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Structures found preserved in many rocks, especially carbonate rocks, that were formed by the activities of algae. They are among the oldest organic structures to have been recognized. Stromatolites have been identified in the gunflint cherts of Ontario, which are about 2000 million years old, and in Precambrian rocks in Australia and Africa some 3000 million years old. Often they consist of a number of white concentric rings, the outer of which may be up to a metre across. Microscopic examination has shown these rings to be composed of numerous blue-green algae that have been preserved due to their ability to secrete calcium carbonate and form large stony cushion-like masses. Stromatolite formation may be observed today in certain coastal zones, tropical streams, and by mineral springs where blue-green algae can flourish due to the absence of grazing invertebrate animals.
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