Did a sheet of ice really scoop out the Great Lakes?
Five lakes that hold more than a fifth of the world's surface fresh water began as hollows dug by ice, with a tectonic head start.
▶ Start the storyMostly yes, though the ice had a head start. The Great Lakes sit in basins that a huge sheet of ice, the , gouged out of the land. When that ice melted and retreated about 14,000 years ago, the hollows it had carved filled with , and the lakes were born.
Picture North America during the last ice age. The covered most of Canada and a large part of the northern United States, and in places it was up to 3 kilometres thick. Over New York, the ice that covered Manhattan was about 2,000 feet high. The ice did not sit still. It flowed, and thick, heavy ice erodes faster than thin ice, so wherever the land already had a low hollow the moving ice scooped it deeper.
2,000 ft
Why there? The ground had already been set up for it. Around 1.1 to 1.2 billion years ago, two previously fused tectonic plates split apart and created a that left a valley, which eventually became the basin of Lake Superior, and a second rift about 570 million years ago laid the groundwork for Lakes Ontario and Erie. The glaciers did not start from nothing. They took existing low ground and made it deeper, wider and more rounded.
When the ice withdrew, the basins were left empty, and from the retreating sheet poured in. Early versions of the lakes were different from today's: one of them, Lake Iroquois, stretched well beyond the shores of present-day Lake Ontario and drained down the Hudson River into the Atlantic. Then the land itself moved. Freed from the weight of the ice, the ground slowly bounced back up.
The result is the largest group of freshwater lakes on Earth by area, holding more than 20 percent of the world's surface fresh water by volume. Paleo-Indian peoples were living around their shores by about 10,000 BC, soon after the ice left.
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Recap
A moving ice sheet scooped and deepened old low ground; when it melted, meltwater filled the basins and the land bounced back.
💡 A trick to remember it · Ice digs the bowl, melt fills the bowl, and the land lifts its head when the weight is gone.
Surprising fact · Land that carried the ice rose back up by more than 150 feet near New York once the weight was gone.
Sources (3)
No source, no claim. Every fact in this lesson (15 claims) cites at least one of these.