Formation of corries by glaciers

Glaciers play a crucial role in shaping mountain landscapes, particularly through the creation of distinctive hollows known as corries. These features, also referred to as cirques or cwms, are formed through specific glacial processes in areas of high altitude.

Conditions and processes leading to corrie formation

• Location of glacier formation - Glaciers typically develop on mountain sides that receive the least sunlight and are exposed to the coldest winds, maximising snow accumulation and minimising melting (ablation).
• Snow to ice transformation - Snow gathers in natural hollows on these slopes, compacting over time into ice as more layers accumulate.
• Ice movement - As the ice thickens, it begins to move through a process called basal sliding, where the ice flows rotationally, deepening the hollow.
• Erosive actions - Two key processes, abrasion (grinding of rock by debris in the ice) and plucking (pulling away of rock fragments), carve out the hollow into a characteristic bowl shape.
• Structural features of corries - The resulting corrie is a steep-sided, bowl-shaped depression with a sharp back wall, often steepened further by frost shattering (freeze-thaw weathering) and plucking.
• Glacier overflow - When the ice mass becomes substantial, it spills over the corrie lip, flowing downwards as a glacier.

Erosional landforms created by glacial activity

Glaciers are powerful agents of erosion, sculpting the landscape as they move through valleys and over bedrock. This results in a variety of distinctive landforms that define glaciated regions.

Key erosional landforms shaped by glaciers

• Arête - A narrow, steep-sided ridge formed between two parallel glacial valleys. As glaciers erode the valleys on either side, the ridge between them becomes sharper and more pronounced.
• Pyramidal peak - A pointed mountain summit with at least three steep sides, created when three or more corries erode back-to-back around a central peak.
• Glacial troughs - Also known as U-shaped valleys, these have steep, near-vertical sides and wide, flat bottoms. They form when glaciers erode pre-existing V-shaped river valleys, deepening and widening them through abrasion and plucking.
• Hanging valleys - These are smaller valleys left at a higher elevation than the main glacial trough. Formed by tributary glaciers that erode less deeply than the main glacier, they often result in waterfalls where they join the main valley after the ice melts.
• Truncated spurs - These occur when protruding ridges of land in a valley are cut off by the advancing glacier, leaving blunt, steep edges.
• Tarns - Small lakes that form in the base of corries after the glacier retreats, filling the depression with meltwater.
• Roche moutonnée - A resistant rock outcrop on the valley floor, shaped by glacial erosion. The upstream (stoss) side is smoothed by abrasion, while the downstream (lee) side is rough and steep due to plucking.

Depositional landforms resulting from glacial retreat

As glaciers melt and retreat, they leave behind vast amounts of debris carried within or on the ice. This material forms various depositional landforms that provide insight into the glacier's past extent and movement.

Types of depositional landforms left by glaciers

• Till - An unsorted mix of boulders, stones, sand, and clay deposited directly by melting glaciers, varying widely in size and composition.
• Moraine - Specific formations of till shaped by glacial movement and deposition, categorised by location:
  • Lateral moraine - Material deposited along the sides of a glacier, forming ridges parallel to the valley walls.
  • Medial moraine - A central ridge of debris in the valley, formed when two glaciers merge, combining their lateral moraines.
  • Terminal moraine - A semicircular mound or hillock of till deposited at the furthest point of a glacier's advance, marking its end.
• Till plain - A broad, gently rolling landscape created when large ice sheets melt in place, leaving behind an extensive layer of till.

Characteristics and formation of drumlins

Drumlins are distinctive depositional landforms associated with glacial activity, often found in areas once covered by ice sheets. Their unique shape and distribution provide clues about past ice movement.

Features and theories of drumlin formation

• Shape and size - Drumlins are half-egg-shaped hills made of till, typically reaching lengths of up to 1500 metres and heights of around 100 metres.
• Orientation - The upstream (stoss) end is broader and taller, while the downstream (lee) end tapers to a narrower, lower point, reflecting the direction of ice flow.
• Group occurrence - They usually appear in clusters, forming what is often described as a 'swarm' across a landscape.
• Formation hypotheses - Two main theories explain their origin:
  • Till may accumulate around natural obstacles under the ice, shaping into streamlined hills.
  • Existing mounds of till are reshaped and streamlined by the overriding ice, creating the characteristic form.

Characteristics and formation of erratics

Erratics are large boulders picked up by glaciers, carried over long distances, and deposited far from their original location.

They often stand out in their new surroundings due to having a completely different rock type or geological composition compared to the local bedrock.