Mudstone (also called mudrock)'s a fine grained sedimentary rock whose original constituents were clays or muds. Grain size's up to 0.0625 mm (0.0025 in) with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the appearance of fissility or parallel layering. This finely bedded material that splits readily into thin layers's called shale, as distinct from mudstone. The lack of fissility or laye… (More on Mudstone)