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.
Hanyuan County, under the jurisdiction of Ya'an City, Sichuan Province, is located in the south of the western part of Sichuan Province, with geographical coordinates between 102 °16 miles east longitude and 29 °05 miles north latitude. It is bordered by Jinkou River District of Leshan City and Hongya County of Meishan City to the east, Ganluo County of Liangshan Yi Autonomous Prefecture in the south, Luding County of Ganzi Tibetan Autonomous Prefecture and Shimian County of Ya'an City to the west, and Yingjing County of Ya'an City to the north; the whole territory is 71.4 km long from east to west and 70.1 km wide from north to south. The county covers an area of 2388 square kilometers, is a subtropical monsoon humid climate, warm in winter and cool in summer, with four distinct seasons. As of 2016, Hanyuan County has jurisdiction over 30 townships (5 ethnic minority townships), and the county seat is located in Fulin Town. As of 2017, Hanyuan County has a total population of 319300. Hanyuan County, the ancient name of Yidu, belongs to