Shan Plateau, crystalline massif forming the eastern part of Myanmar (Burma) and forming part of the Indo-Malayan mountain system. The plateau is crossed by the deep trench of the Salween River in the east and is bordered by the upper course of the Irrawaddy River to the west. The average elevation of the plateau is between 2,500 and 4,000 feet (750 and 1,200 m). It is seamed and ribbed by mountain ranges that split up and run into each other. The mountain ranges have an average elevation of between 4,000 and 5,000 feet (1,200 and 1,500 m), with some peaks rising to more than 8,000 feet (2,400 m). The ranges are interspersed with masses of broken hills, which formerly were heavily forested but which have been extensively denuded because of the shifting (slash-and-burn) cultivation practiced by some of the local inhabitants. In between these uplands are valleys and rolling plains that are covered with grass or are farmed for rice. The rocks in the northern part of the Shan Plateau are the source of rubies, sapphires, and other gems for which Myanmar has long been famous. The plateau is the country’s principal source of lead, zinc, and silver and is an area of major teak forests.