About 15,000 years ago, as the latest ice age was winding to a close, glaciers that covered Canada began to melt. Then, one day, those shrinking glaciers unleashed the waters of a lake the size of Lake Huron, which scoured the land from Montana to the Pacific coast. Here's what we know of the great Ice Age flood.

Over at Ars Technica, Scott K. Johnson has an incredible, in-depth article about his trip to visit the site of this flood, known as the Scablands. Thousands of years ago, a wall of water bashed its way through mountain ridges, turned parts of Idaho into giant, temporary lakes, and left hills of shattered rock debris behind. Unlike other historical floods of similar intensity, this one passed through canyons lined with volcanic basalt rock that had cooled into geometric columns.

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Photo of the basalt columns, by Scott K. Johnson

As it smashed through, the water tore these huge columns from cliff faces and propelled them along with the flood. As a result, we're left with an eerie geological record of the water's route, paved with smashed rock columns.

As Johnson retraces the route of the flood, he tells us about the controversial history of the flood's discovery by rogue geologist J. Harlen Bretz — and shows us why floods are among the world's most terrifying disasters.

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His story begins:

Traveling from the verdant, mossy coastal belt of the Pacific Northwest, one could be forgiven for feeling that the defining characteristic of Eastern Washington is its dryness. It's a land seemingly starved of rain in the shadow of the Cascade Mountains. But the dry landscape known as the "Scablands" actually tells a story about excess—excess of water, water that was torrential and sudden.

The Scablands are essentially wounds, still unhealed by time and erosion. They cut through the land and down into the rock after a series of unfathomably large floods unleashed by the catastrophic draining of great glacial lakes—half the volume of Lake Michigan splashed onto the land in less than a week. If you can imagine that, you've got us beat. The story recorded in this landscape is so incredible, it took one geologist decades to convince his colleagues that he was reading it correctly ...

By chance, I began my trip in the very place that captured the interest of a young J Harlen Bretz when he got his hands on that new USGS map—Potholes Coulee. I was welcomed to the Scablands almost immediately by a rattlesnake eager to make sure I knew which spot of shade it was currently enjoying—a reminder that, despite the geologic feast for the eyes around me, I should also watch my step.

I walked toward the Columbia River, flanked by columns of basalt, occasionally dropping over considerable ledges that were once waterfalls. Fed by runoff from agricultural irrigation, some hosted trickles of water once again.

Like most substances, lava contracts as it cools. A flow of lava that is many miles wide can shrink a bit vertically without complications, but contracting horizontally is another story. So as the rock cools, it breaks into roughly hexagonal columns (plus or minus a side), as at famous locations like Devil's Tower or the Giant's Causeway. Not only are these columns visually appealing, they're key to the dramatic landscape of the Scablands.

Streams carve V-shaped valleys through hills, with a thin ribbon of flowing water cutting downward like a knife. As the walls tumble into the notch of the stream channel, the valley widens but maintains that "V" shape. When it reaches flatlands, the stream slows and meanders, carving a wide, gentle flood plain as it wiggles back and forth over time.

But these coulees aren't like either of those; they're sudden and sharply defined, with flat bottoms and vertical cliffs for sides. No stream winds down their length, and the floor is a jagged mess. They look more like scars left by the talons of a world-eating dragon than the work of some small, if constant, stream.

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You must read the entire story over at Ars Technica

Also, this video may help you visualize just how the flood started, and how it roared through three states — Montana, Idaho, and Washington — before reaching the ocean.

And

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