FRANCES WESTALL / EURELIOS / SCIENCE PHOTO LIBRARY FRANCES WESTALL / EURELIOS / SCIENCE PHOTO LIBRARY
Fossil stomatolite. Cross-section through a rock, showing the concentric layers of a fossilised stromatolite. The rock (white) has inclusions of a coal-like organic material (black). The layers in the rock are due to coverings of blue-green algae (cyanobacteria), which captured grains of sediment and became mineralised. As more layers were added over thousands of years, a stromatolite, or rocky mound, gradually formed. Fossilised stromatolites have been found within rocks as old as 3.5 billion years. This sample is from the Barbeton Greenstone Belt, South Africa, and was photographed in 1999.
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