Unguja is characterised by beautiful sandy beaches with fringing coral reefs. The lagoon is encircled by fringing coral reef. The southern islands are limestone, with level terraces and fringing coral reefs. The coast is bordered by fringing reefs and many fine beaches. They are usually formed from fringing reefs around volcanic islands. He toured the island, examining its volcanic mountains and fringing coral reefs. There are seven named reefs adjacent to the island, known as nearshore fringing reefs. Unlike fringing and barrier reefs which extend only seaward, platform reefs grow in all directions. This fringing seaward reef shelf comprises an area around . Many of the Great Barrier Reef's components are actually fringing reefs. Tidal freshwater marshes are further divided into deltaic and fringing types. Formation takes considerably longer than for a fringing reef, thus barrier reefs are much rarer. Fringing reefs follow coastlines and can extend for many kilometres. It is a small volcanic island with a fringing coral reef. Further offshore lies a peripheral fringing reef.