algae chalk cliff coast England erode erosion face Great Britain green kent landscape low natural nature rock rockpool sea seaweed shore sky stone tide water bare exposed mollusc mussel shellfish winkle beach bladderwrack coastline Cornish cornwall english ocean rocks sand sandy summer surf waves abstract background bacteria bacterium bog canal environment environmental fen goo gooey grunge grungy lake marsh mat plant poison poisonous pond scum Slime slimy stagnant swamp texture thick toxic weed wet holiday leisure resort vacation