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.
Yuanjiang City, which belongs to Yiyang City, Hunan Province, is located in the northeast of Hunan Province, on the shore of Dongting Lake. It was founded in 522 AD (then called Yaoshan County) and changed into a city in October 1988. There are continuous hills in the southwest of the city, alluvial plains in the north and Ludang swamps in the southeast, showing the geomorphological features of the lake and township with "three embankments, three continents, and one water surface." The city has a pleasant climate, rich specialties and convenient transportation, and enjoys the reputation of "Dongting Pearl" and "Jiangnan Treasure Land". In 2012, Yuanjiang City had jurisdiction over 2 streets, 11 towns, 1 township and 2 fields, with a total area of 2177 square kilometers and a total population of 754900, including 328800 urban people. In 2015, according to the township division adjustment plan of Yuanjiang City, 2 streets and 11 towns were under the jurisdiction of the adjustment. two hundred and one