A rocky cushion-like mass formed by the unchecked growth of millions of lime-secreting cyanobacteria. Stromatolites are found only in areas where other organisms that would normally keep down the bacterial numbers cannot survive, such as extremely salty bays. Such bacteria were abundant during the Proterozoic and Archaean eons, from as early as 3500 million years ago. The white rings of fossilized microorganisms found in rocks of this age are the remains of stromatolites.
Subjects: Biological Sciences.