Sedimentary rocks are one of the main three types of rocks that make up The Rock Cycle here on Earth. The other two are igneous and metamorphic, but sedimentary rocks are pretty much the only kind of rock you can find fossils in. The word sediment comes from the Latin sedimentum. Sedimentum means settling and that is exactly how sedimentary rocks form. Different layers of particles like mud, sand or clay settle downwards as more and more layers of sediment pile on top of each other, like a giant stack of pancakes. If dead plants or animals get buried in the sediment, under the perfect conditions, they will fossilize.