beach blue coastal coastline Cornish cornwall England english Great Britain low tide Molluscs mussels Mytilus natural nature rocks sea seascape shellfish shore view waves block boulder coast cracked eroded erosion fissure fractured Gower littoral plant rock shoreline tide wales water betray cliff danger islet Pembrokeshire spray spume summer treacherous united kingdom welsh angry boisterous boulders green rough big Cabo Girão drop enormous harbour high huge landscape large madeira ocean sheer sudden tall village bay hillside surf power bracken plants island isthmus rocky Skomer wild boat trawler