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M1: The Lake District, Snowdonia, the Cairngorms,
Loch Lomond and the Trossachs and the Brecon Beacons.
In the past, many of the UK's great National Parks
were shaped by the action of glaciers.
With corries, U-shaped valleys, pyramid peaks and arêtes,
the large-scale landforms, that dominate these landscapes
are magnificent sculptures carved out by flowing ice.
(light melodic music)
But the more subtle features of these landscapes
also have their own story to tell.
Look down this valley, it's a U-shaped valley
that's been carved out by a glacier.
And down in the centre of the valley,
we can actually see a depression,
where a small glacier sat
just 11,500 years ago,
right at the end of the last ice age.
M2: But if we look more closely, we can see
that the bottom of the U-shaped valley isn't smooth.
We can see these lumps and bumps in the foreground,
we can see these distinct ridges in the background.
What are these features?
Why are they here?
M1: Let's go and take a closer look
and find out what they're made of.
So we're down here on one of these subtle ridges
down in the valley bottom.
And we've actually come right down to the stream,
where the flow of water has been cutting,
or incising down into the material
and we can see what it's made of.
M2: This type of sediment looks really familiar to me.
I've seen similar kinds of sediment,
that are deposited at the margins of modern day glaciers
in places like Iceland, Norway and Greenland.
And if we look closely, we can see how this sediment
is made up of a mixture of fine sediment,
of pebbles, of large boulders,
and we call these types of sediment glacial till.
(light upbeat music)
So how does this glacial till get here?
M1: This is how glaciers begin, as pristine ice,
where snow has accumulated in a depression on a hill slope
and then becomes compressed into ice over time.
M2: As the glacier flows down the valley,
it erodes the valley floor.
It picks up or entrains this sediment
to produce dirty ice at the base of the glacier,
a bit like this block of ice
that we've picked out of the river.
This sediment is then transported
down to the glacier margin or to the terminus.
And at the snout of the glacier, as the ice melts,
this sediment is released to produce this glacial till.
So the glacier acts a bit like a conveyor belt,
eroding, transporting and then depositing sediment.
This glacial sediment accumulates
at the margin or the edge of the glacier
to produce landforms that we call moraines.
M1: Now in this Lake District valley,
we can actually see several different types of moraine
all created by the glacier in slightly different ways.
(light melodic music)
So right at the end of the last ice age,
a little glacier formed here in this valley.
It started up there in the cirque
and it flowed right down the centre of the valley,
and this is where the glacier came to a stop.
This mound or pile of material is made of glacial till
and we call it a terminal moraine,
deposited at the end or the snout of the glacier.
The terminal moraine reaches
right across the centre of the valley.
But since the last ice age,
the river flowing down this valley has been eroding away
or incising the centre of the valley,
and it's cut this moraine in half at its centre.
(light melodic music)
M2: We can actually see two terminal moraine ridges,
which are parallel to each other.
And if we look at Skeiðarárjökull in Iceland,
we can see as many as fifteen terminal moraine ridges.
These illustrate what we call a punctuated retreat:
the glacier retreats during the summer
and experiences little re-advances during the winter.
We call this recessional moraine
and this shows how the glacier retreats and advances,
as it responds to changes in climate.
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M1: As the glacier here flowed down the valley,
it scraped material, tearing rock and soil
away from the valley side.
The glacier also made the sides of the valley steeper,
and mass movements like landslides and rockfalls occurred.
We can actually see all this material from that erosion
and the mass movements, still piled up here
along the valley side.
This long ridge,
this long mound is called a lateral moraine
and it would have run along the side of the glacier.
(light melodic music)
So like most valleys that have been glaciated,
this valley actually has two lateral moraines
running down the sides of it.
The lateral moraines come down either side of the valley
and then they actually curve round
to come together and meet in the middle,
as part of the terminal moraine.
This big, curved deposit of glacial till
tells us where the glacier came up to,
when it was here at the end of the last ice age.
(light upbeat music)
M2: Just like the tributaries of rivers,
two glaciers can come together to produce a larger glacier.
In these situations, the lateral moraines come together
as well to produce a medial moraine,
that flows down the centre line of this larger glacier.
We often see these medial moraines
on the surface of modern day glaciers.
However, they tend to be destroyed
as the glacier recedes and as the ice melts.
So we don't see medial moraines in landscapes like this,
where the ice melted several thousand years ago.
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M1: There's also another type of moraine here,
which is actually less well known.
Look at this chaotic, bumpy ground all around us.
It's not a nice straight line of terminal or lateral moraine,
but it is made of glacial till.
This is called hummocky moraine
and it was formed when a big part of the glacier broke off.
It probably became stuck on some rough ground
or broke away from the main glacier
and came to a stop right here.
The ice then melted,
dropping all the sediment it was carrying,
creating these lumps and bumps
on the surface of the landscape.
This hummocky moraine is really the sign of a dying glacier,
right at the end of its life.
M2: So we've looked at the formation
of different types of moraine.
M1: Terminal moraine, which is deposited
at the snout of the glacier
and it runs right across the middle of the valley.
M2: Recessional moraines,
where we have a sequence of end moraines,
that show how the glacier margin has retreated over time.
M1: Lateral moraines, which have been carved
from the valley sides and they run
right down the valley from top to bottom.
M2: And medial moraines,
where these lateral moraines come together
and the sediment is transported
down the centre of the valley.
M1: And finally hummocky moraine,
where part of the glacier has broken away and melted,
leaving a chaotic mess of sediment
and creating bumpy ground.
M2: By thinking like geographers, we can make sense
of these smaller, more subtle landforms,
that help to reveal the hidden secrets
of our landscape's glacial past.
M1: Every lump and bump in the landscape
is actually there for a reason.
You can get out and explore our National Parks.
Or even start by exploring the entire planet,
using a geographical information system,
and see if you can spot these kinds of landforms
and make sense of the story that they have to tell.
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