Madagascar consists mainly of a block of crystalline rocks. It is generally described as a plateau, rising sharply from the narrow plain of the east coast and descending in a series of steps to the strip of sedimentary rocks along the west coast. The high plateau is much indented and, on the eastern edge, cut by deep gorges and waterfalls. There are numerous volcanic outcrops that produce heights over 1,800 m (6,000 ft); the highest point is Mount Maromokotro (2,876 m/9,436 ft) in the Tsaratanana Massif. The eastern coast is almost straight and has very few anchorages. Behind its coral beaches there is an almost continuous line of lagoons from Foulpointe to Farafangana. These are linked by manmade channels to form an inland waterway called the Pangalanes Canal. The island's major rivers flow westward and are navigable for about 160 km (100 mi) inland.