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  • In the vast, arid landscape of Eastern Washington lie the traces of an ancient disaster.

  • Outside the city of Spokane, massive scour marks run through the rocky ground, creating

  • a strange terrain known as the scablands.

  • A bit to the west, a channel has been carved into the Earth that's as deep as a forty-story

  • building.

  • Elsewhere, miles of rolling hills run across Washington, Montana, and Idaho, resembling

  • enormous ripples up to 15 meters high.

  • These features are all the lingering remains of an epic geological mystery that took nearly

  • half a century to solve.

  • Now, every great mystery requires a great detective, and geologist J Harlen Bretz was

  • a great detective indeed.

  • He researched these strange features in the early 1900's and soon concluded that features

  • like these could only have been made by water.

  • A lot of it.

  • Running fast.

  • But he also knew that a flow of water that could transform the land so drastically would

  • had to have been unimaginably huge.

  • It must've been a flood, of almost biblical proportions.

  • When Bretz presented this hypothesis to fellow geologists in 1927, he was met with ... skepticism,

  • to put it lightly.

  • But ultimately, his research would reveal one of the most powerful and bizarre episodes

  • in recent geologic history.

  • And as a result, it would revolutionize the way geologists understand the world today.

  • Because, Bretz was right: This landscape was the result of flooding.

  • But not just a single flood.

  • Instead it was dozens of major, devastating floods that took place over the course of

  • more than 7,000 years, forever changing the landscape of the Pacific Northwest.

  • What Bretz had discovered was evidence of floods that can only be described in one word:

  • catastrophic.

  • When Bretz first began studying the weird landscape of the Northwest in the 1920s, there

  • was a certain school of thought that most geologists followed.

  • It was known as uniformitarianism, the idea that the present is the key to understanding

  • the past.

  • In this view, all rocks, landforms, and other geological features can only have been created

  • by processes that we can observe today.

  • And except for the occasional volcanic eruption, or river overflowing its banks, all modern

  • processes are gradual, like erosion.

  • So to these geologists, the scablands of Washington could only have been created by glaciers,

  • and the ripples must be deposits of what the glaciers had slowly scraped away.

  • Because, the effects of glaciers had been observed around the world.

  • And through the lens of uniformitarianism, they seemed to most closely resemble the features

  • that Bretz was studying.

  • But Bretz had studied glacial geology, too, and he knew what glaciers could do.

  • And to him, the features he saw just didn't fit.

  • Instead, they looked like scaled-up versions of what happens after a big flood.

  • For Bretz, the most obvious evidence of flooding was the shape of the canyons in the Scablands

  • and other places.

  • These canyons, also called coulees, have flat bottoms and steep, vertical walls - very different

  • from the U shape of valleys that are carved by glaciers, or the V-shaped valleys made

  • by rivers.

  • One especially large coulee, called Dry Falls, appeared to have formed a massive waterfall

  • over 100 meters tall and 3 and a half kilometers wide; that's twice as tall, and five times

  • wider, than Niagara falls!

  • But water doesn't just remove things; it also deposits things.

  • And Bretz saw that the landscape was littered with boulders weighing up to 200 tons, having

  • tumbled miles away from their sources, like pebbles on a beach.

  • He also noted massive ripples in the earth, and gravel bars up to 90 meters high, all

  • typical of deposits made by powerfully flowing water.

  • Finally, Bretz knew that these features couldn't be related to glaciers, because of what was

  • missing: the huge ridges of deposited sand and gravel called moraines, which form

  • around advancing glaciers.

  • Only one tiny moraine was found in the scablands, not nearly enough evidence for the giant glaciers

  • that would have been required to carve features this big.

  • But despite all of this evidence, other scientists weren't convinced that this strange landscape

  • was shaped by an epic flood.

  • They argued that humans had never seen a flood anywhere near as big as the one Bretz proposed,

  • so they were reluctant to believe that such a thing was even possible.

  • Uniformitarianism explained a great deal about geology, and epic floods just didn't fit into

  • it.

  • What giant floods did fit into was the geological mindset that Uniformitarianism had replaced:

  • An older school of thought known as catastrophism.

  • Catastrophism was an idea put forward in the early 1800s by French scientist Georges Cuvier.

  • This theory explained all geologic formations as evidence of large, sudden, unpredictable

  • events -- often events that were referred to in the bible -- like celestial impacts,

  • enormous volcanic eruptions, and ... massive floods.

  • So no matter how good his evidence was, Bretz's hypothesis seemed extremely outdated.

  • And there was still one really big question that Bretz couldn't answer.

  • If all this flooding really happened, then where'd the water come from?

  • And this was something that puzzled Bretz himself.

  • He initially thought that the water had come from some melting glacier.

  • But he couldn't explain how the glacier had melted fast enough to produce so much

  • water all at once.

  • It turns out, Bretz was looking in the wrong place.

  • But someone else knew where the water came from.

  • This half of the mystery was solved by Joseph T. Pardee, a geologist with the U.S. Geological

  • Survey.

  • Pardee had attended a conference where Bretz presented his hypothesis about the Ice Age

  • megaflood, and watched as Bretz defended his claim against a room full of skeptics.

  • And more than 10 years earlier, Pardee had been working in Western Montana, where I am

  • now!, and where he'd found evidence of an enormous, Ice Age lake that had since disappeared.

  • His main piece of evidence?

  • Distinctive lines he saw high on the hillsides.

  • These lines form small benches, much like the shorelines of a reservoir.

  • So Pardee figured that these ancient shorelines were formed by an ancient lake whose source

  • was the Clark Fork River, which still flows today through the valley below.

  • This giant lake came to be known as Glacial Lake Missoula, named after the town -- which is also my

  • hometown! -- where Pardee saw those lines.

  • But a reservoir requires a dam, and a lake this size would've needed a big one.

  • So what had dammed the river to form the lake, and what happened to the dam?

  • To find out, Pardee followed Lake Missoula's shorelines for miles to the west, into the

  • panhandle of Idaho, at which point the linesjust disappeared.

  • But where they ended, he found something else: big, U-shaped valleys and glacial moraines

  • -- both evidence of glaciers in the area.

  • So, the evidence suggested that a glacier had blocked the river to form the lake.

  • Judging by the landforms around it, it must've been about 50 kilometers wide and more 600

  • meters tall.

  • And the reason it didn't exist anymore was just because it was made of ice.

  • So with his missing dam now found, Pardee had a new question to answer: Where'd all

  • the water go?

  • By some accounts, Pardee had already suspected that the scablands that Bretz described were

  • created by the drainage of his lake.

  • But it took more than a decade for Pardee to publish the evidence that linked his lake

  • to Bretz's flood.

  • On a mountain pass in northern Washington, for example, he found massive scour marks.

  • In the river valleys of western Montana, he recorded large bars of debris that had been

  • carried there by currents.

  • And in Montana and Idaho, he studied enormous rippling dunes made of gravel.

  • All of these strange features were consistent with evidence of flooding.

  • And they were all downstream of where the ice dam would have been.

  • So Pardee concluded that, periodically, too much water built up behind the ice dam that

  • held back Glacial Lake Missoula, until it ruptured.

  • After all, ice is less dense than water.

  • So when the water level in Lake Missoula got high enough, it would've caused the dam

  • to float upward.

  • And as the water began to rush out underneath, the enormous pressure would cause the dam

  • to break.

  • Then, by most estimates, about 2500 cubic kilometers of water -- enough to fill half

  • of Lake Michigan -- broke free.

  • The water formed massive waves as it rushed away from Lake Missoula to the west.

  • Along the way, it lifted giant boulders, carved the steep cliffs and rolling hills of Bretz's

  • scablands, and helped shape the vast Columbia River Gorge that today forms the boundary

  • between Washington and Oregon.

  • In 1942, Pardee finally wrote up all of this evidence, detailing what happened to the missing

  • lake, and connecting it to the massive floods that Bretz had postulated.

  • And in the decades after these two intrepid detectives did their work, other

  • geologists used newer techniques to establish that these floods actually happened many,

  • many times.

  • One of the clearest pieces of evidence is in the remains of the bed of Lake Missoula

  • itself.

  • The dark and light bands of sediment on the floor of the lake, known as varves, are like

  • an archive of the years when the lake was full of water.

  • Dark varves correspond to winter deposits, and light ones to summer.

  • But some of these layers are interrupted by beds of gravel -- gravel that was deposited

  • by rapidly moving floodwater.

  • So the number of varves that appear between the layers of gravel tells us that these catastrophic

  • floods happened every 20 to 60 years.

  • And scientists have even been able to track down multiple lines of evidence to estimate

  • when they happened.

  • Over the years, geologists have studied flood deposits in the ocean, where the Columbia

  • River empties into the sea.

  • They've studied the sediments in rocky outcrops, and the chemistry of the giant boulders found

  • along the path of the flood.

  • And together these clues suggest that Glacial Lake Missoula flooded many times within a

  • span of 7,000 years, from around 20,900 to 13,500 years ago.

  • But as freaking massive as the Lake Missoula floods were,

  • they weren't the only megafloods that happened.

  • And they definitely weren't the biggest.

  • For that, let's hear from Stefan Chin at SciShow, where they're talking about the

  • biggest Ice Age flood of them all!

  • There was an even bigger glacial lake in central North America, called Lake Agassiz, and it

  • was at least 8 times the size of Lake Missoula.

  • Because it was so big, its megafloods were even more devastating.

  • The drainage from this lake was so enormous that it disrupted ocean currents.

  • And that in turn may have caused a climate cooling event 13,000 years ago that's at

  • least partly responsible for the extinction of mammoths and other Ice Age megafauna in

  • North America.

  • When you're done here, head on over to SciShow to learn all about Lake Agassiz and how its

  • floods may have changed the climate of the entire planet.

  • The Floods from Lake Missoula didn't change the climate, but they did change the world

  • in other ways.

  • For one thing, they fundamentally changed the landscape of the Pacific Northwest.

  • But perhaps more importantly, they also changed how we understand geology, all over the world,

  • to this day.

  • By 1965, with lots of evidence and finally a source for his floods, Bretz's research

  • was accepted and became part of a new understanding of the processes of the Earth, both big and

  • small.

  • So, today, geologists understand that, although a lot of geology is slow and small, sometimes

  • our world is shaped by huge, catastrophic things.

  • This new framework is sometimes called neo-catastrophism, or modern uniformitarianism.

  • But for me, probably the most fascinating part of all of this is that there may actually

  • have been people around to witness these gigantic floods!

  • The oldest evidence for humans in the Pacific Northwest is about 15,000 years old, old enough

  • that the last of the Missoula floods may have been seen by human eyes.

  • As a person who lives in Montana in the 21st century, all I can do is look up at those

  • lines on the hills around my city, and imagine what it must have felt like to witness one

  • of those floods that changed the world.

  • Thank you to CuriosityStream for supporting PBS Digital Studios

  • CuriosityStream is a subscription streaming service that offers documentaries and

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  • For example, you can learn about a modern day struggle with massive floods

  • inBreakthrough: The FloodGates of Venice

  • the story of how Venice, Italy is trying to keep rising seas at bay with the help of a marvel of engineering.

  • You can learn more at curiositystream.com/Eons

  • Thanks to our friends and colleagues at SciShow who work just down the hall from us.

  • For taking part in this epic ice age collaboration

  • And extra big thanks to our current Eontologists, Jake

  • Hart, Jon Ivy, John Davison Ng and STEVE!

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How 7,000 Years of Epic Floods Changed the World (w/ SciShow!)

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    joey joey に公開 2021 年 05 月 01 日
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