The largest natural arch in Wales — a limestone sea arch where millennia of wave erosion have carved a dramatic bridge connecting two cliff faces, standing 24 meters above the churning Celtic Sea.
The Pembrokeshire coast of southwest Wales is one of the most aggressively eroded coastlines in Britain. The Celtic Sea drives Atlantic swells directly into Carboniferous limestone cliffs, finding every crack and weakness. Over thousands of years this process creates caves, then sea arches, then stacks. The Green Bridge of Wales is a sea arch 24 meters above the sea — it was once a headland. Wave action carved a cave through it, then the cave roof collapsed, leaving the arch. From the cliff path above, you look down at the arch framing a moving rectangle of green-grey sea. The surrounding Castlemartin peninsula is a military firing range, which means access is periodically restricted — unintentionally limiting crowds, and leaving a coastline that has had time to remain exactly as the sea has made it.