A wetland that accumulates more than 30 cm (40 cm in Canada) of highly organic peat. These originate in two major ways: by the filling in of shallow water bodies and their invasion by semiaquatic peat-forming plants, or by the swamping and waterlogging (paludification) of unsaturated mineral soils in upland situations, with the latter being areally more important than the former. See Gorham (1995).