Two kilometers northwest of here, the St. Louis River flows toward Lake Superior through a broad valley in a western extension of the Lake Superior basin. Over the last two million years, kilometer-thick glaciers repeatedly advanced along the basin, scouring it out and eroding the soft sedimentary rocks that had filled it. Near the end of the last glacial period, about 12,000 years ago, the Superior lobe, a tongue-shaped mass of ice in the Lake Superior basin, began to melt and recede northward, and meltwater filled the basin's southwestern end to form Glacial Lake Duluth. This lake carried large amounts of red clay eroded from the basin's red sandstones and shales, and over centuries it deposited more than 100 meters of sediment, mostly red clay, which now forms the banks of the St. Louis River valley. As the ice retreated farther north, primitive Lake Superior drained through newly opened eastern outlets to the lower Great Lakes, and the lake level fell about 60 meters below its present level. The meandering channel of the St. Louis River then removed much of the red clay, shaping the terrain. After the loss of the ice's weight, the earth's crust began slowly rising, with the northeastern basin and its eastern outlet rising faster than the southwest, tilting the basin and flooding the lower course of the St. Louis River from Fond du Lac to the Duluth harbor.