Expansive areas of peat soils are referred to as peatlands. These areas are often located in what were once lakes or oceans. The clay deposits from the former lake provide an impermeable layer so that water accumulates. Plants growing in this wet environment will not be able to decompose because of a lack of oxygen. Accumulations of plants will continue to increase the thickness of the peat deposit until a soil formed entirely of peat is created. These deposits can be 40 feet (12 m) or more thick. Extensive peatlands occur in Minnesota, Wisconsin, Michigan, New England, Russia, England, and Scandinavian countries.