Mudstone

A fine grained sedimentary rock formed from lithified clay and mud, made of silicate grains smaller than 0.063 millimeters, too small to see without magnification. It is a mudrock alongside shale, siltstone and claystone, and is distinguished from shale chiefly by its lack of fissility, the tendency to split along parallel layers. Under close analysis mudstone is found to consist mainly of clay minerals, quartz and feldspar along with minor accessory minerals, and the term is occasionally extended to carbonate rocks made of lime mud or dolomite mud. Its outward appearance often resembles hardened clay, sometimes developing cracks similar to sun-baked mud, and the absence of fissility may reflect either its original depositional texture or disruption of any original layering by burrowing organisms before the sediment hardened.