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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.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.