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Spec mapping (AQA 7037): Paper 1 (Physical), §3.1.4 Glacial systems and landscapes — transport and deposition by ice and meltwater, and the landforms of glacial deposition (till, moraines, drumlins, erratics) and fluvioglacial deposition (outwash/sandur, eskers, kames, kettle holes), distinguishing ice-contact from proglacial environments. This depth lesson is the output/store side of the glacial sediment budget: the debris eroded in lesson 7 is here transported, sorted (or not) and laid down. The defining analytical skill is to read a deposit's properties (sorting, stratification, clast shape, fabric) back to its depositional agent and environment. It links to §3.1.1 systems thinking and to the coastal sediment-budget logic of lesson 1 (deposition = where the budget stores material). Assessment objectives: AO1 (deposit/landform genesis), AO2 (applying ice-contact vs proglacial reasoning to a landscape), AO3 (manipulating till-fabric, clast-roundness and particle-size data).
The single most powerful idea here is the till-versus-outwash diagnosis: ice deposits sediment directly, dumping whatever it carries with no sorting (because ice is a solid, not a fluid, and cannot selectively transport by size); meltwater deposits sediment hydraulically, sorting and stratifying it by flow energy exactly as a river does. Every glacial deposit, and almost every exam question on this topic, turns on telling these two apart from the evidence — so we treat that diagnosis as the organising principle of the whole lesson.
Material is transported by glaciers in three main zones, and where a clast travels determines how it ends up:
The transport pathway is recorded in the clast, which is why this section underpins everything that follows. A supraglacially carried block (rockfall onto the surface, never touching the bed) stays angular and unstriated; a subglacially carried clast is faceted, striated and partly rounded by grinding against the bed. So when you examine till and find a mixture of pristine angular blocks and striated faceted stones, you are reading a deposit that drew on both high (supraglacial) and basal (subglacial) transport — typically an ablation till that combined surface and basal debris as the ice melted. The pathway also predicts position: surface debris concentrated at the margins becomes lateral and (where glaciers merge) medial moraine, while basal debris becomes ground/lodgement till and feeds terminal moraines via thrusting. Always ask, of any clast or deposit, which pathway delivered it — the answer constrains both its character and its landform.
Till (historically called boulder clay) is sediment deposited directly by glacier ice. It has distinctive characteristics:
| 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 |
Because lodgement till is plastered onto the bed by moving ice, its elongated clasts are dragged into a preferred orientation with their long axes parallel to ice flow (and often imbricated, dipping up-glacier). Till-fabric analysis measures the long-axis azimuth (and dip) of ~50 elongated clasts and plots them on a rose diagram; a strong, tight cluster of azimuths indicates lodgement till with a clear flow direction, while a weak, scattered distribution indicates ablation or flow till (deposited passively from melting ice, with no shearing to align the clasts). Fabric strength is quantified by a vector-magnitude statistic (often denoted S1, running from ~0.33 for random to ~1.0 for perfectly aligned). Till fabric is thus the depositional counterpart of striations: both reconstruct ice-flow direction, but fabric also diagnoses the till type from the strength of alignment — a frequent AO3 resource.
An erratic is a rock that has been transported by a glacier and deposited in an area of different geology.
Moraines are accumulations of till deposited by a glacier. They are classified by their position relative to the glacier.
| 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 |
Moraine type maps directly onto the transport pathway of its debris (the section above): lateral and medial moraines are supraglacial (rockfall onto the surface, so angular, unstriated debris), ground moraine is subglacial lodgement till (compact, striated, with fabric), and terminal/recessional moraines combine debris delivered to the snout by all pathways — basal debris thrust up under compressing flow, plus englacial and supraglacial loads released as the ice melts. Hummocky moraine is the signature of stagnant, downwasting ice (no longer flowing, melting in place), distinguishing a glacier that retreated by stagnation from one that retreated actively (which builds tidy recessional ridges instead). So a moraine landscape, read carefully, reveals not just extent but how the ice carried its load and how its margin died — the depositional counterpart to reading erosion from striations and troughs.
Terminal moraines are crucial for reconstructing past glacial extents:
The crucial interpretive point is that a moraine marks a balance, not simply an advance: a terminal moraine builds where the snout position is stationary because forward ice delivery equals frontal melting — the conveyor keeps delivering debris to a fixed line. A rapidly advancing or rapidly retreating snout builds little moraine because it does not dwell. Reading moraine sequences therefore reconstructs not just where the ice reached but how its margin behaved through time — a dynamic history, which is exactly what is needed to test models of how ice sheets respond to climate.
A drumlin is a streamlined, elongated hill of till, shaped like an inverted spoon or half-buried egg.
The exact formation mechanism of drumlins is debated. Leading theories include:
Drumlins rarely occur in isolation. They typically form in groups (swarms or fields) of hundreds or thousands, aligned in the direction of ice flow.
Example: The Ribble Valley drumlin field, Lancashire — one of the finest drumlin swarms in England, with hundreds of drumlins aligned in a broadly southward direction, indicating ice flow from the Lake District towards the Lancashire plain.
Example: The drumlins of Clew Bay, County Mayo, Ireland — partially submerged drumlins forming numerous small islands in the bay.
The unresolved debate over drumlin genesis is itself examinable: the competing depositional, erosional, deformation and (controversial) meltwater-flood hypotheses are not mutually exclusive — drumlins may be equifinal (the same form reachable by different processes), which is precisely why no single theory has won. A strong AO2 answer notes that the evidence (internal till fabric aligned with elongation, occasional bedrock or stratified cores, association with deforming-bed ice streams) best supports subglacial moulding of a deforming bed, while acknowledging the equifinality. Practically, the drumlin's streamlined shape is an unambiguous flow indicator: the blunt stoss end points up-glacier, the tapering lee end down-glacier, and the long-axis orientation maps the ice-flow direction across a whole swarm — so a drumlin field is a regional flow map, just as a single roche moutonnée is a local one. The elongation ratio even hints at flow velocity — more elongated drumlins tend to form under faster ice — adding a quantitative dimension.
flowchart TD
EROS[Eroded debris from lesson 7<br/>carried sub-/en-/supra-glacially] --> AGENT{Depositing agent?}
AGENT -->|directly by ICE| TILL[TILL = unsorted, unstratified,<br/>angular+striated, matrix-supported]
AGENT -->|by MELTWATER| FG[FLUVIOGLACIAL = sorted, stratified,<br/>rounded, clast-supported]
TILL --> TLF[Moraines, drumlins, erratics,<br/>ground/till sheet]
FG --> ICECON[ICE-CONTACT: eskers, kames<br/>deposited on/against ice]
FG --> PRO[PROGLACIAL: outwash/sandur,<br/>kettle holes, varves beyond the ice]
Fluvioglacial (or glaciofluvial) deposits are sediments deposited by meltwater streams. They differ from glacial deposits in important ways:
| Feature | Glacial Till | Fluvioglacial Deposits |
|---|---|---|
| Sorting | Unsorted (mixed sizes) | Sorted (graded by size) |
| Stratification | Unstratified (no layers) | Stratified (distinct layers) |
| Clast shape | Angular to sub-angular | Rounded (water-worn) |
| Fabric | Clasts may be aligned | No preferred alignment |
| Texture | Matrix-supported | Clast-supported or open framework |
An esker is a long, sinuous ridge of sorted sand and gravel deposited by a meltwater stream flowing in a tunnel beneath or within the glacier.
Formation:
Characteristics:
Example: The Trim Esker, County Meath, Ireland — extends for over 14 km.
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