Fossils

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A fossil is formed when an organism is trapped in a soft sediment, like mud. Over a long time as other materials pile on top of the sediment, the heavy weight turns the bottom layers into rock. In time the organism which was alive may decay but the imprint will be moulded in the rock. One of the soft sediments in which we often find fossils is limestone. Limestone is formed when a salt, calcium carbonate, settles out of seawater. Many animals, like corals and clams, have shells made out of calcium carbonate. When these animals die their shells help build up the limestone sediments. For years people could not figure out why they found limestone deposits and fossils of marine plants and animals hundreds of kilometres away from the ocean. We now know much of what is now dry ground or even mountains was once covered by shallow seas. By looking at where certain types of fossils are found, scientists can tell what parts of our world were once underwater and how long ago dry land appeared.

Cambrian Period 570-500 million years ago	Ordovician Period 500-420 million years ago
Cretaceous Period 135-65 million years ago	Miocene Period 26-12 million years ago

Ancient Sea levels
Red indicates dry land.