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.
Helan County (Pinyin: Helan Xian), under the jurisdiction of Yinchuan City, Ningxia Hui Autonomous region, is located in the northern part of Ningxia Hui Autonomous region, bought Lian County in the Western Han Dynasty, and changed its name to Helan County in 1941. As of 2013, Helan County covers an area of 1599 square kilometers. As of 2018, it has jurisdiction over 4 towns, 1 township, 3 fields and 1 management committee. In 2016, the total population of Helan County (resident population) was 255995. Helan County is located in the northwest inland of China's Loess Plateau, in the middle of the Qingtong Gorge irrigation area in Yinchuan Plain, mountainous area in the west and plain in the east. The basic characteristics of the topography trend are high in the southwest and low in the northeast, with a distorted surface tilting from southwest to northeast. The whole area is mainly composed of western mountain and eastern plain.