Making North America: Human

KIRK JOHNSON: North America, the land that we love.

It looks pretty familiar, don't you think?

♪ ♪ (rumbling) Well, think again!

♪ ♪ The ground we walk on is full of surprises, if you know where to look.

As a geologist, the Grand Canyon is perhaps the best place in the world.

Every single one of these layers tells its own story about what North America was like when that layer was deposited.

So, you ready for a little time traveling?

I'm Kirk Johnson, the director of the Smithsonian National Museum of Natural History.

And I'm taking off... Wow.

...on the field trip of a lifetime.

Wow, look at that rock right there.

That thing is crazy!

In this episode: North America is locked behind an enormous wall of ice.

How did the first humans to ever set foot on the continent manage to get in?

This is not the easiest thing in the world.

Once they got here, the challenges were daunting.

(trumpeting) But somehow, we turn the rocks of our homeland... Oh, man!

...into riches.

This thing is phenomenal.

But what challenges lie ahead?

Because our continent may be hiding some pretty dangerous secrets.

(explosion, water crashing) "Making North America: Human," right now on "NOVA."

♪ ♪ gleaming cities.

The incredible wealth of our land, supporting a population of over half a billion.

But rewind the clock just 15,000 years and our continent was truly a wild kingdom filled with amazing Ice Age creatures.

(roaring) Still, there was one animal found nearly everywhere but here... us.

So how, in the blink of a geologic eye, did we turn an unpopulated continent into a rich and dynamic civilization?

And what surprises does our homeland still hold in store?

The story of humans in North America begins with ice.

Alaska's Mendenhall Glacier, a 13-mile-long expanse thousands of feet thick.

This particular ice has been here for centuries.

It may not be as old as rock, but for a geologist, it's no less intriguing... if a bit risky.

Just a bad place to trip.

Yeah, I guess there's that, isn't there?

Yeah.

Once you start falling on this ice you keep falling until you hit something flat.

JOHNSON: 15,000 years ago, this is what much of North America looked like.

By then, humans had migrated from Africa to Europe, Asia, and Australia.

Not yet to the Americas, but that was about to change.

North America was in the grip of an ice age.

So much of the world's water was locked up in ice sheets that sea levels dropped, exposing a thousand-mile-wide expanse of land that connected Siberia to Alaska, the Bering Land Bridge.

In the summer, this would have been a huge, open expanse, allowing ancient hunter-gatherers from Siberia to migrate east into North America.

But the open landscape abruptly ended at the edge of a vast ice sheet in what is now southeastern Alaska.

Would they have tried to cross it?

Ready for this?

I think so.

All right, here we go.

A little bit higher and to your right and you should be good.

That's it, yup.

This is not the easiest thing in the world.

(breathless): You know, it's hard to imagine somebody crossing even one ice field like this, much less an entire ice sheet.

This is insane.

Glaciers are perilous, ever-shifting and often unstable.

Early explorers would have had a tough time getting very far.

Imagine trying to cross big ice sheets 100 miles wide, couple thousand feet high, into North America.

Based on this, I'm pretty sure the people, if they came into North America at that time, didn't go over the ice.

MAN: Looking good, buddy!

JOHNSON: So how did the first Americans wind up on the other side of this daunting barrier?

The traditional view is that when climate began to warm about 13,000 years ago, melting along a seam between two great inland ice sheets opened a corridor.

But there's another potential route to the rich land beyond the ice, first hinted at by a discovery made over 1,000 miles to the south off the coast of California.

25 miles from Santa Barbara lie the remote and windswept Channel Islands.

Here on Santa Rosa Island, a little over 50 years ago, a lone archaeologist stumbled upon some of the very earliest human remains ever found in North America.

Joining me is Joe Watkins, a National Park Service anthropologist and a member of the Native American Choctaw tribe.

He's taking me to a spot that has tremendous meaning to Native peoples and scientists alike.

This is a very important place; it's what brings us to Santa Rosa Island.

Well, that's a nice bluff right there.

Yeah.

JOHNSON: Right here in an area called Arlington Springs back in 1959, archaeologist Phil Orr spotted a couple of distinctive bones revealed in the weathered cliff.

Joe has a replica.

It's clearly a thighbone.

There's the knee joint and there should be a ball on the...

Here it is.

That fits on... that fits on right there.

So I've got a thighbone of a human.

Exactly, and there were two femurs that came out.

They come from a man about five-foot-one-inches tall.

But it's the age that has put the prehistory of North America on its ear.

JOHNSON: Named Arlington Man, these bones date to around 13,000 years ago.

They're among the oldest human remains ever to be discovered in North America.

More than 13,000 years ago puts these bones during the last ice age.

Exactly.

And that's what makes them significant.

JOHNSON: Significant because not only was Arlington Man one of the earliest known inhabitants of North America, but he lived and died here on an island miles off the coast.

WATKINS: That means that Arlington Man-- or his ancestors-- probably got here by boat.

That's an amazing fact.

They didn't walk here, they paddled here.

If boats were in use here, then they were probably in use all along the coastline, from Alaska to South America.

They may have been similar to traditional Inuit boats-- wooden frames covered with walrus hides-- agile and rugged vessels.

This kind of technology could have opened a way south even before the ice sheets melted.

WATKINS: If you figure boats into the equation, it really opens up a whole different set of possibilities.

You don't have to go through the ice, you can just paddle down the coastline.

JOHNSON: Using boats to probe the shore could have opened the way for seafaring hunters to settle along the western edge of the continent.

And as they made their way into this unexplored land, what untapped riches would they have found?

Luckily for us, there are a wealth of clues in the unique geology of Southern California... ...extracted over the years from an iconic tourist attraction, the La Brea Tar Pits.

This is one of the greatest fossil sites in the world, and it's right in the middle of Los Angeles.

Shallow pools of natural sticky asphalt have been bubbling up from below the surface here for tens of thousands of years, creating a lethal trap for unsuspecting animals and a perfect graveyard for their bones.

Hey, Carrie, how's it going?

Hi, Kirk, how are you?

JOHNSON: Carrie Howard is a paleontologist working with an incredible trove of animal fossils all pulled from the pits.

What these bones tell us is that this area was once a great hunting ground-- and not just for ancient people.

Wow, that's a nice skull.

Oh, yeah, this is a saber tooth cat skull.

It sure is!

(laughs) As you can tell.

It would have stabbed or sliced its prey.

In one, in, boom you're done.

Wow, so think about that, just about the time that people showed up in North America for the first time, these guys were waiting to greet them.

(laughing): A bad day in California.

More than a million bones have been found here in the last century-- the largest, from huge mammoths that weighed up to 12,000 pounds.

All told, something like 620 different species of plants and animals have been identified.

Bison scapulae.

Horse neck vertebrae.

More bison leg bones.

These are really great.

Wow, look at those things, that's a huge animal.

Big camel vertebra.

Wow.

Toes of camels and more camels, and more camels!

There are a lot of camels here.

HOWARD: Yeah, not many people know camels originated in North America.

JOHNSON: It's so amazing to hear there were camels here in Los Angeles.

(trumpeting) For ice age hunters, the mammoth would have been especially valued, each one providing thousands of pounds of meat-- if you could bring it down.

What kind of weapon could have evened the odds?

To track down an answer, I head to Boulder, Colorado.

A few years ago, Patrick Mahaffy was having some landscape work done behind his house.

One day, when he checked in with the work crew, he got an unexpected report.

PATRICK MAHAFFY: I got home from work.

It was a very busy day for them, and I asked, you know, like you do on a project, "How did the day go?"

and they went through this list of all the things that they'd done.

And then they said, "Oh, and there's one last thing.

We found something."

JOHNSON: They found what seemed to be an intentionally buried cache of mysteriously shaped stones.

DOUG BAMFORTH: That one's amazing.

That is amazing.

JOHNSON: Archaeologist Doug Bamforth was called in to take a look.

I regularly get phone calls about people's discoveries.

But I don't get very excited, because they usually don't turn into much of anything.

But then I went up the next day and it was just breathtaking.

JOHNSON: An 83-piece stone age tool kit, each piece carved by ancient hands.

BAMFORTH: There's a good sharp edge.

You could only hold it between the tips of your fingers, but you could do light cutting.

JOHNSON: 13,000 years ago, these sharpened stones would have been the most advanced technology on the continent.

BAMFORTH: You have to imagine a world with no metal in it.

All the things that we rely on to cut things and scrape things and do almost all of our physical work, right, the fundamental material that we use, did not exist in the world.

JOHNSON: Like other early human societies, North Americans relied on stone.

So Doug wanted to know how old were the tools in this collection?

To find out, Doug decided to test for DNA residue that might give him a clue about the age.

Incredibly, he found traces of blood from several species of animals.

Two in particular surprised him.

He found DNA from the ancestors of modern-day horses and camels-- species that once lived in North America around the end of the Ice Age, clear evidence that these tools were once used by some of the earliest American hunters.

BAMFORTH: Getting artifacts that have blood residue on them from camels and horses tells us that these are among the oldest tools we know about in North America.

So evidence of some of the very earliest people who were here.

JOHNSON: The most lethal hunting weapon they devised was a spear tip known as a Clovis point, named after the site where it was first discovered in New Mexico.

Clovis points all share a common design: a symmetrical, fluted shape with sharp edges on both faces.

More than 4,000 Clovis points have been found all across North America, a sign that the first Americans traded weapons-grade rock and the latest hunting technology over hundreds of miles.

Bob Patten has been crafting Clovis point replicas for over 40 years, and he's offered to show me just how tricky it can be to coax a sharp-edged weapon from a stone.

BOB PATTEN: These are antler.

Oh, I see.

That catches the blow.

And I want you to reach forward a little bit.

There you go.

Wow, my first flake!

Awesome!

JOHNSON: We're using a rock called chert.

It's similar to flint and was favored by stone age toolmakers because of a very special property: with a bit of persuasion, the rock will break off razor-sharp flakes.

PATTEN: This is better than an X-Acto knife.

Wow, you're cutting leather with a rock.

You can cut with great precision there, too.

I can see the spear point in there, I just can't quite get to it yet.

JOHNSON: Bob's going to put the finishing touches on my Clovis point.

He seems to have a knack for finding just the right spot to hit the stone.

It's a very subtle thing, and you have to train yourself in order to do this consistently.

Now we've got a tool we can go hunt a mammoth with.

Oh, you'd put it on something fairly long because a mammoth is a big animal that you don't want to get too close to until he's dead.

JOHNSON: It's hard to imagine how a relatively small stone could bring down an animal the size of an elephant.

To test its killing power, I'm going to use a block of ballistics gel.

It's a material used to test firearms, because it has the same consistency as flesh.

You've got the mammoth, I've got the spear.

JOHNSON: A piece of animal hide covers the front of the ballistics gel, so I'll be able to see how well the spear point penetrates skin, too.

All right, so the angle is here.

It's like shooting a pool cue.

And now.

The blade went in like it was going into a block of butter, and when it hit the actual shaft that's when it stopped.

Well, I think we know why that style of point was so successful.

Yeah, that thing really worked.

It does cut.

And we should be ready to go and conquer the continent.

JOHNSON: Clovis points have been found embedded within the bones of mammoths, demonstrating just how successful these weapons could be in bringing down even the biggest beasts.

These Clovis spearheads are exquisite artifacts, just beautiful things to behold.

But in reality they were lethal killing machines.

By 12,000 years ago, mammoths, saber-toothed cats, dire wolves, camels and many other large mammals had all but disappeared, likely the victims of an expanding human population and a changing climate.

But others thrived on a landscape that offered more than a million square miles of grass-covered prairie-- home to a North American icon.

WATKINS: On the Great Plains, these oceans of grasses were the primary food for millions of bison.

These bison were the supermarket on the hoofs for the nomadic Native American tribes that hunted them.

The bison hides could be used for housing.

They made them into teepees that could be put up and taken down in a matter of minutes, just so that the Native American tribes could follow the herds from one spot to the next.

For 10,000 years, this was their staple food.

JOHNSON: The geology of North America created a landscape ideally suited to support abundant animal life, sustaining a growing human population.

But not all Native Americans hunted for a living.

In Colorado's Mesa Verde National Park, I've come to see a genuine wonder of the ancient world.

Tucked away in these rocky hills stands an 800-year-old architectural masterpiece called Cliff Palace.

(laughing): Oh man, that is so awesome.

Built from finely laid sandstone slabs, pine beams and mortar beneath natural overhangs, this 150-room complex was home to the ancient Puebloans, forbearers of today's Puebloan peoples.

This is a massive set of structures here.

So these were a people that really invested in real estate.

SCOTT TRAVIS (laughing): Yeah.

JOHNSON: Scott Travis is the park archaeologist.

It's characteristic of cliff dwellings at Mesa Verde to have this type of scale, but nothing approaches the sheer size of this particular site.

JOHNSON: And this original cavelike structure is a natural formation in the sandstone.

TRAVIS: It's just a natural part of the evolution of these canyon systems.

JOHNSON: These formations-- along with others like them-- provided shelter for thousands.

And for centuries, the people here worked the land as farmers, even though conditions were often tough.

TRAVIS: Mesa Verde is a wonderful example of how the Ancestral Puebloans took advantage of their local situation.

Not only did they create remarkable architecture within the alcoves, they were able to farm in one of the harshest environments on this continent.

(thunder) They did this with a combination of expert geological knowledge and the ability to control scarce water resources to farm in a situation that is difficult even today.

JOHNSON: And other farming cultures did even better, using their resource wealth to build big cities and giant places of worship.

Like the builders of the mysterious mounds at Cahokia, Illinois, who raised corn to support a city of 15,000.

Or the Maya of Central America, who built canals and irrigation networks that sustained great stone cities.

Hundreds of thousands lived in the Aztec capital in Central Mexico at the end of the 15th century.

But the story of human interaction with the geology of North America was about to be turned on its head by the arrival of outsiders.

Western European explorers opened the way for conquerors, colonists, and their slaves, who would all ultimately displace most of the Native population.

What drew so many to voyage so far into an unknown world?

To find out, I'm traveling to North Carolina to a research station that studies a native plant that enticed European settlers with a promise of riches.

It's pretty cool.

I have never been in a tobacco field before.

This is the plant that got North America started as a Btish colony.

Sort of the first cash crop.

It was the one that economically could keep the early colonists in business and in great part, because there was a perfect storm of the right plant, brought to the right place, in the right climate, at the right time, with, importantly, the right kind of soil for growing it.

Huh.

JOHNSON: Dave Montgomery studies how the earth's thinnest and most fragile geological layer has shaped the destinies of human societies-- including ours.

So what is soil?

So soil is sort of the frontier between the worlds of geology and the world of biology.

It's that interface, and it's made dominantly of rotten rocks.

JOHNSON: But what kind of rotten rock?

Dave reveals the recipe that makes life on land and all agriculture possible.

The first ingredient is sand.

JOHNSON: Sand is any mineral ground down into tiny grains by natural wear and tear.

This looks like a nice beach sand, it's just little quartz grains.

Little clear grains of quartz is what I am seeing.

Yeah, quartz and feldspar.

It's not unlike a California beach.

JOHNSON: Silt is next.

Made of the same stuff as sand, but ground much finer.

MONTGOMERY: It's too small to really see, but you can feel the grit.

If you put a little on your teeth you'll feel the grit between your teeth.

Oh, yeah.

It's really gritty.

I don't really like eating silt.

No.

JOHNSON: The finest is clay, made up of mineral crystals so tiny, they're visible only with a microscope.

This is all geology still.

This is all geology, this is the basis, the backbone of making a soil.

And the geological part, some combination of these three components makes up about 40% to 50% of the volume of most soils.

So what's the rest?

JOHNSON: How about the rich black stuff we associate with fertile soil?

The stuff of life-- or, actually, of death.

So it's nice and dark.

It looks like coffee grounds.

So it's rotted plant matter.

You've got pieces of plants... Oh, this smells nasty.

I guess it's dead stuff though, right?

It's dead stuff.

That's why it smells so good.

I'm going to put this back in the jar.

So that's it, these four things?

Well, then there's also living matter, which in this case, we have worms.

So you can think of them the way that Charles Darwin did, as God's plowmen.

They basically plow the fields.

Underground, they're like little underground miners.

JOHNSON: There's one last ingredient, a geologist's best friend.

We have time in a jar here?

We got time in a bottle here.

We're not gonna open it.

How much time do you have in that bottle?

There's only one way to find out.

Don't open that jar.

It can be a million years in that jar.

It could be.

JOHNSON: The British colonists got lucky when they chose to plant tobacco here.

For tens of thousands of years, nature had been preparing the soil along the Southeast coast, giving it all the right ingredients, especially an extra helping of sand.

Perfect for growing the original wacky weed.

But they soon paid a price.

It's not hard to see what growing millions of pounds of tobacco to ship back home did to the land.

Even today, comparing soil from the forest to soil from the tobacco field tells the whole story.

So over here on this side, we've got the forest soil, which is probably a lot more like the native soil was like when colonial agriculture arrived in the New World.

Yeah, lots of organic matter in it and roots and twigs and stuff like that.

You notice the dark color relative to the soil next to it, which is from the conventionally plowed tobacco field.

JOHNSON: The sickly yellow color shows the soil's exhaustion.

These fields are lush today thanks to chemical fertilizers.

But the colonists had no such tricks, and growers soon hit a wall.

MONTGOMERY: The erosive effects of colonial agriculture were so apparent on the American landscape that people at the highest levels of American society were very concerned about what it meant for the future of the country.

Washington even wrote in a letter in the 1790s to Alexander Hamilton about his prediction that American society would be compelled to push inland, to push westward, due to the search for fresh and fertile soils after having worn out the soils along the Eastern seaboard.

JOHNSON: But the founding fathers needn't have worried.

Because migrating farmers would soon discover the Great Plains, with some of the best soil in the world.

MONTGOMERY: Dirt is destiny in that sense, where degrading the soils of the Eastern seaboard and then opening up the fresh fertile soils in the American Midwest served like a great magnet pulling people westward towards the source of fertility and prosperity in the heart of the country.

JOHNSON: The great magnet of North American soil exerted an irresistible pull on those with a pioneering spirit.

But the westward expansion also got a major shot of adrenaline with the discovery of a new gift from the geology of the continent: gold.

In 1848, Mexico ceded to the U.S. a territory that included what would become the state of California.

At the time, San Francisco was little more than a military garrison with a population under 1,000.

But that changed when James Marshall found nuggets of gold in a streambed at Sutter's Mill in Coloma, California.

When word got out, California became a hot destination.

In 1849, about 80,000 people, called '49ers, joined the gold rush.

Over the next six years, about 200,000 more followed, turning San Francisco into one of the largest cities on the continent.

By then, prospectors had snatched up pretty much all the gold to be found just lying around, and so the party had to move underground.

Pretty sweet place here.

Yes, it's beautiful here.

So they started mining around here in 1849... JOHNSON: Lisa White is a geologist and a native San Franciscan.

Her passion for California's amazing geological history was bred in the bone.

Let's go have a look.

JOHNSON: Who better to show me this historically preserved mine in the Sierra Nevada mountains?

The miners must have been short guys.

Well, I fit right in here!

Yeah, I guess you do!

Not me.

WHITE: This is such an incredible mine for so many different reasons.

Unbelievably, they dug out the earliest part of this mine by hand.

Wow, look at that, you can actually see the pick marks where they carved this thing.

As Lisa leads me further into the mountain, we spot a change in the rock.

Ah, here we go.

JOHNSON: That's something different.

WHITE: It is, it is.

It's a quartz vein, and it's important and distinctive because when the miners would locate these quartz veins, they knew they were in the area they wanted to be for gold.

So the gold is actually in the quartz?

The gold is in the quartz, yes it is, and so looking for these was the way to the gold.

I've got a quartz sample here in my pocket with some gold flakes in it.

Wow, there's a lot of gold in there actually, wow.

It's really beautiful.

Yeah, if you like gold!

But the gold is in the quartz and not in the rock next to the quartz?

Exactly, so that's key to understanding its formation.

JOHNSON: When Earth was forming, most heavy metals like iron and gold sank to the molten core.

But as Earth cooled, small amounts remained in the rocky mantle.

Later, asteroids deposited more of these metals.

But for gold to work its way back up to the surface, it had to hitch a ride on some kind of geological shake-up, like an earthquake.

WOMAN (on phone): 9-1-1.

MAN: ...Cypress section of the freeway has come down.

JOHNSON: Violent earthquakes like this one in San Francisco cause serious damage above ground.

But they also wreak havoc below the surface.

Most people know that California is earthquake country, but even in the past, earthquakes were key because earthquakes would fracture the rocks.

And in many ways, the cracks, they're like pathways for fluids from deep within the earth.

JOHNSON: Deep below, millions of years of earthquakes and pressure from molten rock have created a network of cracks.

These provide pathways for superheated water full of minerals, like gold from deep in the earth.

When that hot fluid rises up through the fractured rock, it cools down, and the minerals carried within crystalize.

Over time, that builds up a vein of quartz, and trapped inside the quartz are bits of gold.

Over millions of years, they formed the seams found all over California.

WHITE: So every time you look at a vein of quartz you're really seeing an ancient earthquake in many ways.

JOHNSON: I've always thought of earthquakes as very destructive kinds of things, but looking at this, if earthquakes are involved in the formation of gold, maybe there's a silver lining-- I guess a gold lining-- to earthquakes.

The California Gold Rush didn't last very long.

But between 1848 and 1860, it's estimated that more than one million pounds of gold were found in these hills.

Over the following years, much of it made its way right here to the Old Mint in San Francisco, affectionately called The Granite Lady.

This building once held the machinery that turned California gold into beautiful coins.

It's no longer in operation, but there's a vault here holding something that these days is truly rare.

Oh, man.

This thing is phenomenal.

A massive nugget of gold, the largest to be found in California in decades.

Worth around $400,000!

It was probably buried about 40 million years ago, and it was dug up a few months ago.

Gold is so dense that it doesn't look that large, but it feels really heavy.

It's like five pounds packed into the palm of my hand.

You feel both the power of the earth and geology and chemistry, but you also feel the weight of human history.

Gold has driven the behavior of humans since it was first found.

I actually have gold fever right now.

I want to take a bite out of this thing.

It's an amazing thing.

Striking it rich in gold or silver was the dream of thousands of prospectors.

But what other treasures did the geology of the continent have to offer?

Turns out the real money was about to be made back east, in the iron and coal mines poised to power the nation's next great leap.

(whistling) By the 1860s, the industrialization of the American economy was going full steam, literally.

Unbelievable.

Perhaps the one technology that best symbolized the massive transformation underway was this: the steam locomotive.

In 1863, the U.S. government launched an ambitious effort: to unite the coasts with a transcontinental railroad.

Two railroad companies set out to meet in the middle.

Progress from the east was relatively rapid thanks to the wide-open prairie and a natural pass through the Rockies.

But construction of the western line progressed much more slowly, impeded by the Sierra Nevada Mountains.

The only option: cut a pass by hand.

From this logging train, we can see just what they were up against.

This is granite, an incredibly hard rock, and the people that had to build the first railroad across the Sierra Nevadas cut through this rock with hand tools, sledge hamms, steel drills, and explosives.

Incredibly hard work.

(hammer ringing) They relied heavily on Chinese immigrant laborers.

In harsh conditions, they carved 15 tunnels, largely using hand tools and dynamite.

(explosion) The longest stretched about a third of a mile and took 15 months to complete.

Eventually, the two lines met in Utah on May 10, 1869.

It had taken six years, but America's new East-West railroad was open for business.

The building of the transcontinental railroad was an epic endeavor.

But once it was done, it linked the United States from coast to coast, and within 50 years of that, there was a quarter of a million miles of railroad tracks spanning the continent.

And with the two coasts linked, the economy roared, fed by the continent's abundant geological gifts of coal, metals, and other resources, and eventually by a powerful new fuel.

I've come to the great city of Los Angeles.

It's an amazing metropolitan area.

Something like 18 million people live here.

That's a lot of people, and they use a lot of resources.

It's no secret that Los Angeles has an insatiable thirst for oil.

But what's less well known is the area's history as a major producer.

That story takes me back to the La Brea tar pits.

The natural asphalt that trapped so many ancient animals is actually a form of oil.

Paleontologist Carrie Howard takes me into one of the fossil pits to see LA's black gold up close.

I'm just dying to get down in there and scrape some of those bones right now.

But if I did that, I would actually probably be stuck, right?

HOWARD: Yeah, even if you just stepped right there, don't be fooled.

Even, like, an inch could stick your shoe, and then you'd definitely need help.

JOHNSON: This sticky oil was produced by the slow accumulation and burial of marine plankton over millions of years.

Compressed and heated underground and turned into thick oil, it's managed to seep upwards to bubble up here.

It may not seem like much, but there's a lot more where this came from.

These tar pits with their fantastic fossils are just the tip of the iceberg, because deep beneath the streets is a tremendous amount of oil.

Once Californians realized the scale of this geological treasure, they kinda went nuts exploiting it.

What I've got here is a picture from 1901 of this very spot, and I can see a house down there that's in this photograph.

But what's not here today are the dozens of oil derricks that were here in 1901.

This place was an incredible oil field.

Oil is really the DNA of Los Angeles.

It was discovered here first in 1892, and it kicked off an amazing oil boom.

And even today, Los Angeles is defined by oil.

In and around Los Angeles, there are oil wells almost everywhere.

Pretty amazing.

Right here in the middle of a neighborhood is full-on oil rigs.

There's a house and there's a pump jack, and a house and a pump jack.

Looking down, and it's a great view of the city of Los Angeles.

There's oil rigs all over the place down there.

Los Angeles County has more than 3,000 active oil wells tirelessly pumping more than 14 million barrels a year.

But that's just a tiny fraction of the total North American output.

From Mexico to the oil sands of Alberta, Canada, and the Arctic Coast, we're pumping oil and natural gas out of the ground at a record rate.

Which begs the question: should we?

When we burn fossil fuels, the release of carbon dioxide causes Earth's atmosphere and oceans to warm.

As a result, glaciers are melting and sea levels are rising much faster than at the end of the last ice age.

Our impact on the planet is a huge concern, but as a geologist, I also worry about the nasty surprises our continent has in store for us.

In the Pacific Northwest, there are chilling clues about a future natural disaster.

This is an old-growth rainforest.

Some of these trees are more than 500 years old, and they've witnessed events we can barely imagine.

And it's trees like these not too far from here that played a key role in resving a mystery of epic proportions.

Meet geologist Brian Atwater.

He and I are paddling up the Copalis, a coastal river that rises and falls with Pacific Ocean tides.

We're here at low tide.

But at high tide, the river can rise enough to overflow its banks, flooding the surrounding marshes with salt water.

But it wasn't always that way.

Once, this was a lush coastal rainforest.

Now, all that remains are massive roots sticking out of the eroded riverbank and the trunks of long-dead cedar trees-- a ghost forest.

Brian has brought me here to show me evidence of one of the worst earthquakes to hit North America since human beings arrived here.

He's found signs in a place where geologists feel right at home: in the mud.

Slicing into the bank reveals three layers.

They tell a story of change over time.

The lowest once supported a healthy rain forest.

BRIAN ATWATER: So this is almost like a garden soil, right?

But it's got tree roots in it.

It's a forest floor soil.

JOHNSON: Just above is a layer of sand entirely out of place.

Its sharp definition tells Atwater that whatever put it here came fast and furious.

You don't see sign of a gradual change from here because there's such a black and white difference between this and this.

How do we do this combination?

How do we go from forest floor to some kind of muddy flat and have a sand layer brought in first, right at that time?

JOHNSON: The answer lies about 80 miles away at the bottom of the Pacific Ocean.

It's called the Cascadia subduction zone, a 700-mile-long crack in the crust of the planet.

It's where a Pacific Ocean plate is trying to slide under the North American plate.

(plates scraping) But the plates are stuck.

So the story there, you know, is with the down-going oceanic plate and the overriding continental plate stuck together here, they go closer and closer like that, and the overriding plate gets shortened and bulges up.

JOHNSON: The old growth forest that once stood around the Copalis River sat on that bulge.

But then the plates broke free along the fault, causing a violent earthquake, dropping 600 miles of coastline as much as five feet and into the tidal zone, where it sits today.

The local landscape drops.

The ghost forest goes for a swim.

That's it for them.

But the disaster wasn't over.

That same tectonic rupture also drove the edge of the continental plate upward and triggered a series of huge waves-- a tsunami.

The tsunami comes in.

That's the first thing the forest gets to see.

JOHNSON: What the forest sees is a rush of salt water and sand inundating the land, the final blow from a massive fault rupture that turned an old-growth rainforest... ...into this.

We now know that this cataclysmic one-two punch took place in the year 1700.

Today, everyone wants to know: will it happen again?

Is the Pacific Northwest living on borrowed time?

Tsunami is running.

JOHNSON: At Oregon State University, Chris Goldfinger studies deep-sea earthquakes that can cause devastating tsunamis.

He's examined hundreds of sediment cores from the sea floor, sampled all along the 700-mile Cascadia subduction zone.

Analyzing each one, he's assembled a history of earthquakes going back 10,000 years.

And the news is not good.

Chris estimates that severe earthquakes strike somewhere along the fault line about every 240 years, most often in Oregon and Northern California.

We're now 315 years into a 240-year average, and so that drives the probability up quite a bit.

It's up around 37% in the next 50 years.

(rumbling) JOHNSON: Today, seismologists warn that the next big one in the Northwest could be even more destructive than the earthquake and tsunami that struck Japan in 2011.

Luckily, there is time to prepare, thanks to a warning from an ancient disaster written in the land.

Geology gives us many, many examples where what we've learned from interrogating the earth's past prepares us for the earth's future.

The relentless forces beneath the earth will continue shaping our continent far into the future, as they have from the very beginning, since the first land formed from molten rock and the pieces of our homeland slid into place.

As seas and mountains rose and fell, as creatures small and mighty lived and died here, geology has profoundly shaped our destinies ever since we set foot upon these shores-- soil, oil, minerals.

The power of the rocks beneath our feet cannot be ignored.

So what can we expect?

Not just in our lifetimes, but over the serious long haul?

Well, over the next 175 million years, geologists predict that North America will slowly converge with Europe and Africa until eventually, the Atlantic will completely close.

And once again, we'll be part of one giant super continent: Pangea Ultima.

North America today is just the middle of this continent's long and interesting story, and it will go on for tens of millions and hundreds of millions of years into the future.

Because in geology, one thing is for sure: no landscape is permanent.

The investigation continues on line ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪