Castle Rock, a volcanic plug formed in the Carboniferous period (340 million years), towers above Edinburgh’s Old Town and West Princes Street Gardens giving it a clear defensive advantage. The dolerite rock formed as part of an ancient volcanic complex that underlies the heart of the city. The surrounding sedimentary rocks have since been scraped off on all but the eastern side where they form the gently sloping Royal Mile. This is known as a crag-and-tail formation and is testament to the glacial erosion that changed Edinburgh’s landscapes so dramatically. To the north a loch was formed while the south offered stable ground on which to build Edinburgh’s old town and now forms the Grassmarket. These days it is an essential trip for anyone visiting the city.