bog iron

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A soft, porous deposit of impure hydrous oxides, such as goethite, formed in bogs, marshes, shallow lakes, and swamps through precipitation from iron-rich water and the oxidizing action of algae, bacteria, or the atmosphere.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun geology deposits of mixed oxides and hydroxides of iron formed when iron-bearing groundwater emerges onto the surface