Sedimentary rock is formed as layers of sediment are layered on top of the already existent sediments, lower layers of sedimentary rock, and fossils they contain, or normally older than the upper layers. Relative dating allows paleontologists to determine whether a fossil is older or younger than other fossils. To help the paleontologists figure out the relative ages of the fossils, they use index fossils. Index fossils are distinctive fossils used to establish and compare the relative ages of rock layers and the fossils they contain. Useful index fossils can be easily recognized and will only occur in a few rock layers, but these layers will be found in may different places. This means the organism only lived for a short period of time and that the organism was widely distributed.
