Stromatolites are laminated rocks composed of multiple layers of fossilized cyanobacteria. They are a type of boundstone and are produced in low energy marine environments. Cyanobacteria were the first photosynthetic bacteria to contribute to oxygen production in the atmosphere and make up the oldest fossils in the rock record. Cyanobacteria are significant to earth's history because they facilitated the transition from an oxygen-poor atmosphere allowing aerobic life to emerge. Stromatolites form large mounds when precipitated carbonates stick to layers of cyanobacteria. This image is a cross-section of a stromatolite mound.