How are Fossils Formed?

A fossil is the remains of plants and animals preserved in rock. Sometimes they take the form of shells, bones, scales, or other hard parts of animals. Sometimes an entire animal is preserved. Or a fossil can be a film of carbon left by a decaying plants. It might be the track of animals walking across mud. When a plant or animal dies, it gradually disappears.

But if the body is covered by sand or other substances, parts of it may be preserved for a long time, and they may become fossils. Most fossils are remains of plants and animals that lived and died in water. Their bodies became quickly covered by sand carried in the water. Gradually, more sand and mud covered the bodies, pressing down on the layers of sand underneath.

The bottom layers hardened into rock, a form of rock called ‘’sedimentary rock’’. The sedimentary rocks built up very slowly, and the bodies of animals and plants buried in them gradually changed. Minerals from the water filled in the tiny pores in the bones, or the insides of shells. After the shell had crumbled away, a cast of the shell remained in the rock and so a mould of the outside of the shell remained in the rock.

This process may take thousands of years. On land, the bodies of plants and animals may be covered by blowing sand or perhaps ashes from a volcano. Insects and other small animals may be trapped in sticky sap. If this sap hardens into amber, it preserves the bodies of the animals inside. Larger animals may fall into tar pits or quicksand, and their bodies may be preserved for millions of years.

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