字幕表 動画を再生する 英語字幕をプリント Thanks to CuriosityStream for supporting PBS Digital Studios. 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 lines … just 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 non¬fiction titles from a variety of filmmakers, including CuriosityStream originals. For example, you can learn about a modern day struggle with massive floods in “Breakthrough: 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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