The Indonesian archipelago consists of three main regions. One of the regions consists of Sumatra, Java, Kalimantan, and the islands that lie between them, which stand on the Sunda shelf, where the ocean depths are never more than 210 m (700 ft). Another region consists of Irian Jaya and the Aru Isles, which stand on the Sahul shelf, projecting northward from the north coast of Australia at similar depths. Between these two shelves is the remaining region, consisting of the Lesser Sunda Islands, the Maluku Islands (Moluccas), and Sulawesi, which are surrounded by seas with depths that reach 4,570 m (15,000 ft). The large islands have central mountain ranges rising from more or less extensive lowlands and coastal plains. Many inactive and scores of active volcanoes dot the islands, accounting for the predominantly rich volcanic soil that is carried down by the rivers to the plains and lowlands; there are over 100 volcanoes. Peaks rise to 3,650 m (12,000 ft) in Java and Sumatra. Java, Bali, and Lombok have extensive lowland plains and gently sloping cultivable mountainsides. Extensive swamp forests and not very fertile hill country are found in Kalimantan. Sumatra's eastern coastline is bordered by morasses, floodplains, and alluvial terraces suitable for cultivation farther inland. Mountainous areas predominate in Sulawesi.