Limestone is a sedimentary rock composed largely of the minerals calcite and/or aragonite, which are different crystal forms of calcium carbonate (CaCO₃). Limestone was formed from the remains of organisms that lived in ancient seas. The Peak District (England) has a large limestone area. Limestone is partially soluble, especially in acid, and therefore forms many erosional landforms. These include limestone sidewalks, pot holes, cenotes, caves and gorges.