The southern group of islands surrounds Scapa Flow. Hoy, to the west, is the second largest of the Orkney Isles and Ward Hill at its northern end is the highest elevation in the archipelago. The Old Man of Hoy is a well-known seastack. Graemsay and Flotta are both linked by ferry to the Mainland and Hoy, and the latter is known for its large oil terminal. South Walls has a 19th-century Martello tower and is connected to Hoy by the Ayre.