Lake Superior lies over the Midcontinent Rift, a rupture in the North American continent that formed a great rift valley from the Lake Superior region southwest to Kansas about 1.1 billion years ago. For about 20 million years as the valley opened, basaltic lavas erupted into it and accumulated to a thickness of up to 20 kilometers in the Lake Superior region. After rifting and volcanic activity ended, the great thickness of dense basalt depressed the crust into a trough-shaped basin, which was then filled by sediment eroded and washed in from the surrounding heights until the deposits reached a thickness of many kilometers. Over the past two million years, glaciers more than a kilometer thick repeatedly advanced along the buried trough and scoured out much of the soft sedimentary rock that once filled it, while the harder volcanic rocks along the trough margins came to form much of Lake Superior's rocky coastline. Ten thousand years ago, as glacial ice in the basin melted from west to east, water ponded in front of the ice almost to the level of this site to form Glacial Lake Duluth. Eventually the ice melted out of the eastern lake basin and a drainage way opened to the lower Great Lakes; when this eastern outlet first formed, it lay lower than today and drained the lake to 60 meters below its present level. As the earth's crust was relieved of the great weight of glacial ice, it began slowly rising, with the northeastern basin and eastern outlet rising faster because the ice load had been removed there most recently. This tilted the basin toward the southwest and flooded the mouth of the St. Louis River, forming Duluth harbor, which was further enlarged by the formation of Minnesota Point, a baymouth bar of sand washed there by easterly waves and shore currents carrying beach sand from Wisconsin.