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.
Yiyuan County, which belongs to Zibo City, Shandong Province, is located in the middle of Shandong Province, Linqu County of Weifang City to the east, Yishui County to the southeast, Laiwu District and Gangcheng District of Jinan City to the west, Xintai City of Taian City to the southwest, Mengyin County of Linyi City to the south, and Boshan District and Zichuan District to the north. In 1944, it established a county with a total area of 1636 square kilometers, with 10 towns, 2 streets, 1 provincial economic development zone and 643 villages (communities) under its jurisdiction by 2017. As of December 31, 2017, the total population of Yiyuan County is 569900. Yiyuan is the roof of Shandong Province, China, with an average elevation of 401 meters, which is the highest in Shandong Province. There are 1983 famous hills in the territory, and the highest elevation of Mount Lushan is 1108 meters, which is the fourth peak in Shandong Province. There are 1530 large and small rivers.