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We think of coal as a black rock, but at the beginning, more than 200 million years ago it was the stinky, muddy sludge at the bottom of wooded swamplands. The climate changed and the swamps dried out or became huge lakes or seas. Rivers brought down sand and mud and clay which formed layers over the sludge. The climate changed again and new swamps and sludge layers formed and then changed again, repeating the layers of sand, mud and clay. Sometimes the changes were sudden and the layers were thin, other times change was slow and the layers were quite thick.

How coal was formed

The sketch shows what the land may have looked like millions of years ago

As millions of years passed the weight pressing down squeezed out the water and changed the materials creating a huge coal and rock sandwich. We call the layers in this sandwich, strata. Earthquakes twisted and tore the sandwich so that the strata bent and broke and the 'sandwich' was not as easy to recognise. Sometimes the earthquake was so violent that layers of coal showed at the surface, or rock was eroded by weather or seas and rivers to show the coal.

Rock strata

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