Stromatolite slab from the Eocene epoch measures 9" x 4.5" x .75"
Stromatolites or stromatoliths are layered sedimentary formations that are created by photosynthetic cyanobacteria. These microorganisms produce adhesive compounds that cement sand and other rocky materials to form mineral “microbial mats”. In turn, these mats build up layer by layer, growing gradually over time. Wikipedia
Cyanobacteria use water, carbon dioxide, and sunlight to create their food, and expel oxygen as a by-product. The real significance of stromatolites is that they are the earliest fossil evidence of life on Earth. They were the first known organisms to photosynthesis and produce free oxygen.