Stromatolites are laminated sedimentary features forming mound- and pillar-shaped structures. They form in shallow water settings. Stromatolites are formed by sediment trapping and binding by microbes, or by microbial precipitation of minerals. Microbes such as blue-greeen algae (cyanobacteria) live on the surface layer of stromatolites, above the underlying buildup of lithified remains of former microbial surface communities. Because the microbes that build stromatolites are generally consumed by aquatic macroorganisms such as mollusks and arthropods, stromatolites grow to appreciable size only in areas where aquatic macrobiota are excluded. This can happen when the water is too salty (hypersaline) or warm. During the Precambrian (Archean and early Proterozoic eons) before the evolution of complex animals, stromatolites were abundant. Some Precambrian rocks, as in places like Glacier National Park (Montana) are composed entirely of stromatolitic sedimentary rock.