Photo 10, Cambrian shallow marine shelfal sediments, now tilted to an almost vertical orientation, Porth Clais, North Pembrokeshire. These sediments were probably laid down around the south-eastern margin of the Lower Palaeozoic Welsh Basin, but were folded and tilted by the Caledonian (and possibly also the later Hercynian/Armorican) Orogenies. It was the heat and pressure generated by these mountain building events that changed the Lower Palaeozoic mudstone rocks of Wales into low grade