Mudstone is a common sedimentary rock with a very fine-grained texture that is sometimes called by various other names such as mudrock, argillite, claystone and siltstone. Shale is also a type of mudstone rock but differs from most other kinds of mudstones in that it is made up of visible layers. The geological process leading to mudstone formation begins when sediment like clay, mud and silt is deposited in areas such as lakes, the bottom of the ocean, or tidal zones. This material is then buried under more sediment and becomes lithified, meaning the fluids contained in it are removed while the remaining material is compacted under pressure. This type of rock looks like dried clay, and it comes in a wide variety of colors, including black, orange, white, gray, brown, and green.
Sedimentary rock is common in the outermost layer of the Earth’s crust, and an estimated 65% of all sedimentary rock is mudstone. Mudstone is made up of very fine particles no larger than 0.0025 inches (0.0625 mm) in diameter that can only be seen in a microscope. The color of mudstones is determined by its mineral content. Rocks containing iron oxide are commonly red, orange, or yellow, while rocks rich in pyrite or carbon are black.
There are various types of mudstones with somewhat different characteristics when it comes to texture and hardness, depending on various differences in their mineral composition and formation. The term argillite can refer to any type of mudstone, but is sometimes used specifically to refer to deposits that are well-formed and harder than other types of mudstones. Marl is a term used for a specific type of this sedimentary rock that is soft and contains a lot of carbonate. Shale mudstone is characterized by its layered appearance, also referred to by the geological term fissile layering.
Mudstone is generally too soft for construction or similar purposes. However, certain types of argillite have been and continue to be used by some Native American peoples, primarily for carvings and jewelry-making. The general softness of the material, which makes it easy to shape and cut, has contributed to its popularity for such purposes. The Anasazi people in New Mexico commonly used both turquoise and orange argillite for jewelry and inlays. In Canada, the Haida people of British Columbia are famous for their carvings using a hard, very finely textured black argillite that can only be found in one quarry on Graham Island.