The upper portions and cap of many of the hills in Swaledale are made up of the Millstone Grit Group. The name derives from its use in earlier times as a source of millstones used principally in watermills. The group is dominated by very thick-bedded, coarse-grained, pebbly sandstones (formerly referred to as grit). Thinner beds of finer-grained sandstones, mudstones and thin coals can also be found. The transition from the underlying Yoredale Group marks a notable increase in sandstones, coarser grain size and the occurrence of chert. Chert is a fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz. Chert is typically composed of the petrified remains of siliceous ooze, the biogenic sediment that covers large areas of the deep ocean floor, and which contains the silicon skeletal remains of diatoms, silicoflagellates, and radiolarians. Chert mining occurred along Fremington Edge, above Reeth. The blocks of chert were used “…for the grinding of calcined flint, used as a whitening agent in earthenware manufacture. In 1772 the potter Josiah Wedgwood recommended Derbyshire chert as a major improvement over granite millstones, which left annoying black specks in the pure white flint.” Julie Bunting (1995)
Sample of Millstone Grit illustrating the coarse-grained to pebbly sandstone predominantly made of quartz with some feldspar.
