🏔️ Edinburgh: Built on an Extinct Volcano

About this simulation

Around 350 million years ago, during the Carboniferous period, volcanic activity pushed molten basalt up through older sedimentary rock, plugging a vent with hard, erosion-resistant rock. That plug is Castle Rock, the crag on which Edinburgh Castle now stands, and a second vent further east cooled into Arthur's Seat.

How it works

• During the last Ice Age, glaciers ground westward-to-eastward across the land. The soft sedimentary rock eroded away easily, but the hard basalt plug barely wore down at all.
• The plug also sheltered a wedge of softer rock on its downstream (east) side from the scouring ice — this protected wedge is the "tail".
• The result is a classic crag-and-tail landform: a steep cliff on the exposed west side, and a long, gently sloping ridge running east — exactly the shape the Royal Mile now follows from the Castle down to Holyroodhouse.
• The valley gouged out to the north (once the Nor Loch) later became Princes Street Gardens, separating the medieval Old Town from the planned Georgian New Town.

Controls

• Glacial erosion — drags the ice sheet across the landscape and carves the crag-and-tail shape in real time.
• Cutaway cross-section — slices the terrain open to reveal the volcanic pipe cutting through layered sedimentary rock.
• Cutaway position — slides the slice east/west through the model.
• Old / New Town streets — overlays the tightly-packed medieval Old Town along the tail and the planned Georgian grid of the New Town.

Did you know?

Edinburgh's Old Town sits almost entirely on the protected "tail" of the landform, which is why the Royal Mile slopes gently downhill for over a mile from the Castle esplanade to the Palace of Holyroodhouse — you're walking down a 15,000-year-old glacial shadow.