DAVID NUNUK / SCIENCE PHOTO LIBRARY DAVID NUNUK / SCIENCE PHOTO LIBRARY
Concretions. The marine concretions of Red Rock Coulee, Alberta, Canada. These spheroid rock formations are aggregates composed of small crystals that became cemented together. The exact mechanism of their formation is not known. The red colouration is due to the rocks' iron content which has been oxidised on exposure to air. The concretions at Red Rock Coulee are the largest in the world.
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