Glacial Landforms of Erosion (U-Shaped Valleys & Hanging Valleys)

U-Shaped Valleys

U-shaped valleys are distinctive landforms created by the powerful erosive action of glaciers. Unlike river valleys, which are typically V-shaped due to river erosion cutting downwards, glaciers carve out broad, deep valleys with steep sides and a flat floor.

Formation: When a glacier moves through a pre-existing V-shaped river valley, it erodes the sides and floor by two main processes: abrasion and plucking. The glaciers immense weight and movement widen and deepen the valley, smoothing the rough edges left by the river.

Characteristics:

• Steep, straight sides 6 formed as the glacier erodes the valley walls.
• Flat, wide valley floor 6 created by the glacier scraping and smoothing the base.
• Often have a U shape in cross-section, which is much wider and deeper than the original river valley.

This shape contrasts with the narrow, V-shaped valleys formed by rivers, which have a pointed bottom and sloping sides. U-shaped valleys are a key feature of glaciated upland areas in the UK, such as the Lake District and Snowdonia.

For instance, the Great Langdale Valley in the Lake District is a classic example of a U-shaped valley. It was originally a river valley but was transformed by glacial erosion during the last Ice Age.

The glaciers movement widened the valley from a narrow V-shape to a broad U-shape with steep sides and a flat floor. This is because glaciers erode both the valley floor and sides, unlike rivers which mainly erode downwards.

Hanging Valleys

Hanging valleys are smaller valleys that join a main glacial valley but are left hanging above the main valley floor. They are formed when tributary glaciers flow into a larger glacier, but because the main glacier is thicker and erodes more deeply, the tributary valley is left at a higher level.

Formation: Tributary glaciers have less ice and less erosive power than the main glacier. As the main glacier cuts deeply into its valley, the tributary glaciers erode their valleys less deeply. When the ice melts, the tributary valleys remain perched above the main valley floor.

Characteristics:

• Located above the main valley floor.
• Often marked by waterfalls where the tributary valley meets the main valley.
• Steep valley sides and a smaller floor compared to the main valley.

An example of a hanging valley can be seen at Scalebarrow Knott in the Lake District, where smaller valleys join the main U-shaped valley but are left hanging above it, often creating spectacular waterfalls.

Processes of Glacial Erosion

Glacial erosion shapes the landscape primarily through two processes: abrasion and plucking. These processes work together to carve out features like U-shaped valleys and hanging valleys.

Abrasion

Abrasion happens when rocks and debris embedded in the base and sides of a moving glacier scrape against the valley walls and floor. This acts like sandpaper, smoothing and deepening the valley.

The glacier grinds down the rock beneath it, creating striations (scratches) and polishing the surface. Abrasion is responsible for the flat valley floors and steep sides of U-shaped valleys.

Plucking

Plucking occurs when meltwater from the glacier freezes onto cracks in the bedrock. As the glacier moves, it pulls chunks of rock away from the valley sides and floor. This breaks up the rock and helps deepen and widen the valley.

Plucking is especially effective where the glacier encounters fractured or jointed rock, helping to steepen valley sides and create jagged features.

Together, abrasion and plucking allow glaciers to reshape river valleys into the distinctive U-shaped valleys seen in glaciated landscapes.

For example, in the Scottish Highlands, glaciers eroded deep U-shaped valleys such as Glen Coe. The steep sides and flat floors show clear evidence of abrasion and plucking during the last Ice Age.

The combination of these processes also explains why hanging valleys form: tributary glaciers have less erosive power, so their valleys are not eroded as deeply as the main glaciers valley.

Example: If a glacier moves through a valley and erodes the floor by abrasion at a rate of $2\,\mathrm{m}$ per 1000 years, and plucking removes rock from the sides at $1\,\mathrm{m}$ per 1000 years, after 10,000 years the valley floor will have deepened by:

$$10,000 \times \frac{2}{1000} = 20\,\mathrm{m}$$

and the sides will have been steepened by:

$$10,000 \times \frac{1}{1000} = 10\,\mathrm{m}$$

This shows how glaciers can significantly reshape valleys over thousands of years.

For instance, if a glacier erodes the valley floor by $2\,\mathrm{m}$ per 1000 years, then over 5000 years it will deepen the floor by $10\,\mathrm{m}$, demonstrating the gradual but powerful effect of glacial erosion.