Green Bridge of Wales

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 weakness — every crack, every joint plane, every fossil shell that has left a cavity in the rock. The process is hydraulic, chemical, and mechanical simultaneously: water is forced into cracks under wave pressure, compresses the trapped air, and when the wave retreats, the sudden decompression blasts the rock apart from the inside. This has been happening continuously since the last Ice Age uncovered this coastline approximately 12,000 years ago.

The Green Bridge of Wales is what this process leaves behind when it works on a headland over a long period. Wave action first carved a cave through the base of a limestone promontory. The cave enlarged from both sides until its walls met in the middle, creating a sea arch. The arch roof is now 24 meters above the Celtic Sea, and its span is one of the largest natural arches in Wales. The rock below the arch is undercut on both sides — the same erosional process that created the arch is continuing, working on the arch's foundations, with the geological patience that has removed everything else from this coastline that was less resistant than what remains.

The Castlemartin Peninsula, on which the Green Bridge stands, is managed partly as a military firing range by the British Ministry of Defence. The Castlemartin Range has been an active tank training and artillery area since World War II. Access to the coastal path sections near the range is periodically restricted when live firing is in progress — typically announced on the Range's public schedule — and entirely closed during major training exercises. The military use of the peninsula, while inconvenient for coastal walkers, has had the unintended effect of significantly limiting civilian access and development along one of the most dramatic sections of the Pembrokeshire Coast National Park. The military restriction has, over 80 years, protected the coastal landscape from the infrastructure that would otherwise accompany such a viewpoint.

The coastal path above the Green Bridge runs along the cliff edge from Bosherton village, roughly 2 kilometers by the direct route or further along the St Govan's Head circuit. The path offers views of Stack Rocks (the Elegug Stacks — two sea stacks close offshore colonized by guillemots and razorbills) as well as the arch itself. At certain tidal states and in certain sea conditions, waves surge through the arch from south and north simultaneously, meeting in the space below with a collision of energy that sends spray upward through the opening. Watching this from the cliff above is available to anyone who walks the correct distance in the correct direction.

Pembrokeshire's geology at this location is Carboniferous limestone, approximately 300 million years old, laid down in a warm shallow tropical sea when this part of Wales sat south of the equator. The fossils visible in the cliff faces — crinoid stems, brachiopod shells, occasional coral fragments — are evidence of that distant marine environment. The cliff is simultaneously a record of a tropical ocean and a record of its own ongoing destruction. Both are legible simultaneously in the same stone.