>> 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,
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 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 ==