A rock formed from the accumulation and consolidation of sediment, usually in layered deposits. Sedimentary rock is a deposit that has been laid down by water, wind, ice, or gravity. The material of which sedimentary rocks are composed has usually been transported from its source. They have accumulated at or near the Earth’ s surface at normal temperatures and pressures, and are thus distinguished from igneous rocks, which have originated as magma in a molten or semi- molten state, and from metamorphic rocks, which have been produced by the effects of heat or pressure. Many sedimentary rocks were deposited in water in which the transported material was in a state of suspension or solution. After deposition they have been consolidated. Some, such as loess, were deposited by the wind; some, such as boulder clay, are the result of glacial action; some have simply accumulated in place. Sediments can be divided into three groups: clastic rocks, chemical precipitates, and organic sediments. Of these, clastic sediments are the most abundant. They are made up of fragments of pre-existing rocks. Clays, sands, and gravels are in this category. Chemical precipitates include some limestones (the English Chalk, for example) and deposits formed by the evaporation of lakes or seawater. These deposited rocks include gypsum and halite. Organic sediments are those formed largely of the remains of once-living organisms: examples are coal, oil shale, and limestones made mainly of fossil material. By volume, sedimentary rocks make up only five per cent of the known crust of the Earth, compared with 95 per cent of igneous rocks. They are, however, exposed on over two-thirds of the Earth’s land surface, forming thin but extensive deposits. The commonest types are shales, sand-stones, and limestones. The earliest known sedimentary rocks came from the Barberton Mountain Land of South Africa; these have been dated as 3500 million years old.
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