Day 1 - Geological Background

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Precambrian stromatolites.

Stromatolites are finely-layered, bulbous deposits of calcium carbonate built by colonies of cyanobacteria. They were an abundant life form two  to three billion years ago, but now are quite rare. The ideal present-day condition for stromatolite formation is warm, shallow, high-salinity seawater with restricted circulation, under which they grow less than a millimeter a day. This photo shows a stromatolite-rich layer that formed in a shallow sea in Morocco some 600 million years ago and then was buried by volcanic and volcaniclastic deposits of the Ouarzazate Group. The stromatolites were later exposed through uplift and erosion of  the  Anti-Atlas Mountains in the late Palaeozoic and Caenozoic. Their remarkable preservation is due to the modern-day arid climate, making it appear that the sea just receded.

From Bill Burton, U.S. Geological Survey.