The term mudstone included claystones, shales, and siltstones. These are rocks that are categorized by their particular sizes — claystones, for instance, are comprised of individual particles that are less than 4 µ in diameter. The grains in Siltstones are larger, being between 4 and 6.25 µm across. Those examples which split into thin layers are classified as shales, and one which have both clay and silt size particles are generally simply referred to as mudstones. The specimen above is a Triassic Mudstone, and so was laid down as the result of mud deposition in a still water environment somewhere between 200 and 250 million years ago.