An accumulation of dead organic matter, mostly from plants, which becomes preserved mainly by the exclusion of oxygen. It is dark brown or black and partially decomposed, being preserved under anaerobic conditions in an environment of excessive moisture. Peat forms mostly in bogs and fens; the importance of peat to archaeology lies in its preservation of palaeobotanical (palaeoenvironmental) evidence which can be used to reconstruct the ancient environment. The remains can often be radiocarbon dated. Vast beds of this organic fuel occur in Europe, North America, and northern Asia but are worked only where coal is deficient. Peat deposition is the first step in the formation of coal.