Geography and Geology
The Fergana valley is an intermountain depression in Central Asia, between the mountain systems of the Tien-Shan in the north and the Gissar-Alai in the south. The valley is approximately 300 km long and up to 70 km wide, forming an area of 22,000 km2. Its position makes it a separate geographic zone. 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, forming there the Karakchikum. This expanse of moving sands, covering an area of 750 mi², under the influence of south-west winds, encroaches upon the agricultural districts.
The central part of the geological depression that forms the valley is characterized by block subsidence, originally to depths estimated at 6-7 km (4 mi), largely filled with sediments that range in age as far as the Permian-Triassic boundary. Some of the sediments are marine carbonates and clays. The faults are upthrusts and overthrusts. Anticlines associated with these faults form traps for petroleum and natural gas, which has been discovered in 52 small fields.
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