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Spec mapping (AQA 7037): Paper 1, §3.1.4 Glacial Systems and Landscapes — processes of fluvioglacial (meltwater) erosion and deposition; origin and development of fluvioglacial landforms (meltwater channels, outwash plains/sandur, eskers, kames, kettle holes). Meltwater is a major output of the glacial system and an agent that reworks the system's stores, so it links to the systems framework (§3.1.4, §3.1.1) and shares process vocabulary (sorting, stratification, deposition) with fluvial study. Outburst floods (jökulhlaups) and proglacial-lake drainage connect synoptically to §3.1.5 Hazards. The assessment objectives are AO1 (meltwater processes and landforms), AO2 (applying the sorted/stratified signature to explain landform genesis and distinguish from till) and AO3 (interpreting sediment logs and varve records, including dating).
Meltwater is one of the most powerful and underrated agents in glacial environments. The landforms it creates — collectively fluvioglacial (or glaciofluvial) — are diagnostically different from those laid directly by ice, because flowing water sorts and rounds its load whereas ice does not. Mastering this contrast, and the landforms that express it, is one of the most reliably examined parts of the option.
Meltwater is generated by:
It has distinctive, exam-relevant properties:
| Property | Description |
|---|---|
| Discharge | Highly variable — strong seasonal (summer peak, winter minimum) and diurnal rhythm tracking daytime temperature; can spike catastrophically in a jökulhlaup |
| Sediment load | Very high — laden with rock flour from abrasion, giving a milky blue-grey ("glacial milk") colour |
| Energy | High — often flows under hydrostatic pressure in confined subglacial tunnels, so it can flow uphill over bed undulations and erode powerfully |
| Temperature | Close to 0°C year-round |
The flashy, highly seasonal discharge is fundamental to understanding fluvioglacial landforms. Because most meltwater is produced in summer and almost none in winter, proglacial rivers experience enormous swings in flow, with frequent floods that overwhelm their channels. Combined with a colossal sediment supply (far more than the river can carry at low flow), this produces the braided channel pattern so characteristic of glacial outwash: the stream repeatedly chokes on its own load, splits around shifting gravel bars, and rearranges its channels from year to year. The contrast with a "normal" river — which has a relatively steady regime and a single, stable channel — is one of the clearest ways to recognise a glacial meltwater environment, and it is the direct cause of the sorted, graded outwash that the next section describes.
Key Point: Because subglacial meltwater is pressurised, it behaves quite unlike a normal river — it can be forced up adverse gradients and concentrated into fast, erosive conduits. This is why fluvioglacial erosion can incise bedrock gorges and why eskers can run up and over hills.
Meltwater generated at and within the glacier must find its way to the snout, and how it is routed shapes the landforms it leaves. Surface (supraglacial) streams flow across the ice until they plunge down a moulin (a near-vertical shaft) into the body of the glacier, becoming englacial flow; this in turn descends to the bed, joining the subglacial drainage of R-channels and distributed films described in the movement lesson. At the snout the water emerges, often from a dramatic ice cave or portal, as a turbid proglacial river that braids across the outwash plain. Each segment of this journey can deposit or erode: subglacial tunnels build eskers, the ice margin builds kame terraces, and the proglacial zone builds the sandur. Understanding the routing therefore lets you predict which fluvioglacial landform forms where — a higher-order skill than simply listing them.
Meltwater erodes by the same processes as ordinary rivers — hydraulic action, corrasion (abrasion), corrosion (solution), cavitation — but is often more effective because of high pressure, heavy sediment load and extreme peak discharges. The pressurised, sediment-charged nature of subglacial water means it can scour bedrock far more aggressively than a surface stream of the same size, which is why meltwater channels are frequently over-large relative to any present-day flow.
The most spectacular expression of meltwater erosion is the jökulhlaup (an Icelandic term for a glacial outburst flood). These occur when water impounded beneath, within or in front of a glacier is suddenly released — for example when a subglacial volcanic eruption rapidly melts the ice base, when a moraine- or ice-dammed lake fails, or when rising subglacial water floats the ice and drains catastrophically. Discharges can be staggering: the 1996 Skeiðarárhlaup (triggered by the Gjálp eruption under Vatnajökull) peaked at the order of 50,000 m³/s — comparable to the discharge of a large continental river — for a few days, ripping up the outwash plain and destroying bridges.
On geological timescales, jökulhlaups have shaped landscapes on a colossal scale. The Channeled Scablands of Washington State (USA) were carved by the repeated catastrophic drainage of Glacial Lake Missoula at the end of the last glaciation — floods estimated at millions of cubic metres per second that scoured the basalt into a maze of dry channels, giant ripples and dry falls. First proposed by J Harlen Bretz in the 1920s, this idea was fiercely resisted because it seemed too "catastrophist," but is now firmly accepted — a salutary lesson that low-frequency, high-magnitude events can dominate landscape change. For the exam, jökulhlaups are a powerful synoptic link to §3.1.5 Hazards and a reminder that fluvioglacial processes operate across an enormous range of magnitudes.
As meltwater loses energy — through reduced gradient, channel widening, the loss of confinement as it leaves a tunnel, or simply distance from the ice — its competence (the largest particle it can carry) falls and it deposits its load, coarsest material first. Because water deposits selectively by size in this way, the result is diagnostically distinct from till, which ice dumps as an unsorted jumble:
| Feature | Glacial Till | Fluvioglacial Deposits |
|---|---|---|
| Sorting | Unsorted — all sizes mixed | Sorted — graded by size (coarse first, fine last) |
| Stratification | Unstratified — no bedding | Stratified — clearly bedded/layered |
| Clast shape | Angular to sub-rounded | Rounded, smoothed by water transport |
| Surface texture | May be striated/faceted | Smooth, water-worn |
| Depositional agent | Ice (direct) | Meltwater |
Exam Tip: The sorted-vs-unsorted contrast is the single most important diagnostic in this option. Given an unknown deposit, examine particle-size distribution and stratification first — they tell you ice versus water more reliably than anything else.
The landforms divide usefully into proglacial (deposited in front of the ice) and ice-contact (deposited against or beneath the ice, then let down as it melts).
A sandur (plural sandar) is a broad, gently-sloping apron of sorted sediment laid down beyond the snout by braided meltwater streams.
An esker is a long, sinuous ridge of sorted, stratified sand and gravel deposited by a meltwater stream in a subglacial (R-) tunnel; when the ice melts, the tunnel fill is left standing proud.
The formation of an esker neatly demonstrates the inverted relief that fluvioglacial deposition can create: sediment laid down in the lowest point of the system (a tunnel at the bed) ends up as the highest feature on the landscape once the surrounding ice melts away — the opposite of normal river deposition.
Exam Tip: Asked to distinguish an esker from a moraine in the field, key on four things: sorting, stratification, clast roundness, and the sinuous planform. A moraine is unsorted till following the ice margin; an esker is sorted gravel following a former tunnel, often cutting across slope. A fifth clue is orientation relative to ice flow: an esker runs broadly parallel to former ice flow (it follows the down-glacier tunnel), whereas a terminal/recessional moraine runs transverse to flow (across the valley at the former snout). Combining sediment character with planform and orientation lets you separate the two with confidence even from a map alone.
Kames are mounds or terraces of sorted, stratified sand and gravel, all sharing an ice-contact origin that leaves them irregular and prone to collapse:
Together with kettle holes, kames build the distinctive "kame-and-kettle" topography of irregular mounds and hollows that marks areas where a stagnant ice mass disintegrated in place — a landscape type quite different from the orderly moraine sequences of an actively retreating glacier.
A kettle hole forms where a block of dead (stagnant) ice, detached from the retreating glacier, is partly buried by outwash; when it finally melts, the overlying sediment collapses into the void.
Kettle holes are important evidence of stagnant-ice (down-wasting) deglaciation — they show that, rather than the snout retreating cleanly up-valley, large blocks of ice were left behind to melt in situ, buried in their own outwash. A landscape pitted with kettle holes therefore tells a different story from one with neat recessional moraines: it records an ice mass that thinned and disintegrated where it lay.
Varves are paired annual layers of sediment on the floor of a proglacial lake, recording the seasonal rhythm of meltwater supply; one light + one dark couplet = one year. They form because the lake's sediment input swings with the seasons — a flood of coarse, sediment-rich meltwater in summer settling rapidly, followed by the slow settling of the finest clay through still, ice-covered water in winter:
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