Glacial Deposition and Identification of Landforms

From moving ice to deposits

A glacier reshapes a landscape in two connected ways: it removes rock by erosion, then transports and deposits that material elsewhere. Its load can include everything from tiny clay particles to enormous boulders.

Plucking supplies blocks of rock, while abrasion produces smaller fragments. Freeze-thaw weathering also loosens rocks from valley sides, allowing them to fall onto the ice. Moving glaciers carry this debris on their surface, within the ice and beneath their base.

When ice melts, it releases its load. Material deposited directly by ice is called till. It is generally unsorted: clay, sand, gravel and boulders are mixed together rather than separated by size. These deposits can remain long after the glacier has disappeared, providing evidence of former ice movement.

Erratics: rocks carried away from home

An erratic is a rock transported by a glacier and deposited where the local bedrock is a different rock type. Size alone does not identify an erratic: a small rock can be an erratic, while a huge boulder matching the local geology may not be one.

A glacier can pick up a block through plucking and carry it on, within or beneath the ice. Melting releases the block in a new location. By matching its rock type to an original source area, geographers can investigate where the ice came from.

For example, distinctive pink Shap granite erratics occur around Shap in Cumbria. Glaciers transported these rocks away from their source, sometimes for many kilometres, before depositing them. Their distribution provides evidence of former ice movement.

Drumlins: hills streamlined by ice

Drumlins are smooth, elongated hills, commonly made from till. They often occur in groups called drumlin swarms, sometimes described as a ‘basket of eggs’ landscape.

A typical drumlin has a blunt, rounded, steeper end facing the incoming ice and a longer, gentler slope tapering in the direction of movement. Its long axis lies parallel to the former ice flow.

Drumlins develop beneath moving ice. Sediment accumulates beneath the glacier and is moulded into a streamlined shape as ice moves over it. Their formation is complex, so this is a useful general explanation rather than a single mechanism that applies to every drumlin.

The long axis helps identify the alignment of former ice flow. Where the two ends are clearly distinguishable, their contrasting slopes also help indicate which way the ice travelled.

Moraines: debris in different positions

A moraine is an accumulation of debris transported and deposited by a glacier. Moraines are named according to their position and formation. Some form ridges, but others spread across a valley floor.

Lateral moraines form along the sides of a glacier. Rock loosened by freeze-thaw weathering falls from valley sides onto the edges of the ice. The glacier carries it downhill; when the ice melts, it can leave long ridges marking the former glacier margins.

Medial moraines form when two glaciers join. The lateral debris bands on their adjoining sides merge into one band down the centre of the combined glacier. Melting can leave this debris along the valley floor.

Terminal moraine forms at the furthest point reached by a glacier's snout, or end. Ice transports and can push debris towards the snout. If the snout stays in roughly the same position for a time, material builds up there. Retreat leaves a ridge across the valley, marking the glacier's maximum advance.

Recessional moraines also form at the snout, but during pauses in retreat. When forward ice movement and melting are roughly balanced, the snout remains in a similar position and debris accumulates. Several ridges can record successive stages of retreat. Retreat means the glacier's end moves back as ice melts; the ice itself can still flow forwards.

Ground moraine is deposited beneath the glacier as ice melts. It forms a widespread, uneven layer of till rather than necessarily forming a distinct ridge.

Together, these deposits reveal different aspects of a former glacier: lateral moraines mark its edges, terminal moraine marks its furthest advance, and recessional moraines record pauses as it shrank.

Identifying landforms in the Lake District

The Lake District in north-west England is an example of a UK upland area affected by glaciation. During the last ice age, snow accumulated in mountain hollows and became ice. Glaciers flowed outwards from the central uplands through existing valleys. Erosion reshaped the mountains and valleys, while melting ice left deposits behind.

To identify a landform, connect its visible characteristics with its location and the process that produced it.

Erosional features show where ice removed rock; depositional features show where transported material was left. A broad U-shaped valley is therefore evidence of erosion, whereas a streamlined hill of till or a ridge of mixed debris is evidence of deposition. Reading both kinds of landform together helps reconstruct the former glaciated landscape.

Transportation and deposition

• Debris travels on, within and beneath glacial ice.
• Melting releases the load. Till is unsorted, mixing fine sediment and large rocks.
• Erratics: transported rocks unlike the local bedrock; source matching helps reconstruct ice movement.
• Drumlins: elongated, streamlined hills, commonly of till. Ice moved from the blunt, steeper end towards the tapered, gentler end; long axes align with ice flow.

Moraine positions