Mudstone is a type of sedimentary rock made up of fine particles deposited in areas like lakes or the ocean floor. Shale is a type of mudstone with visible layers. Mudstone is commonly red, orange, yellow, black, brown, white, or green. It is too soft for construction but has been used by Native Americans for carving and jewelry making.
Mudstone is a common sedimentary rock with a very fine grained texture that is sometimes called by various other names such as mudrock, shale, clay and siltstone. Shale is also a type of mud rock, but it differs from most other types of mud stone in that it is made up of visible layers. The geological process leading to the formation of mudstone begins when sediments such as clay, mud and silt are deposited in areas such as lakes, the ocean floor or tidal areas. This material is then buried under more sediment and lithifies, meaning the fluids it contains are removed while the remaining material is compacted under pressure. This type of rock looks like dry clay and 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 it is estimated that 65% of all sedimentary rock is mud. Mudstone consists of very fine particles no larger than 0.0025 inches (0.0625 mm) in diameter that can only be seen under 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 slightly different characteristics in terms of texture and hardness, depending on the various differences in mineral composition and formation. The term shale 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 mudstone. Marl is a term used for a specific type of this sedimentary rock that is soft and contains a lot of carbonate. Shale mud 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 shale have been and continue to be used by some Native American peoples, primarily for carving and jewelry making. The general softness of the material, making it easy to shape and cut, has contributed to its popularity for such purposes. The Anasazi of New Mexico commonly used both turquoise and orange argillite for jewelry and inlay work. In Canada, the Haida people of British Columbia are famous for their carvings using a very finely textured, hard black shale that can only be found in a quarry on Graham Island.
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