Coastal erosion of limestone. The limestone, which has been eroded by wave action, dates from the Carboniferous period some 360-300 million years ago. It formed from calcium-containing minerals in the shells of marine organisms. These shells accumulated on the bed of an ancient sea, and over millions of years geological processes formed the rock seen here. Once exposed to the action of wind, rain and sea, the soft rock is steadily being eroded away, forming the shapes seen here. Three limpets are seen at lower centre. Photographed at Rhossili, on the Gower Peninsula, on the South Wales coastline.