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 , 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 , largely filled with sediments that range in age as far back 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. Climate The climate of this valley is dry and continental, being mostly a cool arid climate (Köppen BWk) or a cool semi-arid climate (BSk) in less shielded areas. In March the temperature reaches , and then rapidly rises to in June, July and August. During the five months following April precipitation is rare, but increases in frequency starting in October. Snow and frost, down to , occurs in December and January. History Fergana, on the route to Tarim Basin from the west, remained at the boundaries of a number of classical era empires. Achaemenid Empire As early as 500 BC, the western sections of the Fergana Valley formed part of the Sogdiana region, which was ruled from further west and owed fealty to the Achaemenid Empire at the time of Darius the Great. The independent and warlike Sogdiana formed a border region insulating the Achaemenid Persians from the nomadic Scythians to the north and east. It was forcibly settled by exiled Greeks from the Anatolian coast, who had rebelled or otherwise given Persia trouble. Eventually, it had a significant Greek community. The capital of the region was known to the Greeks as Cyropolis, named after Cyrus the Great. The Sogdian Rock or Rock of Ariamazes, a fortress in Sogdiana, was captured in 327 BC by the forces of Alexander the Great; after an extended campaign putting down Sogdian resistance and founding military outposts manned by his Greek veterans, Alexander united Sogdiana with Bactria into one satrapy. Hellenistic settlement In 329 BC, Alexander the Great founded the city of Alexandria Eschate "The Furthest", probably renaming Cyropolis. This was in the southwestern part of the Fergana Valley, on the southern bank