Deposits of hydrated iron and manganese oxides are common as stalactites, wall crusts, and as thick layers in sediment in caves at Dubuque, Iowa. The metal oxide deposits are concluded to be at least partly the result of organic (bacterial) rather than inorganic precipitation reactions. -from Author

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Journal NSS Bulletin
Peck, S. (1986). Bacterial deposition of iron and manganese oxides in North American caves. NSS Bulletin, 48(1), 26–30.