Fossils are the preserved remains or traces of animals, plants, and other organisms from the past. They can be found in sedimentary rocks, which are formed from the accumulation and cementation of sediments over time. Because fossils are the remains of organisms that once lived, they can be used to date the rocks in which they are found. By comparing the fossils in different rock layers, geologists can determine which layers are the same age and which are older or younger. This process is called correlation.