stromatolite

A dome-shaped structure consisting of alternating layers of carbonate or silicate sediment and fossilized algal mats. Stromatolites are produced over geologic time by the trapping, binding, or precipitating of sediment by groups of microorganisms, primarily cyanobacteria. They are widely distributed in the fossil record and contain some of the oldest recorded forms of life, from over three billion years ago. They continue to form today especially in western Australia.