WEBVTT
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(upbeat music)
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We're on Pen-y-Fan
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in the heart of the Brecon Beacons National Park.
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Like most of Wales, 23,000 years ago
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the valleys of the Brecon Beacons were filled with ice.
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Massive glaciers that completely dominated this landscape.
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Over the next 12,000 years, the glaciers came and went
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as global temperature increased,
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and the last glaciers were still here
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just 11,000 years ago.
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We only have to look around to see the marks
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that these glaciers have left on the landscape.
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Look at this magnificent valley.
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It's not a V-shaped valley created by a river
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cutting down into the landscape,
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but it's a U-shaped valley that has been
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carved out by a glacier.
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With a little help from his freezer,
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Tim has been trying to better understand
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how glaciers have shaped this landscape.
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So to understand how glaciers
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turn V-shaped valleys into U-shaped valleys,
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we need to do a little bit of kitchen geography.
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So the first thing that we need
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is a bottle of water.
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And we're going to put this in the freezer.
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Then we need another bottle of water,
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but in this bottle of water,
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we're going to put some rocks.
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We're going to shake these rocks
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so they're just lying along the bottom of the bottle,
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and we're going to put that flat into the freezer.
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Now we're going to wait.
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After a few hours, we're going to cut away the plastic.
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This is what we've got.
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Two model glaciers, one with no rocks at the bottom
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and one with rocks at the bottom.
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So we've got two halves of a watermelon here.
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We're going to use these to show
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the importance of glacial abrasion.
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If I take the glacier with no rocks at the bottom,
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you can see I could be rubbing this
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over the watermelon for days on end
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and nothing's going to happen to the skin of this at all.
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Whereas if we take the one with the rocks at the bottom,
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let's see what happens.
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So if we rub this over here,
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we can see the contrast is massive.
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The glacier with the rocks at the bottom of it
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has scoured away at the skin of the watermelon,
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and this is very similar to a real glacier.
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(upbeat music)
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Now, as the glacier moves down the hill
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due to the force of gravity,
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it picks up or plucks material from the valley bottom.
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This material is then attached to the glacier,
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and in a process called abrasion
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it scours away the landscape.
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The rocks literally act like sandpaper.
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So we've got another watermelon here.
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And into the watermelon, we've cut a V-shaped valley
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just like you would get from a river.
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And what we're going to do is see what happens
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and how that valley changes when we send a glacier down it.
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So, if we take our glacier away,
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we can see how those processes of plucking
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and abrasion over thousands of years
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have eroded the valley sides
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and the base of the valley,
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to turn that V-shaped valley formed by a river,
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into a U-shaped valley, or glacial trough.
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And here back in the field,
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we can see this glacial trough.
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Along the valley sides are spurs or hills,
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which would have once reached right
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into the centre of the valley.
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These have been chopped off, or truncated
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by the glacier moving down the valley.
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We call these truncated spurs.
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And that,
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is kitchen geography.
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So after a few hours, we'll take them out of the freezer
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cut away the plastic, and then we've got
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two fantastic glaciers.
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(ice breaks)