Bog iron refers to impure iron deposits that develop in bogs or swamps by the chemical or biochemical oxidation of iron carried in the solutions. In general, bog ores consist primarily of iron oxyhydroxides, commonly goethite (FeO(OH)). It was discovered during the Pre-Roman Iron Age, and most Viking era iron was smelted from bog iron.
Iron-bearing groundwater typically emerges as a spring. The iron is oxidized to ferric hydroxide upon encountering the oxic environment of the surface. Bog ore often combines goethite, magnetite and vugs or stained quartz. It is not clear whether the magnetite precipitates upon first contact with oxygen, then oxidizes to ferric compounds, or whether the ferric compounds are reduced when exposed to anoxic conditions upon burial beneath the sediment surface and reoxidized upon exhumation at the surface.
Bog iron was widely sought in colonial America. Lake Massapoag in Massachusetts was drawn down by deepening the outlet channel in a search for bog iron. (Diana Muir, Reflections in Bullough's Pond, University Press of New England, 2000) In New Jersey, bog ore was mined and refined and used for the production of tools, wrought iron rails (many of which still grace stairs in Trenton and Camden), taking advantage of its natural rust-resistance. During the American Revolution, the iron was used for cannon balls for the American forces. Bog iron was also found on the Eastern Shore of Maryland. The remains of a commercial smelting operation near Snow Hill are now a state and national historic site. Known as Furnace Town, it was called the Nassawango Iron Furnace after the nearby creek. The commercial furnace ran from about 1825 to 1850.