How the Earth Was Made (2009) s01e05 Episode Script
New York
Earth, a 4.
5- Billion-year-old planet, still evolving.
As continents shift and clash, volcanoes erupt and 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 450-million-year-old geological history of New York City is explored.
A metropolis pockmarked with strange rocks, haunted by footprints of ancient giant reptiles, and lined with a vast curtain of solidified lava.
Scientists investigate the evidence for fiery volcanoes, massive floods and ice sheets four times as high as the Empire State building.
The clues to understanding New York City's geological past provides a window into the formation of the earth itself.
The investigation into New York City's geological history begins here, with Manhattan's rocky outcrops.
These rocks are clues to how the land was made and how its geology helped it become a dense, thriving, pulsating city.
They're scattered all over Manhattan, poking through the surface of parks and through the concrete between the buildings.
Some, squashed between two apartment blocks, are the size of a whale.
They are the extraordinary survivors of ancient times.
Most importantly, they are the surface tips of the bedrock in which Manhattan's buildings are anchored.
Gigantic skyscrapers stand in two clusters, in downtown and midtown.
In the section between, the buildings are lower.
The clues to the shape of Manhattan's familiar skyline are the rocks beneath the surface.
A leading expert on the rocks in New York is geologist Charles Merguerian.
The entire history of the development of the earth's crust is emblazoned in the rocks beneath us.
The rocks here in New York City harbour an ancestry that dates back over a billion years of time.
Merguerian is searching for evidence to show how the city's bedrock was made.
At Inwood Hill Park in Upper Manhattan, he's found an extremely hard piece of the bedrock known as Manhattan schist.
To the untrained eye, it's just a piece of rock, but to Merguerian, this is his first clue.
The rocks that we're looking at right here are rocks of the Manhattan schist formation and the these rocks are very severely deformed, and the structures here in this rock is a structure that comes up like this, bends around and comes back down on itself as such, and in three-dimensional view, it's a structure that looks something like this.
A very, very tight fold with a plunge towards the south here.
These are rocks that were very, very strongly deformed over protracted periods of time.
And it's the same bedrock that occurs over much of New York City.
This tight fold in the rock suggests New York's bedrock was formed under great pressure.
To confirm this hunch, Merguerian takes a sample to the lab for detailed analysis.
Radiometric dating proves this rock is about 450 million years old.
But the rock has even greater secrets to tell.
It contains a kaleidoscope of minerals, which opens a window into the ancient world.
To me, minerals are like the instrument cluster in your car, they tell you everything about how your car is running.
Merguerian uses a microscope with polarised light to view the minerals.
The examination tells us the former depth regime, how deep the rocks were, they tell you the age of the rocks, they tell you everything you want to know about the development of the earth's crust.
What's striking about these samples is that the minerals inside are elongated.
It is a clue that these rocks must once have been crushed by massive forces.
And the colours support this theory.
Under the polarised light, the sample from Inwood Hill Park shows up blue.
This comes from a mineral called kyanite, which forms at great depths.
It's conclusive evidence that this rock was compressed deep under the surface.
Rocks forged at these depths are much harder, ideal for a city's foundations.
But what gigantic weight was on top? Merguerian believes there is only one answer.
The rock was once buried under the crushing weight of a chain of massive mountains.
The minerals that we find in the bedrock units of New York City tell us that the rocks of New York City were formerly buried when they were formed, under very high pressures, and that those high pressures indicate that these rocks formerly were produced at depths of 20 to 25 miles, and probably the mountains were as high as the Alps are today.
But even the most impressive mountain chains can't survive the ravages of time.
The Rocky Mountains, for example.
Millions of years ago, they soared nearly six miles into the sky.
Today, erosion has halved their size.
The same process happened in New York.
Rain, wind and ice wore the ancient mountains almost flat.
But the microscopic crystals found in the rock in Manhattan testify that they existed in the past.
How did the mountains form? The answer lies in the way the earth's crust moves.
A network of interlocking individual pieces makes up the Earth's surface.
Geologists call them tectonic plates.
Over millions of years, they collide and break apart to form different continents.
surface looked completely different.
North America was much further to the south.
MERGUERIAN: North America was tilted 90 degrees clockwise from its present orientation and it was straddling the equator.
As such, the climate was tropical, the east coast of North America was really experiencing Club Med conditions.
The weather may have been awesome, but the ancient East Coast was heading for trouble.
The plate beneath it was moving.
The East Coast was on a collision course with ancient West Africa.
The impact unleashed geological chaos.
Under intense compression, the land was forced upwards to form a soaring range of mountains.
The collision that took place is the most fundamental and impressive mountain-building event to affect the east coast of North America.
Today, all that remains are their stumps, stumps that form the bedrock of modern-day New York.
The collision that built up the ancient mountains also folded the bedrock into dips and rises.
These folds are responsible for the shape of Manhattan's skyline.
The city boasts two clusters of skyscrapers in downtown and midtown.
Here, the hard bedrock that formed deep underground was forced up.
It is now close to the surface and provides solid anchorage for the high-rise buildings.
In the dip in the middle the rock was folded down.
The area is filled with loose sediments, less suitable for skyscrapers.
MERGUERIAN: When the bedrock is at the Earth's surface where it's actually exposed, then it's pretty easy to build tall buildings 'cause you can root them directly into solid rock.
However, in areas where the bedrock is deep and covered by glacial sediment, in those cases, it's very difficult to build tall buildings because you need to root those buildings either into solid rock or build concrete abutments called caissons that can support tall buildings.
New York's deep history is beginning to take shape.
Building up from tiny crystals in the rock, scientists revealed how New York's bedrock was formed under the crushing weight of a massive ancient mountain range.
The result was hard Manhattan schist, a perfect foundation for the city's skyscrapers.
But New York City still had a long way to go.
The colliding plates created an enormous landmass - the last great supercontinent, called Pangaea.
New York was now trapped in the centre but somehow it made it back to the coast.
Hidden beyond the city's streets is evidence of huge volcanic eruptions, mass extinctions and continents torn into pieces.
Clues that could explain how New York became one of the world's great maritime cities.
Investigators are piecing together how New York City's unique geology was formed.
Much of its early success as a trade and commerce centre is owed to its deep-water harbour and its location at the coast.
But 450 million years ago, things were different.
The area of New York City was landlocked, embedded in the heart of a huge supercontinent.
How did it get to the coast? The investigation fast-forwards In a quarry in New Jersey, paleontologist Paul Olson unearths the first of a string of clues that could explain how New York City reached the coast.
A giant fossilised footprint.
This is the footprint, actually the mud that filled in the footprint, of a four-footed crocodile relative that was the dominant carnivore during the late Triassic.
You can see the toes here have little pads on them and here's the handprint, and these animals would have been, in this case, about the size of a modest crocodile, but some of them became much, much larger, the size even of a T.
Rex.
(ROARS) The footprints are from a huge crocodile-like creature called Postosuchus.
It first appeared on the Earth around 230 million years ago.
Then, some 30 million years later, its footprints suddenly vanished.
But Postosuchus wasn't alone.
Half of all land animals perished at the same time.
The fossil evidence proves it to be one of the biggest mass extinctions ever recorded.
The evidence for this mass extinction is that we have lots and lots of fossils right in this area.
And what you see is especially in the in the reptile footprints, you see one group of forms, the forms that are related to crocodilians, disappear.
Whatever caused the mass extinction must have been a catastrophic event.
Olsen had a hunch that the mass extinction was somehow related to New York's return to the coast.
The ancient area of New York sat on a line of great weakness, the plate boundary where two continents joined to form the supercontinent Pangaea.
And it was unstable, prone to earthquakes and volcanoes.
Olsen's quest - to find the evidence for the natural disaster that finished off Postosuchus A band of dark rock above the footprints in the New Jersey quarry caught his eye.
It was basalt, the smoking gun Olson was looking for.
Basalt is a volcanic rock.
It forms when hot lava erupts onto the surface and cools.
Did the volcanoes that forged this basalt trigger the mass extinction and also rip Pangaea apart? On its own, the evidence at the quarry was unconvincing.
The layer of basalt is only a few feet thick.
To prove mass lava flows caused this global catastrophe, scientists needed corroborative evidence.
High above the Hudson River, geologist Matt Gorring follows another lead.
He's studying the Palisades, a dramatic geologic feature that hugs the Hudson River, beginning across mid Manhattan and running into northeast New Jersey.
They too are made of basaltic rock, the same rock implicated in the mass extinction of land animals.
But the Palisades are on an altogether different scale.
The Palisades are a sheet of basaltic magma, about 1,000 feet thick, it's about 40 miles long, so it's a very prominent set of cliffs that run all the way up the west side of the Hudson River.
Here is proof of massive volcanic activity.
Hot lava flooded out of ruptures in the Earth's crust and covered ancient North America in a mile-deep sheet.
The lava cracked as it cooled.
The vertical ruptures formed regular pencil-shaped columns.
These distinctive rock formations have been known to geologists since the 19th century.
Intriguingly, they appear on both sides of the Atlantic, in North and South America, Europe and Africa.
Geologists suspected that their presence pointed to the spot where Africa and Europe separated from America.
But that was just an unproven theory, until the 1950s, when scientists developed a revolutionary technique called paleomagnetism.
Now, they could study the magnetic properties of rocks.
Many rocks, including basalt, have a distinctive magnetic signature, formed as the rock is born.
Tiny crystals inside the rock act like compass needles.
When the magma that forms the rock is fluid, the crystals align to the Earth's magnetic field, pointing north.
As the rock solidifies, the crystals freeze, forever locked in that fixed magnetic alignment.
As continents move and the rocks travel, the crystals end up pointing in a different direction than north.
Gorring and his team investigate the magnetic signature of the Palisades.
To get a sample, they bore into the rock with a water-cooled drill.
They measure the exact orientation of the crystals today with a compass.
When they offset this reading with magnetic north, they can calculate the original location of the rock.
Uh 14.
One of the useful things that you can do with this rock is you can take it back in the lab and measure its magnetic orientation, and that magnetic orientation will be when this rock crystallised or solidified 200 million years ago.
So this rock would have minerals that would be pointing in some other direction other than north today.
When scientists compared the magnetic orientation of the Palisades with the other basalt outcrops around the Atlantic, they discovered they formed at approximately the same latitude.
Not only did the rocks have the same age, they were also born at the same location.
For example, 200 million years ago, New York and Morocco were neighbours.
The geologists had all the proof they needed.
They could now confidently piece together what happened.
It began with a global volcanic disaster.
About 200 million years ago, North America and Africa began to pull apart from each other.
There were gigantic lava outpourings.
These lava flows erupted along very long cracks in the Earth's crust, that would have produced fountains of lava extending thousands and thousands of feet into the atmosphere.
They covered an almost inconceivably large area, roughly four million square miles, from southwestern France to southwestern Brazil, from New York to central Mali in Africa.
This area was covered in ponded lava flows that in some places ended up being nearly a mile thick.
Volcanic eruptions led to soaring temperatures.
Half of the plants and animals died.
Postosuchus didn't stand a chance.
As enormous forces tore Pangaea apart, a giant sea formed between the separating land masses - the Atlantic Ocean.
The city of New York was now at the coast.
beneath a layer of basalt and the Palisades towering above the Hudson provide evidence that Pangaea split apart to create the east coast of North America.
But the story of New York City was far from over.
After being built by fire, the region was about to be overcome by another destructive force.
Scientists are piecing together the story of New York's violent geological past.
the Atlantic Ocean opened up, leaving the area of New York on the coast.
But the maritime city still had a long way to go.
There was no deep Hudson River channel.
It was nothing but a small stream.
What forces transformed it into the wide river capable of carrying heavy freighters far inland? A clue to how the Hudson Valley was created is the strange boulders that are scattered throughout Manhattan's Central Park.
Some of them weigh several tons.
But they're strangers to these parts, totally unlike the surrounding rocks.
Geologist Charles Merguerian investigates where they came from.
This boulder is a boulder from the Palisade sheet on the other side of the Hudson in New Jersey.
You can see it's very nicely polished.
Compositionally, it's totally different than the surrounding bedrock, which is Manhattan schist, and the Manhattan schist here is very rich in mica.
This rock has no light-coloured mica in it whatsoever.
The Palisade sheet is located to the west and north of us.
The Palisades run for 40 miles along the Hudson River.
Something immensely powerful must have moved the boulders such a great distance.
Merguerian knows the answer is ice.
Scientists have noticed a similar phenomenon 4,000 miles away in the Swiss Alps.
As huge glaciers grind their way across the landscape, they gouge out lumps of rock and carry them along in the base of the ice.
These rocks act like sandpaper and carve out deep scratches.
When the ice melts, it leaves the boulders behind.
Merguerian is convinced the same thing happened in Central Park.
Ice moved the Palisade boulders and carved out grooves in the bedrock underneath.
This bedrock exposure in Central Park shows the profound effects of glaciation in the form of these spectacular glacial grooves that move up the outcrop and show this pattern where glaciers grabbed huge boulders and those huge boulders acted like tools to produce these scratches.
To Merguerian, the rocks in Central Park are compelling evidence that New York once was covered in ice.
Over millions of years, growing and receding ice has repeatedly turned North America into a frozen wilderness.
But the grooves on the rocks in New York don't tell the full story.
The destruction caused by the ice points to a gigantic glacial event that would dwarf the future metropolis.
To find out the extent of the ice, Merguerian travelled to Bear Mountain, some 50 miles north of New York City.
Once again, the clue was in the rocks.
He found glacial marks similar to those in Central Park.
What we're looking at here are chatter marks.
Chatter marks are very diagnostic features of glacial erosion, they're produced by boulders embedded in the base of a thick sheet of glacial ice.
Those boulders impinged on this bedrock surface, polishing it, smoothing it off and then plucking pieces of rock off as the glacial ice moved over with the boulders embedded in the base.
The gouges in the rocks could mean just one thing.
The glacier must have been thousands of feet thick.
In this case, although we're standing at an elevation of about 1,280 feet above sea level, the glacial ice sheet covered Bear Mountain as if it weren't even there.
Scientists have found identical chatter marks on nearby peaks up to a mile above sea level.
It was unmistakable proof.
A glacier at least one mile thick ground its way across these mountains.
Did this same ice sheet also plough through Central Park? Another nearby rock face provided the answer.
The feature that we're looking at here are a series of sub-parallel glacial scratches and grooves and these are, again, are produced by the glacial ice sheet dragging boulders across this very durable granite surface, it's kind of polished the surface.
And, in addition, it's produced these rather subtle but but but obvious, when the lighting is right, striae or grooves in the bedrock.
Now if we measure the orientation of these these these come out about north 20 degrees west, just about identical in orientation to the striae that we measured at Central Park.
It's significant evidence that the ice sheet that covered Bear Mountain also flowed over the surface of Central Park.
Proof that New York City was covered by a glacier four times higher than the Empire State Building.
MERGUERIAN: Just imagine glacial ice, a huge thick ice sheet over a mile thick, exerting tremendous pressure on the surface and sculpting the surface into the landscape that we see today.
The ice sheet's crushing weight bulldozed everything in its path and cut through the remains of the ancient mountains.
Before the ice arrived, the waters of the Hudson River had gently cut down through the landscape to form a V-shaped valley.
But a mile-thick glacier takes no prisoners.
It gouged out the sides and the bottom of the river valley and turned it into a U-shaped riverbed.
The Hudson River was now navigable for big ships.
The picture of modern-day New York was almost complete.
Long grooves in the rocks in Central Park showed scientists that a vast ice sheet flowed over the eroded remains of these mountains.
And marks and glacial striations on Bear Mountain proved this ice sheet was at least four times higher than the Empire State Building.
When the ice melted, it left a vast ridge of debris blocking the Hudson River from the Atlantic.
The final challenge for geologists was to find out how the ridge was destroyed and how New York's harbour opened to the oceans.
New York today boasts one of the largest natural harbours in the world.
But it wasn't always that way.
Towards the end of the last Ice Age, the port's wide entrance was blocked.
left behind a 220-foot-high wall of debris.
The ridge stretched from Long Island to Staten Island and forced the Hudson River through a narrow, more westerly course to the ocean.
What powerful forces destroyed this rock jam? The prime suspect was a flash flood.
But scientists needed evidence to prove that such a flood had happened.
In the 1960s, fishermen made an unexpected find at the mouth of the Hudson River.
They dredged up a giant mammoth tusk from the depths of the sea floor.
(TRUMPETS) Herds of these giant beasts roamed the plains of North America before they became extinct at the end of the last Ice Age.
Finding the odd mammoth tusk here and there is not so surprising.
But since the initial discovery, scientists have found hundreds more tusks and bones in the mouth of the Hudson River.
It was as though a violent torrent swept the mammoths away and dumped their remains off the coast.
And there were more clues nearby.
Huge boulders resting on the sandy sea floor, some of them as big as cars.
The boulders must have been part of the ancient moraine that once ran between Long Island and Staten Island.
Geologist David Franzi knows that only a raging torrent could have shifted them.
Based on the size of the boulders that we see here, we know that that flood must have discharged on the order of 1.
5 million cubic feet per second.
That's three times larger than the largest Mississippi River flood ever recorded.
And all that water had to come from somewhere.
Scientists began looking for the source of this flood, a flood powerful enough to transport huge boulders all the way to the sea.
Rocks to a geologist are like pages in a history book.
For us, erosion oftentimes rips some of the pages out of our history book, so it's the job of the geologist to put together a fragmentary record into a coherent history of the events that happened in the past.
in upstate New York, Franzi tracked what might have been the flood's path to an unusual grove of trees on Covey Hill in the Adirondack Mountains.
The trees are jack pines.
They are rare in this area, where the soil is usually fertile and deep.
But on Covey Hill, their presence shows that there is no more than a few inches of soil on top of the bedrock.
The jack pine is essentially rooted right on the top of a rock's surface here.
This is a bare sandstone surface, very little mineral soil, and it's subject to prolonged periods of dryness during the summertime.
Jack pine's adaptations make it able to survive here where no other tree species can.
What happened to the soil? The jack pines continue to grow at the entrance of a long gorge over 300 feet wide.
Usually, gorges like this are cut down over thousands of years, but in this case, the missing topsoil points to a sudden flood event.
A raging torrent must have ripped away the soil and cut deep into the rock.
In a helicopter, Franzi follows the gorge west.
Eventually, it opens into a vast, empty basin located next to one of the Great Lakes, Lake Ontario.
It doesn't take much of a stretch of the imagination here to imagine this valley filled with water and then with these hills poking up through as islands.
with two billion cubic miles of water - a huge lake geologists call Lake Iroquois.
It formed at the end of the last Ice Age.
As glaciers receded, the melt waters slowly filled up the lake.
The ice dams holding the waters weakened.
Eventually, the dams collapsed, causing sudden and devastating flash floods.
Lake level dropped on the order of 70 feet and about 160 cubic miles of water were released into the Champlain Valley, catastrophically.
That floodwater would have coursed down the Champlain Valley, through the Hudson Valley and ultimately out into the Atlantic Ocean.
The torrent raced towards New York City, 300 miles to the south, then took the straightest course to the sea.
The floodwaters smashed into the ancient moraine, the huge pile of debris blocking the direct exit of the Hudson River.
The bridge we see behind me spans the channel that was cut by the flood event.
When the flood wave came through, it was of sufficient intensity to over-top the dam and very rapidly cut the channel.
The gap that was created by the flood still exists.
It's now a tidal strait called the Narrows.
Today, the gap is spanned by the Verrazano Bridge.
The channel is deep enough for even the biggest ocean-going ships.
It's the most important entrance to New York City's harbour.
Mammoth tusks and huge boulders at the mouth of the river showed scientists that there was a torrent big enough to blast a hole through the ancient moraine.
And a channel leading towards the Great Lakes revealed the source of the flood.
It was this flood that created the Narrows and gave New York a wide entrance to its port.
A unique geology laid down the foundations for New York City.
But the same forces that constructed it may also have sown the seeds for New York's destruction.
Scientists have pieced together the half-billion-year history of New York City.
Huge mountains, volcanic eruptions and glacial ice shaped the area.
But New York's story doesn't end here.
The geology that created one of the greatest cities on Earth also has the potential to destroy it.
Experts have been studying the potential threat to the city.
We're standing here in Lower Manhattan on one of our major thoroughfares, Canal Street.
And it's important, because in 1821, a Category 2 hurricane raised the water level at the Battery 13 feet in one hour and, literally, the Hudson River met the East River and Canal Street was covered by water and Manhattan was actually two islands for three hours, until the water receded.
New York City is vulnerable because of its position on the coast.
Long Island stretches northeast at a right angle from the New Jersey shore.
New York City is nestled behind the western end of Long Island.
Normally, the island protects the city from the sea, but when hurricanes threaten, the opposite is true.
Long Island becomes a dangerous liability.
Hurricanes racing north along the beaches of the Atlantic coast pile up huge bulges of water in front of them.
They're called storm surges.
Hitting the right-angled junction at Long Island, the winds funnel the storm surge in through the Narrows, the gap between Long Island and New Jersey.
This is the place, at the actual apex of the right angle in New York where all the water being pushed by a hurricane would be concentrated.
And in the distance is the Verrazano Bridge, and all that water is gonna go through the passage we call the Narrows and it's gonna be accelerated towards New York City, where it will rise to abnormal heights.
Experts believe that in the United States, New York is the third most vulnerable city after Miami and New Orleans to a hurricane disaster.
If it was hit today, the consequences would be serious.
New York City is hit by hurricanes only infrequently.
Like, in 1821 and in 1893 and in 1938.
However, the point is that the hurricane that will eventually hit New York City again will be catastrophic, and what is going to happen when the utilities are knocked out? What is going to happen when salt water reaches into the subways and ruins the electrical system? We're talking about unbelievable amounts of money to restore the infrastructure.
We're talking about setbacks and delays in commerce and banking and transportation, a catastrophe that's never been seen.
Storm surges are not the only threat to New York's future.
Earthquakes are also part of the vast geological forces that shape this area.
They are still at work today.
Some could change the city in an instant.
November 4th 1884.
New York City was shaken by an earthquake that lasted ten seconds.
The Brooklyn Bridge swayed and people panicked.
The earthquake showed 5.
5 on the Richter scale.
January 17th 2001, New York City was struck again.
This time the quake was relatively small, only 2.
4, but it struck right under 125th Street.
The earthquake in 2001 is the first earthquake that we could confidently locate in Manhattan, that's its claim to fame, it was felt widely.
It's impossible to study the cause of the quakes at the surface.
The evidence is buried beneath the city.
Deep within New York's bedrock, seismologist Leonardo Seeber studies the cause of these quakes.
(HORN BLARES) In a subway tunnel 100 feet beneath the bedrock under the East River, there is a ready-made laboratory.
Here, Seeber can study the rocks up close and personal.
It is the same bedrock Manhattan is built on.
But Seeber fears it isn't as solid as once was thought.
New York area is considered a seismic zone, meaning there is a cluster of known earthquakes that have occurred in this area.
So we are, as geologists, very eager to discover which faults are responsible for these earthquakes.
The majority of earthquakes occur at the boundaries between separate sections of the Earth's crust, the tectonic plates on which the continents sit.
But New York's earthquakes are different.
The city is firmly in the middle of a tectonic plate, halfway between the mid-Atlantic ridge to the east and the San Andreas Fault to the west.
Seeber is anxious to discover what's going on.
As he examines the walls of the tunnel, he comes across a possible clue.
The long fractures in the rock are fault lines that formed when pressure built up.
As the tension was released, the rock cracked and shifted.
This is felt on the surface as an earthquake.
SEEBER: This is an example of a very small fault, but it's a fault that probably did generate some small earthquakes.
When one of these faults generates an earthquake, we think that perhaps other faults of the same family can generate earthquakes.
These faults in New York's bedrock are evidence that the area was hit by earthquakes in the past.
But Seeber has no way of knowing if the faults are still active and dangerous.
If a large earthquake hit New York today, the consequences would be catastrophic.
A large proportion of New York City buildings are simply not built to withstand earthquake shaking.
We worry about transportation tunnels, in particular, tunnels that traverse rivers where parts of the tunnels are rooted in solid rock and other parts are resting on soft sediment.
The oscillation of these two different materials could cause severe cracking and fracturing.
The infrastructure would be severely damaged, it would take tens of years to repair the damage caused by such a large event.
With the evidence geologists have collected, the story of the creation of New York City can now be told.
The city's bedrock was formed under a chain of mountains over a mile high.
Volcanoes and lava fields over millions of square miles split up the ancient supercontinent and created the east coast of North America.
Glacial ice, four times as high as the Empire State building, carved out the deep Hudson River.
A catastrophic flash flood broke through the moraine to form the Narrows and opened up New York City's harbour to the oceans.
Looking ahead to the distant future, geologists see more challenging times for the city.
In 40,000 years, they predict this region will be engulfed by another ice sheet.
And in 250 million years, the Atlantic will start to shrink again.
Europe and Africa will eventually crash back into the American coast.
The fossilised remains of the once great city of New York will become just another layer of rock in a vast new mountain range.
A footnote in the immense, ever-changing story of planet Earth.
5- Billion-year-old planet, still evolving.
As continents shift and clash, volcanoes erupt and 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 450-million-year-old geological history of New York City is explored.
A metropolis pockmarked with strange rocks, haunted by footprints of ancient giant reptiles, and lined with a vast curtain of solidified lava.
Scientists investigate the evidence for fiery volcanoes, massive floods and ice sheets four times as high as the Empire State building.
The clues to understanding New York City's geological past provides a window into the formation of the earth itself.
The investigation into New York City's geological history begins here, with Manhattan's rocky outcrops.
These rocks are clues to how the land was made and how its geology helped it become a dense, thriving, pulsating city.
They're scattered all over Manhattan, poking through the surface of parks and through the concrete between the buildings.
Some, squashed between two apartment blocks, are the size of a whale.
They are the extraordinary survivors of ancient times.
Most importantly, they are the surface tips of the bedrock in which Manhattan's buildings are anchored.
Gigantic skyscrapers stand in two clusters, in downtown and midtown.
In the section between, the buildings are lower.
The clues to the shape of Manhattan's familiar skyline are the rocks beneath the surface.
A leading expert on the rocks in New York is geologist Charles Merguerian.
The entire history of the development of the earth's crust is emblazoned in the rocks beneath us.
The rocks here in New York City harbour an ancestry that dates back over a billion years of time.
Merguerian is searching for evidence to show how the city's bedrock was made.
At Inwood Hill Park in Upper Manhattan, he's found an extremely hard piece of the bedrock known as Manhattan schist.
To the untrained eye, it's just a piece of rock, but to Merguerian, this is his first clue.
The rocks that we're looking at right here are rocks of the Manhattan schist formation and the these rocks are very severely deformed, and the structures here in this rock is a structure that comes up like this, bends around and comes back down on itself as such, and in three-dimensional view, it's a structure that looks something like this.
A very, very tight fold with a plunge towards the south here.
These are rocks that were very, very strongly deformed over protracted periods of time.
And it's the same bedrock that occurs over much of New York City.
This tight fold in the rock suggests New York's bedrock was formed under great pressure.
To confirm this hunch, Merguerian takes a sample to the lab for detailed analysis.
Radiometric dating proves this rock is about 450 million years old.
But the rock has even greater secrets to tell.
It contains a kaleidoscope of minerals, which opens a window into the ancient world.
To me, minerals are like the instrument cluster in your car, they tell you everything about how your car is running.
Merguerian uses a microscope with polarised light to view the minerals.
The examination tells us the former depth regime, how deep the rocks were, they tell you the age of the rocks, they tell you everything you want to know about the development of the earth's crust.
What's striking about these samples is that the minerals inside are elongated.
It is a clue that these rocks must once have been crushed by massive forces.
And the colours support this theory.
Under the polarised light, the sample from Inwood Hill Park shows up blue.
This comes from a mineral called kyanite, which forms at great depths.
It's conclusive evidence that this rock was compressed deep under the surface.
Rocks forged at these depths are much harder, ideal for a city's foundations.
But what gigantic weight was on top? Merguerian believes there is only one answer.
The rock was once buried under the crushing weight of a chain of massive mountains.
The minerals that we find in the bedrock units of New York City tell us that the rocks of New York City were formerly buried when they were formed, under very high pressures, and that those high pressures indicate that these rocks formerly were produced at depths of 20 to 25 miles, and probably the mountains were as high as the Alps are today.
But even the most impressive mountain chains can't survive the ravages of time.
The Rocky Mountains, for example.
Millions of years ago, they soared nearly six miles into the sky.
Today, erosion has halved their size.
The same process happened in New York.
Rain, wind and ice wore the ancient mountains almost flat.
But the microscopic crystals found in the rock in Manhattan testify that they existed in the past.
How did the mountains form? The answer lies in the way the earth's crust moves.
A network of interlocking individual pieces makes up the Earth's surface.
Geologists call them tectonic plates.
Over millions of years, they collide and break apart to form different continents.
surface looked completely different.
North America was much further to the south.
MERGUERIAN: North America was tilted 90 degrees clockwise from its present orientation and it was straddling the equator.
As such, the climate was tropical, the east coast of North America was really experiencing Club Med conditions.
The weather may have been awesome, but the ancient East Coast was heading for trouble.
The plate beneath it was moving.
The East Coast was on a collision course with ancient West Africa.
The impact unleashed geological chaos.
Under intense compression, the land was forced upwards to form a soaring range of mountains.
The collision that took place is the most fundamental and impressive mountain-building event to affect the east coast of North America.
Today, all that remains are their stumps, stumps that form the bedrock of modern-day New York.
The collision that built up the ancient mountains also folded the bedrock into dips and rises.
These folds are responsible for the shape of Manhattan's skyline.
The city boasts two clusters of skyscrapers in downtown and midtown.
Here, the hard bedrock that formed deep underground was forced up.
It is now close to the surface and provides solid anchorage for the high-rise buildings.
In the dip in the middle the rock was folded down.
The area is filled with loose sediments, less suitable for skyscrapers.
MERGUERIAN: When the bedrock is at the Earth's surface where it's actually exposed, then it's pretty easy to build tall buildings 'cause you can root them directly into solid rock.
However, in areas where the bedrock is deep and covered by glacial sediment, in those cases, it's very difficult to build tall buildings because you need to root those buildings either into solid rock or build concrete abutments called caissons that can support tall buildings.
New York's deep history is beginning to take shape.
Building up from tiny crystals in the rock, scientists revealed how New York's bedrock was formed under the crushing weight of a massive ancient mountain range.
The result was hard Manhattan schist, a perfect foundation for the city's skyscrapers.
But New York City still had a long way to go.
The colliding plates created an enormous landmass - the last great supercontinent, called Pangaea.
New York was now trapped in the centre but somehow it made it back to the coast.
Hidden beyond the city's streets is evidence of huge volcanic eruptions, mass extinctions and continents torn into pieces.
Clues that could explain how New York became one of the world's great maritime cities.
Investigators are piecing together how New York City's unique geology was formed.
Much of its early success as a trade and commerce centre is owed to its deep-water harbour and its location at the coast.
But 450 million years ago, things were different.
The area of New York City was landlocked, embedded in the heart of a huge supercontinent.
How did it get to the coast? The investigation fast-forwards In a quarry in New Jersey, paleontologist Paul Olson unearths the first of a string of clues that could explain how New York City reached the coast.
A giant fossilised footprint.
This is the footprint, actually the mud that filled in the footprint, of a four-footed crocodile relative that was the dominant carnivore during the late Triassic.
You can see the toes here have little pads on them and here's the handprint, and these animals would have been, in this case, about the size of a modest crocodile, but some of them became much, much larger, the size even of a T.
Rex.
(ROARS) The footprints are from a huge crocodile-like creature called Postosuchus.
It first appeared on the Earth around 230 million years ago.
Then, some 30 million years later, its footprints suddenly vanished.
But Postosuchus wasn't alone.
Half of all land animals perished at the same time.
The fossil evidence proves it to be one of the biggest mass extinctions ever recorded.
The evidence for this mass extinction is that we have lots and lots of fossils right in this area.
And what you see is especially in the in the reptile footprints, you see one group of forms, the forms that are related to crocodilians, disappear.
Whatever caused the mass extinction must have been a catastrophic event.
Olsen had a hunch that the mass extinction was somehow related to New York's return to the coast.
The ancient area of New York sat on a line of great weakness, the plate boundary where two continents joined to form the supercontinent Pangaea.
And it was unstable, prone to earthquakes and volcanoes.
Olsen's quest - to find the evidence for the natural disaster that finished off Postosuchus A band of dark rock above the footprints in the New Jersey quarry caught his eye.
It was basalt, the smoking gun Olson was looking for.
Basalt is a volcanic rock.
It forms when hot lava erupts onto the surface and cools.
Did the volcanoes that forged this basalt trigger the mass extinction and also rip Pangaea apart? On its own, the evidence at the quarry was unconvincing.
The layer of basalt is only a few feet thick.
To prove mass lava flows caused this global catastrophe, scientists needed corroborative evidence.
High above the Hudson River, geologist Matt Gorring follows another lead.
He's studying the Palisades, a dramatic geologic feature that hugs the Hudson River, beginning across mid Manhattan and running into northeast New Jersey.
They too are made of basaltic rock, the same rock implicated in the mass extinction of land animals.
But the Palisades are on an altogether different scale.
The Palisades are a sheet of basaltic magma, about 1,000 feet thick, it's about 40 miles long, so it's a very prominent set of cliffs that run all the way up the west side of the Hudson River.
Here is proof of massive volcanic activity.
Hot lava flooded out of ruptures in the Earth's crust and covered ancient North America in a mile-deep sheet.
The lava cracked as it cooled.
The vertical ruptures formed regular pencil-shaped columns.
These distinctive rock formations have been known to geologists since the 19th century.
Intriguingly, they appear on both sides of the Atlantic, in North and South America, Europe and Africa.
Geologists suspected that their presence pointed to the spot where Africa and Europe separated from America.
But that was just an unproven theory, until the 1950s, when scientists developed a revolutionary technique called paleomagnetism.
Now, they could study the magnetic properties of rocks.
Many rocks, including basalt, have a distinctive magnetic signature, formed as the rock is born.
Tiny crystals inside the rock act like compass needles.
When the magma that forms the rock is fluid, the crystals align to the Earth's magnetic field, pointing north.
As the rock solidifies, the crystals freeze, forever locked in that fixed magnetic alignment.
As continents move and the rocks travel, the crystals end up pointing in a different direction than north.
Gorring and his team investigate the magnetic signature of the Palisades.
To get a sample, they bore into the rock with a water-cooled drill.
They measure the exact orientation of the crystals today with a compass.
When they offset this reading with magnetic north, they can calculate the original location of the rock.
Uh 14.
One of the useful things that you can do with this rock is you can take it back in the lab and measure its magnetic orientation, and that magnetic orientation will be when this rock crystallised or solidified 200 million years ago.
So this rock would have minerals that would be pointing in some other direction other than north today.
When scientists compared the magnetic orientation of the Palisades with the other basalt outcrops around the Atlantic, they discovered they formed at approximately the same latitude.
Not only did the rocks have the same age, they were also born at the same location.
For example, 200 million years ago, New York and Morocco were neighbours.
The geologists had all the proof they needed.
They could now confidently piece together what happened.
It began with a global volcanic disaster.
About 200 million years ago, North America and Africa began to pull apart from each other.
There were gigantic lava outpourings.
These lava flows erupted along very long cracks in the Earth's crust, that would have produced fountains of lava extending thousands and thousands of feet into the atmosphere.
They covered an almost inconceivably large area, roughly four million square miles, from southwestern France to southwestern Brazil, from New York to central Mali in Africa.
This area was covered in ponded lava flows that in some places ended up being nearly a mile thick.
Volcanic eruptions led to soaring temperatures.
Half of the plants and animals died.
Postosuchus didn't stand a chance.
As enormous forces tore Pangaea apart, a giant sea formed between the separating land masses - the Atlantic Ocean.
The city of New York was now at the coast.
beneath a layer of basalt and the Palisades towering above the Hudson provide evidence that Pangaea split apart to create the east coast of North America.
But the story of New York City was far from over.
After being built by fire, the region was about to be overcome by another destructive force.
Scientists are piecing together the story of New York's violent geological past.
the Atlantic Ocean opened up, leaving the area of New York on the coast.
But the maritime city still had a long way to go.
There was no deep Hudson River channel.
It was nothing but a small stream.
What forces transformed it into the wide river capable of carrying heavy freighters far inland? A clue to how the Hudson Valley was created is the strange boulders that are scattered throughout Manhattan's Central Park.
Some of them weigh several tons.
But they're strangers to these parts, totally unlike the surrounding rocks.
Geologist Charles Merguerian investigates where they came from.
This boulder is a boulder from the Palisade sheet on the other side of the Hudson in New Jersey.
You can see it's very nicely polished.
Compositionally, it's totally different than the surrounding bedrock, which is Manhattan schist, and the Manhattan schist here is very rich in mica.
This rock has no light-coloured mica in it whatsoever.
The Palisade sheet is located to the west and north of us.
The Palisades run for 40 miles along the Hudson River.
Something immensely powerful must have moved the boulders such a great distance.
Merguerian knows the answer is ice.
Scientists have noticed a similar phenomenon 4,000 miles away in the Swiss Alps.
As huge glaciers grind their way across the landscape, they gouge out lumps of rock and carry them along in the base of the ice.
These rocks act like sandpaper and carve out deep scratches.
When the ice melts, it leaves the boulders behind.
Merguerian is convinced the same thing happened in Central Park.
Ice moved the Palisade boulders and carved out grooves in the bedrock underneath.
This bedrock exposure in Central Park shows the profound effects of glaciation in the form of these spectacular glacial grooves that move up the outcrop and show this pattern where glaciers grabbed huge boulders and those huge boulders acted like tools to produce these scratches.
To Merguerian, the rocks in Central Park are compelling evidence that New York once was covered in ice.
Over millions of years, growing and receding ice has repeatedly turned North America into a frozen wilderness.
But the grooves on the rocks in New York don't tell the full story.
The destruction caused by the ice points to a gigantic glacial event that would dwarf the future metropolis.
To find out the extent of the ice, Merguerian travelled to Bear Mountain, some 50 miles north of New York City.
Once again, the clue was in the rocks.
He found glacial marks similar to those in Central Park.
What we're looking at here are chatter marks.
Chatter marks are very diagnostic features of glacial erosion, they're produced by boulders embedded in the base of a thick sheet of glacial ice.
Those boulders impinged on this bedrock surface, polishing it, smoothing it off and then plucking pieces of rock off as the glacial ice moved over with the boulders embedded in the base.
The gouges in the rocks could mean just one thing.
The glacier must have been thousands of feet thick.
In this case, although we're standing at an elevation of about 1,280 feet above sea level, the glacial ice sheet covered Bear Mountain as if it weren't even there.
Scientists have found identical chatter marks on nearby peaks up to a mile above sea level.
It was unmistakable proof.
A glacier at least one mile thick ground its way across these mountains.
Did this same ice sheet also plough through Central Park? Another nearby rock face provided the answer.
The feature that we're looking at here are a series of sub-parallel glacial scratches and grooves and these are, again, are produced by the glacial ice sheet dragging boulders across this very durable granite surface, it's kind of polished the surface.
And, in addition, it's produced these rather subtle but but but obvious, when the lighting is right, striae or grooves in the bedrock.
Now if we measure the orientation of these these these come out about north 20 degrees west, just about identical in orientation to the striae that we measured at Central Park.
It's significant evidence that the ice sheet that covered Bear Mountain also flowed over the surface of Central Park.
Proof that New York City was covered by a glacier four times higher than the Empire State Building.
MERGUERIAN: Just imagine glacial ice, a huge thick ice sheet over a mile thick, exerting tremendous pressure on the surface and sculpting the surface into the landscape that we see today.
The ice sheet's crushing weight bulldozed everything in its path and cut through the remains of the ancient mountains.
Before the ice arrived, the waters of the Hudson River had gently cut down through the landscape to form a V-shaped valley.
But a mile-thick glacier takes no prisoners.
It gouged out the sides and the bottom of the river valley and turned it into a U-shaped riverbed.
The Hudson River was now navigable for big ships.
The picture of modern-day New York was almost complete.
Long grooves in the rocks in Central Park showed scientists that a vast ice sheet flowed over the eroded remains of these mountains.
And marks and glacial striations on Bear Mountain proved this ice sheet was at least four times higher than the Empire State Building.
When the ice melted, it left a vast ridge of debris blocking the Hudson River from the Atlantic.
The final challenge for geologists was to find out how the ridge was destroyed and how New York's harbour opened to the oceans.
New York today boasts one of the largest natural harbours in the world.
But it wasn't always that way.
Towards the end of the last Ice Age, the port's wide entrance was blocked.
left behind a 220-foot-high wall of debris.
The ridge stretched from Long Island to Staten Island and forced the Hudson River through a narrow, more westerly course to the ocean.
What powerful forces destroyed this rock jam? The prime suspect was a flash flood.
But scientists needed evidence to prove that such a flood had happened.
In the 1960s, fishermen made an unexpected find at the mouth of the Hudson River.
They dredged up a giant mammoth tusk from the depths of the sea floor.
(TRUMPETS) Herds of these giant beasts roamed the plains of North America before they became extinct at the end of the last Ice Age.
Finding the odd mammoth tusk here and there is not so surprising.
But since the initial discovery, scientists have found hundreds more tusks and bones in the mouth of the Hudson River.
It was as though a violent torrent swept the mammoths away and dumped their remains off the coast.
And there were more clues nearby.
Huge boulders resting on the sandy sea floor, some of them as big as cars.
The boulders must have been part of the ancient moraine that once ran between Long Island and Staten Island.
Geologist David Franzi knows that only a raging torrent could have shifted them.
Based on the size of the boulders that we see here, we know that that flood must have discharged on the order of 1.
5 million cubic feet per second.
That's three times larger than the largest Mississippi River flood ever recorded.
And all that water had to come from somewhere.
Scientists began looking for the source of this flood, a flood powerful enough to transport huge boulders all the way to the sea.
Rocks to a geologist are like pages in a history book.
For us, erosion oftentimes rips some of the pages out of our history book, so it's the job of the geologist to put together a fragmentary record into a coherent history of the events that happened in the past.
in upstate New York, Franzi tracked what might have been the flood's path to an unusual grove of trees on Covey Hill in the Adirondack Mountains.
The trees are jack pines.
They are rare in this area, where the soil is usually fertile and deep.
But on Covey Hill, their presence shows that there is no more than a few inches of soil on top of the bedrock.
The jack pine is essentially rooted right on the top of a rock's surface here.
This is a bare sandstone surface, very little mineral soil, and it's subject to prolonged periods of dryness during the summertime.
Jack pine's adaptations make it able to survive here where no other tree species can.
What happened to the soil? The jack pines continue to grow at the entrance of a long gorge over 300 feet wide.
Usually, gorges like this are cut down over thousands of years, but in this case, the missing topsoil points to a sudden flood event.
A raging torrent must have ripped away the soil and cut deep into the rock.
In a helicopter, Franzi follows the gorge west.
Eventually, it opens into a vast, empty basin located next to one of the Great Lakes, Lake Ontario.
It doesn't take much of a stretch of the imagination here to imagine this valley filled with water and then with these hills poking up through as islands.
with two billion cubic miles of water - a huge lake geologists call Lake Iroquois.
It formed at the end of the last Ice Age.
As glaciers receded, the melt waters slowly filled up the lake.
The ice dams holding the waters weakened.
Eventually, the dams collapsed, causing sudden and devastating flash floods.
Lake level dropped on the order of 70 feet and about 160 cubic miles of water were released into the Champlain Valley, catastrophically.
That floodwater would have coursed down the Champlain Valley, through the Hudson Valley and ultimately out into the Atlantic Ocean.
The torrent raced towards New York City, 300 miles to the south, then took the straightest course to the sea.
The floodwaters smashed into the ancient moraine, the huge pile of debris blocking the direct exit of the Hudson River.
The bridge we see behind me spans the channel that was cut by the flood event.
When the flood wave came through, it was of sufficient intensity to over-top the dam and very rapidly cut the channel.
The gap that was created by the flood still exists.
It's now a tidal strait called the Narrows.
Today, the gap is spanned by the Verrazano Bridge.
The channel is deep enough for even the biggest ocean-going ships.
It's the most important entrance to New York City's harbour.
Mammoth tusks and huge boulders at the mouth of the river showed scientists that there was a torrent big enough to blast a hole through the ancient moraine.
And a channel leading towards the Great Lakes revealed the source of the flood.
It was this flood that created the Narrows and gave New York a wide entrance to its port.
A unique geology laid down the foundations for New York City.
But the same forces that constructed it may also have sown the seeds for New York's destruction.
Scientists have pieced together the half-billion-year history of New York City.
Huge mountains, volcanic eruptions and glacial ice shaped the area.
But New York's story doesn't end here.
The geology that created one of the greatest cities on Earth also has the potential to destroy it.
Experts have been studying the potential threat to the city.
We're standing here in Lower Manhattan on one of our major thoroughfares, Canal Street.
And it's important, because in 1821, a Category 2 hurricane raised the water level at the Battery 13 feet in one hour and, literally, the Hudson River met the East River and Canal Street was covered by water and Manhattan was actually two islands for three hours, until the water receded.
New York City is vulnerable because of its position on the coast.
Long Island stretches northeast at a right angle from the New Jersey shore.
New York City is nestled behind the western end of Long Island.
Normally, the island protects the city from the sea, but when hurricanes threaten, the opposite is true.
Long Island becomes a dangerous liability.
Hurricanes racing north along the beaches of the Atlantic coast pile up huge bulges of water in front of them.
They're called storm surges.
Hitting the right-angled junction at Long Island, the winds funnel the storm surge in through the Narrows, the gap between Long Island and New Jersey.
This is the place, at the actual apex of the right angle in New York where all the water being pushed by a hurricane would be concentrated.
And in the distance is the Verrazano Bridge, and all that water is gonna go through the passage we call the Narrows and it's gonna be accelerated towards New York City, where it will rise to abnormal heights.
Experts believe that in the United States, New York is the third most vulnerable city after Miami and New Orleans to a hurricane disaster.
If it was hit today, the consequences would be serious.
New York City is hit by hurricanes only infrequently.
Like, in 1821 and in 1893 and in 1938.
However, the point is that the hurricane that will eventually hit New York City again will be catastrophic, and what is going to happen when the utilities are knocked out? What is going to happen when salt water reaches into the subways and ruins the electrical system? We're talking about unbelievable amounts of money to restore the infrastructure.
We're talking about setbacks and delays in commerce and banking and transportation, a catastrophe that's never been seen.
Storm surges are not the only threat to New York's future.
Earthquakes are also part of the vast geological forces that shape this area.
They are still at work today.
Some could change the city in an instant.
November 4th 1884.
New York City was shaken by an earthquake that lasted ten seconds.
The Brooklyn Bridge swayed and people panicked.
The earthquake showed 5.
5 on the Richter scale.
January 17th 2001, New York City was struck again.
This time the quake was relatively small, only 2.
4, but it struck right under 125th Street.
The earthquake in 2001 is the first earthquake that we could confidently locate in Manhattan, that's its claim to fame, it was felt widely.
It's impossible to study the cause of the quakes at the surface.
The evidence is buried beneath the city.
Deep within New York's bedrock, seismologist Leonardo Seeber studies the cause of these quakes.
(HORN BLARES) In a subway tunnel 100 feet beneath the bedrock under the East River, there is a ready-made laboratory.
Here, Seeber can study the rocks up close and personal.
It is the same bedrock Manhattan is built on.
But Seeber fears it isn't as solid as once was thought.
New York area is considered a seismic zone, meaning there is a cluster of known earthquakes that have occurred in this area.
So we are, as geologists, very eager to discover which faults are responsible for these earthquakes.
The majority of earthquakes occur at the boundaries between separate sections of the Earth's crust, the tectonic plates on which the continents sit.
But New York's earthquakes are different.
The city is firmly in the middle of a tectonic plate, halfway between the mid-Atlantic ridge to the east and the San Andreas Fault to the west.
Seeber is anxious to discover what's going on.
As he examines the walls of the tunnel, he comes across a possible clue.
The long fractures in the rock are fault lines that formed when pressure built up.
As the tension was released, the rock cracked and shifted.
This is felt on the surface as an earthquake.
SEEBER: This is an example of a very small fault, but it's a fault that probably did generate some small earthquakes.
When one of these faults generates an earthquake, we think that perhaps other faults of the same family can generate earthquakes.
These faults in New York's bedrock are evidence that the area was hit by earthquakes in the past.
But Seeber has no way of knowing if the faults are still active and dangerous.
If a large earthquake hit New York today, the consequences would be catastrophic.
A large proportion of New York City buildings are simply not built to withstand earthquake shaking.
We worry about transportation tunnels, in particular, tunnels that traverse rivers where parts of the tunnels are rooted in solid rock and other parts are resting on soft sediment.
The oscillation of these two different materials could cause severe cracking and fracturing.
The infrastructure would be severely damaged, it would take tens of years to repair the damage caused by such a large event.
With the evidence geologists have collected, the story of the creation of New York City can now be told.
The city's bedrock was formed under a chain of mountains over a mile high.
Volcanoes and lava fields over millions of square miles split up the ancient supercontinent and created the east coast of North America.
Glacial ice, four times as high as the Empire State building, carved out the deep Hudson River.
A catastrophic flash flood broke through the moraine to form the Narrows and opened up New York City's harbour to the oceans.
Looking ahead to the distant future, geologists see more challenging times for the city.
In 40,000 years, they predict this region will be engulfed by another ice sheet.
And in 250 million years, the Atlantic will start to shrink again.
Europe and Africa will eventually crash back into the American coast.
The fossilised remains of the once great city of New York will become just another layer of rock in a vast new mountain range.
A footnote in the immense, ever-changing story of planet Earth.