A fossil is the preserved remains or trace of an organism that lived long ago. Most fossils form when an organism is buried quickly by sediment, such as mud or sand, before it decays. Over time more layers build up, the sediment is compacted and cemented into sedimentary rock, and minerals in groundwater slowly replace the hard parts.

Body fossils are the remains themselves (bones, shells, leaves). Trace fossils show activity (footprints, burrows). A mold is a hollow imprint; a cast is a mold later filled with minerals.

Law of superposition: in undisturbed layers of sedimentary rock, the oldest layer is at the bottom and the youngest is at the top. Cross-cutting relationships: anything that cuts through rock layers (such as a fault or an igneous dyke) is younger than the layers it cuts.

A cliff face shows these layers from bottom to top: A sandstone; B shale with trilobite fossils; C limestone with shell fossils; E sandstone. A band of igneous rock, D (a dyke), cuts up through layers A, B and C but stops at the bottom of layer E.