The Geological Time Scale outlines the history of the Earth.
William Smith was one of the first to explore geological time in an organised way.
He had always been interested in fossils, but in 1794 he was employed to survey the route of a new canal.
He saw that layers of rock lay at a slight angle on top of each other.
Each layer had its own set of fossils. This is an illustration Smith drew of fossils in a layer he investigated near Bath.
Here is one diagram he drew of part of southern England. The clay layer is a thin blue line over the yellow band of oolite.
It runs from the West, near the River Severn at Tewkesbury, to near London in the East.
These rocks are mainly Jurassic and Cretaceous strata.
In 1815 William Smith published his first Geological Map of England and Wales.
This area covers a similar part of England as the cross-section above.
200 years on, this map will be celebrated by events organised by many different organisations.