fairly thin layer of shale, or slate, where it has been metamorphized, known as the kupferschiefer for its high copper content. In its unmodified form, this layer is high in sulfur compounds that are typical of silt deposited in stagnant shallow marshland. Where faults have allowed mineral-rich groundwater to circulate through this layer, the sulfur has oxidized metal ions to metallic sulfide ores. From the Middle Ages into the modern era, this thin but widely dispersed constellation of ore bodies has been of immense importance as a source of copper across much of northern Europe. The Zechstein salt layer is also used for underground gas storage in England, Germany and France. See also List of stratigraphic units in the Netherlands Marl Slate Formation References Changhsingian Lithostratigraphy of Germany Wuchiapingian Permian System of Europe Sedimentology