Glacial and Fluvioglacial Deposits
Glaciers transport and deposit vast quantities of sediment. At A-Level, AQA requires you to distinguish between deposits laid down directly by ice (glacial) and those deposited by meltwater (fluvioglacial), and to understand the landforms each creates.
Glacial Transport
Material is transported by glaciers in three main zones:
1. Supraglacial Transport
- Material carried on the glacier surface
- Derived from freeze-thaw weathering of rock faces above the glacier and from rockfalls
- Forms lateral moraines (along the sides) and medial moraines (where two glaciers merge and their lateral moraines combine)
- Material is typically angular because it has not been in contact with the bed
2. Englacial Transport
- Material carried within the body of the ice
- Enters through crevasses or is incorporated during ice formation
- Protected from abrasion, so retains angular shape
3. Subglacial Transport
- Material carried at the base of the glacier
- In direct contact with the bedrock — experiences intense abrasion
- Material is ground down, becoming rounded and striated
- Produces fine-grained sediment called rock flour (glacial flour), which gives meltwater streams their characteristic milky-blue colour
Glacial Deposits (Till)
Till (historically called boulder clay) is sediment deposited directly by glacier ice. It has distinctive characteristics:
- Unsorted — contains a chaotic mixture of particle sizes, from clay to boulders
- Unstratified — lacks the layering (stratification) seen in water-deposited sediments
- Angular to sub-angular clasts — especially in supraglacial till
- Striated clasts — pebbles and boulders often show scratches from subglacial transport
- Matrix-supported — larger clasts are set in a fine-grained matrix of clay and silt
Types of Till
| Type | Deposition Mechanism | Characteristics |
|---|
| Lodgement till | Deposited beneath a moving glacier; plastered onto the bed by pressure | Dense, compact, over-consolidated; strong fabric (clasts aligned in flow direction) |
| Ablation till | Released from melting ice at the glacier surface or margin | Looser, less compact; more variable composition; weaker fabric |
| Deformation till | Existing sediment beneath the glacier is remoulded and mixed by ice movement | Mixed, may contain thrust structures and deformed layers |
| Flow till | Saturated till flows from the glacier surface or margin | Poorly sorted, may show flow structures |
Erratics
An erratic is a rock that has been transported by a glacier and deposited in an area of different geology.
- Erratics can be identified because their lithology (rock type) differs from the local bedrock
- They can be traced back to their source area, providing evidence of the direction and extent of glacial transport
- Example: The Norber Erratics, Yorkshire Dales — dark Silurian greywacke boulders resting on pale Carboniferous limestone, transported approximately 0.5 km by a glacier. Some sit on limestone pedestals where the surrounding limestone has been dissolved but the rock beneath the erratic was protected
Moraines
Moraines are accumulations of till deposited by a glacier. They are classified by their position relative to the glacier.
Types of Moraine
| Type | Position | Formation |
|---|
| Lateral moraine | Along the sides of the glacier | Freeze-thaw debris falling from the valley sides onto the glacier margins |
| Medial moraine | Running along the centre of the glacier | Formed where two lateral moraines merge when glaciers join |
| Terminal (end) moraine | At the furthest point of glacial advance | Bulldozed and deposited at the snout; marks maximum extent |
| Recessional moraine | Behind the terminal moraine | Deposited during pauses in glacial retreat; marks stillstand positions |
| Ground moraine | Beneath the glacier, across the valley floor | Lodgement till plastered onto the bed; creates an undulating surface |
| Push moraine | At or near the snout | Formed when a re-advancing glacier pushes previously deposited sediment into a ridge |
| Hummocky moraine | Irregular mounds across the valley floor | Formed by stagnation and in-situ melting of debris-rich ice |
Terminal Moraines as Evidence
Terminal moraines are crucial for reconstructing past glacial extents:
- They mark the maximum extent of a glacier or ice sheet
- Multiple recessional moraines indicate stages of retreat
- Radiocarbon dating of organic material within or beneath moraines provides ages for glacial advances
- Example: The York and Escrick moraines in northern England mark the maximum extent of the Devensian ice sheet (approximately 20,000 years ago)
Drumlins
A drumlin is a streamlined, elongated hill of till, shaped like an inverted spoon or half-buried egg.
Characteristics
- Length: 250 m to 1 km (sometimes longer)
- Height: 15–50 m
- Width: 125–300 m
- Elongation ratio: typically 2:1 to 5:1 (length to width)
- Asymmetric profile: steep stoss end (upstream) and gently tapering lee end (downstream)
- Composed primarily of lodgement till, sometimes with a bedrock core
Formation Theories
The exact formation mechanism of drumlins is debated. Leading theories include:
- Depositional theory — till is deposited and moulded beneath the glacier into a streamlined shape by the flowing ice. Sediment accumulates around a nucleus (e.g., a bedrock knob or a patch of resistant till)
- Erosional theory — a pre-existing deposit of till is shaped and streamlined by erosion from the moving ice
- Deformation theory — subglacial sediment is deformed and moulded by high pressures and shear stresses beneath the ice
- Flood theory (Shaw, 1983) — controversial theory suggesting drumlins formed by catastrophic subglacial meltwater floods
Drumlin Swarms
Drumlins rarely occur in isolation. They typically form in groups (swarms or fields) of hundreds or thousands, aligned in the direction of ice flow.