All language subtitles for How the Earth Was Made HC 720p HDTV S01E07 Great Lakes EN

Earth, a 4.5- Billion-year-old planet,
still evolving.

As continents shift and clash, volcanoes
erupt, glaciers grow and recede,

the Earth's crust is carved
in numerous and fascinating ways,

leaving a trail
of geological mysteries behind.

In this episode,
the Great Lakes of North America,

the largest expanse of freshwater
on the planet, are investigated.

They hold 20% of the world's freshwater

and provide drinking water
for nearly 10% of Americans.

These five lakes are among
the world's greatest natural wonders.

But their origins are a mystery.

Now geologists are investigating,
piecing together the clues that lie hidden

in this extraordinary landscape, delving
deep into a vast underground salt mine

behind the torrential flow of Niagara
Falls, climbing a mile-high glacier,

where clues to understanding the Great
Lakes' formation also provides a window

into the formation of the Earth itself.

The five Great Lakes,
Superior, Michigan, Huron and Erie,

pour over one of the world's great
waterfalls, Niagara, into Lake Ontario.

The mighty torrent of the falls
empties excess water

from four of the five Great Lakes
out to the sea.

For geologists, the lakes are
a natural wonder and a puzzle,

and scientists are on the trail
of how they were formed,

with rocks as their clues, and
ice, lava and water as their suspects.

Their investigation begins at these
seemingly ordinary industrial buildings

beside Lake Huron.

Hundreds of feet below ground here,
there's a remarkable secret.

Deep below Lake Huron, and also
Lake Michigan, are vast salt mines

carved out directly
beneath freshwater lakes.

Right now we're at 1,750 feet
below the surface of the Earth.

We're in the largest underground
salt mine in the world.

And we're below Lake Huron,
a large freshwater lake.

Amazingly, this salt deposit
was uncovered by accident.

They were drilling for oil.

And they hit salt.

And that was the end of
looking for... looking for oil.

They just kept on digging for the salt.

This salt deposit is the
investigator's first clue,

evidence that there was once
an ancient sea here.

Many years ago, the... the salt
was formed in a great salt lake

and the evaporation, dry seasons,
the salt dropped out, evaporated out,

and formed this salt
that we're actually mining in.

There are hundreds of layers of salt,
leading investigators to conclude

the sea must have dried up
and refilled hundreds of times.

Scientists would later prove this sea
finally evaporated millions of years ago.

35% of North America's salt
comes from these mines -

salt used to melt ice on frozen roads
and sidewalks,

salt used to season food -

the remains of million-year-old seas.

All coming from beneath
Lake Huron and Lake Michigan.

McCUE: The salt deposit is massive.

There's probably trillions
of tons of salt in the deposit.

It extends all the way down to Detroit.

All of Lake Huron, the salt is
under it. And all of Michigan.

The salt is soft,

and over millions of years the salt layers
should have worn away.

Why haven't they?

It's because the salt is protected
by a vast impenetrable layer of rock

that lies like a giant basin
beneath Lakes Michigan and Huron

and stretches under Lake Erie.

Like the porcelain lining a bath tub, the
rocky basin holds the lakes' freshwater.

Geologist John Zawiskie and a team
of divers are hunting for clues

to the rocky basin's origins.

They're heading for Thunder Bay, a
small island at the edge of Lake Huron.

As he walked along the beach, Zawiskie
discovered some crucial evidence,

seemingly insignificant rocks
that were overlooked for decades.

But Zawiskie suddenly realised
what he was looking at -

fossilised remains
of ancient sea creatures.

ZAWISKIE: I was seeing something
that many geologists had never seen

when they visited this island.

There were the heads
of giant lime-secreting sponges

that were some of
the main reef builders.

Zawiskie uncovered
a perfectly preserved fossil

of a giant sea sponge that must
have come from an ancient coral reef.

For the past five years, Zawiskie's divers
have been surveying the lake

to discover the size
of the ancient coral reef.

They believe it's hundreds of feet thick
and extends deep below Lake Huron.

And Zawiskie has proof
these rocks are extremely old.

The time period can be
pretty confidently bracketed

at right around 385 million years ago.

America was then a very different place.

385 million years ago,

its land mass
lay in the southern hemisphere,

a land covered
by ancient warm coral seas.

This region was just south
of the equator, in tropical conditions,

and shallow seas had swamped many of
the land areas of the Earth at that time.

Year in, year out,

coral reefs decay naturally
and turn into a soft rock, limestone.

And much of the rock Zawiskie's divers
find under Thunder Bay Island

consists of layer upon layer of this
limestone from successive coral reefs.

But millions of years ago,

some of this soft limestone
near the surface was changed.

When the salty briny sea evaporated,
it turned the limestone

into a second, much harder rock,

something which would decide
the very shape of the Great Lakes.

This rock is limestone.

This other piece was once
the exact same material.

However, it's been converted
by a process of brines

creating the conditions for
recrystallisation into a rock

that we call dolostone.

It's much harder than limestone,
more weathering resistant,

and I can easily demonstrate
the difference between these two.

Calcium carbonate,
calcium magnesium carbonate.

To show the relative hardness
of the two rocks,

Zawiskie uses an essential tool
in the geologist's arsenal.

Let me put a little acid on here.

Acid easily attacks
and dissolves soft rocks.

First, how will the limestone react?

You can see a very
violent reaction there.

Carbon dioxide gas is being
released from the limestone.

Next, the hard dolostone.

Let's go ahead
and do the acid test on it.

And you can see we don't get
this violent reaction.

Almost no reaction at all.

Zawiskie has proved the dolostone layer
is harder and more resistant

than the limestone.

The ancient ocean's salty water
converted the top layer

of the limestone deposit into a cap
of hard, resistant dolostone rock.

It's this that forms the super tough
rock basin under three of the five lakes -

Michigan, Huron and Erie.

Scientists were beginning to piece
together the chain of events that led

to the formation of the Great Lakes.

The clues uncovered so far -

vast salt deposits provide evidence
of an ancient ocean.

The briny ocean changed soft,
fossilised limestone into hard dolostone.

Dolostone makes up the rocky basin
under Lakes Michigan, Huron and Erie.

The tip of the rock basin, the rim,

forms steep cliffs that tower
above these three lakes.

This immense wall of rock,
called the Niagara Escarpment,

forms the boundaries of these lakes,

and makes possible one of the world's
greatest natural spectacles,

Niagara Falls.

Over this hard dolostone cliff,

3,000 tons of water a second tumble
from four of the five lakes.

But it's more than just
a miracle of nature.

Niagara Falls is a vital clue
that helps scientists date

when freshwater first began flowing
into what we now call the Great Lakes.

The Great Lakes of North America.

Geologists have discovered three
of the lakes were formed

in a vast rock-lined basin,
laid down by an ancient lagoon.

The question is, when?
And they think the answer lies here.

Niagara Falls.

Behind this curtain of water lies the
evidence to when the lakes were made.

Like the overflow from a bath tub,
excess water from four of the five lakes,

Superior, Michigan, Huron and Erie,

spills over the falls into Lake Ontario.

And all that water is changing the falls,
change that can be measured

and used to calculate the age
of the lakes themselves.

The falls were first studied

by one of modern geology's
founding fathers, Charles Lyell.

Lyell, who pioneered the early
understanding of Earth's secrets,

was intrigued
by the concept of geological time.

Charles Lyell came to Niagara region
in the 1840s,

and he made very important
observations at Niagara Falls.

Lyell was using the principle that things
that we see are going on today

can be used as examples
for what went on in the past.

Lyell believed the world wasn't shaped
in a few days or even years

but by slow change
over millions and billions of years.

This directly contradicted
the much shorter time

biblical scholars said
the world had been in existence.

Lyell realised that dramatic geological
change was going on in front of his eyes

at Niagara Falls.

If he could measure it he might
be able to calculate the falls' age.

Lyell's technique was brilliantly simple.

He noticed below the falls
was a great gorge

which locals said was
steadily increasing in length

as the water wore away
the ledge of the falls.

The falls, they said, were
moving slowly upstream.

Head to the base of the falls
and you can see why.

The cliff face is being worn away.

The falls are formed by a cliff

capped with a ledge of
the same hard dolostone rock

created, as we've seen, by seawater.

Beneath the tough dolostone cap is a
layer of much softer rock called shale.

As the water crashes over
the dolostone, it erodes out

these soft shales that are
underlying the dolostone,

and the blocks can fall down
from the face.

On the right, then, you can see
these massive blocks of dolostone

that have fallen down at the bottom
of the waterfall.

Each time the dolostone ledge collapses,
the falls move further upstream.

Lyell believed this process had been
going on for thousands of years,

and was still continuing.

It had begun
as the lakes were first formed

when water began wearing away
the hard dolostone ledge of the falls.

To discover the age of the falls,

all Charles Lyell needed
was some simple math.

BURCIK: He realised that the falls
had started at the Niagara Escarpment

which is about 35,000 feet from here,

so if the falls receded at one foot
per year and receded 35,000 feet,

that would give an age for their
present position of 35,000 years.

Lyell's calculation was based on simple
measurements but wrong guesswork.

He thought the falls were receding
by one foot a year.

But today we have much
better records to go on.

This plaque commemorates
Table Rock,

which is where the falls were
at the beginning of the 19th century.

Since that time, they've receded
about 600 feet to my right.

So in the last 200 years,

the falls have steadily retreated
at a rate of not one foot,

but an astonishing three feet a year.

So instead of Lyell's calculation
of 35,000 years old,

the Niagara Falls were a third
of that figure, just 12,000 years old.

A mere blink of an eye
in Earth's 4.5 billion year history.

In the search to find
what created the Great Lakes,

scientists now had a crucial clue,
the age of one of their key features.

Born at the same time, the falls is

the overflow for all the upper lakes

into Lake Ontario and the sea.

So if the falls have only been
around for 12,000 years,

then it means the lakes themselves
must also be incredibly young.

Now that scientists had worked out
when the lakes were created,

the next question was how?

What immense force could have
created not one but five huge lakes?

A force so powerful it must have left
a trail of incriminating evidence

across the region.

Geologist John Menzies scans the
landscape to track the mysterious force

that created the Great Lakes.

And he's spotted something unusual -

strange teardrop-shaped hills,
one after another, called drumlins.

MENZIES: Some are small,
fat and streamlined,

some are extremely elongated.

This one is about... almost a mile
in length, 150 feet high,

and about 200 feet across.

This is the evidence that John Menzies
has been looking for.

There are many drumlin fields
in North America,

but this one is a particularly large field.

It has anywhere
between 60 and 80,000.

So it's truly an enormous
drumlin field.

Each drumlin points
in the same direction, north,

to where an immense force came from.

This tells Menzies they were all created
by the same powerful object,

but what was it?

The answer lies 4,000 miles away,
high in the Swiss Alps.

Here the culprit is plain to see -
snow and ice.

Switzerland is home to some
of Europe's largest glaciers.

They're giant rivers of ice
that flow down mountain valleys.

Glaciologist Dr Andreas Bauder studies

how glaciers can transform
the landscape.

What he discovers here could also point
to how the Great Lakes were made.

We measure
the movement of the ice.

This reflector reflects the laser signal

coming from a theodolite

giving us the position of this stake.

And then we can calculate
the movement.

My colleagues down here
are drilling deep holes

down to the base of the glacier
to install instruments

to understand how the glacier
is changing here.

Bauder's measurements reveal this
glacier moves over ten feet every month.

Here, a seemingly stationary glacier
is shown moving down the mountain,

recorded by time-lapse photography
over a year.

To find out what's driving it,
Bauder climbs high up the glacier.

This glacier is thousands of years old
and almost a mile thick in some places.

Ice that's a mile thick weighs a
colossal 3.8 billion tons per square mile.

That's the weight of 59,000
fully laden supertankers.

And it's this immense weight

that makes the glacier
such a force to be reckoned with.

Its weight is slowly pushing
the glacier down the valley,

gathering anything in its path,
collecting rocks and debris.

The rocks act like the blades of
a giant bulldozer, scouring the ground,

digging up yet more
and more rock and soil.

But when the temperatures rise, the
glacier melts, retreating up the valley,

and leaving rocks and debris behind
in huge piles.

This is how
the teardrop-shaped drumlins

back in North America were formed.

They were bulldozed,
landscaped by a powerful glacier.

A glacier that may also have
gouged out the Great Lakes.

The evidence is coming together.

Niagara Falls,
dated just 12,000 years old.

This suggests the lakes themselves
are very young.

The presence of thousands of drumlins

pointing to ice
that carved out the Great Lakes.

It's a convincing case.

But there's one problem.

The Great Lakes cover an area
five times the size of Switzerland.

No glacier that size
has ever been known to exist.

Geologists were on the hunt
for something even more powerful

that could have created
such huge destruction,

a kind of prehistoric monster
roaming over North America.

Geologists are scouring the landscape,

searching for evidence
of a massive force.

One that was capable of gouging out
12 trillion tons of solid rock,

enough to create
the Great Lakes of North America.

It would be a body of ice so large that it
would break every record, defy all logic.

Geologist John Menzies hunts for
evidence of this prehistoric monster

just south of Niagara Falls.

This whole area was covered
by the ice

with a tremendous torrent
of sediment and water

between the ice and this bedrock.

And as this sediment moved across,

it produced these superb striations
and parallel scratches and marks.

And there's another clue.

Giant boulders of hard
crystalline rock called granite.

These hard, massive rocks
sit in a flat, sandy landscape.

They shouldn't be here.

This is what we refer to
as an erratic boulder.

It's granite.
It weighs some 80 to 100 tons.

It would actually be frozen up into
the base of the ice and then moved,

kind of like a conveyor belt,
along on the base of the ice

down to this part of Southern Ontario,

some 400 or 500 miles to the
south from the Canadian Shield,

where, with ice retreat
and the eventual melting of the ice,

this boulder has been left to sit,
as we see it today.

Erratic boulders moved hundreds
of miles from northern Canada.

Scratches on the bedrock
and drumlin hills -

the evidence is mounting.

There was ice here once - lots of ice.

Geologists map these
glacial features together

and an extraordinary picture emerges.

Not of a glacier, but of
a vast ice sheet one mile thick

and over 2,000 miles long.

It stretched all the way
from the North Pole

as far south as Chicago and New York,
leaving a trail of destruction in its path.

Here was a force powerful enough
to create the Great Lakes.

But even this vast sheet of ice
couldn't have gouged out

basins that are over 1,300 feet deep.

It seemed the culprit
wasn't working alone.

At Scarborough Bluffs,
just 100 feet from Lake Ontario,

John Menzies has spotted an
unusual deposit at the cliff face.

Layers of rock provide him
with a kind of geological time machine.

The deeper he looks,
the further back in time he goes.

You could say that this is a journey
through the last 60,000 years

of geological history
in this part of Canada.

This lower formation is 65,000
to about 40,000 years ago.

The next layer is
between 25,000 and 10,000 years ago.

Menzies focuses on the dark layers
sandwiched between the light ones.

What we have here
is a sequence of sediments

which illustrate the movements
of the ice front, back and forward

across this part of Canada.

These dark layers mark the
exact end of each Ice Age -

formed of organic material when plants
grew again at warmer temperatures.

Here, John Menzies has proof
that Ice Ages returned twice

to this spot during
their cycles of destruction.

In fact, across the Great Lakes region,

geologists have found evidence of up
to ten separate enormous ice sheets.

As each new ice sheet advanced,

it carved the Great Lake basins
deeper and wider,

eventually forming the largest
lake system in the world.

But the ice left vast areas unscathed.

It suggests there was
some other force at play,

something in the lakes' ancient past
that set them apart

from the surrounding landscape,

making them particularly vulnerable
to the ice sheets' attack.

Menzies decided to dig deeper,

down to the landscape that
existed before the Ice Ages.

Going back 2.5 million years,

he found evidence of a chain
of ancient rivers flowing

across what's now
the Great Lakes region.

MENZIES: The pre-glacial topography
of the Great Lakes basin mirrors

the existing Great Lakes system and
Great Lakes basin that we see today.

The ancient rivers' pattern and flow

exactly mirrored the shape
and position of today's lakes.

It's no coincidence.

These rivers formed valleys that affected
the way the ice sheets moved.

MENZIES: As the ice sheet
advanced to the south

it would tend to follow
the pre-glacial rivers,

and so you get these
really fast-moving zones of ice

which create a tremendous amount
of erosion

in these pre-existing depressions.

The ancient river valleys
funnelled the ice sheets

into fast-moving super ice floes.

Menzies believes the coarse
sediments the rivers left behind

dramatically accelerated
the ice sheets' flow.

This sediment acts
as a kind of lubricant,

a bit like ball bearings
underneath the ice.

It would actually speed it up
quite... quite appreciably.

These fast streams of super ice were
even more destructive to the landscape.

The case is coming together.

Drumlins clustered across the landscape

testify to the vast ice sheets'
brutal power.

Dark layers of rock reveal
the ice was a serial attacker,

while a network of ancient rivers

left some areas more vulnerable
to these attacks,

turning slow, lumbering ice
into destructive, fast-moving super ice.

These gouged out
all the loose rock and sediment

down to the hard dolostone layer,
the rocky lake floor.

The result,
the basins of the Great Lakes.

Case closed for three of the five lakes
inside the rocky basin.

But not for the other two. Lakes Ontario
and Superior are outsiders.

The theory doesn't fit.

They're simply too deep.

In an attempt to find out why,

a daring underwater expedition
would investigate Lake Superior,

the largest, deepest, greatest lake of all.

(RADIO CHATTER)
MAN ON RADIO: Roger that.

The hunt is on to discover what formed
the Great Lakes of North America.

Geologists have found compelling
evidence that the central lakes lie

in a vast rock-lined basin
laid down by an ancient lagoon,

gouged out by giant ice sheets.

But when it comes to Lake Superior,
the theory doesn't fit.

The greatest of all the lakes,
at over 1,300 feet deep,

it could almost submerge
the Empire State Building.

And it lies outside the rocky basin.

Lake Superior isn't just deeper
than the other lakes,

its floor is the lowest place
on the North American continent.

Over half of this mighty lake
lies below sea level.

The question is why?

Canadian geologist Henry Halls
was convinced the explanation

could be found
at the very bottom of the lake.

The opportunity came up to study
a very remote part of the lake,

it's almost in the geometrical centre,
and it's called the Superior Shoal.

And people didn't know
what the rocks were there

and they didn't know why it was there.

In the summer of 1987,

Halls led an expedition to the
lake's dark unexplored depths.

HALLS: We went down.
It took us about 15 minutes to go down.

And it gets completely black, apart from
the searchlights of the submersible.

And when we reached the bottom,

the... the pilot, he said,
"This is very strange."

He said, "I'm getting echo sounds
coming back,"

he said, "more or less
from all directions."

MAN: It does look almost vertical.

SECOND MAN: It is vertical.

MAN: More than vertical, we've heard.
In fact, it's hanging over us.

Deep in the centre of the lake, on the
border between Canada and America,

Halls came across
a strange rock formation.

HALLS: The pilot, he said,

"It seems that we were in some sort
of a chimney," or something like this.

He said, "I'm not sure what it is."

Halls and his submersible

were in a deep canyon
1,200 feet below the surface.

Intrigued, he took an even closer look
at the canyon walls.

And as we climbed,
I started to see striations like this.

They were actual glacial striae on the
sides of what presumably was a canyon.

MAN: We are continuing
to move up this vertical face.

Halls had uncovered a vast
canyon lined with striations

or scratches from the glacier
that had carved out the lake.

But it was the type of rock
that was the clue

to Lake Superior's exceptional depth.

He used the sub's robotic arms to take
rock samples from the canyon walls.

The canyon was made
of dark basalt rocks.

The discovery of this rock took the
investigation in a surprising direction.

Basalt could only have been formed
by intense volcanic activity.

Basalt is created when hot magma deep
within the Earth wells up to the surface.

A billion years ago, immense forces
pulled the Earth's crust apart here,

forming a rift valley.

Hot magma seeped up
through the cracks in the thin crust.

As it cooled, it lined the valley
with a layer of hard basalt.

Then, over millions of years, the rift
was filled with soft, sedimentary rocks.

So there's a tremendous
thickness of infill in that lake

lying above those volcanic rocks,
and all of this is relatively soft.

Many geologists believe
the exact same volcanic action

accounts for the formation
of the fifth and final lake.

Ontario, on average,
is the second deepest lake.

A separate rift valley appeared here

much later than the one
under Lake Superior.

The volcanic split in the landscape
stretched as far as the ocean,

creating Lake Ontario
and the St Lawrence Seaway.

Millions of years later,
the mile-high ice sheet

easily carved out
the weakened rift valley structures

under Lake Superior and Lake Ontario.

The extraordinary story of how the Great
Lakes were made is almost complete.

Ice sheets repeatedly
carved out soft rock

down to the hard basins
of the central lakes.

And to the north, ice attacked
billion-year-old rift valleys

to make the deepest lake,
Lake Superior.

The same action was
repeated at Lake Ontario.

When the ice melted for the
last time 14,000 years ago,

it filled the lakes with freshwater.

It sounds straightforward.
But there's a problem.

There's so much ice,

the Great Lakes should be many times
bigger than they are today.

Just when geologists thought
they'd solved the mystery

of how the lakes were formed,
a new puzzle emerges.

Where did all the water go?

Geologist John Menzies
is investigating exactly what happened

at the end of the last Ice Age,
when a vast ice sheet, one mile thick

and stretching to the North Pole,
started to melt.

He believes it was so large
it should have created far bigger lakes

than the ones we see today.

He's looking for evidence
of one of these prehistoric lakes.

As the ice sheet melted,
a vast freshwater lake appeared

that geologists call Iroquois.

Then, later, as Lake Iroquois
dried up, it left beaches

which can still be seen today.

Menzies believes he can detect
these ancient beaches

in the gently sloping landscape
surrounding Lake Ontario.

As Menzies drives uphill,
away from the present-day lake,

he's travelling back in time
across Lake Iroquois' ancient shores.

We're crossing one
shoreline after another.

The reason we know they're shorelines
is that they contain

large zones of sand,

beach sands and beach bars and spits,
the oldest being about 12,000 years ago,

the bottom shoreline
being about 6,000 years ago.

Getting to the top of the hill, 400 feet
above the level of today's Lake Ontario,

Menzies is standing on the ancient
shore of the original lake.

The present-day Lake Ontario
is off there in the mist

and we're sitting about 400 feet plus
on this beach

which is... was formed maybe
10,000, 11,000 years ago,

and then the ultimate oldest beach
is about 12,000 years ago.

These ancient beaches, now
buried under the surrounding landscape,

are evidence
of a colossal freshwater lake.

We're looking at a vast amount of water,
and when you think of the water,

it stretched from here to beyond the
present lake, way into New York State,

beyond into Rochester,
so it's a huge, enormous, inland sea.

Despite their size,

the Great Lakes today are just a small
fraction of these vast prehistoric lakes.

The water has vanished.

Geologists want to know
how they emptied.

50 miles east of Toronto,
at Indian River Canyon,

Menzies picks up the trail
of the missing water torrents.

OK, what we have here is
an enormous subglacial pothole,

formed by subglacial meltwater
exiting underneath the ice sheet,

typically formed with a large
roller ball which rolls around

in these really torrential vortices.

The meltwater is chock full
of... of boulders and sediments,

and in this instance it's drilled itself
the whole way through.

These potholes are evidence
of a catastrophic flood,

of huge volumes of water
moving at high speed.

This flood needed an escape route,

and Menzies believes
he's found the place.

This would be an enormous torrent,
possibly at least a couple of miles across

and could easily have been two,
three, four hundred feet deep,

moving at an incredible velocity.

Nearby, a steep gorge,

yet more evidence of the
floodwater's terrifying power.

The stream that remains today couldn't
have cut such a huge amount of rock.

MENZIES: And what we've got left
is what we call a misfit stream,

which is the fairly small Indian River,
and this, if you like, is the remnant

of that enormous torrential flood.

Geologists believe
as the ice sheet retreated,

it uncovered this ancient
Indian River outlet,

allowing vast amounts of meltwater
to tear down towards the sea.

Finally, 12,000 years ago,

the ice retreated,

freeing the St Lawrence Seaway,

and allowing the lakes
to settle into their present flow.

The story of the Great Lakes
is coming together.

Ice sheets repeatedly
ground out deep basins,

digging out ancient weaknesses
in the Earth's crust.

Prehistoric beaches show that
when the final ice sheet melted,

the water flooded the basin to create
vast superlakes like Iroquois.

And as the ice finally retreated
12,000 years ago,

the excess water drained away to leave
the Great Lakes we know today.

But even now, as we know how
the Great Lakes were formed,

they are still changing.

And scientists predict one day
the lakes might disappear forever.

The Great Lakes evolved
over a billion years.

Today, they're a vital link between the
cities bordering the lakes and the sea.

They provide over 20 million people
with drinking water

and irrigate crops
throughout the Midwest.

But in the past few years, fears have
grown about the Great Lakes' future.

Water levels are falling.

People who have worked the lakes
for years

believe they can already see a change.

We noticed a drastic decrease

in water levels right after
the September long weekend,

where the water in a week
dropped a foot

and, throughout
the... the remaining of the fall,

it went down about another two feet.

And you can notice that by the pinker
or the brighter coloured rock

versus the rock that is typically
exposed to the weather.

And what we saw there
was a clear example

of how the water has dropped
a good three to four feet.

Many have been quick to blame global
warming for the fall in lake levels.

But geologists believe
there is another force at work.

The ice sheet that cut out
the lakes was so heavy,

it pushed down on the Earth's crust.

Now the ice sheet has gone,
the crust is bouncing back.

Incredibly, 9,000 years since
the end of the last Ice Age,

the ground is still lifting.

In the north, where the ice
was thickest, land has risen

by as much as 1,800 feet
since the ice melted away.

Toronto's famous CN Tower
appears to be getting higher.

As the crust bounces back, the land
it's built on, beside Lake Ontario,

rises nearly an inch each year.

The CN Tower is part
of the landmass here,

so in fact, it's rising out of the land,

in fact, the whole land surface
is rising slowly.

Lake Nipissing today is a small body
of water to the north of Lake Huron.

12,000 years ago,
when the ice began to melt

and Lake Nipissing first formed,
it lay at sea level.

MENZIES: Lake Nippising,
an enormous lake there, again,

as the land rebounds,
so the lake eventually drained out,

and the land rose slowly, so the land
is now 400, 450 feet above sea level.

Geologists call this crustal rebound

and it dramatically affects
the delicate balance

of the network of small rivers
that feed the lakes.

This is an interesting example

if we... if we think of trying
to explain crustal rebound,

and we look at this river as it flows out
into the lake at the moment.

If we have crustal rebound,
the land comes back up,

this river, in fact, will cease
flowing out into this lake.

It's this crustal rebound

that's partly responsible
for the fall in level of the lakes.

And as the lakes empty,
their weight decreases,

allowing the crust to bounce up
even faster.

Lake levels will fall so the amount
of water in the basin

will in fact become less,

and the effect of that will be to
increase the rate of crustal rebound.

The land will come up even faster than
it's already doing and continues to do.

As the crust rises,
the lakes slowly empty.

But in a few thousand years,

the lakes will face another,
even more dramatic, change.

One of the exciting things
about geology these days

is not only looking at the past,
but is looking into the future,

in other words, having the ability
to start to predict what might happen

in the next several millennia.

And the future is here at Niagara Falls,
at least in geological terms.

Every year, the falls
are retreating three feet upriver.

Only 12 miles and 21,000 years to go
before they're back into Lake Erie.

When that happens,
everything will change, and fast.

If the falls eroded
all the way back to Lake Erie,

which would take
some thousands of years,

the levels of all the upper Great Lakes,

Huron, Superior and Michigan,

would adjust to the lowered level
of Lake Erie by dropping as well.

The land between the falls
and the lakes acts as a block.

It's the Niagara Escarpment,
topped with hard dolostone rock.

When the falls cuts its way
through this rock,

the water levels
in all the lakes to the west

would drop by a staggering 180 feet,
the height of Niagara Falls.

Almost all of Lake Erie
would drain away.

One day the lakes
may disappear altogether.

But geologists also predict a new cycle
of Ice Ages will begin again.

So an Ice Age will begin,
and this Ice Age would then cover,

we would expect,
at least 30% of the land's surface,

as it did in the previous Ice Ages.

And when the ice returns,

the lake basins will be cut even deeper
before filling again with water.

The largest freshwater lake system in
the world has had an extraordinary past.

A basalt-lined canyon discovered
at the bottom of Lake Superior

shows that two great rifts opened up
below Lakes Superior and Ontario.

Fossilised sea sponges are evidence
of an ancient briny sea

that laid down the rocky bowl that holds
Lakes Michigan, Erie and Huron.

Thousands of drumlin hills
are proof that vast ice sheets

repeatedly scoured out the lake basins.

Born just 12,000 years ago,
the Great Lakes as we know them today

are just a transient feature.

They've only existed
for the geological
blink of an eye.

But their story hasn't ended yet.

The Great Lakes
are changing and evolving -

an endless process, like the Earth itself.