Some of the boxes are named after places where the rocks were studied in detail (eg the Devonian after Devon). Because the world's surface plates are constantly moving, albeit very slowly, the rocks vary at different times. For example the Devonian sandstones of England represent a time when southern England was in the southern latitudes, near the area occupied by the Namibian deserts today. The Carboniferous rocks in England include coals deposited by vast forests when England lay on the Equator, and the Permian sandstones when we were at the latitude of the western Sahara. During the Jurassic we were crossing the latitude of the Mediterranean and moving north through where Spain is now. During the Cretaceous and Palaeogene we were moving through northern Spain and modern France to our current position.
Related: Timeline tools and resources.