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 with the naked eye. If it become finely bedded so that it splits readily into thin layers it is called shale. It tends to be found in older geological formations, being consolidated from muds and clays over time.