Stromatolites () or stromatoliths (from Greek στρώμα, strōma, mattress, bed, stratum, and λίθος, lithos, rock) are layered bio-chemical accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms (microbial mats) of microorganisms, especially cyanobacteria. Fossilized stromatolites provide ancient records of life on Earth by these remains, which might date from more than 3.5 billion years. Lichen stromatolites are a proposed mechanism of formation of some kinds of layered rock structure that is formed above water, where rock meets air, by repeated colonization of the rock by endolithic lichens.