The area around Castleton in the English county of Derbyshire has extensive outcrops of Carboniferous Limestone, showing complex sedimentary facies changes as well as interleaved extrusive basaltic volcanic rocks. Overlying these are the lower parts of the Millstone Grit, ranging from marine shales through a variety of deltaic sandstones. The limestones are hosts to hydrothermal mineral deposits, with veins containing galena, fluorspar, baryte and calcite. A unique variety of fluorspar is Blue John, mined for ornamental purposes since the mid-eighteenth century. Landscape features include a massive landslip, dry valleys, loess, dolines, scree slopes and various karstic landforms. A complex hydrological system encompasses numerous caves, of which four are open to visitors.