Fossils form when a dead animal or plant is covered by sediment. Eventually, the organic matter in the bone or plant, such as the blood vessels and tissue, slowly turns into rock, which becomes known as a fossil. For example, A dinosaur dies and is buried before the remains are completely destroyed.Over time, layers of sediment build up and press down on the buried remains.Dissolved minerals, transported by ground-waters in the sediment, fill tiny spaces in the bones. The combination of pressure, chemical reactions and time eventually turns the sediments into rock and the bones into mineralised fossils.The fossils remain within the rock until uncovered through erosion or excavation.