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Precambrian (Animikean era)
Biwabik Iron Formation
Mary Ellen Mine, Mesabi Iron Range,
Minnesota, U.S.A.
Hematite and limonite, the red and yellow oxides of iron, probably are the predominant minerals in this stromatolite. This specimen is from the Mary Ellen Mine, where large deposits of metallic hematite were produced during photosynthesis by cyanobacteria, most likely Collenia undosa. Cyanobacteria had no predators and multiplied profusely until the arrival of gastropods, particularly snails, which ate them. Today cyanobacteria can only be found in environments such as Antarctica and the tidal pools of Sharks Bay, Australia, that are either too cold or too saline for snails to survive.