Bog Iron

Iron oxide/hydroxide concretions that precipitate out within some peatlands where iron-bearing groundwater reaches the oxygen gradient near the bog surface or margins — dissolved ferrous iron (Fe²⁺) carried in from the water table oxidizes to insoluble ferric compounds (limonite/goethite) on contact with air, accumulating as nodules or hardpan layers within or beneath the peat.

Historical Note

Bog iron was a significant ore source in Iron Age and medieval Northern Europe (Scandinavia, Baltic region), smelted locally before large-scale mined-ore metallurgy took over. It is a well-documented feature of the same peatland landscapes that later became sources of therapeutic and cosmetic peat.

Relevance to Peat Sourcing

Bog iron formation requires groundwater influence, so its presence is a marker of minerotrophic (fen-influenced) hydrology rather than a pure ombrotrophic raised bog — see ombrotrophic vs. minerotrophic. During geological survey of a deposit, bog iron layers are one signal (along with pH and vegetation history) that a site or stratum has fen character, which correlates with the elevated trace iron content documented in some peat analyses (e.g. Estonian deposit data). Bog iron nodules themselves are a distinct mineral deposit, not peat, and are typically excluded during peat extraction and processing.