Located high in the Geneva Creek Basin southwest of Denver, the Geneva Creek Iron Fens are registered as a Colorado State Natural Area. One of 8 iron fens in the state, it is an example of both a rare community type and an unusual geologic process.
A fen is an area of peat-forming wetlands that receives nutrients usually from upslope mineral soils and groundwater movement. It is typically alkaline due to groundwater draining from surrounding calcareous rocks. Fens are different from bogs because bogs are acidic, not alkaline. Most fens have high pH's and have a high mineral content. Furthermore, bogs are rain fed and exist on the Canadian shield (Minnesota, Michigan, Maine, etc.), while fens are fed by groundwater.
Features
Iron fens are a phenomenon unique to Colorado and there are no other known occurrences in the world. The iron fens, due to the iron substrate, are very low in pH (acid) and are high in minerals. The geologic process forming the iron fen begins with a series of springs flowing over highly fractured, highly mineralized bedrock rich in pyrites producing extremely acidic (pH3), mineral-rich water. This process produces limonite (iron saturated peat) ledges and terraces.
Adapted from https://www.co.clear-creek.co.us/index.aspx?NID=257
