), established the establishment of the county. Several changes, to the second year of Tang Guangde (764), because the Yanhe River from the county from west to east into the Yellow River, the name Yanchang County, still in use. The terrain tilts from northwest to southeast, high in the north and south, low in the middle, showing a valley peak type. Is a warm temperate dry continental monsoon climate, spring, summer, autumn and winter four distinct seasons, cold, warm, dry and wet. By 2018, Yanchang County had jurisdiction over one street and seven towns with a resident population of 128500, achieving a GDP of 54.077.