years later, the last volcano went quiet.

Late Precambrian and Paleozoic sedimentary rock

The mountain-building and rifting events left areas of high relief above the low basin of the Midcontinent rift. Over the next 1,100 million years, the uplands were worn down and the rift filled with sediments, forming rock ranging in thickness from several hundred meters near Lake Superior to thousands of meters further south. While the crustal tectonic plates continued their slow drift over the surface of the planet, meeting and separating

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