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Greywacke, a hard, grey, poorly sorted sandstone of Silurian age.
Greywacke or Turbidite ( German Graywacke ) is a type of dark, hard, grey or brown sandstone . It is typically poorly sorted with angular grains of quartz, feldspar & lithic fragments set in a very fine matrix. It is thought to have been deposited from heavily sediment laidened currents ( turbidity currents ) moving very rapidly downslope through water, These are fairly typical of the margins of continental shelves. Greywacke is largely unfossiliferous. These examples are Silurian in age & sourced from around Moffat in Scotland. Specimens are supplied in labelled card trays.