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.
Qingcheng District, Qingyuan Municipal District. Located in the north of central Guangdong Province, the middle and lower reaches of Beijiang River and the southernmost end of Qingyuan City, it is the political, economic and cultural center of Qingyuan City. Qingcheng District is adjacent to Qingxin District in the north, Huadu District in Guangzhou to the south, Fogang County and Conghua City to the east, Sanshui District of Foshan City to the west, and Sanshui District of Foshan City to the west. it has a total area of 927 square kilometers and has jurisdiction over 3 towns, 4 streets and 1 forest farm, with a total population of 620000 in 2008. The Beijing-Guangzhou Railway passes through Yinzhan and Yuantan stations in the region, with a 30-kilometer operation line; 107 National Highway runs through the region in the north, Qingfo Highway in the east and Qingsan Highway in the west. Beijiang River flows from east to west to 100-ton ships, with special deep-water wharf and regular flights to Hong Kong and Macao and Huangpu Port. Qingcheng District has more than 100,000 overseas Chinese and compatriots from Hong Kong and Macao.