This, the largest island of the archipelago, is formed of volcanic rock dipping vertically into the sea. On the slopes, note two curious strata of obsidian, a glazed black volcanic stone. Two beaches, Marina Lunga and Marina Corta, frame the town of Lipari, which is encircled by 13th-14th century walls and dominated by a 16th century Norman castle. Nearby there is a pretty view of the bay. Between the islands of Lipari and Vulcano, in the Bocche di Vulcano, stand picturesque rocks resembling Druidical stones, among them the Pietralunga, a basalt nearly 240 ft, 72m high.