Nova (1974–…): Season 44, Episode 25 - Killer Floods - full transcript
Geologists examine extreme terrain on Iceland, the Scablands in Washington State, and submerged in the English channel to see if they were created by the same process, a sudden titanic flood.
Our planet is capable
of unleashing extreme chaos.
Volcanoes, earthquakes,
hurricanes, and floods
can cause untold devastation.
We may think we've seen
the worst
Mother Nature can throw at us,
but scientists struggling
to understand these disasters
are discovering evidence
that even more extreme events
have struck in the past.
So this is about 13 times
more powerful
than the Pompeii eruption.
They're uncovering clues
that the worst catastrophes
in history
could strike again.
Thousands of years ago,
floods of unimaginable violence.
This water came up 800 feet.
That's huge.
Floods powerful enough
to blast through miles
of solid rock
in just hours.
But how?
Everything in this landscape
was screaming
in terms of its signs or clues
that this was made
by catastrophic flooding.
The clues to some of the biggest
floods ever are here,
carved in mysterious
rock formations,
buried beneath the waves,
or hidden in plain sight,
all around the world.
Now, scientists find new clues
to understand
our volatile Earth...
And unravel the secrets
of "Killer Floods,"
right now, on "NOVA."
Major funding for "NOVA"
is provided by the following:
Floods.
Events of such violence,
they turn oceans, rivers,
and lakes
into devastating walls of water.
On average, around the world,
these powerful surges
kill 25,000 people every year.
In 2004, a deadly tsunami
hits Southeast Asia,
leaving over 200,000 people
dead,
and $10 billion worth of damage
in its wake.
More recently, in 2017,
Hurricane Harvey
slams into Houston.
Heavy rains cause catastrophic
flooding,
killing more than 70,
and leaving tens of thousands
homeless.
That same year,
a third of Bangladesh
is submerged by flooding
which extended
throughout South Asia,
including Nepal, India,
and Pakistan.
The flooding, caused by
an especially strong monsoon,
is thought to be the most severe
in the last hundred years.
But could Mother Nature
have unleashed floods
that were even bigger
and more destructive
in the past?
That's what a series
of discoveries is suggesting.
Scientists are unearthing
what looks like
the scars of cataclysmic floods
that dug deep into the rock,
reshaping the surface
of the Earth itself.
It completely changed the
face of the landscape.
No one has ever witnessed
anything
even close in scale.
Across the world,
three far-flung locations
share an eerie similarity.
In the United States,
16,000 square miles
of dry canyons
and bizarre rock formations
cover the Northwest.
In Iceland,
a 300-foot-deep gorge
appears to have been ripped out
in an instant.
And off the coast of Britain,
a network of mysterious canyons
carved deep into the sea bed
could reveal how this channel
first separated
what is now Britain from France.
Far from eroding gradually,
there's evidence
that vast deluges
tore out these landscapes in
the geological blink of an eye.
But what could have triggered
such killer floods?
And could one strike again?
The trail of clues starts here,
on the plains
of Washington state.
A flat expanse, stretching
for hundreds of miles,
until suddenly, the
landscape changes.
Flat fields give way
to sheer gorges,
some almost a thousand feet
deep.
Rock islands rise to the height
of 30-story buildings,
while in other places,
strange round depressions,
like gargantuan potholes,
plunge 50 feet.
These are the Scablands,
named by settlers who thought
the formations
resembled scabs or wounds
on the rocky terrain.
Located over a hundred miles
east of Seattle,
this mysterious landscape
covers an area
around 16,000 square miles.
For over a century,
geologists have been trying
to understand
what forces created
the Scablands.
When you encounter a landscape,
it's not unlike a detective
encountering a crime scene.
In the case of this landscape,
there are features
that act like clues.
But spotting those clues
takes a trained eye.
You can't really get
a sense of this area
unless you get up high.
This part of the Scablands
is, like, 30, 40 miles across.
It's on a mega scale.
It's the kind of scale that
first led geologists
to suspect that the Scablands
had formed slowly,
eroded over millions of years
by rivers, wind, or ice.
During past ice ages,
as temperatures plummeted,
giant ice sheets and glaciers
carved deep valleys
through solid rock.
Like these, in Glacier
National Park, in Montana.
And rivers,
scouring rock over eons,
helped carve some of the most
dramatic landscapes on Earth,
like the Grand Canyon
in Colorado.
But mapping sediments
left behind by the ice sheet
when it melted 12,000 years ago,
shows the ice only made it
to the northern edge
of the Scablands.
And Vic Baker's bird's eye view
reveals
that fast-flowing water
was the culprit.
The clue?
This curved canyon.
Looking at this from the air,
you can see that the shape
is like a horseshoe,
which is what forms
in waterfalls.
Niagara Falls
and many big waterfalls
have a similar horseshoe shape.
For that reason, this canyon
is called the Dry Falls.
But stretching
three and a half miles,
this formation is five times
the span of Niagara
and twice as tall.
The cliffs behind me
are 400 feet high.
Niagara Falls would fit
just within the alcove here.
There's a similar-sized alcove
and there's an even bigger one
that extends many miles
to the east.
These are the most extensive
falls, wet or dry, known today.
And in a valley below,
Vic finds another clue
that vast amounts of water
once flowed here.
Features that look
like sinkholes, or potholes,
often found along the bottom
of turbulent rivers.
But these potholes
are super-sized.
You can see it's maybe
50 feet deep or so.
Potholes you see
in a normal river
are about the size of a person.
Whereas this would hold
multiple elephants.
To Vic, all the evidence
points to flowing water
on a massive scale.
But there is no water
flowing here now.
Today, the largest rivers
in the region
are the Snake and the Columbia.
But could they
have played a role?
The Columbia River lies
about 30 miles to the north
and isn't big enough
to make this kind of feature.
The Snake River, as well,
is simply too small
to carve out potholes
and waterfalls on this scale.
To get such an immense volume
of water so fast,
we need something spectacular
to happen.
So where did the water
come from?
To answer this question,
scientists are looking
at a distant landscape
3,500 miles away.
Iceland.
This island, on the edge
of the Arctic Circle,
is a land of fire and ice.
In its northeast corner
are scars on the landscape
that bear a striking resemblance
to the Washington Scablands.
Sheer cliffs over 300 feet high.
A towering rock island.
This is the AÁsbyrgi Canyon.
And like scientists
in the Scablands,
geomorphologist Mikael Attal
wants to understand
how it was created.
This really looks
like a dry waterfall.
It's as if
there was a big waterfall here
and it's not there anymore.
Its so similar to the Dry Falls
in the Scablands,
Mikael suspects they were formed
in the same way.
But how long did it take?
To find out, he's using
a relatively new technique
called surface-exposure dating.
Earth's surface
is constantly bombarded
by cosmic rays from outer space.
Rocks buried in the Earth
are sheltered from these rays,
but as soon as the rocks are
exposed, like in these cliffs,
cosmic rays collide with atoms
at their surface.
The force of these collisions
knocks neutrons and protons
out of the atoms
and changes the elements
in the rocks.
This leads to the formation
of new elements,
including a rare form of helium.
These rare helium atoms
build up over time,
at a predictable rate,
so by measuring
their concentration,
it's possible to determine
how long
the rock has been exposed.
It's like starting
the stopwatch.
Mikael samples rocks
from all over the AÁsbyrgi Canyon
and compares the dates
when they were exposed.
His results revealed
something surprising.
All the rocks in this area
were exposed at the same time,
meaning that this entire canyon
was carved out all at once.
This canyon was created
in one event
9,000 years ago.
A slow-moving force,
like a glacier, erosion,
or gradual uplift,
would have exposed the rocks
along the canyon
at different times.
So it had to be a fast-paced
natural disaster,
like a titanic flood.
It would have been a flood
on a scale
far greater than anything
that we have witnessed
in human history.
Thousands of miles away,
scientists in the Scablands
had zeroed in on the same idea.
Everything in this landscape
was screaming,
in terms of its signs or clues,
that this was made
by catastrophic flooding.
A flood big enough to carve
these vast landscapes
seems impossible,
but flowing water can be
surprisingly powerful.
The physical impact of a flood
rises with every increase
in volume, speed, or duration,
and it doesn't take a lot
to pack a punch.
A flood just six inches deep
can knock people
right off their feet,
and a flow of just
seven miles an hour
can have the same force
as a tornado.
But is it possible to blast
through solid rock?
Could a flood carve out
enormous features like these?
The most destructive floods,
from Hurricane Katrina in 2005
to floods in Colorado in 2013,
have etched painful memories.
But they have done little
to make a mark
on the underlying bedrock...
Like we see here in Iceland
and the Scablands.
No one has ever witnessed
anything
even close in scale.
We're talking about floods here
that completely changed the face
of the planet.
Floods powerful enough
to carve whole canyons
out of bedrock
are rarely seen.
But in 2002, one was finally
caught on camera.
On July 4,
after a severe storm
struck central Texas,
Canyon Lake Reservoir flooded,
overtopping its dam.
At its peak,
enough water to fill
an Olympic swimming pool
poured over every two seconds.
And when the floodwaters
subsided,
they revealed a brand-new gorge,
carved into the rock
23 feet deep
and more than a mile long.
This provided proof that floods
can transform whole landscapes
in a matter of days,
as long as there's enough water
flowing quickly enough
to produce the necessary force.
And in Iceland,
geophysicist Magnús Gudmundsson
thinks he's figured out
how such a massive release
of water could occur.
He's come a hundred miles
south of AÁsbyrgi
to the Vatnajoökull ice cap,
the largest glacier in Europe,
similar to glaciers
at the end of the last ice age,
9,000 years ago.
This glacier is the only
possible source of water
to create these floods
that made AÁsbyrgi.
In places, it's 3,000 feet deep,
hundreds of cubic miles of water
locked up as ice.
We have all this ice here,
but how does it become a flood?
Magnús believes the secret lies
in what's hidden
beneath the ice cap:
seven huge volcanoes.
In 1996, one of these,
the volcano called Grímsvoötn,
erupted, triggering the most
catastrophic flood in Iceland
for nearly a century.
From time to time, we have
these very large eruptions
that melt enormous amounts
of ice in a matter of hours.
Almost a cubic mile
of meltwater from the eruption
tore across the landscape
at 16 times the rate
of Niagara Falls,
destroying roads, bridges,
and power lines.
So today, Magnús is monitoring
the volcano.
By recording elevation
and movement,
his team has discovered
there is a lake of meltwater
beneath the ice.
You're actually standing on an
ice shelf floating on the lake.
Magnús now believes
that 9,000 years ago...
...a giant eruption
under the ice cap
unleashed a colossal flood
powerful enough
to carve the AÁsbyrgi Canyon.
So could a chain of events
like this have also triggered
a flood massive enough to carve
the Scablands in North America?
Just a hundred miles
west of the Scablands
are many active volcanoes,
notably Mount St. Helens,
partially covered in ice.
In 1980,
Mount St. Helens erupted,
releasing an enormous amount
of heat,
enough to melt the ice
around its crater
and trigger dramatic floods.
But even if all the ice on this
volcano had suddenly melted...
It would not be enough
to carve out rock
over a 16,000-square-mile area.
But what about in the past?
To find out if ancient ice
were to blame,
Vic Baker needs to find out
when the landscape was created
with the technique
of surface-exposure dating
used in Iceland.
Taking samples from all over
the Scablands,
he discovers most of the rocks
were exposed within a few
thousand years of each other,
and one date in particular
stands out.
Many of the dates we get
are in the range
of about 16,000 years ago.
Although ice sheets covered
much of North America
16,000 years ago,
geologists believe
the ice stopped short
of the volcanoes
of Washington state.
The trail of clues
seemed to dry up...
...until a surprising discovery
200 miles east of the Scablands,
in Missoula, Montana.
Here, geologist Larry Smith
is heading up into the hills,
where he sees a series
of horizontal lines
a thousand feet
above the valley floor.
They look very much
like they'd have been cut
into the hillside
by waves beating
against the rock.
These lines are clearly
lake shorelines,
and show that an immense
body of water temporarily filled
these now-dry valleys
of western Montana.
Tracing these ancient shorelines
for hundreds of miles,
geologists have calculated that
these valleys were once filled
by a body of water
larger than Lake Ontario.
When there was a lake here
at 4,200 feet,
we would have had a beach
right here in front of us,
or a shoreline,
and extending all the way across
to the other side of the valley,
with a thousand feet of water
over what is now
the city of Missoula.
Geologists call it
Glacial Lake Missoula.
And despite being 200 miles away
from the Scablands,
Larry suspects it held
enough water
to tear through the area.
That is a vast amount of water,
and if this lake drained
very rapidly,
it would be fundamental
to carving the channel
in Scabland.
But today, there is no lake here
because the valley
is open-ended.
So where are the formations
that held the water in place?
So the question is, is
where, when, and how did
a dam form to create this lake?
Searching for clues,
Larry travels back down
to Clark Fork,
at the narrow end of the valley.
He sees no signs of landslides
or rock falls
that could have dammed the lake
in the past.
But on the bare rocks, he spots
some tell-tale markings.
You see scratches
within the rock,
geologically it is impossible
to smooth off rock and
scratch it without glacial ice.
As the glacier moves,
rocks embedded within it
scratch the bedrock
like sandpaper.
So these scratches are evidence
that during the last ice age,
a glacier moved
across this valley.
And by mapping where rocks
have been scratched,
geologists have discovered
that the Clark Fork River Valley
was once blocked
by a giant finger of ice
23 miles wide
and half a mile deep.
Larry Smith believes this
ice dam created Lake Missoula.
It blocked the drainage
of the Clark Fork River.
The water had nowhere else
to go,
so backed up a lake behind this
large glacier in this valley.
All the evidence points
to a massive reservoir of water
held in place
by a giant dam of ice,
a lake large enough
to have carved out the canyons
of the Scablands...
...if it were released
in one catastrophic event.
The idea that there
had once been a lake here
that had suddenly drained
also explains one of the other
striking features
of the valley floor...
Giant ripples.
These straight crested hills
are current ripples
that show water flowing
from where we're standing
off to the distance.
Ripples like this
are made by flowing water,
like the tide moving in and out
on a beach.
The faster the flow of water,
the larger
and more widely spaced
the ripples become.
Here, they're giant things
that are spaced
hundreds of feet apart
and they're tens of feet high.
These ripples are so high,
the lake water that created them
must have poured
through this valley
at speeds of up to 80 miles
an hour.
It's evidence that Lake Missoula
was unleashed rapidly
in a massive flood.
But that means the 23-mile-wide
ice dam holding it in place
must have suddenly given way.
How could an ice dam
of this scale
fail so catastrophically?
The exposure dates of the rocks
in the Scablands
reveal the flood occurred well
before the end of the Ice Age.
This rules out gradual melting
from a warming climate.
Could looking at modern
dam failures hold a clue?
You people down the stream
better get out.
In 1976,
the newly constructed
300-foot-high Teton Dam,
in Idaho, failed,
unleashing almost
80 billion gallons of water.
Investigators discovered
that water had seeped
under the earth-filled dam,
eroding it from below.
Larry Smith believes water
seeping under the ice dam
also caused the catastrophic
release of Lake Missoula.
At the bottom
of this 2,000-foot-deep lake,
the water pressures are immense,
and any small cracks in the ice
will get penetrated
by that high-pressure water.
In doing so, that'll expand
that crack network
to form tunnels under the ice.
Lake water began draining
through these tunnels
at a faster and faster rate...
...until the whole ice dam
suddenly collapsed.
It falls within minutes
to hours,
with a cascade of water
coming through the area.
All signs point to Lake Missoula
being the source
of a catastrophic flood.
Still, how likely is it
that floodwater could travel
hundreds of miles southwest
to the Scablands
with enough power to carve out
solid rock
and transform
the entire landscape?
Roger Denlinger studies
fluid dynamics.
He's taken the volume
of ancient Lake Missoula
and 3D maps of the Scablands
to build a computer model
that will predict
where the ancient flood
would have traveled.
Effectively, you're just pouring
water over the landscape.
This is simply water flowing
over the Earth's surface,
and it's going to always head
in the direction
that it sees as downhill.
Roger's model
will also determine
the depth of the water.
And this color bar shows
the flood's erosive power.
If flow lines in the model
turn red,
Roger knows the water
was flowing with enough force
to carve solid rock.
At this point,
we're going to break the dam.
The moment it's released
from the ice dam,
the lake water rushes southwest,
toward what we know today
as the eroded landscape
of the Scablands.
And not only that,
the places that the model
has highlighted in red,
where the power of the flood
is greatest,
exactly match the location
of the most dramatically
transformed landscapes today:
The Dry Falls;
The rock islands,
And the sheer gorges.
We get damage to the surface
in exactly the areas
that we see today.
Roger's model
supports the theory
that a giant flood
from Lake Missoula
carved this landscape,
and reveals that the waters
reached unimaginable heights.
This water came up 800 feet...
That's huge.
Most people think of floods
by watching the TV
and they see the water rising
in a river,
and they see a house
going underwater,
maybe there's a person
on top of the house.
Think of water hundreds of feet
above the house,
that's the difference
in the scale of this flooding.
Bringing all the evidence
together,
scientists can now unpack
the catastrophic flood
blow by blow.
Around 16,000 years ago,
the vast ice dam holding back
Lake Missoula failed,
suddenly unleashing
500 cubic miles of water.
It was equivalent in volume
to ten times all the rivers
of the world's natural flow.
The raging torrent
tears across Washington state,
ripping out billions
of tons of rock
from the once-flat landscape.
There would be blocks of ice,
there would be boulders,
there'd be roiling water,
the sound would be overwhelming.
In a matter of hours,
the flood reaches
the Pacific Ocean,
carrying with it 1,200
cubic miles of rock and earth,
violently torn
from the Scablands.
To do all this landscape change
within a few days to a few weeks
is just mind-expanding.
Even Hollywood disaster movies
do not compare
to what would have happened
as this flood came across
the landscape.
This dramatic event entirely
reshaped the landscape.
But there is one final twist
in the tale.
Further research,
based on core samples
drilled out from the floor
of the Pacific Ocean,
suggests that the Scablands
are a product not of one,
but of many floods.
The evidence reveals
that during the Ice Age,
beginning around
20,000 years ago,
repeated floods
tore across the landscape...
...as the giant ice dam
repeatedly broke,
reformed, and then broke again.
The circumstances that created
this immense volume of water
produced multiple floods.
Decades of geological
detective work
show that the scarred
and eroded landscapes
of Washington state,
as well as Iceland,
both bear the fingerprints
of mega-floods.
And now, this discovery
is helping scientists unravel
a mystery in another part
of the world.
Thousands of miles away
is the channel that separates
what is now England from France.
Today, it links the North Sea
in the east
to the Atlantic Ocean
in the west.
Called the English Channel,
it's the busiest shipping lane
in the world.
And towering more than
350 feet above it,
on the south coast of England,
are the White Cliffs of Dover.
Geologists, like James Lawrence,
now think
these iconic chalk cliffs
hold an extraordinary secret,
and he's going over the edge
to hunt for the evidence.
Because these cliffs
look almost identical to cliffs
on the other side
of the channel,
on the northern coast of France.
People don't realize
that if I was to go over
to France,
we could find similar
chalk cliffs.
This chalk formed
100 million years ago,
when this whole area
was covered by a tropical sea.
The ancient sea teemed
with microscopic organisms.
When they died,
their calcium-rich skeletons
fell to the sea bed.
Over time,
these built up in thick layers
and were compressed into chalk,
a kind of limestone.
We are getting
exactly the same rocks
which have been deposited
in exactly the same environment
on this side of the channel,
and on the French side
of the channel.
And James is discovering
that the connection between
the cliffs in France and England
goes beyond the chalk itself.
Embedded in the white chalk
are a series of horizontal bands
of a dark rock called flint.
Here I have a fantastic band
of flint.
Flint, a form of the
mineral quartz,
is formed by changes
in ocean chemistry.
But these changes occur
only occasionally,
resulting in these
distinctive dark bands.
These flint bands
are continuous
throughout the chalk.
This band of flint
runs through the entire cliff,
and there are dozens
running horizontally...
...each one at a different level
in the chalk.
Taken together, these parallel
bands of dark flint
form a unique geological
fingerprint in the white cliff.
What's extraordinary is that
the same geological fingerprint
is visible on the other side
of the channel.
So the chalk and the flint
in these cliffs forms a bar code
and is exactly the same
as the chalk and the flint
in the cliffs in France.
The spacing and levels of the
flint layers perfectly align.
To James Lawrence, this raises
an extraordinary possibility.
So what we know from this
evidence is that a chalk ridge
once connected England
and France.
These flint layers tell us
that hundreds of thousands
of years ago,
a ridge of chalk
almost seven miles wide
once extended 21 miles
across the channel,
joining what is now Britain
to the European continent.
So it's quite incredible
to think that there
would have been a land mass
stretching across the sea.
But this discovery
raises a brand-new mystery.
Somehow, the cliffs
between England and France
have been separated over time.
If Britain and France
were once joined,
what force separated them and
turned Britain into an island?
Control, Maverick.
While exploring the sea bed
of the English Channel,
geologist Jenny Collier
finds a telling clue.
Four, five, six... Ten
meters in a split second.
We've got a really steep
drop-off in the topography
and it's the edge
of a really unusual landform.
Using sonar to measure
the depth of the channel,
Jenny is surprised to find what
appears to be a steep canyon
carved into solid bedrock.
Sonar works by firing
sound waves at the sea bed.
The deeper the water,
the longer it takes the sound
to make the round trip.
Jenny expected the channel floor
to be flat,
but the sonar has revealed
something far more dramatic.
We've discovered
just an extraordinary
geological event,
right in the middle
of the straits.
To learn more about
this major geological find,
she and her colleagues
took on a massive task.
Using a more advanced
sonar system,
they are mapping
53 square miles of the channel,
to an accuracy of four inches.
What this reveals
is a strange picture
of channels, rock islands,
and valleys,
carved nearly 300 feet down,
into the rock of the sea bed.
I mean, we haven't got anything
like this in Europe.
There's really only one place
that has all of these features.
Without the water, the landscape
beneath the English Channel
appears to have steep valleys
and islands carved into it.
It looks eerily similar
to the channeled Scablands
of Washington state.
But was this underwater
landscape
also created by a mega-flood?
The only way you can dig out
islands into solid bedrock
is to have extreme water flows,
and that basically pointed us
towards,
this was yet another
catastrophic flood terrain.
What the Scablands revealed is
that carving solid rock
requires a huge reservoir
of water to be trapped,
then released in a single
cataclysmic event.
But today, the English Channel
flows between two open seas.
So how could a large enough
volume of water
have built up
to cause a mega-flood?
Geologist Phil Gibbard
believes he has an answer.
And the evidence lies 120 miles
north of the English Channel,
on the coast of the North Sea,
at the bottom of these cliffs.
What we've got here
is a glacial deposit
which is from about
450,000 years ago.
Deep, fine-grained deposits
like this
were laid down
across Northern Europe
as giant ice sheets
ground over rocks.
450,000 years ago, England
was in the grip of an ice age.
Ice sheets, hundreds of miles
across and a mile high,
reached down from Scandinavia.
They would have dammed the
northern edge of the North Sea.
To the south,
the intact ridge of chalk
between what is now
France and England
formed a natural dam.
Phil believes that meltwater
from the ice sheets and rivers
pouring into the North Sea
had nowhere to go.
A vast amount of water built up
behind the chalk ridge.
He sees the evidence
for this ice age reservoir
in the sea cliffs,
as thin horizontal layers
of silt.
The sediments are horizontal,
as you see.
That horizontality
can only be produced
in a lake situation,
a standing-water situation.
And not in a turbulent area,
like an ocean.
Phil has discovered
similar-looking formations
in other places
around the North Sea,
some a hundred feet
above sea level today.
So this was a massive lake
on the scale
of the Great Lakes
in North America,
and this lake provides
the only possible source
for the mega-flood
that formed the Dover Straits.
Could this enormous
reservoir, a glacial lake,
have suddenly drained
to form the dramatic features
on the bed
of the English Channel?
And if so, how?
In order to carve
these features,
this rock ridge must have failed
very, very rapidly.
But what could have caused this?
How could the giant ridge
of solid rock
between France and Britain
have given way
so catastrophically?
A clue lies in the way chalk
reacts to water.
Having a glacial lake
in contact with a chalk ridge
would have saturated the chalk,
making it much weaker
and much more likely to fail.
When water soaks into chalk
and saturates it,
the chalk can lose
half its strength,
making it far more likely
to fail.
One of the problems
with the chalk being so weak
is that it will often lead
to cliff collapses,
like the one we can see
behind us.
Every year, thousands of tons
of rain and wave-soaked chalk
collapse into the channel,
dramatically eroding
the coastline.
Many geologists now believe
that during a previous ice age
almost a half million years ago,
water from the North Sea
reservoir
soaked the chalk ridge,
fatally weakening it.
Once the lake was deep enough,
water began pouring over the top
of the ridge in a waterfall,
rapidly eroding
the waterlogged chalk.
We'd have had initially
a small stream of water
coming over
the top of the rock ridge,
that would have
catastrophically crumbled,
with large amounts of rock
being removed
and more and more water
flooding through,
just running away with itself.
From the shape of the features
on the sonar,
Jenny estimates that
the floodwaters raced through
at a rate of about
264 million gallons a second.
That's almost 60 times the flow
rate of the Mississippi River.
You would have seen
a tidal wave overtopping
and washing a giant gorge
into that landscape.
The deluge crashed on,
breaking through the chalk ridge
linking today's
Britain and France,
before finally reaching
the Atlantic Ocean.
It was this cataclysmic flow
that created the English Channel
and began the process of erosion
that led to what's now Britain
becoming an island
for the first time.
The clues in Iceland,
the English Channel,
and the Channeled Scablands
of Washington state
reveal that floods bigger
and more devastating
than anything we see today
have torn across and
helped shape Earth's surface.
These giant mega-floods
totally shaped a landscape
in a matter of days or weeks.
But the question is:
could a flood on this scale
happen again?
The one thing all these
mega-floods have in common
is that they involve
huge volumes of ice melting
and being released
in one sudden burst.
In Iceland,
a volcano beneath the ice sheet
could trigger a mega-flood
at any moment.
Fortunately, very few people
live in the Icelandic
flood zone,
and the huge volume of ice
needed to create glacial lakes,
on the scale of the ones
that carved the English Channel
and the Scablands,
can only build up
during ice ages.
But there is one region
on Earth today
where stores of melting ice
still pose a major flood risk
to millions:
ice- and snow-covered mountains.
Wherever you have glaciers,
you have a lot of water.
Wherever you have glaciers
in a mountain,
you have the high likelihood of
making a glacially dammed lake,
and those glacially dammed lakes
are unstable
and could drain
catastrophically.
We're not going to get, today,
releases of water
like Lake Missoula,
that was 2,000 feet deep.
But we can get glacial lakes
that are a hundred feet deep,
and these will produce
really dangerous
and spectacular floods.
Today, floods in populated areas
wreak untold devastation.
Faced with Hurricane Harvey's
impact on Houston
and the floods in Bangladesh,
it may seem that floods
could not get any worse.
But the vast floods of the past
carved huge features
into the very bedrock
of continents.
Those scars are a stark reminder
of just how destructive
floods can be.
Look at that guy.
Armed and dangerous.
There's an animal
with an incredible weapon!
And fighting for their lives.
Weapons that are used
in fighting
and weapons that are big.
How did they evolve this way?
And why do only some creatures
have them?
Some have these huge,
spectacular weapons
and others have nothing at all.
An arms race in the wild.
- Wow, look at that!
- Out of nowhere.
"Extreme Animal Weapons."
Coming soon on "NOVA."
This "NOVA" program
is available on DVD.
"NOVA" is also available
for download on iTunes.
of unleashing extreme chaos.
Volcanoes, earthquakes,
hurricanes, and floods
can cause untold devastation.
We may think we've seen
the worst
Mother Nature can throw at us,
but scientists struggling
to understand these disasters
are discovering evidence
that even more extreme events
have struck in the past.
So this is about 13 times
more powerful
than the Pompeii eruption.
They're uncovering clues
that the worst catastrophes
in history
could strike again.
Thousands of years ago,
floods of unimaginable violence.
This water came up 800 feet.
That's huge.
Floods powerful enough
to blast through miles
of solid rock
in just hours.
But how?
Everything in this landscape
was screaming
in terms of its signs or clues
that this was made
by catastrophic flooding.
The clues to some of the biggest
floods ever are here,
carved in mysterious
rock formations,
buried beneath the waves,
or hidden in plain sight,
all around the world.
Now, scientists find new clues
to understand
our volatile Earth...
And unravel the secrets
of "Killer Floods,"
right now, on "NOVA."
Major funding for "NOVA"
is provided by the following:
Floods.
Events of such violence,
they turn oceans, rivers,
and lakes
into devastating walls of water.
On average, around the world,
these powerful surges
kill 25,000 people every year.
In 2004, a deadly tsunami
hits Southeast Asia,
leaving over 200,000 people
dead,
and $10 billion worth of damage
in its wake.
More recently, in 2017,
Hurricane Harvey
slams into Houston.
Heavy rains cause catastrophic
flooding,
killing more than 70,
and leaving tens of thousands
homeless.
That same year,
a third of Bangladesh
is submerged by flooding
which extended
throughout South Asia,
including Nepal, India,
and Pakistan.
The flooding, caused by
an especially strong monsoon,
is thought to be the most severe
in the last hundred years.
But could Mother Nature
have unleashed floods
that were even bigger
and more destructive
in the past?
That's what a series
of discoveries is suggesting.
Scientists are unearthing
what looks like
the scars of cataclysmic floods
that dug deep into the rock,
reshaping the surface
of the Earth itself.
It completely changed the
face of the landscape.
No one has ever witnessed
anything
even close in scale.
Across the world,
three far-flung locations
share an eerie similarity.
In the United States,
16,000 square miles
of dry canyons
and bizarre rock formations
cover the Northwest.
In Iceland,
a 300-foot-deep gorge
appears to have been ripped out
in an instant.
And off the coast of Britain,
a network of mysterious canyons
carved deep into the sea bed
could reveal how this channel
first separated
what is now Britain from France.
Far from eroding gradually,
there's evidence
that vast deluges
tore out these landscapes in
the geological blink of an eye.
But what could have triggered
such killer floods?
And could one strike again?
The trail of clues starts here,
on the plains
of Washington state.
A flat expanse, stretching
for hundreds of miles,
until suddenly, the
landscape changes.
Flat fields give way
to sheer gorges,
some almost a thousand feet
deep.
Rock islands rise to the height
of 30-story buildings,
while in other places,
strange round depressions,
like gargantuan potholes,
plunge 50 feet.
These are the Scablands,
named by settlers who thought
the formations
resembled scabs or wounds
on the rocky terrain.
Located over a hundred miles
east of Seattle,
this mysterious landscape
covers an area
around 16,000 square miles.
For over a century,
geologists have been trying
to understand
what forces created
the Scablands.
When you encounter a landscape,
it's not unlike a detective
encountering a crime scene.
In the case of this landscape,
there are features
that act like clues.
But spotting those clues
takes a trained eye.
You can't really get
a sense of this area
unless you get up high.
This part of the Scablands
is, like, 30, 40 miles across.
It's on a mega scale.
It's the kind of scale that
first led geologists
to suspect that the Scablands
had formed slowly,
eroded over millions of years
by rivers, wind, or ice.
During past ice ages,
as temperatures plummeted,
giant ice sheets and glaciers
carved deep valleys
through solid rock.
Like these, in Glacier
National Park, in Montana.
And rivers,
scouring rock over eons,
helped carve some of the most
dramatic landscapes on Earth,
like the Grand Canyon
in Colorado.
But mapping sediments
left behind by the ice sheet
when it melted 12,000 years ago,
shows the ice only made it
to the northern edge
of the Scablands.
And Vic Baker's bird's eye view
reveals
that fast-flowing water
was the culprit.
The clue?
This curved canyon.
Looking at this from the air,
you can see that the shape
is like a horseshoe,
which is what forms
in waterfalls.
Niagara Falls
and many big waterfalls
have a similar horseshoe shape.
For that reason, this canyon
is called the Dry Falls.
But stretching
three and a half miles,
this formation is five times
the span of Niagara
and twice as tall.
The cliffs behind me
are 400 feet high.
Niagara Falls would fit
just within the alcove here.
There's a similar-sized alcove
and there's an even bigger one
that extends many miles
to the east.
These are the most extensive
falls, wet or dry, known today.
And in a valley below,
Vic finds another clue
that vast amounts of water
once flowed here.
Features that look
like sinkholes, or potholes,
often found along the bottom
of turbulent rivers.
But these potholes
are super-sized.
You can see it's maybe
50 feet deep or so.
Potholes you see
in a normal river
are about the size of a person.
Whereas this would hold
multiple elephants.
To Vic, all the evidence
points to flowing water
on a massive scale.
But there is no water
flowing here now.
Today, the largest rivers
in the region
are the Snake and the Columbia.
But could they
have played a role?
The Columbia River lies
about 30 miles to the north
and isn't big enough
to make this kind of feature.
The Snake River, as well,
is simply too small
to carve out potholes
and waterfalls on this scale.
To get such an immense volume
of water so fast,
we need something spectacular
to happen.
So where did the water
come from?
To answer this question,
scientists are looking
at a distant landscape
3,500 miles away.
Iceland.
This island, on the edge
of the Arctic Circle,
is a land of fire and ice.
In its northeast corner
are scars on the landscape
that bear a striking resemblance
to the Washington Scablands.
Sheer cliffs over 300 feet high.
A towering rock island.
This is the AÁsbyrgi Canyon.
And like scientists
in the Scablands,
geomorphologist Mikael Attal
wants to understand
how it was created.
This really looks
like a dry waterfall.
It's as if
there was a big waterfall here
and it's not there anymore.
Its so similar to the Dry Falls
in the Scablands,
Mikael suspects they were formed
in the same way.
But how long did it take?
To find out, he's using
a relatively new technique
called surface-exposure dating.
Earth's surface
is constantly bombarded
by cosmic rays from outer space.
Rocks buried in the Earth
are sheltered from these rays,
but as soon as the rocks are
exposed, like in these cliffs,
cosmic rays collide with atoms
at their surface.
The force of these collisions
knocks neutrons and protons
out of the atoms
and changes the elements
in the rocks.
This leads to the formation
of new elements,
including a rare form of helium.
These rare helium atoms
build up over time,
at a predictable rate,
so by measuring
their concentration,
it's possible to determine
how long
the rock has been exposed.
It's like starting
the stopwatch.
Mikael samples rocks
from all over the AÁsbyrgi Canyon
and compares the dates
when they were exposed.
His results revealed
something surprising.
All the rocks in this area
were exposed at the same time,
meaning that this entire canyon
was carved out all at once.
This canyon was created
in one event
9,000 years ago.
A slow-moving force,
like a glacier, erosion,
or gradual uplift,
would have exposed the rocks
along the canyon
at different times.
So it had to be a fast-paced
natural disaster,
like a titanic flood.
It would have been a flood
on a scale
far greater than anything
that we have witnessed
in human history.
Thousands of miles away,
scientists in the Scablands
had zeroed in on the same idea.
Everything in this landscape
was screaming,
in terms of its signs or clues,
that this was made
by catastrophic flooding.
A flood big enough to carve
these vast landscapes
seems impossible,
but flowing water can be
surprisingly powerful.
The physical impact of a flood
rises with every increase
in volume, speed, or duration,
and it doesn't take a lot
to pack a punch.
A flood just six inches deep
can knock people
right off their feet,
and a flow of just
seven miles an hour
can have the same force
as a tornado.
But is it possible to blast
through solid rock?
Could a flood carve out
enormous features like these?
The most destructive floods,
from Hurricane Katrina in 2005
to floods in Colorado in 2013,
have etched painful memories.
But they have done little
to make a mark
on the underlying bedrock...
Like we see here in Iceland
and the Scablands.
No one has ever witnessed
anything
even close in scale.
We're talking about floods here
that completely changed the face
of the planet.
Floods powerful enough
to carve whole canyons
out of bedrock
are rarely seen.
But in 2002, one was finally
caught on camera.
On July 4,
after a severe storm
struck central Texas,
Canyon Lake Reservoir flooded,
overtopping its dam.
At its peak,
enough water to fill
an Olympic swimming pool
poured over every two seconds.
And when the floodwaters
subsided,
they revealed a brand-new gorge,
carved into the rock
23 feet deep
and more than a mile long.
This provided proof that floods
can transform whole landscapes
in a matter of days,
as long as there's enough water
flowing quickly enough
to produce the necessary force.
And in Iceland,
geophysicist Magnús Gudmundsson
thinks he's figured out
how such a massive release
of water could occur.
He's come a hundred miles
south of AÁsbyrgi
to the Vatnajoökull ice cap,
the largest glacier in Europe,
similar to glaciers
at the end of the last ice age,
9,000 years ago.
This glacier is the only
possible source of water
to create these floods
that made AÁsbyrgi.
In places, it's 3,000 feet deep,
hundreds of cubic miles of water
locked up as ice.
We have all this ice here,
but how does it become a flood?
Magnús believes the secret lies
in what's hidden
beneath the ice cap:
seven huge volcanoes.
In 1996, one of these,
the volcano called Grímsvoötn,
erupted, triggering the most
catastrophic flood in Iceland
for nearly a century.
From time to time, we have
these very large eruptions
that melt enormous amounts
of ice in a matter of hours.
Almost a cubic mile
of meltwater from the eruption
tore across the landscape
at 16 times the rate
of Niagara Falls,
destroying roads, bridges,
and power lines.
So today, Magnús is monitoring
the volcano.
By recording elevation
and movement,
his team has discovered
there is a lake of meltwater
beneath the ice.
You're actually standing on an
ice shelf floating on the lake.
Magnús now believes
that 9,000 years ago...
...a giant eruption
under the ice cap
unleashed a colossal flood
powerful enough
to carve the AÁsbyrgi Canyon.
So could a chain of events
like this have also triggered
a flood massive enough to carve
the Scablands in North America?
Just a hundred miles
west of the Scablands
are many active volcanoes,
notably Mount St. Helens,
partially covered in ice.
In 1980,
Mount St. Helens erupted,
releasing an enormous amount
of heat,
enough to melt the ice
around its crater
and trigger dramatic floods.
But even if all the ice on this
volcano had suddenly melted...
It would not be enough
to carve out rock
over a 16,000-square-mile area.
But what about in the past?
To find out if ancient ice
were to blame,
Vic Baker needs to find out
when the landscape was created
with the technique
of surface-exposure dating
used in Iceland.
Taking samples from all over
the Scablands,
he discovers most of the rocks
were exposed within a few
thousand years of each other,
and one date in particular
stands out.
Many of the dates we get
are in the range
of about 16,000 years ago.
Although ice sheets covered
much of North America
16,000 years ago,
geologists believe
the ice stopped short
of the volcanoes
of Washington state.
The trail of clues
seemed to dry up...
...until a surprising discovery
200 miles east of the Scablands,
in Missoula, Montana.
Here, geologist Larry Smith
is heading up into the hills,
where he sees a series
of horizontal lines
a thousand feet
above the valley floor.
They look very much
like they'd have been cut
into the hillside
by waves beating
against the rock.
These lines are clearly
lake shorelines,
and show that an immense
body of water temporarily filled
these now-dry valleys
of western Montana.
Tracing these ancient shorelines
for hundreds of miles,
geologists have calculated that
these valleys were once filled
by a body of water
larger than Lake Ontario.
When there was a lake here
at 4,200 feet,
we would have had a beach
right here in front of us,
or a shoreline,
and extending all the way across
to the other side of the valley,
with a thousand feet of water
over what is now
the city of Missoula.
Geologists call it
Glacial Lake Missoula.
And despite being 200 miles away
from the Scablands,
Larry suspects it held
enough water
to tear through the area.
That is a vast amount of water,
and if this lake drained
very rapidly,
it would be fundamental
to carving the channel
in Scabland.
But today, there is no lake here
because the valley
is open-ended.
So where are the formations
that held the water in place?
So the question is, is
where, when, and how did
a dam form to create this lake?
Searching for clues,
Larry travels back down
to Clark Fork,
at the narrow end of the valley.
He sees no signs of landslides
or rock falls
that could have dammed the lake
in the past.
But on the bare rocks, he spots
some tell-tale markings.
You see scratches
within the rock,
geologically it is impossible
to smooth off rock and
scratch it without glacial ice.
As the glacier moves,
rocks embedded within it
scratch the bedrock
like sandpaper.
So these scratches are evidence
that during the last ice age,
a glacier moved
across this valley.
And by mapping where rocks
have been scratched,
geologists have discovered
that the Clark Fork River Valley
was once blocked
by a giant finger of ice
23 miles wide
and half a mile deep.
Larry Smith believes this
ice dam created Lake Missoula.
It blocked the drainage
of the Clark Fork River.
The water had nowhere else
to go,
so backed up a lake behind this
large glacier in this valley.
All the evidence points
to a massive reservoir of water
held in place
by a giant dam of ice,
a lake large enough
to have carved out the canyons
of the Scablands...
...if it were released
in one catastrophic event.
The idea that there
had once been a lake here
that had suddenly drained
also explains one of the other
striking features
of the valley floor...
Giant ripples.
These straight crested hills
are current ripples
that show water flowing
from where we're standing
off to the distance.
Ripples like this
are made by flowing water,
like the tide moving in and out
on a beach.
The faster the flow of water,
the larger
and more widely spaced
the ripples become.
Here, they're giant things
that are spaced
hundreds of feet apart
and they're tens of feet high.
These ripples are so high,
the lake water that created them
must have poured
through this valley
at speeds of up to 80 miles
an hour.
It's evidence that Lake Missoula
was unleashed rapidly
in a massive flood.
But that means the 23-mile-wide
ice dam holding it in place
must have suddenly given way.
How could an ice dam
of this scale
fail so catastrophically?
The exposure dates of the rocks
in the Scablands
reveal the flood occurred well
before the end of the Ice Age.
This rules out gradual melting
from a warming climate.
Could looking at modern
dam failures hold a clue?
You people down the stream
better get out.
In 1976,
the newly constructed
300-foot-high Teton Dam,
in Idaho, failed,
unleashing almost
80 billion gallons of water.
Investigators discovered
that water had seeped
under the earth-filled dam,
eroding it from below.
Larry Smith believes water
seeping under the ice dam
also caused the catastrophic
release of Lake Missoula.
At the bottom
of this 2,000-foot-deep lake,
the water pressures are immense,
and any small cracks in the ice
will get penetrated
by that high-pressure water.
In doing so, that'll expand
that crack network
to form tunnels under the ice.
Lake water began draining
through these tunnels
at a faster and faster rate...
...until the whole ice dam
suddenly collapsed.
It falls within minutes
to hours,
with a cascade of water
coming through the area.
All signs point to Lake Missoula
being the source
of a catastrophic flood.
Still, how likely is it
that floodwater could travel
hundreds of miles southwest
to the Scablands
with enough power to carve out
solid rock
and transform
the entire landscape?
Roger Denlinger studies
fluid dynamics.
He's taken the volume
of ancient Lake Missoula
and 3D maps of the Scablands
to build a computer model
that will predict
where the ancient flood
would have traveled.
Effectively, you're just pouring
water over the landscape.
This is simply water flowing
over the Earth's surface,
and it's going to always head
in the direction
that it sees as downhill.
Roger's model
will also determine
the depth of the water.
And this color bar shows
the flood's erosive power.
If flow lines in the model
turn red,
Roger knows the water
was flowing with enough force
to carve solid rock.
At this point,
we're going to break the dam.
The moment it's released
from the ice dam,
the lake water rushes southwest,
toward what we know today
as the eroded landscape
of the Scablands.
And not only that,
the places that the model
has highlighted in red,
where the power of the flood
is greatest,
exactly match the location
of the most dramatically
transformed landscapes today:
The Dry Falls;
The rock islands,
And the sheer gorges.
We get damage to the surface
in exactly the areas
that we see today.
Roger's model
supports the theory
that a giant flood
from Lake Missoula
carved this landscape,
and reveals that the waters
reached unimaginable heights.
This water came up 800 feet...
That's huge.
Most people think of floods
by watching the TV
and they see the water rising
in a river,
and they see a house
going underwater,
maybe there's a person
on top of the house.
Think of water hundreds of feet
above the house,
that's the difference
in the scale of this flooding.
Bringing all the evidence
together,
scientists can now unpack
the catastrophic flood
blow by blow.
Around 16,000 years ago,
the vast ice dam holding back
Lake Missoula failed,
suddenly unleashing
500 cubic miles of water.
It was equivalent in volume
to ten times all the rivers
of the world's natural flow.
The raging torrent
tears across Washington state,
ripping out billions
of tons of rock
from the once-flat landscape.
There would be blocks of ice,
there would be boulders,
there'd be roiling water,
the sound would be overwhelming.
In a matter of hours,
the flood reaches
the Pacific Ocean,
carrying with it 1,200
cubic miles of rock and earth,
violently torn
from the Scablands.
To do all this landscape change
within a few days to a few weeks
is just mind-expanding.
Even Hollywood disaster movies
do not compare
to what would have happened
as this flood came across
the landscape.
This dramatic event entirely
reshaped the landscape.
But there is one final twist
in the tale.
Further research,
based on core samples
drilled out from the floor
of the Pacific Ocean,
suggests that the Scablands
are a product not of one,
but of many floods.
The evidence reveals
that during the Ice Age,
beginning around
20,000 years ago,
repeated floods
tore across the landscape...
...as the giant ice dam
repeatedly broke,
reformed, and then broke again.
The circumstances that created
this immense volume of water
produced multiple floods.
Decades of geological
detective work
show that the scarred
and eroded landscapes
of Washington state,
as well as Iceland,
both bear the fingerprints
of mega-floods.
And now, this discovery
is helping scientists unravel
a mystery in another part
of the world.
Thousands of miles away
is the channel that separates
what is now England from France.
Today, it links the North Sea
in the east
to the Atlantic Ocean
in the west.
Called the English Channel,
it's the busiest shipping lane
in the world.
And towering more than
350 feet above it,
on the south coast of England,
are the White Cliffs of Dover.
Geologists, like James Lawrence,
now think
these iconic chalk cliffs
hold an extraordinary secret,
and he's going over the edge
to hunt for the evidence.
Because these cliffs
look almost identical to cliffs
on the other side
of the channel,
on the northern coast of France.
People don't realize
that if I was to go over
to France,
we could find similar
chalk cliffs.
This chalk formed
100 million years ago,
when this whole area
was covered by a tropical sea.
The ancient sea teemed
with microscopic organisms.
When they died,
their calcium-rich skeletons
fell to the sea bed.
Over time,
these built up in thick layers
and were compressed into chalk,
a kind of limestone.
We are getting
exactly the same rocks
which have been deposited
in exactly the same environment
on this side of the channel,
and on the French side
of the channel.
And James is discovering
that the connection between
the cliffs in France and England
goes beyond the chalk itself.
Embedded in the white chalk
are a series of horizontal bands
of a dark rock called flint.
Here I have a fantastic band
of flint.
Flint, a form of the
mineral quartz,
is formed by changes
in ocean chemistry.
But these changes occur
only occasionally,
resulting in these
distinctive dark bands.
These flint bands
are continuous
throughout the chalk.
This band of flint
runs through the entire cliff,
and there are dozens
running horizontally...
...each one at a different level
in the chalk.
Taken together, these parallel
bands of dark flint
form a unique geological
fingerprint in the white cliff.
What's extraordinary is that
the same geological fingerprint
is visible on the other side
of the channel.
So the chalk and the flint
in these cliffs forms a bar code
and is exactly the same
as the chalk and the flint
in the cliffs in France.
The spacing and levels of the
flint layers perfectly align.
To James Lawrence, this raises
an extraordinary possibility.
So what we know from this
evidence is that a chalk ridge
once connected England
and France.
These flint layers tell us
that hundreds of thousands
of years ago,
a ridge of chalk
almost seven miles wide
once extended 21 miles
across the channel,
joining what is now Britain
to the European continent.
So it's quite incredible
to think that there
would have been a land mass
stretching across the sea.
But this discovery
raises a brand-new mystery.
Somehow, the cliffs
between England and France
have been separated over time.
If Britain and France
were once joined,
what force separated them and
turned Britain into an island?
Control, Maverick.
While exploring the sea bed
of the English Channel,
geologist Jenny Collier
finds a telling clue.
Four, five, six... Ten
meters in a split second.
We've got a really steep
drop-off in the topography
and it's the edge
of a really unusual landform.
Using sonar to measure
the depth of the channel,
Jenny is surprised to find what
appears to be a steep canyon
carved into solid bedrock.
Sonar works by firing
sound waves at the sea bed.
The deeper the water,
the longer it takes the sound
to make the round trip.
Jenny expected the channel floor
to be flat,
but the sonar has revealed
something far more dramatic.
We've discovered
just an extraordinary
geological event,
right in the middle
of the straits.
To learn more about
this major geological find,
she and her colleagues
took on a massive task.
Using a more advanced
sonar system,
they are mapping
53 square miles of the channel,
to an accuracy of four inches.
What this reveals
is a strange picture
of channels, rock islands,
and valleys,
carved nearly 300 feet down,
into the rock of the sea bed.
I mean, we haven't got anything
like this in Europe.
There's really only one place
that has all of these features.
Without the water, the landscape
beneath the English Channel
appears to have steep valleys
and islands carved into it.
It looks eerily similar
to the channeled Scablands
of Washington state.
But was this underwater
landscape
also created by a mega-flood?
The only way you can dig out
islands into solid bedrock
is to have extreme water flows,
and that basically pointed us
towards,
this was yet another
catastrophic flood terrain.
What the Scablands revealed is
that carving solid rock
requires a huge reservoir
of water to be trapped,
then released in a single
cataclysmic event.
But today, the English Channel
flows between two open seas.
So how could a large enough
volume of water
have built up
to cause a mega-flood?
Geologist Phil Gibbard
believes he has an answer.
And the evidence lies 120 miles
north of the English Channel,
on the coast of the North Sea,
at the bottom of these cliffs.
What we've got here
is a glacial deposit
which is from about
450,000 years ago.
Deep, fine-grained deposits
like this
were laid down
across Northern Europe
as giant ice sheets
ground over rocks.
450,000 years ago, England
was in the grip of an ice age.
Ice sheets, hundreds of miles
across and a mile high,
reached down from Scandinavia.
They would have dammed the
northern edge of the North Sea.
To the south,
the intact ridge of chalk
between what is now
France and England
formed a natural dam.
Phil believes that meltwater
from the ice sheets and rivers
pouring into the North Sea
had nowhere to go.
A vast amount of water built up
behind the chalk ridge.
He sees the evidence
for this ice age reservoir
in the sea cliffs,
as thin horizontal layers
of silt.
The sediments are horizontal,
as you see.
That horizontality
can only be produced
in a lake situation,
a standing-water situation.
And not in a turbulent area,
like an ocean.
Phil has discovered
similar-looking formations
in other places
around the North Sea,
some a hundred feet
above sea level today.
So this was a massive lake
on the scale
of the Great Lakes
in North America,
and this lake provides
the only possible source
for the mega-flood
that formed the Dover Straits.
Could this enormous
reservoir, a glacial lake,
have suddenly drained
to form the dramatic features
on the bed
of the English Channel?
And if so, how?
In order to carve
these features,
this rock ridge must have failed
very, very rapidly.
But what could have caused this?
How could the giant ridge
of solid rock
between France and Britain
have given way
so catastrophically?
A clue lies in the way chalk
reacts to water.
Having a glacial lake
in contact with a chalk ridge
would have saturated the chalk,
making it much weaker
and much more likely to fail.
When water soaks into chalk
and saturates it,
the chalk can lose
half its strength,
making it far more likely
to fail.
One of the problems
with the chalk being so weak
is that it will often lead
to cliff collapses,
like the one we can see
behind us.
Every year, thousands of tons
of rain and wave-soaked chalk
collapse into the channel,
dramatically eroding
the coastline.
Many geologists now believe
that during a previous ice age
almost a half million years ago,
water from the North Sea
reservoir
soaked the chalk ridge,
fatally weakening it.
Once the lake was deep enough,
water began pouring over the top
of the ridge in a waterfall,
rapidly eroding
the waterlogged chalk.
We'd have had initially
a small stream of water
coming over
the top of the rock ridge,
that would have
catastrophically crumbled,
with large amounts of rock
being removed
and more and more water
flooding through,
just running away with itself.
From the shape of the features
on the sonar,
Jenny estimates that
the floodwaters raced through
at a rate of about
264 million gallons a second.
That's almost 60 times the flow
rate of the Mississippi River.
You would have seen
a tidal wave overtopping
and washing a giant gorge
into that landscape.
The deluge crashed on,
breaking through the chalk ridge
linking today's
Britain and France,
before finally reaching
the Atlantic Ocean.
It was this cataclysmic flow
that created the English Channel
and began the process of erosion
that led to what's now Britain
becoming an island
for the first time.
The clues in Iceland,
the English Channel,
and the Channeled Scablands
of Washington state
reveal that floods bigger
and more devastating
than anything we see today
have torn across and
helped shape Earth's surface.
These giant mega-floods
totally shaped a landscape
in a matter of days or weeks.
But the question is:
could a flood on this scale
happen again?
The one thing all these
mega-floods have in common
is that they involve
huge volumes of ice melting
and being released
in one sudden burst.
In Iceland,
a volcano beneath the ice sheet
could trigger a mega-flood
at any moment.
Fortunately, very few people
live in the Icelandic
flood zone,
and the huge volume of ice
needed to create glacial lakes,
on the scale of the ones
that carved the English Channel
and the Scablands,
can only build up
during ice ages.
But there is one region
on Earth today
where stores of melting ice
still pose a major flood risk
to millions:
ice- and snow-covered mountains.
Wherever you have glaciers,
you have a lot of water.
Wherever you have glaciers
in a mountain,
you have the high likelihood of
making a glacially dammed lake,
and those glacially dammed lakes
are unstable
and could drain
catastrophically.
We're not going to get, today,
releases of water
like Lake Missoula,
that was 2,000 feet deep.
But we can get glacial lakes
that are a hundred feet deep,
and these will produce
really dangerous
and spectacular floods.
Today, floods in populated areas
wreak untold devastation.
Faced with Hurricane Harvey's
impact on Houston
and the floods in Bangladesh,
it may seem that floods
could not get any worse.
But the vast floods of the past
carved huge features
into the very bedrock
of continents.
Those scars are a stark reminder
of just how destructive
floods can be.
Look at that guy.
Armed and dangerous.
There's an animal
with an incredible weapon!
And fighting for their lives.
Weapons that are used
in fighting
and weapons that are big.
How did they evolve this way?
And why do only some creatures
have them?
Some have these huge,
spectacular weapons
and others have nothing at all.
An arms race in the wild.
- Wow, look at that!
- Out of nowhere.
"Extreme Animal Weapons."
Coming soon on "NOVA."
This "NOVA" program
is available on DVD.
"NOVA" is also available
for download on iTunes.