Glacial Erosion, Transport and Deposition (AQA A-Level Geography): Revision Notes

Glacial Erosion, Transport and Deposition

How glaciers erode the landscape

Glaciers are powerful agents of erosion. As warm-based glaciers move downhill, they reshape the landscape through their immense weight and movement. This erosive power is particularly strong in upland regions where glaciers flow. Two main processes work together to erode the bedrock beneath and beside glaciers.

Abrasion

Abrasion occurs when debris embedded in the glacier grinds against the bedrock below and at the valley sides. Think of this process like sandpaper - the ice itself doesn't erode the rock, but rather acts as a backing that holds the abrasive material in place. The rocks, stones and sediment frozen into the base of the glacier do the actual erosive work as the ice moves forward.

This grinding action produces several distinctive features:

• Striations - scratches and grooves carved into bedrock by harder, coarser rocks dragged across the surface
• Smoothed and polished surfaces - created by finer particles that buff the rock
• Rock flour - extremely fine sediment produced when debris is ground down to a powder-like consistency

Rock flour is often carried away from the glacier by meltwater streams, giving these waters a distinctive milky appearance.

Plucking

Plucking is a freeze-thaw process that removes chunks of bedrock from beneath the glacier. This occurs when:

This process is most effective at the glacier base where:

• Pressure and friction generate heat, creating meltwater
• The melting point of ice is reached or exceeded
• Rock surfaces have been weakened by freeze-thaw weathering (frost shattering)

The weight and movement of the glacier can also simply break away rock fragments, incorporating them into the ice. Plucking typically occurs on well-jointed rocks and surfaces already weakened by weathering, resulting in a jagged, rough landscape.

How glaciers transport material

Valley glaciers are highly effective transport systems, capable of moving enormous quantities of debris. This material comes from two main sources: rock eroded from beneath and beside the glacier, and rockfall debris from valley sides. The position where debris is transported determines what it is called.

The glacier carries material in three distinct zones:

• Supra-glacial debris - material transported on the glacier's surface
• En-glacial debris - material buried within the ice itself
• Sub-glacial debris - material at the glacier's base, which may include rock fragments from plucking, material washed down crevasses (called moulins), and debris eroded from the bedrock

These different transport zones create distinctive ridges of debris called moraine. Three main types form during transport:

• Lateral moraine - debris carried along the glacier's edges
• Medial moraine - forms where two glaciers merge, combining their lateral moraines
• Ground moraine - material beneath the glacier

Glacial deposition

The vast quantities of material transported by a glacier must eventually be deposited. This happens in two main situations:

1. At the snout - when the glacier melts and releases its debris load
2. During flow changes - when the glacier becomes overloaded with material and can no longer transport it all, or when the glacier alternates between compressing and extending flow, reducing the ice mass at specific locations

Till deposits

The material deposited directly by ice is known as till. This term has largely replaced the older term "boulder clay," though boulder clay remains useful for describing certain specific till types.

Till has distinctive characteristics that reveal information about its source:

• Shape - stones are angular to sub-angular because they haven't been rounded by water transport
• Composition - reflects the geology over which the ice passed
• Unsorted nature - all sizes mixed together, from clay particles to large boulders

Case study evidence

Till deposits provide evidence of past ice movement and source areas. For example: