The Wrekin was created over 566 million years ago during a process of mountain building known as the 'Caledonian Orogeny', an event which in itself unfolded over a period of at least 200 million years. At the time the hill was formed, England and Wales were situated in the southern hemisphere and separated from Scotland by the Iapetus Ocean, which gradually contracted and caused the continental blocks of the two land masses to collide. The impact of this event led to highly explosive, volcanic eruptions that were responsible for the creation of The Wrekin and produced uplift that eventually brought Britain above sea-level. The hill itself is not a volcano but is made from layers of solidified magma and lava that are over a mile thick. The Wrekin has some of the best and most well preserved Uriconian lavas in the country, exposed examples of which can be seen on the summit, at the main entrances to the hill fort.