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Spec mapping (AQA 7037): Paper 1 (Physical), §3.1.3 Coastal systems and landscapes — landforms of coastal erosion and their development over time, the role of geological structure and lithology, and the production of emergent and submergent landscapes by sea-level change. This depth lesson assumes the basic erosional sequence is known and develops three advanced themes: landform sequences as time-transgressive responses, structural and lithological control on where and how landforms form, and the reading of polygenetic, relict and inherited landscapes. It draws on the marine processes of the wave/erosion lessons, the systems framing of §3.1.1, and the sea-level history shared with the glacial system of §3.1.4 (eustasy is a direct consequence of glaciation). Assessment objectives: AO1 (landform genesis and structural control), AO2 (applying lithology/structure to a real coast or geological map), AO3 (manipulating long-profile, dip and dating data).
The intellectual step at depth is to stop treating landforms as fixed objects and start treating them as snapshots of a process working through a structure over time. A stack is not a thing; it is the headland erosion process caught at a particular stage, on a particular rock, under a particular sea level. Change any of those three — process intensity, structure, or sea level — and the snapshot changes. This is why the same chalk produces vertical cliffs at Beachy Head, an arch-and-stack assemblage at Old Harry, and a wave-cut platform at Flamborough: one lithology, three expressions, set by structure and stage.
Many coastal erosional landforms are stages in a single progressive sequence; at depth, the examinable subtlety is that the stages are time-transgressive — they migrate landward as the cliff retreats, so a coast preserves several stages simultaneously at different points along it.
flowchart LR
A[Joint / fault<br/>exploited by<br/>hydraulic action] --> B[Cave<br/>at cliff base]
B --> C[Arch<br/>caves meet through<br/>a narrow headland]
C --> D[Stack<br/>arch roof collapses]
D --> E[Stump<br/>stack undercut<br/>and reduced]
E --> F[Reef<br/>planed to platform<br/>below wave base]
Stage 1 — Structural exploitation. Erosion is selective: hydraulic action (shock pressure and air compression) and corrosion attack joints, faults and bedding planes because these are the weakest available paths. The spacing and orientation of discontinuities therefore set where a cave will form — a fault crossing a headland is a near-guarantee of cave initiation.
Stage 2 — Cave. Sustained quarrying along the weakness opens a cave at the high-energy zone between high and low water. Wave energy is focused at the cave back by reflection and air compression, so the cave deepens faster than it widens. A cave open to the sky, or one cut along a vertical fault, is a geo (Old Norse) or zawn (Cornish).
Stage 3 — Arch. Where caves on opposite sides of a narrow headland are excavated back to back along the same weakness, they meet to form an arch. The arch is inherently unstable: continued basal erosion widens its legs while sub-aerial weathering and gravity attack the roof from above. Durdle Door (Dorset) is the type example — an arch cut through near-vertical Portland limestone.
Stage 4 — Stack. The arch roof eventually exceeds the strength of the spanning rock and collapses, isolating a stack seaward of the cliff line. Old Harry (Studland, Dorset) is a chalk stack; The Old Man of Hoy (Orkney, ~137 m, Old Red Sandstone on a basalt plinth) is among the tallest in Britain and shows how a resistant cap and a hard base can preserve a tall column.
Stage 5 — Stump and reef. Marine erosion and weathering undercut and lower the stack to a stump exposed only at low water (Old Harry's Wife is the stump beside Old Harry), and ultimately to a planed reef below wave base. The whole sequence is one process — selective marine erosion — caught at five stages.
Two refinements distinguish a depth answer. First, the sequence is not deterministic in its timing: a resistant cap or a particularly massive joint block can preserve a stack for centuries, while a heavily fractured arch may collapse within decades, so the rate of progression through the stages is set by structure and energy, not by a fixed schedule. Second, because the cliff is retreating while the sequence runs, the stack and stump are progressively isolated seaward of the present cliff line — Old Harry stands offshore of Ballard Down precisely because the chalk cliff has retreated landward since the stack was cut from it. The spatial separation between a stack and its parent cliff is therefore itself a measure of cliff retreat since the stack formed, turning the landform into a crude retreat-rate indicator.
A blowhole forms where hydraulic action and air compression in a cave roof exploit a vertical joint, driving a shaft up to the cliff top; storm waves then jet spray through it. It is the headland sequence operating vertically rather than horizontally, and like the rest of the sequence it is structurally located — the shaft follows a pre-existing joint. The example at Trevose Head (Cornwall) is cut in slate.
A shore (wave-cut) platform is the gently seaward-sloping rock surface left as a cliff retreats; it is the cumulative record of that retreat and a key negative-feedback brake on it.
| Type | Gradient | Dominant process | Setting |
|---|---|---|---|
| Type A (sloping) | 1–5° | Mechanical wave erosion; gradient reflects the erosion/energy balance | Exposed, higher-energy, resistant rock |
| Type B (sub-horizontal) | < 1° | Weathering + bioerosion lowering the surface to a water-controlled datum | Sheltered, often carbonate or warmer coasts |
| Plunging cliff | No platform | Deep water lets waves reflect without breaking; no notch cut | Steeply shelving, structurally controlled coasts |
The crucial concept is the negative-feedback width limit: as a platform widens, an ever-greater share of incoming wave energy is dissipated by friction and breaking crossing the platform before it reaches the cliff foot, so the rate of basal erosion — and hence retreat — declines. This is why hard-rock platforms tend toward a characteristic maximum width (a few hundred metres) and then near-stability, and it is the mechanism behind Flamborough Head's wide chalk platform fronting a near-static cliff. A platform is therefore not just a relict surface but an active regulator of the cliff system, and reading its width tells you how far the negative-feedback brake has engaged.
Read closely, a platform's surface records the processes that lowered it. Runnels and rock pools etched along joints and bedding mark structurally-guided solution and abrasion; potholes record swirling abrasion by trapped clasts; a scarp or ramp at the seaward edge marks the transition to the actively-breaking zone; and a biological zonation (lichens above, then barnacles, then fucoid algae, then kelp toward low water) maps the tidal frame onto the rock and, through grazing and boring, contributes to lowering. On a macrotidal coast the platform is wide because erosion is spread over a large vertical band; on a microtidal coast it is narrow and the notch sharply defined because marine action is concentrated. Thus platform width reflects tidal range and rock resistance, while platform gradient (Type A vs Type B) reflects the balance of mechanical wave erosion against weathering/bioerosion — two independent signals you can read from one surface.
A cliff's profile records the balance between basal marine erosion and sub-aerial face processes — the same coupling developed in the erosion lesson, here expressed as form. Where the sea undercuts faster than the face can degrade, the cliff is steep, active and "fresh" (a notch at the base, bare rock, rockfall — Beachy Head chalk). Where basal protection (a wide beach or platform) lets sub-aerial weathering and mass movement dominate, the cliff declines to a lower angle, accumulates talus and vegetates — a degraded, relict cliff. The same lithology can therefore present opposite profiles depending on whether the marine "conveyor" at its foot is running. Reading a cliff profile back to its dominant process — and hence to whether the coast is advancing into or retreating from active erosion — is one of the commonest AO2 demands at this level.
At depth, AQA expects you to explain not merely that landforms form but why they form where they do, which is a question about geological structure and lithology.
The Dorset coast is the textbook case because it shows both on one peninsula: the east-facing Studland frontage is discordant (the Purbeck strata are cut across at a high angle, giving Ballard Down chalk headland and Studland Bay), while the south-facing frontage at Lulworth is concordant (the same strata run parallel to the coast, giving the straight Portland-limestone wall breached only at Lulworth Cove and Stair Hole). The single geological structure produces opposite coastal styles depending on the angle the coast happens to cut across it — a perfect AO2 illustration that "concordant" and "discordant" describe a relationship between geology and coastline orientation, not a property of the rock itself. Where refraction then concentrates energy on the headlands and disperses it into the bays (see the wave lesson), the discordant frontage tends, very slowly, to straighten — but resistant headland lithology means the discordance persists for a very long time, so the equilibrium plan form is rarely reached.
| Dip of strata | Cliff profile | Dominant failure |
|---|---|---|
| Horizontal | Steep, stepped/vertical | Rockfall along bedding |
| Seaward (with slope) | Lower-angle, planar | Translational slide along dip-parallel planes |
| Landward (against slope) | Steep, often overhanging | Rockfall/toppling as toe undermined |
| Steeply inclined | Asymmetric | Joint-set-controlled |
The dip determines whether the rock "leans into" or "leans away from" the sea, which sets both the cliff angle and the dominant mass-movement style — a frequent AO2 demand when you are given a cliff sketch or a geological cross-section and asked to predict failure mechanism.
A recurrent AO3/AO2 skill is to predict landforms from a geological map. The workflow is: (1) read the key to rank lithologies by likely resistance (igneous/well-cemented = resistant; clays/unconsolidated = weak); (2) note the orientation of geological boundaries relative to the coast to classify it as discordant (boundaries meet the coast at a high angle → expect headland–bay) or concordant (boundaries run parallel → expect a straight coast or cove); (3) infer dip direction from the "rule of V's" (geological boundaries V upstream in valleys when dipping with the slope, and the displacement of boundaries across contours reveals dip); (4) locate faults (bold lines, often with tick marks or displacement) as probable sites of caves, geos and embayments; (5) synthesise — mark predicted headlands on resistant outcrops, bays on weak ones, and cave/geo sites on faults. Doing this before describing a coastline lets you explain, not just list, its landforms — exactly the analytical move AQA rewards. Spring lines at permeable-over-impermeable contacts (e.g. chalk over Gault clay) are a bonus prediction: they mark where groundwater emerges to lubricate failure planes and concentrate mass movement.
A polygenetic landscape bears the imprint of more than one set of formative processes acting at different times. Many UK coasts are polygenetic in a specific way: they are paraglacial — fashioned partly by Pleistocene glacial/periglacial processes and now being reworked by marine processes under a different (Holocene) climate and sea level. Holderness is the cleanest case: its cliffs are glacial till (deposited by a Devensian ice sheet) now attacked by the sea, so the landform is glacial in material but marine in process — a sediment supply inherited from one system being consumed by another. Recognising inheritance — that a coast's material may pre-date its processes by tens of thousands of years — is a hallmark of top-band synoptic reasoning and the explicit link between §3.1.3 and §3.1.4.
The paraglacial concept (Church and Ryder, 1972) is worth stating precisely because it is the conceptual key to so much British coastal geomorphology: it describes the transient period after deglaciation during which a landscape, destabilised and over-supplied with sediment by the departed ice, adjusts toward a new equilibrium under non-glacial processes. During this adjustment, sediment yields are anomalously high because glacial and periglacial deposits (till, head, outwash) are readily eroded and reworked — and then yields decline as those finite inherited stores are exhausted. A great deal of the sediment in UK sediment cells is paraglacial: the shingle of southern beaches is reworked outwash; the rapidly-eroding cliffs of Holderness, north Norfolk and the Yorkshire coast are reworked till. The strategic implication for the rest of this course is profound: these coasts are not in steady state but are running down a sediment inheritance, so their high present erosion rates and abundant beaches may not be sustainable once the Pleistocene legacy is consumed — a sobering frame for long-term management and a direct, examinable bridge between the coastal and glacial halves of Paper 1.
Relative sea level is the net of global (eustatic) and local (isostatic) change, and its history is written into the coast.
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