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".