In Our Time - The Geological Formation of Britain

Episode Date: October 22, 2009

Melvyn Bragg and guests Richard Corfield, Jane Francis and Sanjeev Gupta discuss the geological formation of Britain.Around 600 million years ago the island that we now call Britain was in two parts, far to the south of the Equator. Scotland and north-western Ireland were part of a continent (Laurentia) that also included what is now North America. To the south-east, near the Antarctic Circle, meanwhile, you would have found southern Ireland, England and Wales. They formed a mini-continent (Avalonia) with what is now Newfoundland.Over the course of hundreds of millions of years, as they inched their way north, the two parts came together - first as part of a vast unitary continent (Pangaea), later as a promontory on the edge of Europe, and eventually, as sea levels rose, as an island. The story of how Britain came to be where it is now, in its current shape - from the separation of North America and Europe to the carving out of the English Channel - is still being uncovered today.Richard Corfield is Visiting Senior Resarch Fellow at Oxford University; Jane Francis is Professor of Palaeoclimatology at the University of Leeds; Sanjeev Gupta is a Royal Society-Leverhulme Trust Research Fellow at Imperial College London.

This BBC podcast is supported by ads outside the UK. Thanks for downloading the In Our Time podcast. For more details about In Our Time and for our terms of use, please go to BBC.co.com.uk forward slash radio 4. I hope you enjoy the programme. Hello, 600 million years ago, Britain was in two parts, far to the south of the equator. Scotland and the north of Ireland were part of a continent

that also included what is now North America. To the southeast near the Antarctic Circle, you would have found England and Wales and Southern Ireland. Over the course of hundreds of millions of years, they were pushed north, and the two parts came together, and gradually, in the course of this amazing journey, to where we are now, these islands have taken on their current shape. It's a story in a vast scale.

Oceans, open and close, continents, split and collide. It takes us from the separation of North America and Europe to the carving out of the English Channel. With me to discuss the geological formation of Britain, Richard Corfield, visiting senior research fellow at Oxford University, Jane Francis, Professor of Paleoclamatology at Leeds University, and Sanjeev Gupta, Royal Society Leibahum Trust Research Fellow at Imperial College, London. Richard Corfield, can you give us a sense of how varied the geology of Britain is

and how unusual that is in such a small space? Yes, the geology of Britain is incredibly varied, both in absolute and relative terms. By that I mean that you can travel a relatively small distance and cross a very great deal of geological time, not necessarily in one direction. You can go up column, up geological column and down geological column. And it's that variation in the geology of Britain,

which in fact is the reason why Britain is the spiritual home of the science of geology. Because of this variation, 19th century geologists were able to travel the length and breadth of the country and see a great variety of rocks and rock types, igneous rocks, metamorphic rocks, sedimentary rocks. So why is the geology of Britain so varied compared to places like, for example, Canada and Australia, which is extremely monotonous geologically? Basically, as you said, it's because Britain has travelled a long way from near the South Pole. It's travelled through various climatic zones during its long history. Britain has been near continental margins, so it's been subjected to tectonic forces and sea level change.

It's also been subject to mountain building episodes on a grand scale. On top of that, it's been subject to many ice ages, and all of this has been subject to various rates of erosion, so that the face of Britain has been built up and ground down many, many times. If you like, Britain is a bit like a geological Doctor Who. It's very old. It's been to a lot of places and it's seen a lot of things. Now, how unusual is that? You made a passing reference to what you call the monotony of various other continent, various places,

massively bigger than these islands. How unusual is that diversity? It is very unusual to have such a great diversity of rock types and rock ages in one relatively small place like Great Britain. The first rock, I understand, would eventually form this island was laid down 2.7 billion years ago. What happens between then and 600 million years ago? Well, the history of the geological history of Britain really can be traced

from about 1 billion years ago. The difference between the 2.7 billion-year age of the Louisiana Nices, so-called, because they're found on the Isle of Lille of Lusian. in northwest Scotland. Nice is a geological term which refers to sediments which have been, in this case, Earth's crust, which have been subject to reheating and repressurization. And this is a piece of the ancient Earth crust, which is just been thrown up through random forces and is visible on the northwest of Scotland, but is not necessarily part of Britain per se. If I can put it this way, it's like a house which incorporates some old bricks. But that is there, and then what happened before we got to get to the start of our story in 600 million years? I'm a bit, you know, we've got to be sort of straightforward.

We have some of the oldest rocks here, and we have some of the youngest. And the oldest are up in Scotland, and however it got there, is still there, and it's from 2.7 billion years ago, and so that's that. And then we start our story 600 million years ago. So what happened briefly in between, so we can get crack. Okay, well the first, there was a great supercontinent in the southern hemisphere called Rodinia at about 1 billion years old. And this supercontinent eventually broke up and began to drift northwards and then re-coerlessed into another supercontinent, which is where our story starts at about 600 million years.

Okay, Jane. Jane Francis, so we've got continents and ocean, six or six or six. many years ago. Can you give us a map of the planet then at that time? What was where? It would have looked very different from what we see today. Many of the continents were

in the southern hemisphere in a big landmass called Gondwana but they were all massed together and then there were a few blocks near the equator. So in fact in the story of the UK that we're interested in today the British Isles part of Scotland and Northern Ireland were actually near the equator

on the blocks of continents there. and England and Southern Ireland were right down south, almost near the Antarctic Circle. In terms of land, there's almost nothing in the northern hemisphere? No, I mean, if you went to the sort of North Pole, as it is today, I don't think you'd seen any land masses. So big oceans. Big oceans there and most of the land south of the equator.

Yeah. And the story, really, of geological evolution of British Is and the rest of the world is about how these continents moved around and coalesced and moved apart again, as Richard said, and then go through different climatic zones. And the movement is tectonic. Can you refresh listeners' notions of the tectonic shifts? We've done a programme on it, but that was some time ago.

Well, basically, I guess if you think of the earth as a... I like to think of it as an egg, hard-boiled egg with the eggshell, cracked eggshell on it. Those are the tectonic plates and they move around. So there are currents in what we call the mantle below, and they move the plates around. So they're continuously moving. today. And there are how many of them are there? Oh, hundreds. Well, there are big plates.

And there are big plates on which whole continents are built. Yeah, so there are whole continents on some plates. So these plates are shifting whole continents around the place. They are indeed. Yeah. They're sort of like a biscuit crust on the earth. And then there are places where there are very small plates. So there are places where, say, not the UK, the UK is pretty stable tectonically, but if you go somewhere like Asia where we've seen recent earthquakes, I mean, those are tiny little bits of plates that are always jostling together. And these tectonic plates are crashing into each other and being pulled under each other and eating each other up

over a period of time but that is what has been happening and still is happening. It's a continual process of movement and some of the big continents are stable and some of the big tectonic blocks have continents on them that are moving around. Some are very, very old centres. These are old pre-Cambrian cratons.

That's pre-600 million years. Yeah, very, very old. And then there's new, mountain ranges on the edges of them. So if we come back to our story, at that time, what will become Britain is on two different continents. What would we have seen

if we'd been able to look at them then? What would there have been? Let's go to say the southern part of the British Isles, so about 400 million years ago, 4 to 600 million years ago. So we get the evidence for that from rocks.

So if you go, if you went to say Pembrokeshire in south-west Wales, you'd see a whole suite of rocks of that age, which are about 400 million years old, which are really black shales, very fine-grained rocks. And in it you'd find fossils, marine fossils, that tell us those rocks were formed in deep seas, deep oceans. So you couldn't walk on the land at that time.

You'd be snorkeling in very deep seas. And then we also find evidence in the Lake District and again in Wales of volcanoes. So there were volcanoes erupting, spewing out lavas, and ashes that were falling into these oceans. So we can look at the rocks there and reconstruct what was happening in England and Southern Ireland when it was near the Antarctic.

And there are desert places, or do they come later? They're later, yeah, much younger. We know that Scotland, how do we know, how do we know, that Scotland was originally quite separate from England and Wales? Well, that's a beautiful reconstruction by geologists, as much like historians, we read the rock. archive. And one of the first things that geologists did was that they looked at fossil faunas, essentially, and they discovered that fossil faunas were different in these different continental

landmasses. And particularly they looked at, the key things to look at are organisms that live in shallow, shellful areas that feed at the sea bed. These can't travel very far. And what they found was these organisms like trilobites and brachyopods, which were shelly things like, these are very, very separate in the different land masses. And slowly through time, they come together and they become similar. So that suggests that they're able to cross the oceans. The oceans have become much smaller. Secondly, most importantly, we're able to reconstruct where we are,

where the continent landmasses are with relation to the poles. And this is, again, beautiful in that rocks can preserve a record of the magnetic field at the time. So, for example, volcanic rocks record the magnetic field, a paleo-magnetic field or sediment grains, magnetic grains deposit aligned to that, and this magnetic field, the Earth's magnetic field, is a little bit like a bar magnet. And that is related, so that's got a latitudal relationship, so we can reconstruct the paleo-latitude of the continents.

And so roughly, so that people listening, you know, Scotland was a long way from England was, It seemed inevitable they would join up. Was it a series of accidents? Or can you give us some idea of what's going on out there in the past? Scotland was part of this big North American continent and this was sort of situated around the equator, or just south the equator, whereas England and Wales were further south

and separated by a huge ocean around 400 million years ago. And these slowly became together, at least probably about 5,000 kilometres apart. They slowly came together. And it's not inevitable that they came together, but the forces, the plate forces that were driving. We don't fully understand what drives these things. That's likely related to motions of the continents basically float on this dense mantle,

which makes up 80% of the earth. And it's the motions and the mantle that drive these plates. And Jane has talked about the tectonic shifting of the plates, which is going on, I'm sitting here now, but the other things driving the shift. changes are to do with climate and the rise and fall of sea levels. Can you tell us the other factors, the big factors involved? Well, certainly, firstly, tectonics also controls climate and sea levels.

For example, when you have oceans opening, you have lots of less dense, light ocean crust rising to the surface and displaces water, so it causes sea levels to rise. But also, climates are controlled by the orbital variation of the earth. So how the Earth shifts, moves around the sun, there's three different factors that influence. And this influences the amount of solar radiation the Earth's surface receives. And this overall impacts the climate.

So, for example, in the last 600 million years, Earth has experienced three major episodes of glaciation, large-scale glaciation, one about 400 million years ago, one in the... And this is all to do with the way, the tilting, to do with the way that the sun here. Nothing to do. That's what it's to do. That's right. That's right. So for example, one of the main things is that the orbit of the Earth around the sun can change from being a circle to an ellipse. And this will obviously change the amount of sunlight solar radiation being received. And this can have a big factor in terms of climate change. So the things buffeted in these landmasses are the tectonic shifts, the changes in climate, therefore the changes in sea levels, therefore and changing climate bringing glaciers.

So these are the big factors. Is there anything else going on? And then we can get on our own sort of journey of how Britain came to be where it is now. But these are the things that are moving around. Is it an exact science? I mean, you've said once it was we think and perhaps and so on. I mean, could you, or I go like I would say,

this is how it got here. We can map it. Yeah, actually, it's fairly exact, but it becomes less exact the further back you go in time. I mean, you rightly pressed me about what happened between 2.7 billion and 1 billion. And the reason is that we don't know very well because, as Sanjeeva said,

where we know things were in the past is because of these paleo-magnetic reconstructions. And there is very little rock of that kind of age which is assemblable, if I can put it in that way, to actually find out where Britain was prior to about one billion years before present. There is another complicating factor as well, which is that

The Earth's history moves in cycles. The longest cycle is about 500 million years, half a billion years, which is the rate at which supercontinants form and break apart. Then there seems to be another cycle of about 30 million years. And then there's three short period cycles, which Sanjeeves talked about, 100,000 years, 42,000 years, 23,000 years. And these are related to the way the Earth's geometry, its orbital geometry, varies around the size.

sun. And that's always happened, but it doesn't always find expression in terms of climate change. The way it finds expression in terms of climate change is if there's land masses over the north and south poles. If you have that, then you are preconditioned for ice ages. And that's when these orbital cycles become important. Jane? Well, we've mentioned climate. And actually, one of the most important factors about influencing Earth's past climate is where the continents have been positioned over the latitudes. So, I mean, we see a fairly logical pattern of climate in the past. If the continents over the equator, then we see evidence of deserts or tropical conditions

in the rock record and if it's over the poles. And in fact, I think the big glaciers themselves are more likely to have been caused by variations in carbon dioxide. And that carbon dioxide gas would have come from volcanoes. So that's intimately related to tectonics. So if we've got a big tectonic episode when the plates are moving around and our continents are moving around quite fast

then there's more volcanic activity, more carbon dioxide and then we get a really hot climate, hot earth and then at times that carbon dioxide levels have fallen and that's when we get the big glaciers. And then once we've got a glaciation then the orbital impact starts. And this really influences sea level significantly. So for example, if we go to the Cretaceous time

just before. The Cretaceous time, let's see, just before 70 million years, sort of between 145 and 70 million years. What we see is really high sea levels. Many of the continent land masses were covered in seawaters. That's the time when we see the chalk, shallow seas extending out. And this is a time when many of the oceans, this is the breakup of this big supercontinent, Pangea,

the oceans are breaking up and we've got the Atlantic forming. We have mid-ocean ridges forming, at the centres of these new oceans. And these basically, because it's young crust, that's less dense, it rises up, it displaces the seawater across onto these continental land masses, and this is why we have globally high sea levels. When you talk about globally high sea levels,

I was reading those, they can go up by 100 metres or more. This is way before people were talking about, well, there weren't many people around to have a talk to about global level. Yes, it's right. But there's still massive. massive rises, aren't that? The dinosaurs did very, very, very nicely. Yeah. And the other thing is that, obviously,

when we have these glacial periods and we have continents at the poles, we can lock up that seawater into those continental ice sheets, and that can rapidly reduce sea levels. So we can have very abrupt shifts in sea level by hundreds of meters. Right. I think the listeners will have a view of what's going on

the sense of the tectonic shifts, climate changes. It's all speeded up, and you've been very helpful in speeding it all up, but volcanoes and glaciers and so on. Let's try to follow this story through. So let's try to get a grip on where Britain, as we are now, started from, and how it came to be. Now, let's devote the rest of the programme for that. So starting with you, Richard, let's start with when I said about tectonic, shunted north. When did this northern shunt take place of England and Wales, and how did it happen?

Well, that started about half a billion years ago, 500 million. At this time, as Sanjeeva said, Scotland was relatively close to the equator, and England and Wales was a lot further south, about 40 degrees further south, which is several thousand miles. And there was an ocean between them, and this ocean was a sort of proto-Atlantic. My colleagues will shoot me for summarizing it like that, but it was called iapetus.

And what happened is that southern Britain and Wales broke off the second of these big supercontinents, which we've talked about, which was called Gondwana Land. And this Britain and Wales broke off and drifted north as this ocean iapetus closed. And it closed because the northern margin was being pulled underneath, a tectonic plate to the north. So if you imagine this entire ocean shutting, and eventually by the Devonian, which is broadly 400-ish million years ago,

southern England and Wales slammed into Scotland and Nova Scotia, which was on the southern margin of this continent, which was to the north of this ocean. And this was a major interval of mountain building in the British arms. And they're on the equator at this time, huh? They're still south of the equator, considerably south of the equator.

To simplify, if you imagine Scotland being a bit south of the equator, say just south of latitude of Central Africa at the moment, and England and Wales being much further south, say Tierra del Fuego type latitudes, and there's an ocean between them. The ocean disappears and they slam together. this mountain-building episode was called the Caledonian orogeny, and the southern uplands of Scotland are the margin where this collision took place.

It's one of the two big mountain-building episodes in British geological history. Sanji, England, they collide, and it's called the closing of the oceans. Can you, which your rich is referred to, can you develop that a little? Well, if we go back to a more recent analogy, If you look at the Himalayas today, the Himalayas were formed by the collision of a small continent, India, that was floating in an ocean, and it moved northwards and slammed into Eurasia to form this huge mountain belt, the Himalayas.

Also, the European Alps were formed by collision of Africa into Europe. So these are present-day mountain belts built by mountain building processes 50 million years ago to present. these older mountain belts, we can't see them now. We've only got the degradation complexes, the eroded remnants of these, but they would have looked a little bit like the Himalayas look like today. Jane, so we're still south of the equator.

They've slammed into each other, but they're not isolated then, are they? They're still part of England, Scotland, Ireland and Wales. They're still part of a landmass. Yes, you wouldn't have recognised the shape of the British Isles at that time. They would just form part of this big landmass. How big was the landmass? How small were they?

I mean, say the landmass is six feet long, what were they, six inches? Oh, tiny, tiny, because we're talking about the whole of North America. Give us an inch. Give you a centimetre. No, I mean, they've got the whole of North America and the whole of Asia all joined together in one big landmass. And so what was to become, what we know of the British Shiles, was just a tiny blip in that whole landmass, just a tiny blip. but they held together

and they're in the middle of that landmass and so what is the next movement? I know I'm not rushing I'm just trying to get it. We're still below the equator now we've got a way to go haven't we? We're on the equator now, we're on the equator How long ago is this?

So let's go to what we call the carboniferous period which is about 300 million years ago which I think is the next step in the history and the British Isles area is on the equator, the whole of northern hemisphere continents are on the equator And so because we're on the equator in the carboniferous,

we see, I'm going to talk about climates again now, we saw really hot tropical climates. So if you go to, let's go on a field trip again to the UK, if you go to Wales, if you go to Scotland, if you go to Leeds, where I am, northern England, you'll see lots of coal measures, lots of coal pits. And that's because these rocks are of carboniferous age and they were formed on the equator

and they were really lush forests growing there. And all those plants were buried in waterlog conditions and they form coal pits. So that's a really good evidence that we, that the Bischarles was around the equator of that time. And the whole of the northern hemisphere, that's when all the major, major coal fields formed. And at the same time, there was a sea level rise, a so-called transgression, which meant that these swamps, which Jane has been talking about, were continuously flooded, then unflooded, and so that they built up in peat layers, which became compressed and formed the coal.

Is there any, sorry, Jan, have to you? I was just going to carry on with the story. And then the continents are still moving, and they're all, all, all, beginning to merge together into this big landmass, which we've referred to as Panjia, but it's a big landmass. It's an unusual time when all the continents on earth were all packed together. And they spread almost from the North Pole to the South Pole, sort of equally spread across the equator. And because then we had this big landmass, we had a time of deserts. So it's a big, big change from a time of swamps to a time of

really of deserts. And the only wet climates we see at the poles. So again, if we go and look in the rocks, in the UK of about, say, 200 million years old. It's called a sort of Triassic, Permian Triassic period. We see red bits. It's the red rocks that formed in desert. So go to Central Australia, go to the Namib Desert today and see sand dunes with lots of red colouring due to one.

And you see red here too. Yeah, so if you go somewhere in the Midlands and you look at the rocks around you, if you drive up the M1 and have a look, you'll see red rocks around you. And that's because they were formed when the British Shiles was really a desert. just one big desert.

We're already talking, Sangeva, about the British House. Was there any sense of shape at that time? Or was it undistinguishable from the rest of the stuff around? Was it just part of? Yes, it was undistinguishable from the west of it. We were really right in the centre of this big supercontinent that lasted about 100 million years. Richard, Richard, Ritchcockfield,

do you mind if we jump forward to when the Atlantic Ocean begins to form the effect that that had? That's about, well, it says 200 to 100 million years ago, I mean, I'm lost in these figures, but then away you go. Okay, well, to put it in perspective, the Atlantic Ocean began to form when this supercontinent, which is the third of the big supercontinants we've been talking about in this program, the youngest, started to break apart.

And that was, as you say, about 200 million years ago, which would be, if you think about timescale, this is a 24-hour clock, about 22-11 at night. Okay? So our story started at 9 o'clock at night, we're now at 22.11. Humankind appeared about half a second towards midnight. So what happens is that this new supercontinent starts to break apart. It doesn't break apart in a simple fashion, you'll be unsurprised to hear,

because just to the east was a gigantic ocean called Tethys. And this ocean was pushing westwards and was about to separate north and south. America and Eurasia from Africa, which means that by this time Britain is well and truly in the northern hemisphere. And so by the time you get to the time of the dinosaurs, the Cretaceous broadly 100 million years ago, you've got an ocean which goes right around the world, around the equator. At the same time, you've got a new ocean forming north to south, unzipping from the south northwards like a zipper being unzipped. And this is a

is the Proto Atlantic. And it's this Proto Atlantic, which is splitting this supercontinent into North and South America, on the one hand, on my left, and on my right, Eurasia and Africa. So can you take us up from that? I mean, Richard's put us graphically to about 100 million years ago. Where is Britain then? So the British Charles are still...

By that time, just let's summarize. What sort of characteristics of rocks has it picked up on its journey so far? You've talked about the evidence of deserts, you've talked about the evidence of being in swamp lands or the coal fields. What else has it picked up? We had its claim at the top, at the beginning of the programme, great diversity, longest, youngest and so and so forth. It's picked a lot up on the way, which is holding in it,

in this particular what becomes a small landmass. So we had all the mountains that Richard mentioned at the beginning. And then actually, after that sort of mountain building phase and the joining of the two parts to make the British Isles, Actually, things became relatively quiet and this landmass is moving north. There's a little bit of mountain building, but not too much. It's fairly quiet, tectonically.

And still, the British Isles is fairly quiet, even though the Atlantic is opening. And the next big event, really, is the opening of the North Atlantic, and that happened about 60 million years ago. And so the North Atlantic splits and pushes North America away from the UK so North America moves west

and you can see that in the UK because that big splitting, tectonic splitting of the North Atlantic is accompanied by lots of volcanic activity so on the Isle of Sky and on rum they're all volcanoes associated with the rifting of this big ocean and North Atlantic so it's part of a big area of volcanic activity Sunji

But we're left, but Britain is still part of Europe though There's no... Yes, Britain's still part of Europe so we've got an o'clock forming to the west. But what's also happened during this time is that the crust has been separated. It's rifting apart.

So we get lots of highs and lows. So for example, the North Sea, which is part of our big petroleum province, is essentially formed by this separation of the continent. It doesn't go to full ocean spreading, but you form these basins. So lots of small-scale topography with small mountains being eroded and filling these basins.

And subsequently, we go into the Cretaceous, we have this period of time when we have these high sea levels that I've talked about earlier. So we get this blanket of chalk spreading out across Europe all the way out to Asia.

And then we start seeing the next bit of plate tectonics happening and that is we see around about 50 million years ago we start seeing the closing of this ocean that Richard talked about Tethys, the convergence of India and Africa with Eurasia.

and particularly in Europe we see Africa moving towards Europe. We get a little microcontinent within that ocean which collides with Europe that forms the European Alps, so a big mountain building period. And we actually see the effects of that in Britain, particularly in southern England, by a rippling of the landscape, if you like. So if you imagine if you push on a carpet at one edge of the carpet,

at the other edge, you'll see the carpet fold over into some gentle folds. And the landscape of Southern England are these gentle folds, essentially. So the wheeled that we see in southern Kent is a gentle fall and that sets the next stage, this sort of low-lying, small-scale topography. And what does then, what happens in the next stage, Jane?

Well, if you come up to sort of what we call, where geologists will call the sort of modern day will be the glaciation I think in the Bichars. So that started a couple of million years ago. Well, gradually the climate of the earth started cooling about 40 million years ago

and then gradually the northern hemisphere got cooler and cooler in the British Isles the climate around here would have cooled down considerably so that big ice sheets formed in the north in the hall of the northern continents. An ice spread over the UK right down towards southern England. So it covered in big ice sheets

and only the southern part of the country at the time when it was really cold was just free of ice. That's very recent really, isn't it? I mean, that's just the last two billion years. It's at this time, at the English, Are we beginning, is there any formation of the English Channel there? Is that later than two million years ago? That was it.

Yeah, I mean, that sort of really sets off at about half a million years ago. And the key thing is that with the ice age there, the ice sheets present, we get these fluctuations, again, due to orbital variations, we get abrupt changes between glacial periods and interglacial periods. And during these times, during the glacial periods, the seawater is locked up in the ice sheets. So Britain is basically attached to the continent because there's no shallow seas. When we go into these little warming blips, when the ice sheets melt, sea levels rise.

Now the key to this and why Britain we see the shape of it at the moment is that during these glacial periods, Britain is attached. And I've talked about these folds that were formed during the alpine collision. And these form topography. So, for example, the wheeled, these rock ridges, would have extended out into Europe. What happened to make Britain separate is that we had a breach in that rock ridge. And we know that the closest that Britain is to Europe are the Straits of Dover. And this is where this rock ridge was breached. And that was a precondition to how Britain became an island.

How is it breached? Well, that's some work we've been doing recently. We've been mapping in the sea floor of Britain. We've discovered evidence for huge megaflads. So Jane has talked about these ice sheets. Well, the ice sheets extended from Britain across into Scandinavia. And it basically went right across the southern North Sea, forming a wall of ice.

And this ended up forming a lake where we see the southern North Sea, you know, east of East Anglia, basically a large lake that was basically dammed to the south by this rock ridge. And for some reason, this rock ridge failed, releasing all this lake water into the dry land that's the English Channel and carving the sally system. Subsequently, once we'd created this breach and the ice sheet started melting during one of these interglacial blips,

sea level rose, and allowed the English Channel Sea to actually connect with the North Sea, and that's when Britain finally became an island. Just to complete this before we move on, Jane, how did Ireland become separated from England, Wales and Scotland? It's the same story, more or less.

I mean, Ireland is actually attached to the UK and just became the Irish Sea just formed as ice sheets melted and filtered big channel, lowland area between Ireland and England and Wales. So it's all to do with sea levels at this stage, isn't it? That's what forms the islands. But if you took all the water away, we'd just still be on one big lamass, all joined together. We'd just be a big promontory of Europe, essentially. Richard Goughkevon, how regularly does the coast of Britain fluctuate?

I was actually just going to say that really because I think that the message to take away from this at the moment is just how transient everything is. I mean what you think of as the topography of Britain today was nothing like what it was even a million years ago and with sea level... Did you elaborate on that? That would be interesting.

What was it like a million years ago as you put it? Well, if we were say, say the highest of the last ice age was what 18,000 years ago? So if before that, as the ice sheets came southwards, you could imagine a kilometre high sheet of ice grinding its way across Birmingham, pretty much as far south as Coventry, say. And then it stopped. And so you would have a situation where Britain was in the thrall of very cold,

cold climates. The Gulf stream had not started. It is the Gulf stream which keeps Britain artificially warm and that's a function of the closure of Isthmus of Panama basically. And then natural climate cycles, these cycles at San Jeevas talked about, these

variations in the Earth's orbital geometry dictate that probably through some modulation related to carbon dioxide release from the continental shells, climate warm and so these ice sheets then retreated back towards the North Pole. As I said earlier, a precondition for glacial conditions on this planet

is almost certainly to have a continent on one or preferably both poles. And the precondition for the latest series of glacial ages, what we consider the latest series of glacial ages is just one of about four, which have happened in the last billion years. We talked about the Ediacaran a little while, a mile ago. That was a snowball Earth and then there have been two ice ages in between and now there's the latest ice ages. And they are separated relatively widely according to whether or not there's land over the poles. So once you have land over the poles, you can in fact

express these variations in Earth's orbital geometry by warm and cooling episodes, probably through the mechanism, the in-between mechanism of carbon dioxide. Well, I mean you've been very gallant in spelling this out so clearly in such a short time. Can you just give us an idea of where we are now in the history of things in the broad history

of things? Because you speak in very broad terms, it's very exhilarating for most of us who wonder what's going to happen tomorrow morning. So where are we now? Where's the planet now in terms of glaciation, sea level rises, phonic... James raising a hand, my goodness.

That's a bit of it. This is what I work on. I mean, at the moment, the really interesting research, and I think is really interesting research that's going on is what's happening to the big ice sheets in the globe. I'm sure you've seen that in the press, and particularly in Antarctica, because Antarctica has a huge amount of water stored on it,

and believe it or not, what goes on in Antarctica actually affects the British Isles now. And so if the climate is warming and sea levels and the ice melts, then sea levels rise again, which will affect us in the UK. So, I mean, I live in Leeds on a hill, and I think in a few years' time, if the Antarctic ice sheets melt,

I'll have a nice beachside property. What do you mean? No, seriously, I know you live in Leeds. I know you live in Leeds. What do you mean by a few years' time? Oh, well, that's the big question. That's a big question. How long is it going to take to melt all the ice?

Because for most of us, a few years does mean next birthday, yeah. For you, it could mean anything from 10,000, to half a million years. I'm not going to talk about a million years in terms of melting the ice at the present rate of warming, then that's what we're trying to work out. But we're probably talking now

about some change in sea level in the next hundreds of years. Significant, I mean, sea level rise all the time, but we could see major sea level rises in thousands of years. Richard and then Sanjew. I just want to make a statement that this is not necessarily a conversation

about man-made global warming. We are in the middle of an interglacial anyhow. And if we've demonstrated anything, I hope it's that climate change changes through natural mechanisms. The question about whether or not and how much a man-induced carbon dioxide into the atmosphere is going to change things is still moot and probably beyond the scope of this programme.

But in the longer term, in 250 million years, then the continents will coalesce once more into another supercontinent. This one probably centred in the northern hemisphere, just as it did 250 million. years ago because it's the continental super cycle. So I think you're sure of that, I am? Pretty sure, yes. It's a easy bit. It's 250 million years ahead.

The continental super cycle is about 500 million years, which is... So that will happen? Yes. Well, there we are. Somebody to look forward to you. People use physical laws and they make computer programs showing how all the plates act together. And we know some of the laws

that govern the movement of these plates. And so you can... I mean, as we've hopefully we've demonstrated, there is a continual movement and continual change in plate motions. And as that changes, climate changes, we're talking on geological time scales now, not what's going to happen in the next few years. But over geological time, the continents will move

and we can pretty much work out where things are going to go. And that's all really governed by these movements of the mantle. So you have convection cycles. The mantle under the crust. You've got heat transport. And this is driving these supercontents. We don't fully understand the processes. they operate that.

But that's fundamentally what drives the Earth. It's the Earth's engine. But remember that Richard was talking about 250 million years' time, which is a rather long time, even for geologists. My money's still on cats to inherit the Earth. But how is Britain changing now? How is it changing in shape now? I mean, in reasonable time.

In reasonable time, sea levels are rising, and so you're going to get coastal erosion. And that's the period we're in. How long have we been in that period? the last 18,000 years. And it's been sort of steady, one step forward, half a step back, more step? No, it's pretty much one step forward.

We also know that some of parts of Southern England are subsiding, you know, just the weight of the rocks in Southern England a depressing land. So, you know, there is erosion around the coastline, especially on the East Coastline because the rocks are so soft there because they're so young, but they're also sinking. in say Scotland where the ice sheets have taken off. So since the last ice age age sheets

have been taken off. So the rocks are rebounding. They're moving up because the weight of the ice has been taken away. So that's going on. It's very, very slow, but it's still going on. And then there'll be another ice age and the sea level were decreasing. Well, that's the big point. So if we entered another ice age, then again we would drop our sea levels and Britain would again become connected to the continent.

But this is all to look forward to. immensely reassuring, actually. Well, it depends for whom. For bacteria and that for the rest of us. So what's to look forward to is it's very slow. That's the nice thing about it. It's as far as we living in

2000 and whatever it is are concerned. Yeah. But it is going on. Jay, finally. The UK, as you mentioned before, the UK is fairly stable tectonically. I mean, you mentioned this is that things are going slow. I mean, in some parts of the world, things move very fast in terms of earthquakes and volcanoes. And that happens. So some bits of geology happen extremely fast and are major hazards in the British arts. We're

quite lucky. Well, thank you very much, Jane Francis, Shangjib Gupta and Richard Corfield. Next week we'll be talking about Schopenhauer. Thanks for listening. If you've enjoyed this Radio 4 podcast, why not try others, such as Thinking Aloud, where Laurie Taylor discusses the latest social science research. To find out more, visit BBC.com .co.uk forward slash radio four.