Bioko and Annobón are volcanic islands that are part of the chain starting with the Cameroon Highlands and outcropping into the Atlantic as far as St. Helena. Río Muni is a fluvial mainland plateau, except for the sandy shore and the ridges of the Sierra Cristal range that separate the coast from the interior plateau. The Muni and Ntem rivers, on the south and north boundaries of Río Muni, are estuaries navigable for about 20 km (12 mi); the Mbini River, midway between them, is typical of the cascading streams that drain all of Río Muni. Bioko has short cascading streams; Annobón has only storm arroyos. Most of the country, including the islands, is tropical rain forest. On Annobón, volcanic deposits restrict agriculture, and the Muni estuarial islands are sandy, but the rest of the country has tropical humus conducive to agriculture.