Glacial Processes: Weathering, Erosion & Transportation Revision Notes | GCSE AQA Higher Geography

1. Ice as a Landscape-Shaping Force

Picture the Scottish Highlands, the Lake District, or Snowdonia. Those dramatic valleys, sharp ridges, and deep lake basins were not carved by rivers. They were carved by ice. During the last glacial period, which ended roughly 10,000 years ago, vast glaciers covered much of the UK and reshaped the land in ways that are still visible today.

Glaciers are not passive sheets of frozen water. They move, grind, rip, and carry. To understand the landforms they leave behind, you first need to understand the three key groups of processes: weathering, erosion, and transportation. Each one does a different job, and together they explain almost every glacial feature you will meet in an exam question.

2. Freeze-Thaw Weathering: Breaking Rock Apart

Before a glacier can erode rock, the rock often needs to be weakened first. That is where freeze-thaw weathering comes in. Water is unusual because it expands by roughly 9% when it freezes, and that expansion generates enormous pressure inside any crack it occupies.

The process works in a cycle. During the day, temperatures rise above 0°C and meltwater seeps into cracks in exposed rock. At night, temperatures drop below 0°C, the water freezes, and the expanding ice forces the crack open a little wider. Over many cycles, the crack grows until a fragment of rock — called scree — breaks away entirely. You will find scree slopes at the base of many glacial features, and they are direct evidence of this process at work.

• Water enters a crack in rock when temperatures are above 0°C
• Temperatures drop below 0°C and the water freezes, expanding by approximately 9%
• The expanding ice exerts pressure on the surrounding rock, widening the crack
• Temperatures rise again, the ice melts, and more water enters the now-wider crack
• Repeated cycles cause the crack to grow until a rock fragment breaks off as scree

Freeze-thaw weathering is most effective where temperatures fluctuate repeatedly around 0°C — not in the coldest polar regions where water stays permanently frozen, but in upland areas where daily temperature swings cross the freezing point regularly. This is exactly the kind of climate that existed across upland Britain during glacial periods.

3. Glacial Erosion: Abrasion and Plucking

Once a glacier is moving across the landscape, it erodes the rock beneath and around it through two main processes: abrasion and plucking. These two processes often work together, but they operate in very different ways, and examiners frequently ask you to distinguish between them.

Think of abrasion like rubbing coarse sandpaper across a wooden surface. The glacier itself is the sandpaper holder, and the fragments of rock frozen into its base are the abrasive grains. The bedrock gets worn down, and the fragments doing the scraping also get ground into finer material called rock flour. This rock flour gives glacial meltwater streams their distinctive milky appearance.

Plucking is most effective where the bedrock is well-jointed — meaning it already has natural cracks running through it. The meltwater exploits those joints, freezes around the rock blocks, and the glacier's forward movement does the rest. Plucking tends to produce a rougher, more jagged surface than abrasion, and it removes much larger chunks of material.

4. Glacial Transportation: Rotational Slip and Bulldozing

Erosion produces material, but that material has to go somewhere. Glaciers are remarkably effective at moving large quantities of rock and sediment — sometimes for hundreds of kilometres. Two key processes govern how glaciers move and transport their load: rotational slip and bulldozing.

Rotational slip is the mechanism that creates corries — the armchair-shaped hollows you find high on glaciated mountainsides. Snow accumulates in a sheltered hollow, compacts into ice, and begins to move in a rotating arc, like a slow-motion spoon scooping out the rock beneath. The combination of abrasion at the base and plucking at the back wall deepens and steepens the hollow over thousands of years.

Bulldozing works on a much more straightforward principle. As a glacier advances, its snout — the front edge — pushes loose material ahead of it, much like a bulldozer blade pushing soil. This material accumulates at the glacier's margin and, when the glacier eventually retreats, it is left behind as a ridge of unsorted sediment called a moraine.

Glaciers also carry material in other ways — frozen within the ice itself (englacial transport), on top of the ice surface (supraglacial transport), and along the base (subglacial transport). For AQA GCSE, the key distinction is between rotational slip as the mechanism of movement within a corrie, and bulldozing as the mechanism of pushing material at the glacier's margin.

5. How These Processes Create Distinctive Landforms

Now that you have the three process groups — weathering, erosion, and transportation — you can see how they combine to produce the landforms that make glaciated uplands so distinctive. Each landform is essentially a record of which process dominated in that location.

A corrie, for example, is the product of rotational slip combined with abrasion at the base and plucking at the back wall. The freeze-thaw weathering that shatters rock on the exposed back wall feeds loose material into the glacier, giving it more tools for abrasion. These processes reinforce each other. An arête — the knife-edged ridge between two corries — forms when two corries erode back-to-back on either side of a ridge; glacial erosion (abrasion and plucking) from both corries, combined with freeze-thaw weathering, narrows the ridge into a knife-edged shape. A pyramidal peak, such as the Matterhorn in the Alps or Snowdon in Wales, forms where three or more corries erode back-to-back into the same mountain.

Glacial troughs — the wide, flat-floored, steep-sided U-shaped valleys — are the product of sustained abrasion and plucking as a glacier moves through a pre-existing river valley. The glacier widens and deepens the valley floor while steepening the sides, transforming the original V-shape of a river valley into the characteristic U-shape. In the UK, Borrowdale in the Lake District and Nant Ffrancon in Snowdonia are classic examples.

When a glacier retreats, it deposits the material it has been carrying. Bulldozed material left at the furthest point of advance forms a terminal moraine — a ridge that marks the glacier's maximum extent. Material deposited beneath the glacier as it retreats forms a layer of unsorted sediment called till or boulder clay. These depositional landforms are just as important as erosional ones for understanding the glacial history of the UK.

6. Ice and the UK Landscape: The Bigger Picture

The AQA specification makes a specific point worth locking in: ice was a powerful force in shaping the physical landscape of the UK. This is not background context — it is an examinable statement. The UK's upland areas bear the clearest evidence, but glacial influence extends further than many students realise.

During the Pleistocene epoch, ice sheets and valley glaciers covered Scotland, Wales, northern England, and parts of the Midlands. The southern limit of the ice sheet ran roughly through the Thames Valley. North of that line, the landscape was directly shaped by glacial erosion and deposition. South of it, periglacial conditions — extreme cold without permanent ice cover — still influenced the landscape through processes like freeze-thaw weathering.

The result is a landscape that tells a story. The Lake District's radiating valleys were carved by glaciers moving outward from a central dome of ice. The Scottish Highlands' lochs occupy over-deepened glacial troughs. The drumlin fields of County Down in Northern Ireland record the direction of ice movement. Every one of these features connects back to the processes you have studied in this lesson.

7. Exam Technique: Writing About Glacial Processes

Glacial processes questions reward precision. The difference between a 2-mark answer and a 4-mark answer is usually the difference between naming a process and actually explaining the mechanism. Here is how to approach the most common command words you will encounter on Paper 1.

When a question says describe, give a sequence of what happens — no 'because', no explanation, just the observable steps in order. When it says explain, you must include cause and effect. Use the word 'because' or 'therefore' to link your observation to the underlying process. When it says suggest, apply your knowledge to a new scenario — there may be more than one valid answer, so commit to one and justify it clearly.

Worked example: Answering a 4-Mark 'Explain' Question on Corrie Formation

Explain how a corrie is formed. [4 marks]

1. Snow accumulates in a sheltered hollow on a north- or north-east-facing slope where it is protected from direct sunlight. — Setting the scene earns the first mark — examiners want to know where and why snow persists. [B1]
2. The snow compacts into ice and begins to move in a curved, rotational arc — a process called rotational slip. This scooping movement deepens the hollow. — Naming rotational slip and describing the curved movement earns the second mark. [B1]
3. Abrasion at the base grinds down the floor of the hollow, while plucking at the back wall steepens and extends it as the glacier tears away jointed rock. — Linking both abrasion and plucking to specific parts of the corrie earns the third mark. [B1]
4. Over thousands of years, these processes produce a deep, armchair-shaped hollow with a steep back wall, a rock lip at the front, and often a corrie lake (tarn) on the floor. — Describing the final landform with precise terminology earns the fourth mark. [B1]

Answer: A four-point answer covering: sheltered snow accumulation, rotational slip deepening the hollow, abrasion at the base and plucking at the back wall, and the final armchair shape with a rock lip.

Notice how each step in that model answer connects a process to a specific part of the landform. That process-to-outcome link is what separates a full-mark response from a partial one. Examiners are not rewarding description alone — they are rewarding your ability to explain the mechanism behind the shape.

Across this lesson, every process you have studied — freeze-thaw weathering, abrasion, plucking, rotational slip, and bulldozing — is a tool that ice used to carve and shape the UK's upland landscapes. The landforms those tools produced are still there, waiting to be read like a geological record of the last ice age.