Fen Valley - 2022

Fens are among the rarest type of wetland found in Iowa. These ecosystems, though often small, support large numbers of rare plants and animals. Fens are unusual because they are formed under unique geological settings and are actually fed by groundwater that slowly seeps out of the soil. By comparison most other wetland types are formed from pooled surface water (i.e., rain, surface run-off). Typically in fens, a layer of sand and gravel covers a less permeable stratigraphic layer. Rain water permeates the sandy upper layer and accumulates above an impermeable layer. Fens often form on or along hillsides. As water flows underground it dissolves carbonate minerals and becomes alkaline. In extremely alkaline fens a crusty, yellowish-white calcium carbonate deposit called marl or tufa forms at the surface on the soil.

-Iowa Department of Natural Resources