mudstone

plural mudstones

1. countableuncountablegeographyA fine-grained sedimentary rock whose original constituents were clays or muds.First known use 18th c.

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

   2018 March 23, Susannah Lydon, The Guardian

   1 more example

   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.

   2022 December 14, Paul Stephen, “HS2's Dorothy starts to dig second tunnel bore”, in RAIL, number 972, page 22