Glacial Transportation (Bulldozing & Freeze-In)

Glacial Transportation Overview

Glaciers are powerful agents of landscape change, moving vast amounts of rock and sediment as they slowly flow downhill. This movement of debris by glaciers is called glacial transportation. It plays a vital role in shaping the physical landscapes of the UK, especially in upland areas like the Lake District and the Scottish Highlands.

Glacial transportation occurs as glaciers pick up, carry, and eventually deposit material ranging from fine silt to large boulders. The debris transported is called glacial till. There are several processes by which glaciers transport this material, including:

• Bulldozing 6 pushing debris at the glacier front
• Freeze-in 6 sediment freezing onto the base of the glacier and being carried within the ice
• Other processes like traction and saltation (covered in other topics)

Understanding bulldozing and freeze-in helps explain how glaciers move material and contribute to landscape features.

Bulldozing Process

Bulldozing happens at the front, or snout, of a glacier. As the glacier advances, it pushes loose rock and sediment in front of it, much like a bulldozer pushing earth. This material is moved forward and accumulates in ridges called push moraines (see Glacial Landforms of Deposition for more on moraines).

Key points about bulldozing:

• Occurs at the glacier snout where the ice meets the ground
• Debris is pushed forward rather than carried within the ice
• Builds up ridges of material at the glaciers edge
• Important in shaping the front edge of glaciers and influencing landscape features

For example, during the last Ice Age, glaciers in the UK pushed large amounts of sediment ahead of them, forming ridges that are still visible in places like the Vale of Eden in Cumbria.

Bulldozing is a mechanical process, and the amount of material pushed depends on the glaciers size, speed, and the amount of loose sediment available.

For instance, if a glacier pushes a ridge of sediment 10 metres wide, 2 metres high, and 50 metres long, the volume of sediment bulldozed can be calculated as:

$$\text{Volume} = \text{width} \times \text{height} \times \text{length} = 10\,\mathrm{m} \times 2\,\mathrm{m} \times 50\,\mathrm{m} = 1000\,\mathrm{m^{3}}$$

Freeze-In Process

Freeze-in is a process where sediment and rock fragments become frozen onto the base of a glacier. This happens when meltwater at the glacier bed refreezes, trapping debris in the ice. The glacier then transports this material within its body as it moves downhill.

Key features of freeze-in include:

• Occurs at the glacier base where meltwater refreezes
• Sediment is carried within the glacier ice, not just pushed in front
• Material is released when the glacier melts, often far from its original source

This process explains how glaciers can transport large boulders and sediment over long distances. When the glacier melts, the debris is deposited, sometimes creating isolated boulders known as erratics (covered in another topic). Erratics are large rocks that have been transported and deposited by glaciers, often far from their source.

Freeze-in is important because it allows glaciers to carry material beneath the ice, contributing to the erosion and deposition seen in glacial landscapes.

For example, if a glacier moves at a rate of 5 metres per year and carries sediment frozen into its base, over 100 years it can transport debris:

$$\text{Distance} = \text{rate} \times \text{time} = 5\,\mathrm{m\,yr^{-1}} \times 100\,\mathrm{yr} = 500\,\mathrm{m}$$

This shows how sediment can be moved hundreds of metres from its original location by freeze-in.