NARRATOR: Earth, a unique
planet, restless and dynamic. Continents shift and clash. Volcanoes erupt. Glaciers grow and recede,
titanic forces that are constantly at work, leaving
a trail of geological mysteries behind. This episode journeys
back through time to explore how ice has
shaped modern North America. Geologists discover how
America's temperatures plunged to an average of 26 degrees
Fahrenheit all year round, how a wall of ice eight times
the height of the Empire State Building covered much
of North America, and how ice created some
of the country's most stunning geological wonders,
a deep frozen episode in the continuing drama of
"How the Earth Was Made." One awesome physical force has
had more impact than any other on the shaping of
modern North America-- the dynamic, unstoppable
power of ice. Over millions of years, ice
sheets a mile or more thick have decapitated the
continent's mountains and dammed and
diverted its rivers. But the full extent of how
ice has carved and sculpted the landscape of the
US is only now becoming clear as scientists investigate
the overwhelming geological impact of America's Ice Age. For centuries, America's
early geologists were puzzled by mysterious marks
etched into the bedrock of much of North America and by
lonely mounds of rock randomly scattered across the land. Many believed such features
were evidence of a massive flood in the long distant past. But then, in 1847, a newly
appointed Harvard University professor came up with
a radical new idea. Geologist Louis Agassiz compared
North America's landscape and rocks with the geology
of mountain glaciers in his native
Switzerland, and he built on theories he had
developed in the Swiss alps a decade earlier to declare
that North America had once been buried deep under ice. Agassiz's work at Harvard made
him world famous in his day. But the final proof
of his Ice Age theory had to wait until the 1950s
when new evidence came from the seabed of
the Atlantic Ocean. That evidence is still
stored to this day in this refrigerated
laboratory at Miami University. These thousands of
plastic tubes offer scientists a unique window to
look back at what the weather was like on the ancient Earth. These records which you see
in this room here basically are the primary
information we have now about the climate changes
we have had over the last 2 million years. So these are really
starting points that open people's eyes
about changes which occurred during glacial periods. NARRATOR: The mud cores were
collected by oceanographer Cesare Emiliani. The deeper he drilled,
the further back in time he went and the older the
sample he pulled to the surface. PETER STEWART: These are
cores of deep sea mud, and within the mud are
small organisms which record the temperature
and the salinity and the other conditions
which were prevalent when that organism lived. NARRATOR: Those tiny
undersea creatures are called foraminifera. Their chemical makeup varies
depending on the temperature of the water in which they grow. So the sea bugs in
different layers of mud act as tiny thermometers
recording how hot or cold the earth was at different
times in its past. They reveal that just
over 2 million years ago the average annual temperature
across the lower 48 states was around 56 degrees
Fahrenheit, about 3 degrees warmer than it is today. Then the temperature regularly
started crashing below zero and staying there for
month after month. Huge changes happen with
what, in geological terms, was astonishing speed. Within 10,000 years, the
year-round average temperature bottomed out at just
26 degrees Fahrenheit, the same as the average
winter temperature in Chicago. Even today, scientists
are uncertain what caused the temperature
to drop so quickly. Variations in the
Earth's orbit, a change in the tilt of its axis,
and even the creation of the Himalayan mountain
range are all prime suspects. But one thing is certain. With subzero temperatures all
year round, snow never melted. A major storm can drop more
than 40 million tons of snow. The ever-increasing
weight compacted down into thick layers of solid ice. America's Ice Age had begun. An important clue to
what happened next lays in a rock quarry
just outside the town of Champaign, Illinois. In quarries like
this, rocks normally hidden deep underground or
exposed by commercial blasting. [explosion] That gives geologists the
chance to examine rocks they could never normally see. This rock in the
sandy, cobbly bank is a very important
significant piece of evidence for the
history of North America during the last
several hundred thousand years. It's what we call an erratic. It's a rock that
doesn't belong here. This rock traveled
several hundred miles to get to this place. NARRATOR: In fact, this is a
type of granite usually found in Canada. Somehow it has
traveled to Illinois. But it's not unique. All over the northern states
rocks of all shapes and sizes have been dumped in
places they don't belong. This one the size of an elevator
is in Central Park, New York. It was originally
from New Jersey. And this one the
size of an automobile and weighing just as much
comes from 500 miles north of Ontario. Scientists have figured out just
how such huge rocks must have been transported over
such long distances. The ice sheets which built
up 2 million years ago began to move, flowing
as huge glaciers across the northern US. STEVE BROWN: Some of
the very large rocks can be literally frozen on and
plucked off from the rock where they came from, and transported
right into the glacier and moved that distance
of hundreds of miles in the glacier, like
a conveyor belt, out to where we finally
find it on the landscape or in an outcrop like this. NARRATOR: The investigators'
next challenge is finding out just how far the ice
flowed, and for that task they are taking to the air. In the early
morning light, Brown spots a mysterious
small hill standing out from the flat Illinois plane. Stripped of its camouflage layer
of trees, the shape of the hill tells Brown the secrets
of its creation. STEVE BROWN: This
gently rolling landscape extends for about 400 miles. We look at the landforms
and how they're shaped, and so we can use that to
unravel the history of the Ice Age. NARRATOR: The land form Brown
is studying is a mound of rubble that geologists call a moraine. It is an accumulation of debris
that a glacier has plucked up from the Earth, dragged along,
and dumped at the front edge. This hill marks the spot
where the glacier finally stopped flowing southward. Geologists all over the US
have recorded the positions of other moraine hills, helping
them map out precisely how far north America's
ice sheets spread. Nearly 2/3 of the North American
continent was buried under ice. So far, scientists
studying America's Ice Age have discovered tiny
temperature recording sea bugs in ocean sediments showing
the US chill down 27 degrees at the start of the Ice
Age, and moraine hills of rock and debris mapping out
where the ice sheets halted. But the frozen chaos
did not stop there. The challenge now
facing geologists is figuring out how ice created
some of modern North America's most famous features. 2 million years ago,
temperatures fell and massive ice sheets
spread out as glaciers across much of North America. Geologists set out to discover
what the moving ice did to the underlying land. Everywhere the ice had
flowed, investigators discovered the same puzzling
feature, lines of stones all laid out in the
same particular way. This is a really good example
of a pavement of stones. This concentration of
small rocks and cobbles that are collected
here in a line is just one stone
or two stones thick. NARRATOR: Scientists realized
that the stone pavements must have been laid down at
the base of a glacier. And even the toughest rocks
had scratches and grooves carved into their surface. STEVE BROWN: Glacier carries
along other fragments of rock, and as it's doing so,
it scratches, polishes, and creates grooves in the
rock called striations. NARRATOR: Those scratches
offered vital clues to help uncover the history
of how the Ice Age changed America. STEVE BROWN: We can
measure their orientation, and they can tell us something
about the direction of ice flow. So I'm aligning my
compass parallel to the striations on this rock. The compass tells me that
the ice is flowing from about the northeast, from this
direction to this direction. NARRATOR: But other
scratches on rocks found all over the northern
parts of the US run in puzzlingly
different directions. These ice-scraped rocks
in New York's Central Park show scratches made
by a glacier that arrived from the northwest. There's only one explanation
for the different ways in which the ice appears to have moved. The scratches were not all
made by a single ice sheet, as first thought, but
by many different ones. STEVE BROWN: This phenomenon
of advance and retreat and changing of that
landscape happened many times, not just once, but many times. NARRATOR: Everywhere
the ice flowed, it marked the underlying rocks. That's given
investigators plenty of clues to help establish
a pattern of ice movements. But understanding
the full picture involves a lot of
geological detective work. STEVE BROWN: This
cycle of glaciation, of advance and retreat, leaves
an incredibly complicated record for us to solve. We only see fragments of
this puzzle in any place we look at, like in a quarry,
and so it's a great challenge for us to figure out how each
of these fragments fit together. The ice changed the existing
conditions on the landscape. Those changes include
erosion of glacial debris in a way that changes the
whole way these areas look. NARRATOR: Grinding over
the landscape at speeds of around 2 feet per day,
the massive glaciers diverted the course of ancient rivers. Even the mighty Mississippi
was no match for the ice. It moved the position
of the Mississippi River. The Mississippi
River used to flow through the middle of Illinois. NARRATOR: A glacier
advanced across the state and created an ice dam across
the course of the old river. A huge lake grew at the
edge of the ice, eventually overspilled, and cut a
new river channel 80 miles to the west, the same course
that it follows to this day. And when the ice
sheets melted away, their impact on North
America was almost as great. They produced immense
floods of water, which all had to drain away somewhere. STEVE BROWN: It formed a
completely new landscape with new river valleys, new
channels, new places where rivers flow, completely
rearranging the character of the existing landscape. NARRATOR: Where rocks were
soft or weakened by faults, the glaciers had carved out
huge basins in the landscape. When the floods came,
these depressions rapidly filled up with water. STEVE BROWN: Every natural
lake here in Indiana, Illinois, Wisconsin were all
created by glaciers. NARRATOR: The biggest ice
scoured basins became what we now call the Great Lakes. These water-filled craters in
the North American landscape are nearly 5 and 1/2
thousand cubic miles in size. The water they hold could drown
the whole of New York state to a depth of more
than 600 feet. But even that is just a tiny
fraction of the total flood unleashed as the retreating
ice poured out nearly 39,000 cubic miles of water. That's over 100
times as much water as is found today in every
single river on Earth. The flood rushed down
the Niagara escarpment at the rate of 42
million gallons a minute, 1 and 1/2 Olympic-sized
swimming pools every second. And that cut a gorge 7
miles long and 180 feet deep through solid rock. The Ice Age had created
one of the modern wonders of the world, Niagara Falls. The investigators'
next challenge was figuring out why the
ice advanced and retreated so often. It wasn't until the mid 1960s
that the first real clues were discovered. The US military was building
bases in some of the Earth's most remote locations. NARRATOR (IN VIDEO): This is
an ideal arctic laboratory, permanently frozen under
a polar ice cap which covers all but a few
coastal areas of the island. NARRATOR: Geologists realized
that the cores of ice being extracted by hollow drills held
a unique historical record. Those cores and many
others extracted since are housed in a
laboratory in Denver, Colorado. JIM WHITE: We're in the
Fort Knox of paleo climate. What you see in
these tubes is gold in terms of what we
know about climate and what we've learned about
climate over the last 50 years. NARRATOR: These tubes contain
3-foot-long plugs of ice, each dug out from layer after
layer of compacted snow. But it is what is
trapped inside the ice that most fascinates
the scientists. As each year's snow
falls onto the last, it builds up in loose
packed layers that trap air between the snowflakes. JIM WHITE: Ice cores are
the only archive we have, really good archive we
have, of past atmosphere. In an ice core, you
can see tiny bubbles. And those bubbles are
air, unadulterated air from as much as 100,000
to a million years ago. NARRATOR: The deeper the
bubble, the older the air. It contains not only
a temperature signal, but it contains dust
signals that tell us about atmospheric circulation. It contains a record
of the atmosphere, and specifically the
greenhouse gases. NARRATOR: Each ice layer
corresponds with a precise time in Earth's history when
its snow first fell. So by analyzing the
bubbles from any one layer, scientists can figure out
what Earth's atmosphere was like at any point in the past. JIM WHITE: Imagine you're
writing a book and every year you add a page to that book, and
that page stacks up and stacks and stacks up. The climate scientists
come along later and read that page
by page by page as we go down
through an ice sheet. NARRATOR: Reading this frozen
record of Earth's climate has revealed that the growth
and retreat of America's ice followed a distinct pattern. JIM WHITE: Over the last
million years or so, we see a fairly regular pacing
of major, major glaciations, roughly one every 100,000 years. NARRATOR: Some theories suggest
those colder spells coincide with variations in Earth's
orbit around the sun. But that's not the only
thing the ice cores revealed. The cores that White
has been studying suggest that the longer
the Ice Age went on, the worse it became. JIM WHITE: Somewhere
around 400,000 years ago, we went from a period that
had lesser glaciations, if you will, to ones that
were very, very big. NARRATOR: So far, the
geology detectives have uncovered evidence of how
America's ever-worsening Ice Age created some of
the continent's most significant features. Scratched rocks show how the
glaciers ground their way across North America and ice
cores record a 100,000-year cycle of freeze and thaw, and
temperatures that grew ever colder as the Ice Age continued. But new clues suggest that the
awesome impact of America's Ice Age didn't stop there. The investigation
next explores how ice distorted the
shape of the Earth and transformed the coastal
outline of the entire North American continent. 2 million years ago, the
average annual temperature fell dramatically. Moving glaciers spread
sheets of solid ice as far south as Illinois. Investigators set out to
discover how thick those ice sheets were and what the
weight of all that ice did to North America. Their first clue came in New
York State on Bear Mountain. No one knows for sure
how thick the ice sheet was that came from the northwest,
but right now we're standing on the top of Bear
Mountain, about 1,280 feet above sea level. What we're looking at here
are a series of chatter marks. These are features
of glacial erosion. They're produced by large
blocks of solid rock embedded in the base of a
very thick glacier. Those boulders impinged
on this bedrock surface, plucking pieces of rock off as
the glacial ice moved over Bear Mountain. NARRATOR: Charles Merguerian's
mountaintop discovery shows how thick the
advancing ice must've been. CHARLES MERGUERIAN: The glacial
ice sheet could care less about this large mountain. There's no question that
the glacial ice sheet travel over Bear Mountain as if
it weren't even there, and most people think that
this glacial ice sheet was about a mile thick or more. NARRATOR: The marks are clear
evidence that the New York ice sheet must have towered
about four times higher than the
Empire State Building. Figures like this have
enabled Merguerian to estimate the total amount of
ice that buried North America. It's a staggering 17
million cubic miles of ice. Imagine this block of ice--
it's about 6 inches thick-- being the ice sheet that
covered North America during the last glaciation. You need several billion of
these, maybe 100 billion. It's a number almost
too big to imagine. NARRATOR: This block
of ice weighs 27 pounds and is just half a
cubic foot in size. But there were 17 million cubic
miles of ice covering America. When scientists calculated
the total weight of the ice, it was an almost
incomprehensibly large figure-- 68,000 trillion tons. Unsurprisingly, such
a vast weight of ice did not just affect America,
it distorted the shape of the planet. I've got a beach
ball here that has a depiction of the whole Earth
and every continent on it. And right here is the
outline of North America. And during the Ice
Age, snow started to accumulate in Canada. And as that ice
thickened, it started to flow from the centers of
snow accumulation southward into North America, reaching
its maximum extent right here in Illinois. And when it did that,
this massive ice depress the continent. NARRATOR: Earth's solid
crust sits on top of a layer that scientists call the mantle,
semi-molten rock which behaves a little like hot,
sticky plastic. The weight of the
vast plains of ice pushed down inland areas
and simultaneously made parts of the coastline
bulge up from the sea. But investigators have also
found evidence of other ways that the ice altered the
outline of America's coasts, the borders with the sea that
defined the continent we see today. A vital clue came
in downtown Miami in the unlikely setting
of a municipal courthouse. KEVIN HELMLE:
We're here in front of this impressive building
built in the early 1900s because of the materials
used to face its surface. These stones are constructed
of fossilized coral reefs. And if you know what
you're looking at, it can actually tell you quite
a bit about Florida's past and maybe its future. NARRATOR: The coral stone is
made from the hard calcium carbonate skeletons of tiny sea
creatures, animals that only grow in very shallow water. But this courthouse
stone was quarried out of an ancient
coral reef that now lies a mile from the present day
ocean and way above sea level. KEVIN HELMLE: I'm standing here
about 5 or 6 feet above sea level and walking alongside
this rock face, which is about 6 feet tall. The interesting
thing about this rock face, it's actually composed
of fossilized corals. And the fact that we're
standing here today amongst these fossil corals
tells us that, in the past, we would have been under 10
or 15 feet of water right now. NARRATOR: The coral
stones have been dated to around 130,000
years ago, which means the seat
level around Miami must then have been
far higher than today. As experts tried to figure out
why the sea level was so high then, new evidence emerged
about Florida's coral reefs, and the mystery
deepened even further. Divers off the coast of
Miami found a coral reef now submerged 450 feet
under the water. These corals grew
just 25,000 years ago. And because a reef like this
only forms in shallow water, the scientists realized
that the sea level back then must have been far
lower than it is today. And that means the coastline
would have been in a completely different location. BERNHARD RIEGL: Right now we're
about 2 miles offshore of Fort Lauderdale, on the edge
of the Gulf stream. But 20,000 to 25,000 years
ago at the time of sea level lowest end, we would be sitting
right next to a beach here. We'd be looking-- where
we see today the ocean and the high rises
in the back, we'd be looking towards a
sandy beach in front of a gently rolling hillside. NARRATOR: Investigators studying
the coral records figured out that the up and down sea levels
would repeatedly have altered the entire shape of America. If you look at the outline
of North America, the outline that I learned to draw
in elementary school, if we were living years ago, I
would draw Florida differently. NARRATOR: 25,000 years ago,
when sea levels were low, Florida was triple the size it
is today because more land was out of the water. Geologists realized that
the changes in sea level around the coast were directly
related to the different levels of ice on land. That discovery was the key
to understanding what the Ice Age had done. Water evaporated from the oceans
and condensed in the atmosphere to form snow. But in the colder times, that
snow built up as ice sheets on the land. And with so much of the
world's total water supply locked up as ice, the oceans
simply ran short of water and the sea levels fell. When the Earth
is cold, there's more ice on the land and sea
level is lower because of that. When the Earth is warm, there's
less ice on the land and sea level is higher. And that makes
sense because when that ice, when it
melts off the land, has got to go in the ocean. NARRATOR: The story of how ice
sculpted modern North America is drawing closer
to the present day. Chatter marks in
upstate New York reveal that ice sheets
grew a mile or more thick, and ancient reefs off
the coast of Florida prove that water levels in the
ocean changed again and again. Ice had shaped the continent. Now scientists discovered
that it had also uncovered a bridge
to other lands and shaped the
future of mankind. North America's Ice Age
began 2 million years ago. As the Earth moved
around the sun, America's temperatures
fluctuated. 130,000 years ago, it enjoyed
a warm, tropical climate. Then, 25,000 years ago,
it grew colder than ever. Glaciers covered 2/3 of North
America with ice up to a mile deep. Now the investigation examines
how these freezing temperatures altered the course of human
history in North America. At Paisley Caves in
Oregon, scientists are unearthing vital clues
that humans left behind. Here's your coprolite. Beautiful specimen. NARRATOR: The
evidence is coprolite, fossilized human excrement. This site is the location
of the find of the oldest human remains that have
been directly dated here in the United States with DNA
in them that indicates people from probably Siberia came
here at least 14,300 years ago. NARRATOR: Today, North America
is separated from Siberia by open water. But the presence of men from
Siberia on American soil is compelling evidence that
around 14,000 years ago, a land bridge must have
linked the two continents. Geologists know that the
amount of water locked up as ice on land meant that there
was less water in the oceans. More importantly for
the future of the US, the retreating Pacific
and arctic oceans left the seabed between America
and Siberia high and dry. But a doubt still remains. The men who lived
in Paisley Caves 14,000 years ago
could conceivably have reached America by sea. But that theory is diminished by
the discovery of this artifact. This is a tool that has been
carved out of a bone belonging to a camelops, more commonly
called the Western camel. DENNIS JENKINS: You can see
the curvature of the bone here. This is the outside of the bone. It would have continued on in
quite a large arch all the way around. What they have done is
to saw this piece out, and then they have sawn in teeth
right here, running all the way across. NARRATOR: The animal that died
here was the ancient ancestor to the two-humped camel
now found in Asia. Few people know it
evolved in the US. DENNIS JENKINS:
Camels have been here for about 40 million years. So what has been
surprising to me is the fact that people didn't
know that camels were here. NARRATOR: In fact, the earliest
camel fossils ever found were discovered in South
Dakota and pre-date finds elsewhere in the world
by millions of years. The finds are clear evidence
that camels evolved in America and once lived nowhere
else on the planet. So camels, which couldn't
have migrated by sea, could only have crossed
into Asia via a land bridge. But a mystery remains. Fossil records show that camels
disappeared from North America 12,000 years ago. Scientists want to know why. DENNIS JENKINS:
There is no doubt, at the end of the last Ice Age
we saw a substantial reduction in the number of species here
in the Western hemisphere. So what causes those things? It's possible that weather
changed so dramatically, so quickly that they
could not adapt. NARRATOR: Back in Denver
in the ice core lab, Jim White is examining the piece
of evidence which could confirm that the Ice Age
killed off the camels. JIM WHITE: This is a really
fascinating piece of ice and one of the most
important pieces of ice that we have in the collection. This is the aha moment
of climate change. NARRATOR: The evidence is an
ice core, a frozen time capsule recording the temperature of
North America 12,000 years ago. It shows that the US
suddenly got even colder. JIM WHITE: We've opened
our book of climate. And if we read the book from
today, going back in time, that's page 12,000. And imagine, if you will,
from this moment back in time, we had 1,000 years of cold. NARRATOR: It's evidence
of the last gasp of the Ice Age, another
rapid freezing of America. JIM WHITE: These
abrupt climate changes can be a 10 degrees Celsius
change in temperature in 50 years or less, temperature
changes of a degree Celsius per year sustained
over several years. NARRATOR: That is a 16-degree
Fahrenheit fall in temperature. You're looking at going
from a Miami to Montreal kind of change in climate. NARRATOR: The big
chill occurred quickly, from mild to freezing cold
in less than a century. But it lasted
nearly 1,600 years. It was a deep freeze so
great that it proved too much for North America's
camels, mammoths, and other large animals. We're on a roller
coaster of climate change that must have been extremely
stressful for the plants and animals that were
living during that time. NARRATOR: This final deadly
big chill of the Ice Age ended 10,000 years ago. It was the last time ice
advanced across the continent. Scientists investigating
the legacy of the advance of America's ice sheets
have discovered coprolite, strong evidence that glaciers
caused the sea level to drop, exposing a land bridge that
linked Siberia to North America, and this ice
core which records the final deadly deep
freeze of the Ice Age. Next, scientists investigate
how the end of the big chill marks the beginning of the warm
period America enjoys today, and how that may spell disaster
for some of her greatest cities. Deep freezes over
2 million years have seen giant ice sheets
rampaging across North America. That ice crushed, scarred,
and flooded the land to create some of
America's most recognizable geological features. The Great Lakes, the mighty
Mississippi, and Niagara Falls were all created by the
action of advancing and then retreating ice. For the last 10,000
years, the Earth has experienced warmer
and warmer temperatures, and the passing of its ice has
impacted the North American continent in other
equally significant ways. STEVE BROWN: The ice changed
the existing conditions on the landscape. It's changed the way we
live in this environment. We get our drinking water
from these materials. Our rich agricultural soils
are a part of that legacy. The most productive
farmland in the world is right here in North America. From an economic standpoint
alone, the billions of dollars worth of
productivity of agriculture are a direct result
of the Ice Age. NARRATOR: During its Ice Age,
vast areas of North America were covered by glaciers. Today, they cover just
29,000 square miles. America's dwindling ice
is a precious resource. Each summer in
Washington state alone, melting glaciers provide
470 billion gallons of drinking water. But it comes at a price. The melting ice has pushed
up sea levels by a foot each century, and that
rate is increasing. And the scientists
who life the oceans are seeing firsthand
the impact it is having. BERNHARD RIEGL: The coral animal
itself builds a city, very much like we humans do. The coral had to move
as sea level changed. As sea level went down, it had
to follow that sea level down. As sea level came up, it had
to follow that sea level up. The question we have to ask
ourselves as humans, as sea level changes, will our
cities have to adapt just like the coral reef? NARRATOR: Currently, about
10% of the world's total land surface, approximately
6 million square miles, is covered by ice sheets. But this ice is melting fast. If present warming
trends continue, all the glaciers in Montana's
Glacier National Park will be gone by 2030. Today, North America's
longest glacier is the Bering Glacier in Alaska. It is 130 miles long and 10
miles wide, with solid ice up to half a mile deep. But over the past
20 years, that ice has become thinner, as much as
600 feet thinner at the point where the glacier
reaches the sea. So much ice is now melting
that every year it pours out at least 7 cubic
miles of floodwater, twice as much water as there
is in the entire length of the Colorado River. Over the same time,
Arctic sea ice has shrunk by 250 million
acres, an area the size of Texas and New Mexico combined. If Earth's ice continues
to melt at this rate, it will once again redraw the
map of America's coastline. This is what Florida
looked like in 1995. Here is what it could look like
in 100 years, a loss of more than four million acres,
where 1 and 1/2 million people lived in 2010. And at the ocean's
present rate of rise, this is the coast
of Florida as it might look at the end of the
next century, in the year 2200. This is Miami, or
what's left of it. By then, the city's
Ocean Drive might be a drive under the
ocean, a return to the sea levels of 130,000 years ago. The geologists investigating
America's ice age have found evidence
that the power of ice has been crucial in helping to
create modern North America. Bugs in sea mud acted
as tiny thermometers recording the start
of America's Ice Age. Moraine hills prove that moving
glaciers flowed far enough to cover 2/3 of the
continent under ice. Crescent-shaped marks on a
mountaintop in upstate New York showed that the ice sheets grew
to be more than a mile thick, and ancient coral
reefs were evidence that sea levels repeatedly
changed throughout America's 2 million-year-long Ice Age. The investigation has uncovered
the full picture of how ice sculpted the geology of
the US, but that process is not yet finished. As north America's
ice melts back, it uncovers a landscape that's
been buried for thousands, sometimes millions of
years, living proof that the Earth is never at rest.
