calmstone

calmstone (countable and uncountable, plural calmstones)

1. (colloquial) A sandstone, shale, or mudstone that forms in layers that are easily split into flagstones.
   • 1841, Transactions of the Highland and Agricultural Society of Scotland, page 380:

     The mineralogical characters of both sandstone and calmstone are identically the same, as well as the fossil organic remains, which occur so abundantly in the latter.

   • 1885, Sir Charles Lyell, The Student's Elements of Geology, page 403:

     The grey flags and thin grey and olive shales and ' calmstones ' are almost confined to Forfarshire, and in the north-east part of the basin are known as Arbroath flags.

   • 1901, Geological Survey of Great Britain, Summary of Progress of the Geological Survey of Great Britain and The Museum of Practical Geology:

     If this view be correct there must have been a great difference in the sedimentation of the two areas, as the thick beds consist of alternations of flaggy sandstone with occasional true sandstone, almost pure limestones, calmstones, and few or no real flagstones.

   • 1937, Thomas Robertson, David Haldane, The Economic Geology of the Central Coalfield:

     The dolerite in the railway cutting, a short distance east of this quarry, shows a white trap top under calmstone.