mous regions"), covers 720,000 km2, but its high, rugged terrain—80% of which lies at or above 3000 m—supports a population of only 4.3 million, or 6 persons per square kilometer. On the other hand, with more than 30 million hectares of usable grassland, it supports many sheep (13.5 million in 1989), cattle and yaks (5.2 million), and other livestock. Animal husbandry accounts for 10% of this province's gross value of output. Qinghai leads the nation in wool production per household.3
The interior of Qinghai consists of extensive river and lake systems, which include the large inland Qaidam basin and the headwaters of the Yangtze, Mekong, and Yellow rivers. Qinghai's high plateau terminates on the north at the Altun and Qilian mountains, which reach more than above 4000 m before descending into the Hexi Corridor in neighboring Gansu. Large-scale uplift since the Quaternary raised the plateau by 1000 meters in the early Pleistocene, and by more than 1000 meters in the mid to late Pleistocene. Glaciers developed during the early Pleistocene uplift were first replaced by freshwater lakes. As the climate grew more arid in the late Pleistocene, these lakes became salty, and sand-gravel deserts formed, creating a high alpine desert.
Gansu province occupies a natural corridor between eastern China and Xinjiang province in the west. Eastern Gansu, which lies within the Ordos Plateau, is drained by the Yellow River. The Hexi Corridor runs through the center of the province, bounded by the Qilian Mountains on the south and the Alashan Plateau, a high desert, on the north. To the west, Gansu merges with the Tarim Basin and Beishan range of Xinjiang.
Although the climate of Gansu is arid, the longer growing season and the supply of irrigation water from the Qinghai Plateau support a dense population of 21.4 million people in an area of 450,000 km2. The Hexi Corridor, gateway to the ancient Silk Road, is the grain belt of the northwest. Because the annual precipitation in the region is only 50–200 mm, 80% of the farmland is irrigated by melting snow from the Qilian Mountains that border the corridor on the south. Farming in the corridor is productive, although 10% of the cropland is so salinized that it is useless for agriculture. In 1987, Gansu reported 14.5 million hectares of grasslands, of which 8.5 million hectares were deemed usable. These grasslands, the largest and most fertile of which lie north and south of the Hexi Corridor, are home to more than 5 million cattle and nearly 9 million sheep. Animal husbandry accounts for slightly more than 5% of the Gansu economy.4 The vast alpine grassland on the Qilian Mountain is inhabited by minority groups, chiefly Yokous, Tibetans, and Mongols. On the northern fringe of the corridor are the Mazong and Heshou mountains, surrounded by desert (mostly gobi or gravel) and semidesert rangeland.
The Ningxia Hui Autonomous Region occupies a small wedge of land along the Yellow River, east of Gansu. Northern Ningxia includes an alluvial plain formed by loess deposits laid down by flooding of the Yellow River. Southern Ningxia extends into the loess area of the Ordos Plateau that rises to 2000 m.