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.
Congyang County, which belongs to Tongling City, Anhui Province, is located in the south-central part of Anhui Province, the north bank of the lower reaches of the Yangtze River, the southeast foot of the Dabie Mountains, between 30 °38 °N and 31 °05'N and 117 °35'E, with a total area of 1473.43 square kilometers. Congyang County is high in the north and low in the south, low and flat in the middle, surrounded by the river and the lake. Congyang County has a warm and moist monsoon climate in the northern subtropics, with four distinct seasons, sufficient sunshine and abundant rainfall. By the end of 2018, Congyang County has jurisdiction over 15 towns and 4 townships, with another development zone. The county people's government is stationed in Fushan Road, Congyang Town, with a registered population of 970000. Congyang has a history of more than 2100 years and is one of the birthplaces of Tongcheng School. It was the patriarchal kingdom in the Western Zhou Dynasty, and the county was bought in the fifth year of Yuan Feng in the Western Han Dynasty (106 BC).