Glacial Processes and Landforms

UK ice extent during the last ice age: The last glacial maximum in Britain (part of the Devensian glaciation) peaked approximately 18,000–20,000 years ago. At this time:

• A continuous ice sheet covered Scotland, most of Wales, Ireland, and northern England as far south as approximately the Thames–Severn line
• Ice was up to 1,000 m thick over the Scottish Highlands
• South of the ice margin, periglacial conditions (permafrost, freeze-thaw) shaped the landscape
• When temperatures rose from approximately 15,000 years ago, ice retreated northward; the UK has been ice-free (except for small corrie glaciers in Scotland, now melted) for approximately 10,000 years
• The landforms created by this ice — U-shaped valleys, corries, ribbon lakes, moraines — are the dominant physical feature of the UK's upland landscapes today

How glaciers form:

1. Snow accumulates in a hollow on a sheltered slope (north- or northeast-facing in the Northern Hemisphere)
2. Successive layers compress earlier snow; air is expelled; snow becomes granular (névé — extra context — beyond AQA 8035 spec), then dense glacial ice
3. Where snowfall exceeds melting each year, the glacier grows
4. When ice mass becomes sufficient, the glacier flows downhill under gravity

Freeze-thaw weathering (frost shattering): Water enters cracks in exposed bedrock above and around the glacier; repeated freeze-thaw cycles expand ice in cracks (~9%), progressively widening them; angular rock fragments break off and fall onto the glacier surface. Supplies debris that enables abrasion below.

Erosion — abrasion: Angular rock fragments carried in the glacier's base grind against the bedrock, smoothing and deepening the rock floor. Can produce scratches and grooves (striations — extra context — beyond AQA 8035 spec) in the direction of ice flow.

Erosion — plucking: The glacier's base freezes onto jointed rock; as the glacier advances, it rips out blocks of rock, leaving a jagged surface. Most effective where rock is well-jointed (e.g. granite, limestone).

Movement and transportation — rotational slip: In corries and bowl-shaped hollows, the glacier moves in a curved, rotating arc, pivoting around a central point within the hollow. Rotational slip deepens the corrie floor through abrasion while the back wall is steepened by plucking, producing the characteristic armchair shape.

Movement and transportation — bulldozing: As the glacier advances, it pushes loose material (soil, sediment, and previously deposited debris) ahead of it, accumulating a mound at the ice front. Bulldozing produces terminal moraines and ridge-like deposits at the glacier's snout.

Deposition: When a glacier melts, it deposits all carried material. Till (unsorted, mixed debris deposited directly by ice) and sorted outwash (deposited by meltwater streams) are the two main deposit types.

Corrie (cirque, cwm): A corrie is an armchair-shaped hollow in a mountainside, open on the downslope side and with a steep back wall and a deeper basin:

1. Snow and ice accumulate in a pre-existing hollow on a sheltered (typically north or northeast facing) slope
2. Freeze-thaw weathering steepens the back wall; plucking removes rock from the back and sides
3. Abrasion deepens the floor of the hollow; the glacier moves by rotational slip within the hollow, maximising erosion of the floor
4. A rock lip forms at the front (ablation zone, less erosion)
5. When the glacier melts, a corrie lake (tarn) may form behind the rock lip

The cross-section below shows the armchair shape: a steep plucked back wall, the deepened basin worn by rotational slip, the rock lip, and the tarn left behind.

• UK examples: Red Tarn (Lake District, beneath Helvellyn), Glaslyn (beneath Snowdon, Wales)

Arête: A narrow, knife-edged ridge formed when two corries develop back-to-back on opposite sides of a mountain:

• Freeze-thaw weathering and plucking progressively narrow the ridge between the two corries
• UK example: Striding Edge (Lake District, leading to Helvellyn summit; 950 m)

Pyramidal peak (horn): Three or more corries developing around a central peak attack it from multiple sides; the residual peak becomes very sharp and pointed:

• UK example: Snowdon (Yr Wyddfa) (Wales, 1,085 m) — its summit is the remnant of corries eroding from several sides
• Classic Alpine example: the Matterhorn (Switzerland/Italy, 4,478 m)

Glacial trough (U-shaped valley): A pre-glacial V-shaped river valley is transformed by glacial erosion into a wide, flat-floored, steep-sided U-shaped valley:

• The glacier fills and overflows the original valley, eroding downward and outward through abrasion and plucking
• Removes interlocking spurs (turning them into truncated spurs — cliffs cut off abruptly by ice)
• Post-glacial: the flat floor is used for farming, roads, and settlement; the steep sides remain as dramatic rock faces
• UK example: Great Langdale (Lake District); Glencoe (Scottish Highlands)

The cross-valley profile below shows the wide, flat-floored, steep-sided U-shape carved by the glacier. The dashed line marks the original V-shaped river valley before the ice arrived.

Truncated spurs: Interlocking spurs that once wound between the meanders of a pre-glacial river are cut off by the advancing glacier, leaving steep cliff faces on either side of the straightened glacial trough.

Ribbon lake: A long, narrow lake occupying the floor of a glacial trough:

• Forms where glacial erosion was particularly intense (e.g. over a patch of weaker rock, or where two glaciers merged and discharge increased)
• Deepened below sea level in some cases (over-deepening)
• UK examples: Windermere (Lake District, England's largest lake; 16.9 km long), Ullswater (Lake District), Loch Ness (Scottish Highlands; 37 km long, 230 m deep)

Hanging valley: A tributary glacier occupying a smaller side valley cannot erode as deeply as the much larger main valley glacier. When both glaciers melt, the floor of the tributary valley is left high above the main valley floor — a hanging valley. The stream now tumbles down a waterfall into the main valley below.

• UK example: Sourmilk Gill (Lake District) — a waterfall dropping from a hanging valley into the Ennerdale Valley

When glaciers melt, they deposit the material they have been carrying — till (unsorted mixture of all particle sizes from clay to boulders, deposited directly by ice) or outwash (sorted sediment deposited by meltwater streams beyond the ice margin).

Erratics: Boulders or blocks of rock transported by a glacier and deposited far from their original source, often resting on bedrock of a completely different rock type:

• The rock type of an erratic allows geologists to trace its source and the direction the glacier travelled
• UK example: Norber erratics (Yorkshire Dales) — dark Silurian greywacke boulders perched on pedestals of pale limestone; transported approximately 1 km by glacier ice from Crummack Dale and deposited as the ice melted
• The size of erratics can be remarkable — some in Scotland are the size of houses, transported hundreds of kilometres

Moraines:

Type	Description	Location
Lateral moraine	Ridge of debris along the sides of the glacier (derived from freeze-thaw weathering of valley walls above the ice)	Deposited along valley sides; visible as raised ridges on valley slopes after the glacier melts
Medial moraine	Ridge of debris running along the centre of the glacier (formed where two glaciers and their lateral moraines merge)	Forms a dark central stripe on the glacier surface
Terminal moraine	Ridge of till marking the furthest extent (snout) of the glacier — deposited when the glacier stalls at its maximum advance	Forms a curved ridge across the valley mouth; can act as a natural dam impounding a lake
Ground moraine	Till spread across the valley floor as the glacier retreats	Produces an uneven, hummocky landscape of low hills and boggy hollows

Drumlins: Smooth, whale-back shaped hills of till, streamlined in the direction of ice flow:

• Long axis parallel to ice flow; steep end (stoss face — extra context) facing up-glacier; gentle tail (lee face — extra context) downstream
• Form in swarms, creating a "basket of eggs" topography
• Exact formation debated, but associated with fast-moving ice over soft sediment
• UK example: Eden Valley, Cumbria — drumlins formed during deglaciation after the last ice age

1. Describing a U-shaped valley as simply a V-shaped valley made bigger

The correct description is: a pre-glacial V-shaped valley is transformed by glacial erosion (abrasion and plucking) into a wide, flat-floored, steep-sided U-shaped trough. The shape change is fundamental — the floor is flattened and the sides steepened, not simply widened. Include that interlocking spurs become truncated spurs.

2. Confusing the processes of abrasion and plucking

Abrasion smooths and grooves rock — the glacier's debris grinds against the bed. Plucking tears out chunks of rock — the glacier freezes onto jointed rock and rips it away. Both occur simultaneously but produce opposite textures: smooth on the up-glacier side (abrasion), jagged on the down-glacier side (plucking).

3. Not naming a specific UK example for corrie, arête and U-shaped valley

The spec requires located examples. "A valley in the Lake District" is not enough. Use: Red Tarn (corrie lake, Lake District), Striding Edge (arête, Lake District), Great Langdale (glacial trough, Lake District), Windermere (ribbon lake, Lake District). The Lake District as a single named upland area covers most requirements.

4. Describing moraines without specifying which type

There are four distinct moraine types. In a question about glacial deposition, name the type (terminal, lateral, medial, ground) and describe its position relative to the glacier. Answering "a moraine is a ridge of debris deposited by a glacier" identifies only one mark — naming the type and its location earns more.

5. Treating freeze-thaw as a glacial erosion process

Freeze-thaw is subaerial weathering — it breaks up rock above and around the glacier. It is an important preparatory process that supplies the angular debris that then enables abrasion and plucking. But freeze-thaw itself is not glacial erosion. The two glacial erosion processes are abrasion and plucking.