Lesson video

Hello, geographers.

My name's Mrs. Hormigo.

I'm really looking forward to teaching you today.

I hope you're going to learn lots and really enjoy the lesson.

Let's get started.

Today's lesson looks at the landforms created by erosion, transportation and deposition.

By the end of the lesson, you'll understand the characteristics and the formation of glacial landforms. There are four key words for today's lesson, plucking, abrasion, glacial trough, and moraine.

Plucking is when a glacier pulls chunks of rock from the bedrock as it moves over it.

Abrasion is when rocks and debris carried by a glacier scrape and wear away the surface beneath it.

Glacial trough is a valley reshaped by glacial erosion into a U-shape, and moraine is accumulation of unsorted debris, such as soil and rock, that is deposited by a glacier.

There are two parts to today's lesson.

What landform are created by glacial erosion and what landform are created by glacial deposition? Let's start by looking at what landform are created by glacial erosion.

Glacial erosion creates dramatic landforms by processes, such as plucking and abrasion, as glaciers carve through landscapes.

In this image here, you can see a pyramidal peak an arete, and a corrie, all landforms created by glacial erosion.

A corrie is a bowl-shaped hollow that is found on the upper slopes of glaciated valleys, and it often contains a small lake, known as a tarn or a cwm after glaciation.

You can see here in this image, this is Cwm Idwal in North Wales created by glaciers.

Let's look now at how a corrie develops.

So at the start of a glacial period, this is when the corrie begins to develop.

It often happens on north-facing slopes in the northern hemisphere, and this is because snow doesn't melt in the summer.

It builds up instead.

So snow accumulates in sheltered hollows on the mountain side.

You can see there in that image.

Over time, it compresses into ice.

So the layers build up, and it compresses into ice.

During the glacial period, there'll be some freeze-thaw weathering, steepening the back wall and adding debris.

Abrasion and plucking will deepen the hollow as the glacier moves slowly downhill, and three, rotational slip will increase the erosion at the base of the corrie, making it deeper.

At the front of the corrie, the ice becomes thinner, and erosion decreases as there's less energy.

A rock lip will develop.

After the glacial period, so as the glacier retreats, a steep-sided hollow will remain, and this is the corrie.

It will often feature a lake called a tarn.

Erosion and weathering processes will continue to act on the steep back wall, and after the glacier melts, the rock lip traps water, forming a small lake.

Check for you now.

Can you use numbers to correctly order the process of corrie formation? Pause the video, and come back when you've decided.

I hope you put the third one as number one, the first one as number two, and the middle one as your final number three.

Well done.

An arete is a sharp, knife-edged ridge that forms between two corries or glaciated valleys, and you can see on this air diagram here, there are three aretes separating these two different valleys.

A pyramidal peak is a pointed mountain peak that's formed when three or more corries erode back into a central point.

Mount Everest is an example of a pyramidal peak with aretes leading up to it.

Can you look at this photograph and identify the pyramidal peak and the aretes? Pause the video when you've labelled them correctly.

You should have picked out that there are two aretes leading up to Mount Everest in this image in Nepal and a pyramidal peak right at the summit.

Well done.

Glacial troughs, or U-shaped valleys, are formed when glaciers transform pre-existing V-shaped river valleys.

So as the glacier moves downhill, processes of erosion carve out the V-shaped valley to create a larger U-shaped valley.

You can see there in that first image, before glaciation takes place, a river has created a V-shaped valley.

During glaciation, as a glacier moves, its immense weight and erosional processes deepen and widen the valley.

So it goes from that V-shaped to a U-shape, a wider bottom with steeper sides.

Then after glaciation, you can see there that, as the ice retreats, you're left with that U-shaped valley or glacial trough.

Let's have a look at the characteristics of a glacial trough.

Steep, straight sites, wide, flat valley floors, and that characteristic U-shape.

Truncated spurs are found in glacial troughs.

They're formed when the interlocking spurs, which we can see in this image that are created by river erosion, are eroded away as the glacier advances.

So these interlocking spurs are part of the V-shaped valley formation before a glacier moves through.

After the glacier has been, you have the steep, cliff-like edges on the sides of the valley, and these are known as truncated spurs.

This is Llanberis Pass in North Wales, a very famous glacial trough.

Can you add annotations to describe its key characteristics? And can you label the truncated spurs? Pause the video, and come back when you've had a go.

Your labels may look like this.

The flat, wide valley air bottom, the U-shape, the steep, straight valley sides, and the truncated spurs, so the vertical cliff-like edges to the valley.

Well done.

Hanging valleys are formed by smaller tributary glaciers as they join the larger main one.

They erode their valleys less deeply.

So when the ice melts, these valleys are almost left hanging above the main valley floor.

They're often marked by waterfalls where the river flowing through that tributary valley flows into the main valley.

Ribbon lakes are another key landform.

They're long, narrow lakes that form in the base of a glacier trough, and they're found in areas where the glacier carved more deeply due to softer bedrock or increased ice pressure in a particular location.

And you can see there it's identified.

It's got long, it's deep, and they're often in the lowest parts of a U-shaped valley.

Check for you again.

Can you label the glacial land forms that we would find in this glacial trough pointed out by these arrows? Pause the video, and come back when you've had a go.

You should have added a hanging valley, a truncated spur, and a ribbon lake.

Well done.

Let's move now to our first task.

For the first question, can you label the glacial features on this glacial trough? And for the second question, can you explain the formation of a corrie? And for this, you may wish to use annotated diagrams in your answer.

Jacob says, "This question asks about the formation of a landform, so I must make sure I sequence my answer correctly and be sure to explain each stage carefully.

I must not forget the processes involved." Have a go, and come back when you're ready.

Your label should look something like this.

On the top left, we had three aretes, a truncated spur, and the main U-shaped valley.

In the bottom of the U-shaped valley, we've got a ribbon lake, a hanging valley, a corrie at the very top, and a pyramidal peak at the summit.

Well done if you got all these labels correct.

For question number two, "Explain the formation of a corrie," your answer may have included this.

One, "Snow accumulates in a sheltered hollow, and over time it compresses into ice." And remember, it's often on the northern side of a mountain in the northern hemisphere, as the snow doesn't melt over the summer.

Number two, free-thaw weathering will steepen the back wall.

Erosion will deepen the hollow, and rotational slip causes more erosion at the base.

The glacier will move downhill due to gravity, and at the front of the corrie, the ice thins, and erosion decreases, forming a rock lip.

After the glacier melts, a lake, known sometimes as a tarn or a cwn, will remain in the hollow.

Well done.

Hopefully, you can see in this answer we've got a clear sequence, and we've got processes involved, as well as the actual landforms noted.

Check your answer and see how you did.

Well done.

Let's move now to our second learning cycle.

What landforms are created by glacial deposition? So glacial deposition occurs when a glacier melts.

It no longer is able to carry material, and so it deposits its sediment, or glacial till, that it has been carrying.

And we can see here in this image of Glen Lochay in Scotland, there are moraines, glacial moraines, so where sediment has been deposited as the glacier melts.

Check for you now.

Glacial erosion is the only process that creates new glaciated landscapes.

Pause the video, and decide whether this is true or false and why.

Hopefully, you said false, and your reason may have been something similar to this.

Glacial erosion creates new landforms, for example, glacier troughs, corries and aretes, but also glaciers transport and then deposit material, which can create new landforms, for example, the moraines that we just saw on that image.

Well done.

Drumlins are elongated hills made of deposited moraine.

They vary in size, but they're often found in clusters, as we can see in this image, that are known as drumlin swarms. So those long, rounded hills that you can see in this image are what we call drumlins.

The landscape they form is often referred to as a basket of eggs due to the distinct appearance.

As a glacier flows, it transports unsorted sediment at the base.

If the glacier encounters an obstacle, such as rock, ice and sediment start to accumulate around it.

The pressure and movement of the glacier compact and mould this sediment into an elongated hill, which we call a drumlin.

So that movement and pressure of the glacier downhill moulds it into this shape, and we can see there the direction of the ice flow and the formation of the shape of a drumlin.

When the glacier then melts and retreats, the drumlin remains.

So the material that have been deposited remains in place, and we call that a landform of glacial deposition.

It's a characteristic shape.

It's got a steeper stoss end, which faces up the valley, and a gentler lee loop, which faces the direction of the ice flow.

Moraines.

Another example of a landform created by glacial deposition are ridges of material deposited by glaciers.

They're made up of unsorted rocks and clay and debris.

There are several different types of moraine, the first are ground moraine.

So this is formed from material that is deposited underneath the glacier as it melts, so material that was carried by the base of the glacier.

But as the glacier then melts, it's deposited and forms a ground moraine.

A terminal moraine will form at the glacier's snout, and this marks its maximum extent, so the furthest point that the glacier reached.

Lateral moraines form along the sides of a glacier, and they may be formed of material that has been eroded from the valley walls and was carried on the edge of the glacier until it retreated.

And finally, medial moraine.

These moraines are found at the centre of a glacier, and they'll be created when two glaciers join together with their lateral moraines combining to form that medial moraine, and that's pointed out, as you can see, in that image of those two glaciers joining to form a medial moraine in the middle.

Check for you now then.

Can you add labels to this diagram to identify the terminal, lateral and medial moraine? Pause the video, and have a go at your labelling.

Come back when you're ready.

Did you identify them correctly? Terminal moraine at the snout of the glacier, marking its maximum extent, a lateral moraine formed by material eroded from the valley sides, and your medial moraine, running down the middle of the glacier where two smaller glaciers have joined together.

Erratics are large boulders deposited by glaciers, and they're often out of place.

So their geology will not match the geology of the area where we find them.

And this is because the boulder could have been transported hundreds of kilometres by that glacier before it is eventually deposited.

Check for you then.

How would you identify this boulder as an erratic? Pause the video, and come back when you've decided.

It's made up of different rock to the surrounding rock.

So its geology is different to where it's located.

First task for you now.

Can you match the glacial landforms created by deposition with their correct definitions? Read them carefully.

And for question number two, can you name and describe the glacial landform shown in this diagram? And finally, for question number three, can you explain its formation? Pause the video, and come back when you've had a go at all three questions.

Let's see if you've matched yours up correctly.

So a ground moraine is formed from material deposited underneath the glacier as it melts.

A lateral moraine forms along the sides of a glacier.

A medial moraine is found in the centre of a glacier when two glaciers merge.

A terminal moraine forms at the glacier's snout.

A drumlin is an elongated hill made of deposited moraine, and an erratic is a large boulder deposited by glaciers.

Well done if you got all of those definitions matched correctly.

Question number two, you should have identified this as a drumlin that is made from unsorted glacial sediment.

It's shaped like an elongated hill.

It's got a steeper stoss end, which faces up the valley, and a gentler lee slope facing down the valley.

So you can see there that we've named it and then described it.

And then for question number three, we had to explain its formation.

You may have written something like this.

"As the glacier moves downhill, it carries unsorted sediment beneath it.

An obstacle, such as a rock or a mound of debris, in the glacier's path causes ice and sediment to build up around it.

Ice flow and pressure from the glacier compacts and shapes the sediment into an elongated hill.

It creates a steeper slope, stoss end, which faces up the valley, and a gentler slope, lee slope, facing the direction of ice flow.

When the glacier melts and retreats, the drumlin is left behind as a landform of glacial deposition.

Well done if your answer was similar to this.

Let's have a look now at the summary.

Glacial erosion processes, such as plucking and abrasion, form landforms, such as corries, aretes, and pyramidal peaks.

Glaciers erode existing valleys to create glacial troughs, U-shaped valleys that have steep sides and flat bases.

Glacial troughs often contain truncated spurs, hanging valleys, and ribbon lakes that are created by glacial erosion.

Glaciers transport material, and when they melt, this material is deposited, forming moraines, drumlins and erratics.

Well done.

Hopefully, you now feel more confident on the characteristics and the formation of glacial landforms that are created by erosion and deposition.

I look forward to seeing you again soon.

Well done.