The British Isles lie at the juncture of several regions with past episodes of tectonic mountain building. These orogenic belts form a complex geology that records a huge and varied span of Earth's history. Of particular note was the Caledonian Orogeny during the Ordovician Period, c. 488–444 Ma and early Silurian period, when the craton Baltica collided with the terrane Avalonia to form the mountains and hills in northern Britain and Ireland. Baltica formed roughly the northwestern half of Ireland and Scotland. Further collisions caused the Variscan orogeny in the Devonian and Carboniferous periods, forming the hills of Munster, southwest England, and southern Wales. Over the last 500 million years the land that forms the islands has drifted northwest from around 30°S, crossing the equator around 370 million years ago to reach its present northern latitude.

When did the Caledonian Orogeny occur?