Erosional Landforms

Definition of erosional landforms

An erosional landform is a landform that is created by erosional processes, such as plucking and abrasion, during periods of glacial advance. They are left behind after periods of glaciation and can be found in relict landscapes.

What types of erosional landforms are there?

There are many different types of erosional landforms found in relict landscapes. The ones we focus on in this explanation are roches moutonnées, glacial troughs, and corries because these are the most used landforms for reconstructing past ice mass extent and movement.

Roches moutonées

Roches moutonnées (also known as sheepbacks) are asymmetric hills or composites of bedrock found in post-glacial landscapes. They range in size from less than a meter to several hundreds of meters in length and are often found in clusters.

Glacial troughs

Glacial troughs, also known as glaciated valleys or U-shaped valleys, are long valleys shaped like a ‘u’. They are created by glaciers that have either receded or disappeared. These troughs typically have flat floors and sides that are straight and steep. Fjords, like those found in Norway, are a good example of coastal troughs created by moving glaciers.

Corries

Corries are armchair-shaped hollows found on the sides of mountains, typically on slopes facing north in the Northern Hemisphere. Cwm Clyd and Cwm Idwal in Snowdonia, Wales, are good examples of corries.

How are erosional landforms created?

Roches moutonnées, glacial troughs, and corries are formed in different ways. As mentioned at the beginning, erosional landforms are created by erosional processes that occur in periods of glacial advance. The two main types of erosion are abrasion and plucking.

How are roches moutonnées created?

Roches moutonnées are formed by erosional glacial processes, namely abrasion and plucking.

Abrasion

When a glacier moves over a protruding lump of bedrock on flatter terrain, it will exert a lot of pressure on the bedrock. This leads to an increase in the pressure melting point (PMP) of the ice. As a result of this increased pressure, the ice of the glacier melts at lower temperatures, which causes meltwater to form at the boundary of the glacier and the slope. This, in turn, decreases the friction between the slope and the glacier, which allows the glacier to move smoothly uphill and over the slope.

Due to this high pressure and the glacier’s movement, the rubbing effect of the glacier on the slope will cause this part of the slope (known as the shock end, which always points upslope) to erode. This process is known as abrasion.

Fig. 1 - The process of abrasion creates roches mountonees

Plucking

Towards the end of the glacier’s movement over the slope, there will generally be a sharp increase in the gradient of the slope going downwards - this is known as the lee end of the slope. As the glacier moves downhill, the amount of pressure exerted onto the slope decreases, which leads to a reduction in the PMP. This causes the meltwater to freeze and become ice once again.

This former meltwater is now attached to both the slope and the glacier, but the glacier keeps moving, which results in parts of the slope being plucked out by the glacier, leading to cracks in the rock. This process is known as plucking.

Together, abrasion and plucking lead to the formation of a roche moutonnée.

Fig. 2 - Roche mountoneé in Scotland, Chris Upson CC BY-SA 2.0, Wikimedia Commons

How are glacial troughs created?

As glaciers flow through valleys (which are carved by rivers), the force of the glacier moving downslope erodes the entire valley floor and valley sides. This leads to a widening of the valley floor and a steepening of the valley walls. Once the glacier retreats, only the widened and flattened valley is left behind, sometimes with a small river, known as a misfit stream, flowing down it.

How are corries created?

Corries form when there is an extensive collection of snow in a hollow (something like a pit) on the side of a mountain. These hollows are usually north-facing in the Northern Hemisphere. In the summer, the snow does not melt because at high altitudes, temperatures are lower, and the hollow shelters the snow from direct sunlight.

As more snow collects, the hollow becomes compacted and the air is squeezed out, leaving only ice. Freeze-thaw weathering and plucking cause the back wall of the hollow to steepen, and the forces of abrasion also deepen the base. As the glacier gets heavier, it moves downhill and exits the hollow via a process called rotational slip.

Eventually, increased temperatures will cause the ice to melt and form a lake in the hollow, which is known as a corrie lake (sometimes called a tarn).

Fig. 3 - Erosional landforms are created by the glacier

How to reconstruct past glacial landscapes with erosional landforms

The purpose of reconstructing glacial landscapes is to determine the past extent of ice mass and/or movement at any given point in time. This is important because these reconstructions can give key information about former glacier changes and glacial behaviour over much longer periods.

Below we look at how helpful each erosional landform is in reconstructing past glacial landscapes. Note that each landform has its own unique indicators for reconstructing former ice masses.

Are roches moutonnées useful in reconstructing past glacial landscapes?

These erosional landforms are very useful for reconstructing past ice mass movement. There are a few ways that we can use a roche moutonnée to reconstruct past glacial landscapes:

1. identifying shock slope and lee slope

We can use a roche moutonnée to reconstruct ice mass movement by identifying its shock slope and lee slope. The shock end always points upslope (against the direction of glacial movement) and can be identified as the smooth and long end of the slope. In contrast, the lee end always points downslope (in the direction of glacial movement) and can be identified by rough and jagged edges.

2. Identifying striations

Another way to reconstruct past ice mass movement is through identifying striations on the roche moutonnée. Striations resemble scratch marks and are usually parallel to the direction of glacial movement.

But the problem with this strategy is that striations can sometimes be hard to identify because similar scratch-like marks can form through other processes. Striations also only show local movement, and this may not always represent the overall movement of the former ice mass.

Fig. 4 - Glacial striations on granite, Public Domain

Are glacial troughs useful in reconstructing past glacial landscapes?

It is possible to identify a trimline on the valley sides on some glacial troughs. The trimline indicates the greatest extent to which the glacial trough was filled with ice. Below this line, we can see evidence of glacial abrasions, such as striations and polished rock surfaces.

One of the limitations of this method, however, is due to compressional and extensional flow.

• Compressional flow is when the glacier moves at a slower rate and is more compact. This causes the glacier to be taller, making the trimline higher than usual.
• Extensional flow is when the glacier moves at a faster rate. This causes it to be longer, making the trimline appear lower than usual.

If the glacier was going through one of these two phases as it was flowing down the slope, the height of the trimline could be misleading.

When it comes to ice mass movement, the glacier would have travelled down the slope of the valley due to gravitational forces. Still, we could also observe the directions of striations for extra certainty. It’s important to note that we can only reconstruct ice mass movement for the time when the glacier was in the valley using glacial troughs. This is because the troughs don’t have any indicators that could inform the movement of the glacier if or when it left the valley.

Are corries useful in reconstructing past glacial landscapes?

In most places in the Northern Hemisphere, corries are nearly always oriented between the north-west through to the north-east to the south-east. Corrie orientation shows the direction of ice movement (ice usually flows to the opposite direction of the corrie wall). As the glacier moves and leaves the valley, it turns larger till rock fragments (clasts). We can use till fabric analysis to measure the orientation of these clasts and identify the direction of glacial movement.

Fig. 5 - Example of a tarn (or corrie loch), pixabay