depression in western Kazakhstan. Some 12–125 miles (20–200 km) wide, it runs roughly north-south for about 375 miles (600 km) through the middle of the Torghay Plateau. It was formed by a caving-in of the ancient foundation, and in the Ice Age, water flowed along it from the West Siberian Plain to the Turan Plain. Today it contains a chain of fresh and saline lakes, with the Ubagan River flowing north and the Torghay south. The valley is a major spring and autumn migration route for birds.