betray blue cliff coastline danger Great Britain islet natural nature Pembrokeshire rock sea shore spray spume summer treacherous united kingdom wales water waves welsh angry boisterous boulders coast green rocks rough big Cabo Girão coastal drop enormous harbour high huge landscape large madeira ocean seascape sheer sudden tall village bay Cornish cornwall England english hillside surf tide power bracken plants view island isthmus rocky Skomer wild boat trawler algae bare bivalve exposed kent mollusc mussel Mytilus edulis rockpool seaweed shellfish stone