The Geological Time Scale has been put together over the last 200 years.
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 particular layers of rock lay at a slight angle on top of each other, and each layer had its own set of fossils.
Here is one diagram he drew of part of southern England.
It runs from the West, near the River Severn at Tewkesbury, to near London at the East.
These rocks fit into the modern version of the time scale - they are 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 will be marked by events organised by many different organisations.