fine grained sedimentary rock whose original constituents were clays or muds

A mudstone is a siliciclastic sedimentary rock which has a mixture of silt- and clay-sized particles (at least 1/3 of each).[1] So, a mudstone is a type of mudrock with a more varied grain size than either siltstones or claystones.

These are some of the oldest rocks in the Dingle Peninsula, being Silurian in date. They consist of mudstones and siltstones with some volcanic material (County Kerry)

Alternatively, some sources say it is a synonym for mudrock.[2]

Claystone in Slovakia



A claystone is a mudrock which does not split easily.

In order for a rock to be a claystone, it must be up to half (50%) clay, whose particles measure less than 1/256th of a millimeter.



  1. Blatt H; Middleton G & Murray R. 1980. Origin of sedimentary rocks. 2nd ed, Prentice Hall, Englewood Cliffs, N.J. ISBN 0-13-642702-2
  2. Kearey P. The new Penguin dictionary of geology. London: Penguin Books. p176