Depositional Landforms: Drumlins and Erratics

🪨 Depositional Landforms: Drumlins and Erratics

While erosional landforms are carved by the glacier, depositional landforms are built by the debris the glacier carries and then drops. All glacial deposits — whether forming drumlins, moraines, or erratics — consist of till: unsorted, unstratified rock debris of all sizes mixed together, from fine rock flour to large boulders. The fact that till is unsorted (particles of all sizes jumbled together) is how you can identify it in the field and distinguish it from water-sorted sediment.

Drumlins — smooth hills that reveal the direction of past ice movement

Drumlins are smooth, elongated hills of glacial till. In plan view (from above) they are oval-shaped; in cross-section along their long axis they are asymmetric — one end is blunter and steeper, the other is more gently tapering. They typically occur in groups called drumlin swarms or drumlin fields, and a landscape covered in them is described as having a "basket of eggs" topography — it looks exactly like a basket full of eggs when viewed from higher ground.

How drumlins form — the stoss and lee model

The exact mechanism of drumlin formation is debated among geographers, but the most widely accepted explanation is that moving glacier ice reshapes existing till or deposits it around a rock obstacle (a whaleback of bedrock). As the ice flows over a mound of till, it streamlines it — moulding the shape in the direction of flow. The blunt, steep end (the stoss face) faces into the direction from which the ice came; the tapered, gentler slope (the lee face) points in the direction the ice was travelling. Think of it like a teardrop: the blunt end faces the oncoming flow, the pointed tail streams behind it.

This orientation is critical for exam questions: drumlins act as a compass needle for past ice flow. If all the drumlins in a field point northeast–southwest, the ice flowed from the northeast towards the southwest (the stoss faces point northeast). This makes drumlins one of the key pieces of evidence geographers use to reconstruct the patterns of ice movement during the last glaciation.

UK named example: Eden Valley, Cumbria — one of the UK's best-known drumlin fields. Hundreds of drumlins in the Eden Valley are elongated in a NNW–SSE direction, showing the direction of former ice flow. Also: County Down, Northern Ireland — the classic "basket of eggs" drumlin landscape, with drumlins forming the hills and the depressions between them flooded by the sea to create the drumlins' characteristic partially submerged island appearance in Strangford Lough.

Erratics — rocks that don't belong

Erratics are rocks transported by a glacier and deposited far from their source. Because glaciers are capable of carrying debris for hundreds of kilometres before dropping it, an erratic may be sitting on bedrock of a completely different rock type. They are identified by the mismatch between the rock type of the boulder and the rock type of the local bedrock beneath it.

Erratics are invaluable evidence for reconstructing past ice flow paths. If geologists can identify the source rock of an erratic and know where that rock type occurs, they can trace the route the glacier must have taken to transport the boulder from its origin to where it now rests.

UK named example: The Norber Erratics, Yorkshire Dales. Large boulders of Silurian greywacke (a hard, grey metamorphic rock) are sitting on pedestals of Carboniferous limestone. The greywacke comes from outcrops approximately 2.5 km to the north — ice transported these boulders southward and dropped them on the limestone surface. The limestone around the base of each boulder has been chemically weathered (dissolved by rainwater) while the boulder protected the limestone beneath it — leaving the boulders perched on limestone pedestals up to 30 cm high. The pedestals tell us how much limestone has dissolved since the ice retreated approximately 10,000 years ago.

Moraines — the debris ridges

Moraines are ridges or sheets of till deposited by glaciers. You should know four types:

• Lateral moraine — a ridge of till along the sides of a glacier. Material falls from the valley walls (through freeze-thaw weathering and rockfall) onto the glacier edges and is carried at the glacier's margins. When the ice melts, this debris is deposited as a ridge running parallel to the direction of ice flow, along the valley sides.
• Medial moraine — a ridge of till running along the centre of a glacier. Forms when two glaciers merge: the lateral moraines of each glacier join together in the middle of the combined glacier, producing a central stripe of debris that becomes a medial moraine when deposited.
• Terminal moraine — the most important moraine for landscape formation. A curved ridge of till deposited at the snout (front) of the glacier where it stopped advancing. This marks the maximum extent of the glaciation. Terminal moraines often dam ribbon lakes by blocking the downstream end of a glacial trough (e.g., the moraine at the foot of Thirlmere, Lake District).
• Ground moraine (till plain) — a sheet of till deposited across the floor of the valley as the glacier retreated. Creates a low-relief, undulating landscape of gentle mounds and hollows. Found widely across lowland Britain, including much of East Anglia.