one of the world's largest regions of continuous flatland, central Russia. It occupies an area of nearly 1,200,000 square miles (3,000,000 square km) between the Ural Mountains in the west and the Yenisey River Valley in the east. On the north the West Siberian Plain is bounded by the Kara Sea and in the south by the Torghay Plateau, the Saryarqa (Kazak Uplands), and the Altai Mountains. The plain is drained by the Ob, Irtysh (Ertis), and Yenisey rivers and their tributaries. It is a region of the Earth's crust that has undergone prolonged subsidence and is composed of horizontal deposits from as much as 65,000,000 years ago. Glacial deposits extend as far south as the Ob–Irtysh confluence, forming occasional low hills and ridges, but otherwise the plain is exceedingly flat and featureless and has very poor drainage. Much of it lies within the zone of coniferous forest. Parts of the lowland are underlain by an extensive oil field that also has large gas deposits.