Mudstone is a fine-grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm (0.0025 in) with individual grains too small to be distinguished without a microscope. With increased pressure and time the platey clay minerals may become aligned and a fissility or parallel layering appears. This finely bedded material that splits readily into thin layers it is called shale. The lack of fisility or layering in mustone may be due to original texture or the disruption of layering by burrowing organisms in the sediment prior to lithification. Mudrocks, mudstone and shale, comprise some 65% of all sedimentary rocks. Looks like clay.

References

• Blatt, Harvey, and Robert J. Tracy, 1996, Petrology, W. H. Freeman, 2nd ed. ISBN 0716724383

Sedimentary rocks