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.
Yangshan County, which belongs to Qingyuan City, Guangdong Province, is located in the northwest of Guangdong Province, the middle part of Qingyuan, the southern foot of Nanling Mountains and the middle reaches of Lianjiang River. it is located between longitude 112 °22 °01mm east and latitude 23 °58470 °24 °5552' north. It is one of the 21 key counties of poverty alleviation and development in Guangdong Province. There was Yangyu State in the warring States period, Yangshanguan was established at the end of the Qin Dynasty, and Yangshan County was located in Gaozu of the Western Han Dynasty, which has a history of more than 2200 years. The county has a total area of 3418 square kilometers and has jurisdiction over 12 towns and one Yao township. As of 2018, Yangshan County has a total population of 569000. Yangshan County, 160 kilometers away from Guangzhou, is an important part of the "two-hour life circle" in the Pearl River Delta. The waterway is convenient. Lianjiang flows through the county from north to south, all the year round.