What is coal?
Coal is just old wood, twigs and leaves. It began as forests, which grew in swampy land more than 300 million years ago. When the trees died they fell into the swamp water, which prevented them from breaking down properly. They turned into peat, a soft brown substance. The huge pressure of rock layers building up above turned the peat into coal.
Animals are not the only things that can turn into fossils. Coal sometimes contains the fossilised bark and leaves of giant ferns and clubmosses, trees that grew in swampy forests more than 300 million years ago.
Tracks through time
Animal footprints can be preserved in the rocks, and so can insect trails or even the traces left on the seabed by worms and jellyfish. However, these small, delicate structures can only be preserved in rock that originally comes from mud or sand.