calmstone

1. (colloquial) A sandstone, shale, or mudstone that forms in layers that are easily split into flagstones.Examples: "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."; "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."; "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."colloquialcountableuncountable