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.
Yimen County is located in the central and western part of Yunnan Province, northwest of Yuxi City, connected with Anning City and Jinning District in the east, Eshan in the south, across the Luzhe River in the west and Shuangbai, and bordered by Lufeng County and Anning City in the north. It belongs to Yuxi City. The southeast is 146 kilometers away from Hongta District of Yuxi City, and 94 kilometers northeast from the provincial capital Kunming. The high-grade highway with a mileage of 43 kilometers can reach Kunming and Chuxiong from the county seat. The county has a distance of 44 kilometers from east to west and 57 kilometers from north to south, with a total area of 1571 square kilometers, ranking fourth in Yuxi City. The total population is 177110 (2010). The government is stationed in Longquan Town. Yimen Longquan National Forest Park, a national forest park approved by the State Forestry Administration in 1992.