(vii) Duripans (Bqm or Cqm) are subsurface soil horizons cemented by illuvial silica, usually opal or microcrystalline forms, to the degree that less than 50 % of the volume of air-dry fragments will slake in water or HCl. Often the duripans in Aridisols have a considerable content of calcium carbonate and can be distinguished only by the test described above.