Original subtitles

>> 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 percent of the

world's freshwater and

provide drinking water for

nearly ten percent 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.

S01x07 Great Lakes Original Air Date on March 24, 2009

== sync, corrected by elderman ==

The five Great Lakes,

Superior, Michigan, Huron and

Erie, pour over one of the

world's great waterfalls,

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 percent 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

realized what he was looking

at - fossilized 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

recrystallization 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, fossilized 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 help scientists date,

when fresh water, 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 realized 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 realized 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 4000 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' attacks.

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.

>> As the ice sheets 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

funneled the ice sheets into

fast-moving super ice flows.

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.

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 center, 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

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 center 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.

>> There's a tremendous

thickness of insul 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 believed 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 traveling back in

time, across Lake Iroquois's

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 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 sub glacial

pothole, formed by sub glacial

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, 400 feet deep, moving

at an incredible velocity.

>> Nearby, a steep gorge,

yet more evidence, of the

flood waters 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

colored rock versus

the rock that is typically

exposed to the weather.

And what we saw the 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 1800 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 a, 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 rounds, 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--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 percent 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.

Fossilized 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

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.

== sync, corrected by elderman ==