Winnipeg lies at the bottom of the Red River Valley, a low-lying flood plain with an extremely flat topography. This valley was formed by the ancient glacial Lake Agassiz which has rich deposits of black soil. Winnipeg is on the eastern edge of the Canadian Prairies in Western Canada; it is known as the 'Gateway to the West'. It is relatively close to many large Canadian Shield lakes and parks, as well as Lake Winnipeg (the Earth's twelfth largest freshwater lake). Winnipeg is bordered by tallgrass prairie to the west and south and the aspen parkland to the northeast.