Rock formed by the deposition and compression of mineral and rock particles, but often including material of organic origin and exposed by various agencies of denudation. Sedimentary rocks may be classified as terrigenous (i.e. derived from the breakdown of pre-existing rocks exposed on the land), organic (i.e. produced either directly or indirectly by organic processes such as shell production or peat formation), chemical (i.e. produced by precipitation from water, e.g. some carbonates and all evaporites), or volcanogenic (pyroclastic, e.g. tuffs and bentonites). They may also be described according to their chemical properties and behaviour and their environmental deposition, and each scheme complements the others.