Noun
mudstone (plural mudstones)
(petrology) A fine-grained sedimentary rock whose original constituents were clays or muds. [from 18th c.]
The mudstone was located in between the Lokochot Tuff and the Tulu Bor Tuff. Source: Internet
The surrounding strata, if shale or mudstone, often contain slightly more than average and this may also be reflected in the ash content of 'dirty' coals. Source: Internet