Ferruginous Stromatolites (IMAGE) Geological Society of America Caption These are approximately 1.8-billion-year-old ferruginous stromatolites from the Biwabik Iron Formation, Minn., USA. Credit E. Calvin Alexander Jr. for GSA Special Paper 504 Usage Restrictions Credit to GSA and the photographer, E. Calvin Alexander Jr. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.