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Spec mapping (AQA 7037): Paper 1, §3.1.4 Glacial Systems and Landscapes — processes of glacial transport (entrainment, supraglacial/englacial/subglacial debris) and deposition; origin and development of glacial depositional landforms (till, moraines, drumlins, erratics). This completes the transfer → output end of the glacial system framework (§3.1.4, linked to systems thinking in §3.1.1), and the diagnostic distinction between ice-laid and water-laid sediment sets up the fluvioglacial lesson that follows. The assessment objectives are AO1 (transport pathways, till types, depositional landforms), AO2 (applying process to explain landform morphology and the till/outwash contrast) and AO3 (interpreting sediment data and till-fabric measurements).
Glaciers transport astonishing quantities of rock debris — everything from clay-sized rock flour to boulders the size of houses. Where erosional landforms dominate the uplands, depositional landforms dominate the lowlands and valley floors, and they preserve some of our best evidence of past ice extent and flow direction. Reading these deposits correctly — distinguishing material dumped directly by ice from material reworked by meltwater — is one of the most heavily examined skills in this option.
Transport and deposition complete the sediment cascade of the glacial system: material eroded from the bed and walls (lesson 3) is carried through the system and ultimately released, either directly by the ice (till) or, after reworking, by meltwater (the fluvioglacial deposits of lesson 5). A central theme of this lesson is therefore the diagnostic contrast between ice-laid and water-laid sediment — a contrast that runs through the exam specification and underpins almost every data-response question on glacial deposition. Master it here and the fluvioglacial lesson follows naturally.
Before a glacier can transport debris it must entrain it. Debris enters the ice in three main ways: (1) rockfall of frost-shattered material from valley walls onto the surface; (2) plucking and abrasion generating basal debris at the bed; and (3) burial — surface debris is covered by later snowfall, or falls into crevasses, and is incorporated into the ice. Basal debris can also be entrained by freezing-on, where regelation ice refreezes around bed material and lifts it into the basal layer. Once entrained, debris is carried in one of three positions.
The quantity of debris a glacier carries depends on its setting. A valley glacier confined between steep, frost-shattered rock walls receives a heavy supply of supraglacial rockfall, so it carries — and ultimately deposits — large volumes of angular debris and builds prominent lateral and medial moraines. An ice sheet or ice cap, by contrast, buries the whole landscape and has few exposed rock walls above it, so almost all of its debris is basal (derived from plucking and abrasion of the bed) and is therefore sub-rounded and striated. This difference explains why valley-glacier deposits tend to be coarser and more angular, while ice-sheet tills are dominated by fine, abraded basal material — a useful discriminator when interpreting an ancient deposit's origin.
graph TD
D["Frost shattering of valley walls"] --> A["SUPRAGLACIAL<br/>Angular, unsorted, on the surface"]
E["Plucking & abrasion of the bed"] --> C["SUBGLACIAL<br/>Sub-rounded, faceted, striated"]
A -->|"buried by snow / falls into crevasses"| B["ENGLACIAL<br/>Angular, within the ice"]
B -->|"works down / melts out"| C
B -->|"thrust to surface under compressing flow"| A
Key Point: The position of transport leaves a fingerprint on the debris. Angular, unscratched clasts suggest a supraglacial/englacial history; sub-rounded, striated, faceted clasts suggest subglacial transport. This is how sedimentologists reconstruct a stone's journey.
It is helpful to think of debris moving through the glacier as a cascading subsystem — the sediment counterpart of the energy and mass cascade in the systems lesson. Material is input (rockfall from walls; plucking/abrasion at the bed), stored (in the supra-, en- and subglacial positions), transferred (carried passively in the moving ice, or routed down through crevasses and up along thrust planes), and finally output as till or, where meltwater intervenes, as sorted fluvioglacial sediment. The residence time of a clast can be years to millennia, and its pathway determines its final character.
graph TD
I1["Rockfall from frost-shattered walls"] --> S["STORES:<br/>supraglacial / englacial / subglacial debris"]
I2["Plucking + abrasion of the bed"] --> S
S --> T["TRANSFER: passive carriage in moving ice;<br/>routing via crevasses & thrust planes"]
T --> O1["OUTPUT (ice): TILL — unsorted"]
T --> O2["OUTPUT (meltwater): SORTED fluvioglacial sediment"]
A crucial implication is that the same rock fragment can end up in very different deposits depending on whether it is released directly by melting ice (→ till) or picked up and re-laid by meltwater (→ outwash). This is why glaciated landscapes contain interleaved sorted and unsorted sediment, and why careful sedimentology — not just "it's near a glacier" — is needed to read them.
When a glacier slows, thins or melts, it releases its load. All glacial and fluvioglacial sediment is collectively drift — a term that survives from the 19th-century "drift theory" that once attributed these deposits to icebergs and floods before Agassiz established their glacial origin. Drift is subdivided into sediment laid directly by ice (not reworked by water), called till, and sediment reworked and re-laid by meltwater, called fluvioglacial (stratified) drift (the subject of the next lesson). The whole of glacial deposition therefore turns on this one diagnostic split: was the agent ice, or water?
| Characteristic | Description |
|---|---|
| Sorting | Unsorted — a chaotic mix of clay, silt, sand, gravel and boulders together |
| Stratification | Unstratified — no bedding or layering |
| Clast shape | Variable — subglacial clasts sub-rounded, faceted and striated; supraglacial clasts angular |
| Fabric | Lodgement till often shows a preferred clast orientation parallel to ice flow |
| Composition | Reflects source-area geology; may contain far-travelled erratics |
Two principal types:
A third category, deformation (glaciotectonic) till, is increasingly recognised: where the bed itself shears, the till is pervasively deformed and may incorporate rafts of underlying material. The distinction between these till types is not merely academic — lodgement till is compact and relatively stable, with low permeability (important for groundwater and for the foundations of buildings and roads on glaciated lowlands), whereas ablation till is looser and more prone to settlement. Engineers and hydrogeologists working in formerly glaciated regions therefore care a great deal about which till they are dealing with, giving this apparently dry sedimentological distinction a direct practical relevance.
Exam Tip: The most reliable exam discriminator is till vs fluvioglacial deposit: till is unsorted and unstratified; fluvioglacial sediment is sorted and stratified with rounded clasts. Memorise this contrast — it underpins data-response questions in this and the next lesson. A useful mental model is to ask "could water have done this?": water cannot transport a clay particle and a boulder together and lay them down in the same unsorted heap, so an unsorted, unstratified deposit must be ice-laid till. The presence of striations or facets on clasts further confirms subglacial transport, since water-worn clasts are smoothly rounded and unscratched.
An erratic is a clast carried by ice and dumped on bedrock of different geology from its source — direct evidence of ice path and extent. Erratics were historically pivotal: it was their occurrence far from any possible source rock that helped convince 19th-century geologists (following Agassiz) that ice, not a flood, had transported them, overturning the earlier diluvialist theory. Because an erratic's lithology can be matched to a specific, mappable source outcrop, a trail of erratics — an "erratic train" — traces the path and reach of former ice with great precision.
Moraines are accumulations of till at the margins or terminus of a glacier. The word "moraine" is used both for the debris itself and for the landform it builds, which can cause confusion: a medial moraine, for instance, is a striking dark stripe of debris on an active glacier's surface but makes a poor landform once the ice melts (the debris simply collapses to the floor). Moraines are classified principally by position relative to the glacier, but it is worth remembering that the same lateral or terminal ridge may be built by several processes — dumping, pushing and squeezing — acting together over the life of the glacier. They are classified by position as follows:
| Moraine Type | Position | Formation | Characteristics |
|---|---|---|---|
| Lateral | Along the valley sides | Frost-shattered wall debris builds up at the ice margins | Embankment of angular, unsorted debris, often surviving as ridges |
| Medial | Down the glacier centre | Two inner lateral moraines merge where glaciers converge | Dark central stripe on the ice; subdued as a landform after deglaciation |
| Terminal | Maximum advance limit | Debris bulldozed and dumped at the snout | Cross-valley ridge, sometimes tens of metres high; can dam lakes |
| Recessional | Up-valley of the terminal | Deposited at temporary still-stands during retreat | Series of smaller ridges parallel to the terminal moraine |
| Ground | Across the floor | Lodgement till laid beneath moving ice | Undulating, hummocky blanket of variable thickness |
| Push | At/near the snout | A re-advancing glacier shoves earlier deposits | Steep-fronted ridge with up-tilted, deformed internal structure |
Case Study — Buttermere & Crummock Water (Lake District): a terminal (and associated recessional) moraine ridge of till crosses the valley floor between these two lakes, dividing what was once a single over-deepened ribbon lake into two. The hummocky till barrier is clearly visible and is a textbook example of a moraine-dammed lake division.
A sequence of terminal and recessional moraines is invaluable evidence: the outermost ridge marks the maximum extent, and successive inner ridges mark pauses during retreat, so the morphology effectively maps the deglaciation history.
It is worth distinguishing the processes that build moraines, because exam questions often probe mechanism rather than position:
A single terminal-moraine complex can combine all three, which is why these landforms are sedimentologically messy. A particularly important compound landform is the moraine-dammed lake: a terminal moraine blocks a valley and impounds water (the Buttermere–Crummock case, or many newly-forming proglacial lakes today), creating a GLOF hazard if the moraine dam fails — a forward link to the climate-change and hazards material.
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