Stromatolites are large formations of rocks that have been formed by the growth of multiple layers of bacteria over a very long period of time. These usually take the shape of mounds, columns and sheets. Cyanobacteria, single-celled microbes that can perform photosynthesis, develop into these stromatolites. There are many different layers to the various types of formations that stromatolites can take.
Stromatolites are formed when the bacteria that exist in certain regions continue to pile up and form layer upon layer. This usually occurs in areas where there are many different cyanobacteria present and where the natural movements of the surroundings lend themselves to this type of layer development. Places with lots of water, such as lakes, rivers and even some types of soil, are often home to cyanobacteria. Although stromatolites are made up of bacteria, they are completely prokaryotic, so they do not have any DNA.
It takes a very long time for stromatolite formations to be created, as it is an extremely gradual process requiring thousands and thousands of these bacteria. Modern stromatolites face destruction as a result of grazing by snails, worms and other types of animals that can survive on the single-celled organisms of which they are composed.