Rising from the coastal lowlands, which range in width from 30 to 200 km (20 to 125 mi), is a band more than 96 km (60 mi) wide forming a rocky escarpment, which ranges in height from 450 to 600 m (1,480 to 1,970 ft). This plateau covers the north and east and most of the south. Rivers descending from the interior have carved deep channels in the face of the escarpment, dividing it into distinct blocks, such as the Crystal Mountains (Monts de Cristal) and the Chaillu Massif. There are mountains in various parts of Gabon, the highest peak being Mt. Iboundji (1,575 m/5,167 ft). The northern coastline is deeply indented with bays, estuaries, and deltas as far south as the mouth of the Ogooué River, forming excellent natural shelters. Farther south, the coast becomes more precipitous, but there are also coastal areas bordered by lagoons and mangrove swamps. Virtually the entire territory is contained in the basin of the Ogooué River, which is about 1,100 km (690 mi) long and navigable for about 400 km (250 mi). Its two major tributaries are the Ivindo and the Ngounié, which are navigable for 80–160 km (50–100 mi) into the interior.