Glacial Landforms of Deposition (Erratics)

Definition of Erratics

Erratics are large rocks or boulders that have been transported and deposited by glaciers. They are distinctive because they differ in type and composition from the local bedrock where they are found. This means they have been carried over long distances by ice and left behind when the glacier melts during retreat.

Unlike moraines or drumlins, erratics are individual rocks, not accumulations of smaller debris or streamlined hills. They provide clear evidence of glacial movement because their origin can often be traced back to a different geological area.

Formation Process

Erratics form through a process involving glacial plucking and transport:

• Plucking: As a glacier moves, it freezes onto rocks and pulls them away from the ground.
• Transport: These rocks become embedded in the ice and are carried along as the glacier flows, sometimes for many kilometres.
• Deposition: When the glacier melts or retreats, the ice can no longer carry the rocks, so they are dropped onto the landscape.

Because glaciers can transport rocks far from their source, erratics often stand out clearly against the local geology.

For instance, a granite boulder found in an area of limestone bedrock is likely an erratic, carried there by glacial ice from a granite-rich region.

Examples in the UK

Erratics are scattered across many parts of the UK, especially in Scotland and northern England where ice sheets covered the land during the last Ice Age.

• Scotland: Large granite erratics from the Cairngorm Mountains have been found far south of their origin.
• England: Erratics of volcanic rock from the Lake District are found in areas of sedimentary rock further east.

Some famous erratics include the 2 Bowder Stone2 in the Lake District, a massive boulder weighing around 2,000 tonnes, which was transported by glacial ice.

The presence and location of erratics help geographers and geologists map the direction of ice flow during glaciation periods, showing how ice sheets moved across the UK landscape.

Importance of Erratics

Erratics are important for several reasons:

• Indicators of Ice Flow Direction: By identifying the source of an erratic, scientists can determine the path the glacier took.
• Evidence of Past Glaciation: Erratics provide clear proof that glaciers once covered areas where they are found today.
• Impact on Soil and Landscape: Erratics can affect soil composition and local ecosystems by introducing different rock types.

They also serve as natural landmarks and are sometimes used in education and tourism to explain glacial history.

Learning Example

If a granite erratic is found 50 BCkm away from the nearest granite outcrop, this shows the glacier transported the rock at least 50 BCkm before depositing it. This helps reconstruct the glaciers movement and extent.