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At the Dee Estuary, an imaginary line in the mud
marks the boundary between the English and the Welsh.
You soon hit a high spot of Victorian resort building, Llandudno.
The town's nestled in the shelter of the Great Orme's imposing cliffs, which point our way westward.
Out towards my destination, the largest island in Wales, Anglesey.
Many make their way to these cliffs for the glorious sights looking out
to sea, but what's brought me here are the rocks beneath my feet.
On the island's edge you see a slice right through the Earth's geological history,
an extraordinary collection of rocks are exposed here.
Just to show you how different Anglesey is,
look at this geological map of southern Britain.
Great swathes of it are all the same colour, meaning they're all the same rock type.
Here's this great band of chalk running up here in green,
there's another huge band of limestone running down here.
But up here on Anglesey something different is happening.
There's an intense mosaic of different colours, meaning there are many different rock types.
Much of the mystery of Anglesey's formation is buried below the turf,
but the coast reveals the island's subterranean secrets.
The most stunning geological feature is the long channel of water
that separates Anglesey from the mainland, the Menai Strait.
To understand its significance I'm with David Schofield from the British Geological Survey.
What part does this gulf play in Anglesey geology?
Well, this is actually a long fault zone which we call the Menai Strait fault system.
It separates very much
older rocks to the north west than those to the south east.
It's a fundamental geological divide, which we know is still active
cos we're seeing some of Britain's biggest earthquakes happening along this fault line.
-Where we're standing?
-Right where we're standing, yes.
So the shore we're on here is moving in relation to the shore over there.
It certainly is, at a very slow rate every year,
and every now and then it takes a bit of a jump and there's an earthquake.
Around 300 small earthquakes shake Britain each year, often felt
most strongly here, caused as the mainland grinds against Anglesey.
It's part of the bigger movement of landmasses around the globe.
The Earth's crust is made up of separate distinct plates
which are constantly moving against each other.
Where the edges of the plates move apart, new crust is created about as fast as your fingernails grow.
Deep on the ocean floor, as the plates tear apart, lava can ooze out.
This fiery business of planet building is exposed beautifully
on a small strip of Anglesey at Llanddwyn Island.
Local geologist Margaret Wood is my guide.
These are the world-famous pillow lavas of Llanddwyn.
All I can see is a grey rock.
It's beautifully bluey-grey, though.
We're looking at pillows which are lava which came up on the ocean bed.
They get into the water and immediately the outside will crack.
These huge great big rounded lumps here?
Each one of those is called a pillow.
It is astonishing the way that raw nature can produce these symmetries and shapes.
But having looked at those,
something even more extraordinary, on the other end of the island,
you've got material that has actually gone down back into the crust,
and the fantastic thing is Llanddwyn Island is a complete mini-plate.
But that's amazing. I always thought that these plates on the surface
of the Earth really were the size of continents or oceans.
You're telling me that here on this beach in Anglesey there's an entire plate.
This tiny island tells a big tale of how the Earth's built.
The plates of crust pull apart at one edge, but collide at the other edge.
As they crush into each other a jumble of different rocks is left behind,
which remarkably you can also see on Llanddwyn Island.
Wow, just look at that! Those colours, Margaret!
-It's fantastic, isn't it?
-It looks like a great, big blancmange.
It's wonderful, isn't it? Those are quartz-rich rocks, you've got limestone over there,
and you've got schists, you've got conglomerate, and the colours are fantastic, aren't they?
-So this is two plates of the Earth crust colliding?
In the hundreds of millions of years Anglesey has been moving around the globe,
collisions and splits in the Earth's crust have created an astonishing array of rocks.
It's not just geologists who love this landscape,
it's a paradise for climbers, too.
The sea's worked away at the weaker rocks to create some of Britain's toughest cliff climbs.
Now I'm taking up the challenge to see these rocks as only climbers can.
But before the ascent, I've got an exhilarating 100-foot descent in prospect.
Fortunately, Libby Peter and Graham Desroy know their ropes.
Is it the nature of cliff climbing that you're always going
to start by going down before you can come up?
Normally you climb a mountain
and then abseil down again, but sea cliffs, it's the reverse.
You commit yourself by abseiling in and then you have to climb out again.
It does look amazing when you just disappear into the...
Yeah, it's like you're abseiling straight into the sea.
Yeah, it does. See you down there.
OK, will do.
Here goes. It's a very long way down.
The rock is now very dry and storm battered.
It's as if it's been scoured clear of vegetation.
That's pretty exciting.
-Is this where we start traversing round or...
You know you're close to the sea when the spray starts whacking you in the face.
What do you think?
Well, it beats sitting on a beach!
Just awesome, it's architecturally massive.
Takes your breath away.
The old heart's going.
The pros rate this climb as "very severe".
I can't tell you what I call it.
I can see all the incredible folds of rock, it's been bent like a piece of paper.
I mustn't get too distracted, I'm meant to be climbing.
I've been following this band of quartz all the way up.
Here it is,
glistening white in the sunshine. It's very beautiful.
Thank you so much.
That was sensational.
Thank you so much. It's such an honour to be taken up by the two of you.
I was so impressed with the way you climbed it. It was brilliant, it really was.
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