A rocky column or mound consisting of layers of trapped sediment and precipitated limestone formed by the unchecked growth of millions of 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 photosynthetic bacteria were abundant during the Proterozoic and Archaean eons, from as early as 3500 million years ago. They began to generate oxygen, which ultimately led to the dominance of life forms dependent on aerobic respiration as a means of obtaining energy. The white rings of fossilized microorganisms found in rocks of this age are the remains of stromatolites.