mudstone

noun

1. (countable, uncountable)A fine-grained sedimentary rock whose original constituents were clays or muds.

   “The new fossils come from mudstones in central Poland, in beds that have been dated using other, much more common and cosmopolitan, fossils.”

   “Excavation of the twin bore tunnel will produce around 500,000 tonnes of mudstone and soil, which will be processed on site and then reused to build embankments elsewhere along the route.”