heart of the central lowland of northeastern China ( Manchuria). It has a surface area of about 135,000 square miles (350,000 square km), all of which lies below 1,000 feet (300 metres) above sea level. The plain, largely the product of erosion from the surrounding highlands, is mostly undulating, with fertile black soils. It is bordered on the west by the Da Hinggan Range, on the north by the Xiao Hinggan Range, and on the east by the Changbai (Changbai Mountains) ranges, but on the south it is open to the Gulf of Liaodong. It is drained by the Sungari (Songhua) River (Sungari River) and its tributary, the Nen River, in the north and by the Liao River in the south. It connects via a narrow strip of coastal plain with the great alluvial North China Plain to the southwest. The Northeast Plain is China's major soybean-growing area, and it also produces corn (maize), rice, wheat, sorghum, sugar beets, and flax. After 1949, large state farms were established and land reclamation projects begun. It is also an important base of heavy industry with an extensive system of railways, and it is rich in natural resources (iron ore, coal, and oil). The major industrial cities on the plain are Harbin (Ha'erbin), Shenyang (Mukden), and Changchun. The Sanjiang Plain at the confluence of the Sungari, Amur (Amur River), and Ussuri (Ussuri River) rivers in the far northeast is generally considered to be part of the Northeast Plain.