0:00:25 > 0:00:29At the Dee Estuary, an imaginary line in the mud
0:00:29 > 0:00:33marks the boundary between the English and the Welsh.
0:00:36 > 0:00:42You soon hit a high spot of Victorian resort building, Llandudno.
0:00:42 > 0:00:49The town's nestled in the shelter of the Great Orme's imposing cliffs, which point our way westward.
0:00:52 > 0:00:57Out towards my destination, the largest island in Wales, Anglesey.
0:01:00 > 0:01:05Many make their way to these cliffs for the glorious sights looking out
0:01:05 > 0:01:12to sea, but what's brought me here are the rocks beneath my feet.
0:01:12 > 0:01:16On the island's edge you see a slice right through the Earth's geological history,
0:01:16 > 0:01:21an extraordinary collection of rocks are exposed here.
0:01:21 > 0:01:24Just to show you how different Anglesey is,
0:01:24 > 0:01:27look at this geological map of southern Britain.
0:01:27 > 0:01:31Great swathes of it are all the same colour, meaning they're all the same rock type.
0:01:31 > 0:01:34Here's this great band of chalk running up here in green,
0:01:34 > 0:01:37there's another huge band of limestone running down here.
0:01:37 > 0:01:41But up here on Anglesey something different is happening.
0:01:41 > 0:01:48There's an intense mosaic of different colours, meaning there are many different rock types.
0:01:48 > 0:01:53Much of the mystery of Anglesey's formation is buried below the turf,
0:01:53 > 0:01:57but the coast reveals the island's subterranean secrets.
0:01:57 > 0:02:02The most stunning geological feature is the long channel of water
0:02:02 > 0:02:06that separates Anglesey from the mainland, the Menai Strait.
0:02:08 > 0:02:14To understand its significance I'm with David Schofield from the British Geological Survey.
0:02:16 > 0:02:20What part does this gulf play in Anglesey geology?
0:02:20 > 0:02:24Well, this is actually a long fault zone which we call the Menai Strait fault system.
0:02:24 > 0:02:26It separates very much
0:02:26 > 0:02:30older rocks to the north west than those to the south east.
0:02:30 > 0:02:33It's a fundamental geological divide, which we know is still active
0:02:33 > 0:02:37cos we're seeing some of Britain's biggest earthquakes happening along this fault line.
0:02:37 > 0:02:40- Where we're standing? - Right where we're standing, yes.
0:02:40 > 0:02:44So the shore we're on here is moving in relation to the shore over there.
0:02:44 > 0:02:46It certainly is, at a very slow rate every year,
0:02:46 > 0:02:50and every now and then it takes a bit of a jump and there's an earthquake.
0:02:51 > 0:02:57Around 300 small earthquakes shake Britain each year, often felt
0:02:57 > 0:03:02most strongly here, caused as the mainland grinds against Anglesey.
0:03:02 > 0:03:07It's part of the bigger movement of landmasses around the globe.
0:03:07 > 0:03:11The Earth's crust is made up of separate distinct plates
0:03:11 > 0:03:14which are constantly moving against each other.
0:03:18 > 0:03:25Where the edges of the plates move apart, new crust is created about as fast as your fingernails grow.
0:03:25 > 0:03:32Deep on the ocean floor, as the plates tear apart, lava can ooze out.
0:03:32 > 0:03:37This fiery business of planet building is exposed beautifully
0:03:37 > 0:03:41on a small strip of Anglesey at Llanddwyn Island.
0:03:41 > 0:03:46Local geologist Margaret Wood is my guide.
0:03:46 > 0:03:50These are the world-famous pillow lavas of Llanddwyn.
0:03:50 > 0:03:52All I can see is a grey rock.
0:03:52 > 0:03:53It's beautifully bluey-grey, though.
0:03:53 > 0:03:58We're looking at pillows which are lava which came up on the ocean bed.
0:03:58 > 0:04:01They get into the water and immediately the outside will crack.
0:04:01 > 0:04:03These huge great big rounded lumps here?
0:04:03 > 0:04:05Each one of those is called a pillow.
0:04:05 > 0:04:10It is astonishing the way that raw nature can produce these symmetries and shapes.
0:04:10 > 0:04:13But having looked at those,
0:04:13 > 0:04:17something even more extraordinary, on the other end of the island,
0:04:17 > 0:04:20you've got material that has actually gone down back into the crust,
0:04:20 > 0:04:27and the fantastic thing is Llanddwyn Island is a complete mini-plate.
0:04:27 > 0:04:30But that's amazing. I always thought that these plates on the surface
0:04:30 > 0:04:33of the Earth really were the size of continents or oceans.
0:04:33 > 0:04:38You're telling me that here on this beach in Anglesey there's an entire plate.
0:04:38 > 0:04:39Absolutely.
0:04:39 > 0:04:45This tiny island tells a big tale of how the Earth's built.
0:04:45 > 0:04:50The plates of crust pull apart at one edge, but collide at the other edge.
0:04:50 > 0:04:55As they crush into each other a jumble of different rocks is left behind,
0:04:55 > 0:04:59which remarkably you can also see on Llanddwyn Island.
0:05:02 > 0:05:07Wow, just look at that! Those colours, Margaret!
0:05:07 > 0:05:10- It's fantastic, isn't it? - It looks like a great, big blancmange.
0:05:10 > 0:05:16It's wonderful, isn't it? Those are quartz-rich rocks, you've got limestone over there,
0:05:16 > 0:05:22and you've got schists, you've got conglomerate, and the colours are fantastic, aren't they?
0:05:22 > 0:05:26- So this is two plates of the Earth crust colliding?- Exactly.
0:05:26 > 0:05:31In the hundreds of millions of years Anglesey has been moving around the globe,
0:05:31 > 0:05:37collisions and splits in the Earth's crust have created an astonishing array of rocks.
0:05:37 > 0:05:42It's not just geologists who love this landscape,
0:05:42 > 0:05:45it's a paradise for climbers, too.
0:05:45 > 0:05:53The sea's worked away at the weaker rocks to create some of Britain's toughest cliff climbs.
0:05:53 > 0:05:59Now I'm taking up the challenge to see these rocks as only climbers can.
0:05:59 > 0:06:05But before the ascent, I've got an exhilarating 100-foot descent in prospect.
0:06:05 > 0:06:11Fortunately, Libby Peter and Graham Desroy know their ropes.
0:06:11 > 0:06:14Is it the nature of cliff climbing that you're always going
0:06:14 > 0:06:16to start by going down before you can come up?
0:06:16 > 0:06:18Normally you climb a mountain
0:06:18 > 0:06:23and then abseil down again, but sea cliffs, it's the reverse.
0:06:23 > 0:06:28You commit yourself by abseiling in and then you have to climb out again.
0:06:28 > 0:06:32It does look amazing when you just disappear into the...
0:06:32 > 0:06:34Yeah, it's like you're abseiling straight into the sea.
0:06:34 > 0:06:36Yeah, it does. See you down there.
0:06:36 > 0:06:38OK, will do.
0:06:38 > 0:06:42Here goes. It's a very long way down.
0:06:48 > 0:06:52The rock is now very dry and storm battered.
0:06:52 > 0:06:55It's as if it's been scoured clear of vegetation.
0:07:00 > 0:07:01That's pretty exciting.
0:07:01 > 0:07:05- Is this where we start traversing round or...- That's right.
0:07:08 > 0:07:12You know you're close to the sea when the spray starts whacking you in the face.
0:07:12 > 0:07:14Hello, Libby.
0:07:14 > 0:07:16What do you think?
0:07:16 > 0:07:18Well, it beats sitting on a beach!
0:07:18 > 0:07:22Just awesome, it's architecturally massive.
0:07:22 > 0:07:24Takes your breath away.
0:07:24 > 0:07:27The old heart's going.
0:07:29 > 0:07:33The pros rate this climb as "very severe".
0:07:37 > 0:07:38I can't tell you what I call it.
0:07:42 > 0:07:48I can see all the incredible folds of rock, it's been bent like a piece of paper.
0:07:48 > 0:07:51I mustn't get too distracted, I'm meant to be climbing.
0:07:52 > 0:07:56I've been following this band of quartz all the way up.
0:07:56 > 0:07:58Here it is,
0:07:58 > 0:08:01glistening white in the sunshine. It's very beautiful.
0:08:19 > 0:08:21Thank you so much.
0:08:23 > 0:08:25That was sensational.
0:08:29 > 0:08:34Thank you so much. It's such an honour to be taken up by the two of you.
0:08:34 > 0:08:38I was so impressed with the way you climbed it. It was brilliant, it really was.
0:08:38 > 0:08:42Subtitles by Red Bee Media Ltd
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