attention danger falling rocks germany isolated mountains pictogram rock fall rock slide rockfall sign stones traffic warning cliffs coastline erosion natural nature Pembrokeshire rockfalls rocks sea shore stone wales water welsh accident fall hexagon hill hillside lichen mesh metal net netting pattern prevent rock safety secure texture wire boulders green hazard Lofoten Islands norway norwegian red road roadside sheep signs summer triangle triangular beach boulder cliff England english Great Britain Hastings Sussex unstable wild Birling Gap chalk coast cream evening face height high white