North America’s Great Lakes are great indeed.

They contain 20 percent of the planet’s surface freshwater—enough to cover the continental U.S. 10 feet deep.

They formed at the end of the last Ice Age, when Canada was under an ice sheet 3,000 to 9,000 feet thick.

This massively heavy continental glacier had flowed southward, depressing and carving the Earth as it passed over, pushing ahead of it huge piles of eroded rock and gravel called glacial moraines.

20,000 years ago, the ice sheet finally began to melt. As the glacier receded northward, floods of meltwater filled the deep depressions it had carved and were trapped in place by the banks of moraines it left behind.

Over centuries, this formed the Great Lakes.

But the areas the glacier eroded, and therefore the shape of the lakes, were determined by geology long before that.

Lake Superior began life a billion years ago as a huge rift, a crack in the continental crust. Over millions of years it filled with sediment, which was soft and easy for the glacier to eventually scour away.

Lakes Michigan and Huron were likewise carved out of softer sedimentary rock that surrounds the harder rock that makes up the state of Michigan.

Lakes Erie and Ontario are the shallowest and smallest, both carved into weaker shales and connected by Niagara Falls, which flows over a harder dolomite shelf.

The Great Lakes are truly one of America’s great places, in natural beauty and history.