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Spec mapping (AQA 7037): Paper 1 (Physical), §3.1.3 Coastal systems and landscapes — marine erosion processes, sub-aerial processes (weathering and mass movement) and the factors controlling rates of recession, plus measurement and modelling of cliff retreat. It builds on the energy inputs of the wave/tide lesson and the systems/budget framing of §3.1.1, and feeds directly into management (§3.1.3) and hazard (§3.1.6) content. Assessment objectives: AO1 (process and mechanism), AO2 (applying controls to located coasts), AO3 (measuring, modelling and evaluating retreat-rate data).
At depth, "erosion" is not a single process but a coupled system: marine processes attack the cliff base while sub-aerial weathering and mass movement degrade the cliff face, and the two are linked through the cliff-foot sediment budget. The headline figure for this lesson — Holderness retreating at ~1.8 m yr⁻¹ — is explained not by exceptional waves but by the interaction of weak lithology, beach starvation and saturation-driven mass movement.
The mechanical force of water (and the air it traps) on rock. Two distinct mechanisms:
Most effective on well-jointed or faulted rock and high-energy coasts. Hydraulic action is the dominant process on resistant but fractured lithologies (chalk, jointed sandstone) because it exploits the discontinuities rather than the intact rock.
A useful way to see why structure beats raw strength here is to compare the rock-mass strength with the intact-rock strength. A chalk hand-specimen tested with a Schmidt hammer may register a high rebound (the intact rock is reasonably strong), yet a chalk cliff retreats far faster than a weaker but massive mudstone, because the chalk is cut by closely-spaced joints and bedding planes along which hydraulic action can lever out blocks. The relevant property for erosion is therefore the strength of the jointed rock mass, which depends on discontinuity spacing, orientation and aperture as much as on the strength of the rock between the joints. This is the resolution of the apparent paradox that "hard" chalk erodes faster than "soft"-sounding clay-rich mudstone in some settings: erosion attacks the weakest available path, and a dense joint network supplies that path. It also explains why wave attack is so sharply concentrated where a fault crosses the coast — the fault zone is a pre-shattered weakness that focuses quarrying into caves, geos and ultimately the cell-bounding embayments.
Sediment hurled by waves scours the rock — the "sandpaper" process. Effective work depends on the tools available (coarse, hard clasts abrade fastest) and the energy delivering them. Evidence: polished platforms, scratches, potholes. On harder rock, abrasion lowers surfaces at the order of 1–5 mm yr⁻¹. Note the paradox: abrasion requires sediment, so a stripped beach may reduce abrasion even as it increases hydraulic action — the processes can trade off.
Wears down the sediment, not the cliff: clasts collide and reduce in size and angularity (boulders → cobbles → pebbles → sand → silt) and become rounded. Rounding is quantified with the Cailleux or Power's roundness index; progressive rounding and size reduction downdrift is classic evidence of longshore transport. Attrition matters to the budget because it converts coarse beach material into fine sediment that is lost offshore — a one-way "fining" of the cell's sediment that, over centuries, depletes the coarse fraction that beaches need to form. On a shingle coast living on a finite Pleistocene inheritance, attrition is therefore a slow drain on the budget as well as a transport indicator.
The Power's roundness scale runs in six classes from very angular (freshly plucked, score ~0.12) through sub-angular and sub-rounded to well-rounded (score ~0.84). In fieldwork you sample clasts at intervals along a beach, classify each against a roundness chart, and test whether mean roundness increases downdrift — a classic AO3 enquiry combining a measured index with a hypothesis about transport direction. The same data also distinguish marine shingle (well-rounded) from freshly delivered cliff debris (angular) and from fluvioglacial gravel (rounded but often with different lithology), so a single roundness survey can fingerprint a beach's sediment sources.
Chemical dissolution, dominant on carbonates. Limestone dissolves in slightly acidic seawater:
CaCO3+H2O+CO2→Ca(HCO3)2
Seawater is normally mildly alkaline (~pH 8.1) but local acidification (organic decay, CO₂) and biological activity drive solution; the rate rises with temperature and acidity. Produces solution pools, pitting and enlarged joints, and contributes to platform lowering on chalk and limestone coasts. Much of the "solution" on temperate carbonate coasts is in fact biologically mediated: grazing molluscs (limpets, periwinkles) rasp the rock as they feed on algae, boring organisms (piddocks, Polydora worms, boring sponges) tunnel into it, and the metabolic CO₂ and acids of the intertidal biofilm locally depress pH at the rock surface. This bioerosion can lower a platform at rates comparable to abrasion, and it is concentrated in the mid-intertidal where the grazing fauna is densest — one reason shore platforms often show a distinct biologically-controlled notch and a subhorizontal "Type B" form on sheltered carbonate coasts where mechanical wave erosion is weak.
flowchart TD
W[Wave energy + sediment load] --> HA[Hydraulic action: shock + air compression]
W --> AB[Abrasion: sediment scours rock]
W --> AT[Attrition: sediment self-reduces]
SEA[Seawater chemistry] --> CO[Corrosion: carbonate solution]
HA --> CLIFF[Cliff-base erosion → notch]
AB --> CLIFF
CO --> CLIFF
CLIFF --> COLLAPSE[Undercut → mass movement of face]
Mechanical (physical):
Chemical: carbonation (carbonic-acid solution of limestone), oxidation (iron minerals → weaker oxides, orange staining), hydrolysis (feldspar → clay in granite).
Biological: root prising, boring organisms (piddocks, Polydora worms) on soft rock, and organic acids from algae/lichen.
Gravity-driven downslope movement of weathered/undercut material, classified by mechanism, water content and material:
| Type | Material | Water | Speed | Typical cliff |
|---|---|---|---|---|
| Rockfall | Hard jointed rock | Low | Very fast | Vertical chalk/limestone (Beachy Head) |
| Topple | Columnar/vertically jointed | Low | Fast | Vertically jointed faces |
| Translational (planar) slide | Rock/debris | Variable | Moderate–fast | Seaward-dipping bedding planes |
| Rotational slump | Clay / soft rock | High | Slow–moderate | London Clay, till (Holderness) |
| Mudflow / earthflow | Saturated clay/silt | Very high | Moderate | Impermeable clay after rain |
| Soil creep / solifluction | Soil/regolith | Moderate | Very slow | Gentle clay slopes |
The rotational slump dominates on clay-rich cliffs and is governed by pore-water pressure: rainfall raises the water table, increasing pore pressure on the potential failure surface, reducing effective stress and shear strength until the factor of safety (ratio of resisting to driving forces) falls below 1 and the cliff fails along a curved (listric) surface. This is why clay-cliff failures cluster after wet periods and why cliff drainage is a key (if limited) management tool.
The slope-stability logic is worth stating formally because it underpins every clay-cliff case study. The factor of safety is:
F=shear stress (driving forces)shear strength (resisting forces)
The driving force is gravity acting on the slope mass; the resisting force is the material's shear strength, which the Mohr–Coulomb relationship gives as cohesion plus frictional resistance proportional to the effective normal stress (total stress minus pore-water pressure). The crucial term is that subtraction: as pore-water pressure rises, effective stress falls, frictional resistance falls, and so the resisting force shrinks even though nothing about the slope's geometry has changed. When F>1 the slope is stable; at F=1 it is on the point of failure; when F<1 it fails. This single inequality explains a remarkable amount: why failures lag rainfall by days to weeks (the time for water to raise the water table); why marine undercutting at the toe is so destabilising (it removes resisting mass from the foot of the failure surface, like pulling the support from under a stack of books); and why cliff-top development that adds soakaway, septic or irrigation water can trigger failure on a slope that was previously stable. Cliff drainage works by attacking the pore-pressure term directly — lowering the water table to restore effective stress — which is why it is often a more cost-effective intervention than ever-larger sea walls, though it cannot prevent failure where the toe is still being actively eroded by the sea.
| Rock | Retreat (m yr⁻¹) | Resistance | Reason |
|---|---|---|---|
| Granite | < 0.01 | Very high | Crystalline, low porosity, few open joints |
| Carboniferous limestone | 0.01–0.1 | High | Coherent, but solution-prone |
| Chalk | 0.1–0.5 | Moderate | Porous, jointed; fails by rockfall |
| Sandstone | 0.1–1.0 | Variable | Depends on cement and bedding |
| London Clay | 0.5–1.5 | Low | Slakes; slumps when wet |
| Glacial till (boulder clay) | 1–2 (locally >5 in storms) | Very low | Unconsolidated, no cementation |
The two-order-of-magnitude span (granite to till) is far wider than the variation in wave energy between sites, so lithology is usually the first-order control on retreat-rate contrasts.
Joints/faults give hydraulic action its leverage; bedding-plane dip controls failure style (seaward dip → planar slides); permeability contrasts create spring lines that lubricate slide planes (permeable chalk over impermeable Gault clay is a classic landslide setting).
Set by fetch, wind, water depth and orientation. Holderness faces ~800 km of North Sea fetch; deep water close inshore (little platform) lets waves reach the cliff with energy undissipated.
A wide, high beach dissipates wave energy before it reaches the cliff foot (negative feedback). Beach loss — by storms, longshore drift, terminal-groyne starvation or aggregate extraction — re-exposes the cliff. The presence or absence of a beach is often the proximate switch between slow and rapid retreat on a given lithology.
Defences protect locally but can starve downdrift frontages (positive feedback elsewhere); cliff-top drainage reduces pore pressure and slows slumping; cliff-top building and irrigation/septic discharge raise pore pressure and can accelerate failure. Human action can therefore appear on both sides of the ledger — stabilising one cliff (drainage, toe protection) while destabilising another (sediment starvation, added cliff-top water) — which is why "human activity" is never a single-signed control but a set of specific, locatable interventions whose effect depends entirely on which term of the system they alter. Marine-aggregate dredging offshore is a further, often-overlooked human factor: by lowering the nearshore seabed it allows larger waves to reach the cliff before breaking, indirectly accelerating retreat far from the dredge site.
The five controls do not act independently; they interact multiplicatively. A weak lithology with a seaward dip, a long fetch, no protective beach and a wet cliff-top will retreat catastrophically, whereas weak lithology alone — buffered by a wide beach and drained at the top — may be near-stable. Ranking the controls for a given coast (rather than in the abstract) is the analytical skill AQA tests: at Holderness lithology and beach absence dominate; on a chalk cliff structure (joint spacing) and basal undercutting dominate; on a London Clay cliff pore-water pressure dominates. The same five factors, reweighted by place, explain the entire two-order-of-magnitude span of UK retreat rates.
A cliff's profile records the balance of basal marine erosion against sub-aerial face processes.
Dip control on profile: horizontal strata → stepped/vertical; landward dip → steep/overhanging (base undermined, rockfall); seaward dip → lower-angle, planar-slide-prone; oblique dip → asymmetric. Reading a cliff profile back to its dominant process is a frequent AO2 demand.
The deepest idea in this lesson is that marine and sub-aerial processes are not two independent attackers but a single coupled feedback system mediated by the cliff-foot sediment budget. The coupling runs through the debris that mass movement deposits at the cliff base:
flowchart LR
RAIN[Rainfall raises pore pressure<br/>F drops below 1] --> SLUMP[Rotational slump /<br/>mudflow delivers debris to toe]
SLUMP --> APRON[Debris apron protects cliff base<br/>negative feedback: erosion slows]
APRON --> REMOVE[Marine processes remove the apron<br/>over weeks to months]
REMOVE --> STEEPEN[Cliff toe re-exposed and re-steepened<br/>F driven down again]
STEEPEN --> RAIN
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