As the climate warmed at the end of the Pleistocene epoch, the Superior Lobe shrank northeastward. The deep basin gouged by the ice filled with meltwater around 12,000 years ago as Lake Duluth. Since the eastern shore was formed by the towering wall of the ice sheet, the lake was higher than modern Lake Superior. The St. Louis River, flowing into Lake Duluth, dropped its sediment load onto the lakebed, forming layers of silt and sand alternating with clay. The clay layers include incongruous cobbles and boulders, dropstones which melted free from icebergs floating above.