Sea arch. Waves crashing at the base of The Green Bridge of Wales. This limestone arch is the largest natural sea arch in the British Isles. Natural arches most often form as a narrow ridge, walled by cliffs, becomes narrower from erosion. A softer rock layer under the top of the cliff, gradually erodes to form two caves on either side. The caves then erode further until they meet underneath the ridge to form an arch. The Filmed in Castlemartin, South Pembrokeshire, Wales, UK. Filmed at 200 fps.