IF THE HISTORY OF THE PLANET took place in a day, humans would appear at 23:59:56, and the Great Lakes would take their current shape across the northland a fraction of a second before midnight.
IF THE HISTORY OF THE PLANET took place in a day, humans would appear at 23: 59: 56, and the Great Lakes would take their current shape across the northland a fraction of a second before midnight. The Great Lakes basin is set in the Canadian Shield, the geological core of North America. The shield floated on a sea of magma around the planet before ending up where it is today. At one point, it straddled the equator on the ancient continent of Laurentia. It was an ocean floor to several prehistoric seas; then it collided with South America and West Africa and created the Appalachian and Adirondack Mountains. Volcanic ash buried it, then glaciers scraped it clean. Geologists predict that in a hundred million years, it will merge with Asia and South America, creating another supercontinent they call Amasia.
The Laurentide Ice Sheet gouged the bottom of Lake Superior thirteen hundred feet deep during the last Ice Age. It cut Lake Michigan nine hundred feet down. When the ice retreated fourteen thousand years ago, the basins filled with freshwater. Lake Superior topped out first and spilled into Lakes Michigan and Huron. The Saint Lawrence River was still dammed with ice, so water flowed north through the North Bay Outlet and south down the Mississippi and Hudson Rivers. When the last of the ice melted, water flowed to Lakes Erie and Ontario, plunged over Niagara Falls, and drained through the Saint Lawrence to the Atlantic.
