Glacial Processes and Landforms of Erosion

Glacial Weathering and Erosion

• Every time you put a full glass bottle in the freezer, you risk it shattering. This exact same principle breaks down solid mountain rock.
• Freeze-thaw weathering is a mechanical process where water enters cracks or joints in the rock. When the temperature drops below 0°C, the water freezes and expands by 9%, widening the crack.
• Repeated cycles of freezing and thawing eventually cause the rock to shatter, creating loose, angular rock fragments known as scree.
• As a glacier moves, it erodes the landscape through two main mechanisms:
  1. Plucking: Meltwater at the base of the glacier freezes onto the underlying bedrock. As the glacier moves forward, it "tears" away angular chunks of rock (most effective on well-jointed rocks).
  2. Abrasion: This is the "sandpapering effect". Rocks embedded in the base of the ice grind against the bedrock, leaving deep scratches called striations and producing fine, pulverized rock flour.

Glacial Movement and Deposition

• How does a massive, solid block of ice actually move? Gravity is the primary force, but temperature dictates the exact mechanism.
• Temperate (warm-based) glaciers move faster via basal slip. High pressure at the base lowers the Pressure Melting Point, allowing meltwater to exist below 0°C and lubricate the glacier's slide over the bedrock.
• Polar (cold-based) glaciers move very slowly (1–2 cm per day) via internal deformation. The sheer weight and gravity cause individual ice crystals to slide past each other like plastic.
• When a glacier loses energy or melts (usually at the snout or when overloaded), it drops its sediment. This process of deposition creates till (also known as boulder clay).
• Glacial till is unsorted (mixed sizes), unstratified (no distinct layers), and contains angular rocks. In contrast, material deposited by glacial meltwater, known as outwash, is sorted and rounded.
• The glacier can also push loose debris forward at its snout, a process known as bulldozing.

Erosional Landforms: Corries, Arêtes, and Pyramidal Peaks

• You can easily snap a piece of chalk, but try snapping a diamond. Glaciers act like diamond-tipped tools, carving incredibly jagged peaks out of solid mountain rock.
• A corrie is an armchair-shaped hollow with a steep back wall, a deep basin, and a rock lip. In the UK, they typically form on North or North-East facing slopes where less sunlight allows snow to accumulate.
• Formation of a corrie:
  1. Snow collects in a hollow and compresses into glacial ice.
  2. Rotational slip (circular movement of the ice) deepens the basin through abrasion.
  3. Freeze-thaw weathering and plucking steepen the back wall.
  4. After the ice melts, a circular lake called a tarn often remains (e.g., Red Tarn in the Lake District).
• When two corries erode back-to-back, they leave a knife-edged ridge between them called an arête (e.g., Striding Edge).
• When three or more corries erode back-to-back, they carve a sharply pointed summit known as a pyramidal peak (e.g., Snowdon / Yr Wyddfa).

Erosional Landforms: Glacial Troughs and Ribbon Lakes

• Look at a typical river valley, and you will see a 'V' shape. Look at a glacially carved valley, and you will see a massive 'U'.
• Formation of a U-shaped valley (glacial trough):
  1. A pre-existing V-shaped river valley contains interlocking spurs.
  2. A large valley glacier moves downslope, filling the valley.
  3. Through powerful plucking and abrasion, the glacier vertically and laterally erodes the valley.
  4. It bulldozes straight through the interlocking spurs, leaving blunt-ended ridges called truncated spurs.
  5. The result is a wide, deep U-shaped trough with a flat floor and steep sides (e.g., Nant Ffrancon in Snowdonia).
• Smaller tributary glaciers do not have the mass to erode as deeply as the main glacier. They are left high above the main trough as hanging valleys, often featuring dramatic waterfalls.
• Ribbon lakes form on the floor of the glacial trough where softer rock has been over-deepened by abrasion, or where a moraine has dammed the valley (e.g., Lake Windermere).
• The small rivers that flow through these massive troughs today are called misfit streams, because they are far too small to have carved the U-shaped valley themselves.

Depositional Landforms

• Imagine dropping a mixed handful of sand, pebbles, and boulders all at once. That chaotic mix is exactly how glaciers dump their load.
• A terminal moraine is a prominent ridge of till marking the glacier's furthest extent.
• A lateral moraine is a ridge of till deposited along the sides of the glacier, while a medial moraine forms down the centre of a valley when two glaciers meet and their lateral moraines merge.
• A drumlin is an elongated, egg-shaped hill made of till. Its steep, blunt side (the stoss) faces the direction the ice came from, while its gently tapered side (the lee) points in the direction of ice flow.
• An erratic is a massive boulder deposited by the ice in an area of completely different geology.