Fossils are the remains of animals or plants that once lived but have now died. In most cases the animal or plant would decay, however in some circumstances when the remains are buried quickly the process of fossilisation can take place. The plant or animal is covered over with layers of sediment and over millions of years this sediment is compressed so that layers of sedimentary rock are formed. Over time this rock sinks down and turns into a hard rock. The original skelton is now dissolved leaving a hole or cavity that looks like a mould into which minerals are deposited. These minerals take on the shape of the mould and therefore the shape of the original plant or animal. Over further millions of years the rock and now fossil rise to the surface, the outer rock erodes in wind and water exposing the fossil.
