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.
Pingliang, prefecture-level city of Gansu province. Located in the east of Gansu Province, the eastern foot of Liupan Mountain and the upper reaches of Jinghe River, it is the geometric center of Shaanxi-Gansu-Ningxia "Golden Triangle", across Longshan (Guanshan), east of Xianyang, Shaanxi, west of Dingxi and Baiyin of Gansu, south of Baoji of Shaanxi and Tianshui of Gansu, and adjacent to Guyuan of Ningxia and Qingyang of Gansu to the north. Pingliang is an important animal husbandry base and fur distribution center in northwest China, and the main agricultural and forestry product production base and animal husbandry in Gansu Province. Pingliang, known as "Longshang dry Wharf", is an important city that must pass through the ancient "Silk Road". It is historically known as "the first city of Chang'an in the west". Since ancient times, Pingliang has been an important town for the barrier of the three Qin dynasties and the control of Wuyuan. It is a "must-contend place for soldiers" and a traditional commodity distribution center in Longdong. The traffic and military focal point of the Central Plains leading to the western region and the eastern end of the ancient Silk Road, not only in the northwest region.