Dzungarian Gate
A traveller going west from China must go either north of the Tian Shan mountains through Dzungaria or south of the mountains through the Tarim Basin. Trade usually took the south side and migrations the north. This is most likely because the Tarim leads to the Ferghana Valley and Iran, while Dzungaria leads only to the open steppe. The difficulty with south side was the high mountains between the Tarim and Ferghana. There is also another reason. The Taklamakan is too dry to support much grass, and therefore nomads when they are not robbing caravans. Its inhabitants live mostly in oases formed where rivers run out of the mountains into the desert. These are inhabited by peasants who are unwarlike and merchants who have an interest in keeping trade running smoothly. Dzungaria has a fair amount of grass, few towns to base soldiers in and no significant mountain barriers to the west. Therefore, trade went south and migrations north. Today most trade is north of the mountains (Dzungarian Gate and Khorgas in the Ili valley) to avoid the mountains west of the Tarim and because Russia is currently more developed.