Sedimentary rocks are derived from the breakdown of rock at the Earth’s surface by weathering and erosion. These sediments are deposited, usually in water, in horizontal layers called beds. Sedimentary rocks may yield fossils.
Clastic sedimentary rocks are composed of cemented fragments of rock and can be classified according to the size (coarse to fine), shape (angular or rounded), composition and degree of sorting of the fragments or ‘clasts’. Some varieties from coarse to fine grained are featured below.
Sedimentary rocks can form by precipitation from solution, or by replacement of one mineral by another. Chemical sediments include evaporites, which form due to the evaporation of a body of water, resulting in the precipitation of salts and carbonates. Travertine limestone is another example, formed by the precipitation of calcium carbonate directly from solution. Rocks composed primarily of dolomite (a calcium-magnesium carbonate) and some kinds of chert may also be chemical sediments.
The most common biochemical rock is a limestone, which comes from the carbonate skeletons of a wide variety of organisms – from macroscopic animals like corals and clams, to microscopic organisms living in the water column. Chert may also be formed biochemically – for example, from the siliceous skeletons of the planktonic microscopic radiolaria.
Organic rocks like coal and oil shales form from the organic parts of the organism, rather than the mineralized shells. Coal is composed of 40 to 90% organic matter – the most organic-rich and compact form of coal is known as anthracite, which has a metallic lustre.
Sedimentary rocks are deposited in a variety of environments – marine, freshwater and terrestrial. The nature of the environment partially controls what sediments are deposited (i.e. clastic sediments become finer as distance from the shoreline increases: coarse conglomerates and sandstones prevail inshore while dark muds are found in deep sea basins). All features such as sedimentary rock type, sedimentary structures (e.g. bedding), and fossil content which characterizes a particular rock as having been deposited in a particular environment is referred to as facies. Studies of facies are useful in building up a picture of past landscapes and their environmental conditions. All scale bars in the slideshow below are 1cm.