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Sedimentary Rocks

Sedimentary rocks are the most abundant rock type exposed at the surface of the earth and cover about 99 percent of Kentucky. They were formed by the accumulation of rock fragments, shell fragments, plant material, muds, and sands, which were transported to their present locations by water, air, wind, or ice to form clays, sandstones, shales, coals, and conglomerates. Clays are ultra fine-grained deposits generally composed of complex silicates. Coal is a very common sedimentary rock formed from plant material, and different coals are formed from variations in temperature and pressure. Limestone was formed in marine depositional environments and as a chemical precipitate from seawater. Dolostone is a limestone that has been chemically altered. Evaporites such as anhydrite, gypsum, and salt were precipitated by the evaporation of water in arid environments.

Other sedimentary rocks accumulated because of physical processes where rivers, deltas, and beaches produced sand deposits and conglomerates. Many of these types of sedimentary rocks accumulate to thousands of feet of thickness in large depositional basins known as sedimentary basins. Under the pressure of their increasing weight, the sediments were compacted, hardened, and cemented together to form solid rock. By compaction and cementation, gravels became conglomerates, muds became shales, sands became sandstones, and lime sediments became limestones. Most sedimentary rocks preserved in Kentucky were originally deposited underwater and are therefore nearly flat lying. However, subsequent movements of the earth's crust have deformed or tilted them in some places, causing them to dip steeply.