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 accumulated to thousands of feet 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.
The term "clay" is applied to various earthy materials composed dominantly of hydrous aluminum magnesium silicate minerals. The most familiar characteristic of clay is plasticity or the ability of moist clay to be fashioned into a desired shape. The physical properties of a clay are plasticity, strength, and refractoriness. Plasticity enables the clay to be molded; strength permits it to be handled during the forming, drying, and burning processes; and refractoriness permits it to be burned into a hard body of permanent form. Many types of clay are found in Kentucky. The most important ones are ball, bentonite, flint, fuller's earth, and halloysite. Mineralogy of the clays is described in Minerals.Read more
Shale is the most abundant of all sedimentary rocks. It is composed primarily of soft clay minerals, but may include variable amounts of organic matter, calcareous material, and quartz grains. Read more
Sandstone is composed of mineral grains (commonly quartz) cemented together by silica, iron oxide, or calcium carbonate. Sandstones are typically white, gray, brown, or red. Read more
Conglomerate is a sedimentary rock usually composed of rounded quartz pebbles, cobbles, and boulders surrounded by a matrix of sand and finer material, and cemented with silica, iron oxide, or calcium carbonate.Read more
More than 50 percent of the surface rocks in Kentucky are limestones. Limestone is chiefly composed of the mineral calcite (calcium carbonate), but may contain small amounts of clay, silt, chert, and dolomite. Read more
Dolostone is a sedimentary rock composed chiefly of the mineral dolomite, which is magnesium calcium carbonate.Read more
Coal is formed from peat, which is an accumulation of decayed vegetation usually associated with swamps. The process of the transformation from peat to coal is called coalification. In the stages of coalification, peat is altered to lignite, lignite is altered to sub-bituminous coal, sub-bituminous coal is altered to bituminous coal, and bituminous coal is altered to anthracite coal. Most Kentucky coal is bituminous, but some lignite occurs in western Kentucky. Read more
Geodes are spherical or oblong rocks filled or partially filled with minerals. When a geode is broken, as in the example above, the minerals inside are revealed. Read more
On July 14, 2000, agate was officially designated Kentucky's State Rock (Kentucky Acts Ch. 146, sec. 1). The Kentucky Geological Survey was not consulted prior to this designation, which is unfortunate, because agate is a microscopically crystalline variety of the mineral quartz, and not a rock. Rocks are composed of many minerals and are formed through sedimentary, igneous, or metamorphic processes. Kentucky has a state rock that is really a mineral, and a state mineral (coal) that is really a rock! Read more
Glacial drift is a special type of sedimentary deposit that has brought many different rock types to Kentucky that were formed outside of Kentucky. Read more
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