Stromatolite

Stromatolites or stromatoliths (/strɵˈmætoʊlaɪts/; from Greek στρώμα, strōma, mattress, bed, stratum, and λίθος, lithos, rock) are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria (commonly known as blue-green algae). Stromatolites provide some of the most ancient records of life on Earth by fossil remains which date back more than 3.5 billion years ago.