This gently rolling landscape is part of the Glaciated Plains, a vast area extending south from Manitoba and Saskatchewan through northern and eastern North Dakota into eastern South Dakota. Glaciers flowed south through this area 12,500 years ago, depositing up to several hundred feet of sediment on top of a much older bedrock surface, and when the glaciers melted they formed the landscape seen today. As the glacier advanced over the bedrock surface, it picked up materials that were later deposited as glacial sediment, a mixture of clay, silt, sand, and rocks of all sizes called till. Because till is composed of sediments from many different sources and contains many different kinds of rocks and minerals, soils developed on it are rich and fertile. At Valley City, the Sheyenne River flows as a small stream through a large valley known as a meltwater trench, which was eroded in a short time by floodwaters released when ice-dammed lakes in northern North Dakota and Saskatchewan drained as the glacier melted back. While it was being eroded, the Sheyenne River valley flowed nearly full of water, in contrast to the small stream in it today. A similar meltwater trench is crossed at Jamestown. Fifteen miles to the east, the Glaciated Plains give way to the flat Red River Valley of the North, the former floor of Glacial Lake Agassiz, which was flooded at the end of the Ice Age between 12,000 and 9,000 years ago.