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.
Gengma Dai WA Autonomous County, which belongs to Lincang City, Yunnan Province, spans 98 °48 "99 °54" east longitude and 23 °20 "24 °02 'north latitude. The town of Gengma is located in the middle of the county, with a maximum horizontal distance of 112 kilometers northeast of Kunming, the capital of Yunnan, and a maximum vertical distance of 76 kilometers from north to south. By 2013, the population density will be 70 people per square kilometer. Located in the southwest border of China, connected with the mountains and rivers of Myanmar, the border is 47.35km long. It is the most convenient land passage for Lincang and Kunming to Yangon and the coast of Myanmar in the Indian Ocean. It is 15 km and 24 km from the border Qingshui River to Huban and Gunnong, an important town in Myanmar, to La Xu 161 km in northern Myanmar, and 1162 km to Yangon, Myanmar. Gengma Dai WA Autonomous County established