and, depending upon circumstances under which it was formed, it may show cracks or fissures, like a sun-baked clay deposit. They can be separated into 5 categories: Siltstone - greater than half of the composition is silt-sized particles. Claystone - greater than half of the composition is clay-sized particles. Mudstone - hardened mud; a mix of silt and clay sized particles. Shale - exhibits lamination or fissility. Argillite - has undergone low grade metamorphism.… (More on Mudstone)