evidence: Fergana Valley is a valley in Central Asia, lying mostly in eastern Uzbekistan, extending into southern Kyrgyzstan and northern Tajikistan. [...] The Fergana Valley is an intermountain depression in Central Asia, between the mountain systems of the Tien Shan in the north and the Alay in the south. The valley is approximately 300 kilometres (190 mi) long and up to 70 kilometres (43 mi) wide, forming an area covering 22,000 square kilometres (8,500 sq mi). Its position makes it a separate geographic zone.[2] The valley owes its fertility to two rivers, the Naryn and the Kara Darya, which unite in the valley, near Namangan, to form the Syr Darya. Numerous other tributaries of these rivers exist in the valley including the Sokh River. The streams, and their numerous mountain effluents, not only supply water for irrigation, but also bring down vast quantities of sand, which is deposited alongside their courses, more especially alongside the Syr Darya where it cuts its way through the Khujand-Ajar ridge and forms the valley. This expanse of quicksand, covering an area of 1,900 km2 (750 sq mi), under the influence of south-west winds, encroaches upon the agricultural districts.[3]