Glacial Erosion

If you hold some ice, it's pretty smooth, right? Well, glacial ice isn't. It's often full of rocks and other materials that rub against the mountainside, grinding away, making some interesting marks on the landscape. Let's learn about how ice can shape landscapes and what kind of landforms can still be seen today as a result of glacial erosion. This explanation goes hand in hand with the Glacial Depositional Landforms explanation, as well as our explanations on Glacial Landscapes, Glacial Landforms and Glacial Processes, so don't forget to read all of those too! They will give you a firm understanding of the topic we are learning about today! If you haven't read these, you might come across some words you don't know in this explanation! So, what are the types of glacial erosion? What landforms are created by these processes? Let's dive in!

Types of glacial erosion

Ice is a pretty powerful thing, and through the physical process of erosion, particular landforms can be created. We'll talk more about these landforms later on, but first, let's understand what the process of erosion is, and the different types of glacial erosion that take place in glacial environments.

During a glacial period, the power of gravity can pull glaciers down mountainsides, and as they move, the process of erosion takes place, i.e., as the ice moves, it scrapes against the mountainside. There are actually a couple of different erosion processes, so let's take a look at them.

Abrasion

This type of erosion occurs when there are rocks of all shapes and sizes inside a glacier. As a glacier moves downwards, these rocks scrape on the rocks underneath, eroding them away. Think of when you use sandpaper on some wood; it's a similar kind of action.

Plucking

As the ice is moving over a mountainside, some of the rocks can be looser than others; this allows meltwater to make its way underneath those loose rocks.

As this meltwater freezes, it allows the loose rock to break away, or, it's plucked off the mountainside, as the ice continues its movement downwards.

Glacial landforms caused by erosion

Landforms of glacial erosion definition

What exactly is the definition of landforms of glacial erosion? Well, as the processes of abrasion, plucking, and freeze-thaw weathering occur, they can create certain things or distinct marks that stand out on a landscape. These landforms can include things like lakes, peaks, or landforms with interesting names like arêtes. Let's explore some of these landforms, their characteristics, and their formation, and offer some examples along the way!

Corries

Arêtes are when two corries are formed on either side of each other, creating a kind of spiky ridge between the two. If you have two eroding glaciers on either side, that ridge is going to become sharper and steeper! Add another corrie into the mix on another side of the mountain, alongside the arête, and a pyramidal peak will form. With increasing erosion, the peak will become more and more pointy.

Fig. 1 - the Carn Mòr Dearg arête, Scotland

U-shaped valleys/glacial troughs

On a non-glaciated mountain landscape, rivers flow down mountainsides, eroding as they make their way down, typically creating a 'V'-shaped valley. (You'll learn about this in the explanations on rivers!) As ice became dominant, it found its way into these valleys from the expansion of small corries and was able to erode these valleys further. Through plucking and abrasion, as well as the sheer power of the ice, a U-shaped valley, or glacial trough, is formed. These valleys are typically incredibly deep, with steep valley sides.

Fig. 2 - U-shaped valley in Grey Mare's Tail, Scotland

Hanging valleys

Tributaries are smaller streams that lead to or from rivers. In non-glaciated times, these tributaries would flow down the valley sides to meet the river at the bottom of the valley. With ice presence, as the ice erodes down the mountain, it leaves these tributaries behind on the steep valley sides, ending pretty sharply. They are essentially left hanging. Once the ice melts, waterfalls from those tributaries fall from sky-high into the U-shaped valley, forming a hanging valley.

Truncated spurs

On a river landscape, interlocking spurs are a major characteristic. As a river flows, it erodes different types of rocks differently, depending on whether they are easy or hard to erode. This creates a kind of zigzag in the landscape, known as interlocking spurs. During a glaciated period, the sheer power of the ice, cuts right through these spurs, creating truncated spurs.

Ribbon lakes

Glaciers erode the landscape as they move. However, some of the rock may be softer than others, which means glaciers can erode these more intensely. This can mean that small troughs (like a ditch) are created, and when the ice finally melts, these troughs are filled with water, creating a ribbon lake.

Fig. 4 - Lake Windermere ribbon lake, the Lake District

When the glacier meets rock that is harder to erode, it can actually move over it and abrade the rock below. This can create other kinds of landforms; Roches moutonnées and Crag and Tails.

Roches moutonnée

Cool name for a landform, right? Roches moutonnées are big chunks of rock on the landscape, with one side that is quite smooth, eroded by abrasion. The other side is much more rough, eroded by plucking. It is formed when ice moves over the rock that is more difficult to erode; with the direction of ice flow, the side of the rock hit first by the glacier is abraded (the stoss side). As it moves over the rock onto the other side, it plucks the rock away (on the lee side) as it moves further through the landscape, creating some pretty cool rock outcrops. They may also be covered in striations. Examples can be seen at Dulnain Bridge, Cairngorms National Park.

Crag and Tail

Although similar to a roche moutonnée, a crag and tail is a little different. This is where a glacier travels over a piece of resistant rock, with less resistant rock on either side. Erosional processes erode the softer rocks on the stoss side, leaving the chunk of resistant rock behind. This chunk of rock is the crag. On the lee side, the resistant rock (crag) actually protects the less resistant rocks here from erosion, leaving behind softer rocks, or the tail. Edinburgh is a great example of a Crag and Tail; the castle sits directly on top of the Crag, and The Royal Mile is the Tail!