(upbeat music)
F1: We're on Pen-y-Fan
in the heart of the Brecon Beacons National Park.
M1: Like most of Wales, 23,000 years ago
the valleys of the Brecon Beacons were filled with ice.
Massive glaciers that completely dominated this landscape.
Over the next 12,000 years, the glaciers came and went
as global temperature increased,
and the last glaciers were still here
just 11,000 years ago.
F1: We only have to look around to see the marks
that these glaciers have left on the landscape.
Look at this magnificent valley.
It's not a V-shaped valley created by a river
cutting down into the landscape,
but it's a U-shaped valley that has been
carved out by a glacier.
M1: With a little help from his freezer,
Tim has been trying to better understand
how glaciers have shaped this landscape.
M2: So to understand how glaciers
turn V-shaped valleys into U-shaped valleys,
we need to do a little bit of kitchen geography.
So the first thing that we need
is a bottle of water.
And we're going to put this in the freezer.
Then we need another bottle of water,
but in this bottle of water,
we're going to put some rocks.
We're going to shake these rocks
so they're just lying along the bottom of the bottle,
and we're going to put that flat into the freezer.
Now we're going to wait.
After a few hours, we're going to cut away the plastic.
This is what we've got.
Two model glaciers, one with no rocks at the bottom
and one with rocks at the bottom.
So we've got two halves of a watermelon here.
We're going to use these to show
the importance of glacial abrasion.
If I take the glacier with no rocks at the bottom,
you can see I could be rubbing this
over the watermelon for days on end
and nothing's going to happen to the skin of this at all.
Whereas if we take the one with the rocks at the bottom,
let's see what happens.
So if we rub this over here,
we can see the contrast is massive.
The glacier with the rocks at the bottom of it
has scoured away at the skin of the watermelon,
and this is very similar to a real glacier.
(upbeat music)
Now, as the glacier moves down the hill
due to the force of gravity,
it picks up or plucks material from the valley bottom.
This material is then attached to the glacier,
and in a process called abrasion
it scours away the landscape.
The rocks literally act like sandpaper.
So we've got another watermelon here.
And into the watermelon, we've cut a V-shaped valley
just like you would get from a river.
And what we're going to do is see what happens
and how that valley changes when we send a glacier down it.
So, if we take our glacier away,
we can see how those processes of plucking
and abrasion over thousands of years
have eroded the valley sides
and the base of the valley,
to turn that V-shaped valley formed by a river,
into a U-shaped valley, or glacial trough.
M1: And here back in the field,
we can see this glacial trough.
Along the valley sides are spurs or hills,
which would have once reached right
into the centre of the valley.
These have been chopped off, or truncated
by the glacier moving down the valley.
We call these truncated spurs.
M2: And that,
is kitchen geography.
So after a few hours, we'll take them out of the freezer
cut away the plastic, and then we've got
two fantastic glaciers.
(ice breaks)