algae arch chalk cliff coast England face Great Britain green hole kent landscape low natural nature peephole rock sea seaweed shore sky stone tide white beach bird fun gull holiday lookout perch play post sand seagull vacation watch wave bank brassicaceae coastal Crambe maritima Cruciferae Cuckmere Haven ecology english gravel habitat halophyte halophytic plants sea kale shingle summer erode erosion rockpool water bare exposed mollusc mussel shellfish winkle coastline Cornish cornwall current flow mouth ocean reach river surf waves yellow atlantic blue danger dangerous distant dizzy dizzying drop edge fall far high madeira sheer vertigo view