The east-west Tien Shan Mountains divide it into Dzungaria in the north and the Tarim Basin to the south. Dzungaria is bounded by the Tarbagatai Mountains on the west and the Mongolian Altai Mountains on the east, neither of which is a significant barrier. Dzungaria has good grassland around the edges and a central desert. It often behaved as a westward extension of Mongolia and connected Mongolia to the Kazakh steppe. To the north of Dzungaria are mountains and the Siberian forest. To the south and west of Dzungaria, and separated from it by the Tian Shan mountains, is an area about twice the size of Dzungaria, the oval Tarim Basin. The Tarim Basin is too dry to support even a nomadic population, but around its edges rivers flow down from the mountains giving rise to a ring of cities which lived by irrigation agriculture and east-west trade. The Tarim Basin formed an island of near civilization in the center of the steppe. The Northern Silk Road went along the north and south sides of the Tarim Basin and then crossed the mountains west to the Fergana Valley. At the west end of the basin the Pamir Mountains connect the Tien Shan Mountains to the Himalayas. To the south, the Kunlun Mountains separate the Tarim Basin from the thinly peopled Tibetan Plateau.