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(upbeat electronic rock music)
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(light piano music)
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One of the most exciting things about physical geography,
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is that it can help reveal some
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of the hidden secrets of our planet.
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And landscapes like this are full of hidden secrets.
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But they're easy to miss if we're not looking for them.
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We're here in Teesdale, in North Yorkshire,
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and on this wintery day a great mystery
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lies hidden right beneath our feet.
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In the midst of this broad U-shape valley,
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we have this hill, about 20 metres high.
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But why is it here?
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What could possibly have created this landform?
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Let's go and see if we can find some hidden clues.
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The first clue is in the shape of the hill.
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If we look at the side facing up the valley,
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we can see that it is steeply sloping.
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But on the side facing down the valley,
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the slope is much gentler.
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It almost looks like an egg that's
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been half buried in the ground.
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The second clue comes in the shape of the valley overall.
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Rather than having a narrow, V-shaped valley
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carved by a river, which is common in upland environments,
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what we have here is this big, broad U-shaped valley.
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And what this tells us is that
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this valley has been carved by ice.
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As recently as 14 or 15 thousand years ago,
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this valley would have been filled with ice,
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that would have been flowing down the valley.
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So what do we know about glaciers?
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Glaciers form in very cold environments, where snow piles up
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over time and becomes compressed into ice.
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That ice then slowly flows down slope,
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and it carves away at the valley floor
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through abrasion and plucking.
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But the base of the glacier also sculpts and moulds
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big piles of sediment as it flows down the valley.
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Just like the ripples on the bed of a river.
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(soft guitar music)
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This is what the sediment at
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the base of a glacier looks like.
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You can see here we have this sediment
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which is made up of a mixture of
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fine sediment particles, of these small pebbles,
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and of these large boulders.
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And this chaotic jumble of material
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is known as "glacial till".
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As the ice melts at the base of the glacier,
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it saturates the sediment, and makes it easy for it to
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be moulded and shaped by the force
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of the glacier flowing over the top of it.
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We call this process "subglacial sediment deformation".
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So "sub-", like submarine which would be under the water,
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here "subglacial" means under the glacier.
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Subglacial sediment deformation
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refers to the moulding, or deformation,
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of sediment underneath the glacier.
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If we look closely at one of the boulders
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that we've taken out of this glacial till,
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we can find one final clue that shows us
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that the sediment has been deposited underneath a glacier.
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If we look closely on the surface of the boulder,
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we find it is covered in these scratches,
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or these striations.
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These would have been created
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as the boulders within this till,
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rubbed against each other as the till was moulded
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and sculpted underneath the moving glacier.
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This is some glacial till,
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and its saturated with water, just like we'd find
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at the base of a glacier where the ice is melting.
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The glacier would be flowing over
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the top of this glacial till.
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As the glacier flows down the valley,
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sometimes it comes into contact with
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a stiffer, sticky, heavier piece
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of sediment on the valley floor.
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When that happens, all the sediment bunches up,
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the ice flows over the top of it
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and then continues down the valley creating this hill,
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with a steep side facing up the valley,
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and a smoother, streamlined side facing down the valley
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where the ice continued to flow away.
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Through all these clues, the landscape has revealed to us
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the secret of how this hill was formed.
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It was created by a glacier flowing over the ground,
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moulding and sculpting the sediment underneath it,
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in a process that we call subglacial sediment deformation.
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And we call these landforms "drumlins".
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The length of drumlins can range from
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about 50 metres to several kilometres long.
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And their height ranges from a few metres tall
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to up to 100 metres tall, or five times as big
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as this drumlin here behind me.
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Here at Holwick, this drumlin looks
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like it just occurs on its own.
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But in reality, this drumlin forms part of a much larger
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group of drumlins that covers the whole of the valley floor.
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The physical landscapes of our planet
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are shaped by different processes.
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Now, although this drumlin was created by
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subglacial sediment deformation,
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this may not be the only process that can create drumlins.
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When a glacier melts quickly, this can result
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in huge amounts of water flowing beneath the glacier.
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This happens during hot volcanic eruptions,
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when lava melts the ice, or when water becomes trapped
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beneath the glacier and the ice dam bursts.
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This can create a subglacial mega-flood, or jökulhlaup,
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generating powerful flows of water that flow under the ice,
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to erode the sediment and to create the drumlins.
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So drumlins can form both through
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deposition and through erosion:
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deposition associated with subglacial sediment deformation,
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and erosion associated with catastrophic
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meltwater floods occurring underneath the ice.
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Like topics right across geography,
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the formation of drumlins is still
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a subject of continuing research.
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The main theory at the moment is that
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drumlins are formed by subglacial sediment deformation:
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the moulding and sculpting of sediment beneath the glacier
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by the force of the flowing ice.
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Alternatively, some drumlins could be eroded
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by flowing meltwater underneath glaciers,
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or perhaps they could be formed
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by a combination of the two processes working together.