Coastal Landforms

Erosional landforms:

• Headlands and bays
• Cliffs (and wave-cut platforms)
• Caves, arches, stacks, stumps

Depositional landforms:

• Beaches
• Spits and bars

• Spit — a long, narrow ridge of sand or pebbles deposited by longshore drift, projecting from the coast into open water (e.g. Spurn Head, Humber, UK).

• Bar — a ridge of sand or pebbles that extends across a bay, sometimes connecting two headlands or enclosing a lagoon (e.g. Slapton Sands, Devon).

• Beach — a depositional area along the coast where sand or pebbles accumulate, built by constructive waves and longshore drift.

On a DISCORDANT coastline, rock bands run PERPENDICULAR to the coast — alternating hard and soft rocks meet the sea at right angles.

Differential erosion acts on the two rock types. The SOFTER rock (clay, soft sandstone) is eroded FASTER by hydraulic action, abrasion and solution. It RETREATS inward → forms a curved BAY. The HARDER rock (chalk, limestone, granite) is more RESISTANT → it juts out as a HEADLAND.

Once formed, wave REFRACTION concentrates wave energy on the headland (waves bend around it from both sides → focus energy on the exposed tip) while leaving bays sheltered. This INTENSIFIES the difference over time: headlands are attacked while bays accumulate beach sediment.

Named example: Swanage, Dorset (UK). Bands of hard CHALK and soft Wealden CLAY run perpendicular to the coast. Chalk forms the headland of Old Harry Rocks; clay has retreated to form Studland Bay with its sandy beach. The same process at smaller scale produced the chalk headland at Ballard Down.

The sequence develops over time on a headland made of resistant rock with lines of weakness (joints, faults, cracks).

1. Cave. Hydraulic action + abrasion enlarge a line of weakness in the headland → form a CAVE that extends inwards.

2. Arch. Continued erosion through the headland from BOTH SIDES eventually breaks through, leaving an opening through the headland — an ARCH. Durdle Door (Dorset, UK) is a famous limestone arch.

3. Stack. Weathering (freeze-thaw, salt) weakens the top of the arch; continued wave erosion at the base undermines it; eventually the arch ROOF COLLAPSES → leaves an isolated pillar of rock offshore = STACK. Old Harry (Dorset, UK) is a chalk stack created by collapse of an earlier arch.

4. Stump. Continued wave erosion + weathering + mass movement reduce the stack to a low, often-submerged remnant — a STUMP. Old Harry's wife (Dorset, UK) is a stump nearby Old Harry; visible mostly at low tide.

The sequence shows how erosional processes progressively destroy a headland over thousands to tens of thousands of years.

A SPIT is a long, narrow ridge of sand or pebbles projecting from the coast into open water.

Step 1 — longshore drift. Waves approach the coast at an ANGLE set by the prevailing wind. The swash carries sediment diagonally UP the beach; the backwash returns it STRAIGHT DOWN under gravity. The zig-zag movement transports sediment ALONG the coast in the direction of the prevailing wind.

Step 2 — change in coast direction. Where the coastline changes direction sharply (e.g. at a river estuary), the longshore-drift current continues OUT into the open water in the original direction. Sediment is deposited just beyond the corner.

Step 3 — spit builds out. The deposit grows as a long ridge of sand or pebbles, EXTENDING OUT INTO THE SEA along the original direction of longshore drift. The spit grows year by year as more sediment arrives.

Step 4 — recurve at the end. If a SECONDARY wave direction is present (e.g. occasional waves from a different angle), the free end of the spit curves around — forming a RECURVED SPIT.

Step 5 — sheltered area behind. The spit creates a SHELTERED ZONE on its landward side. Fine sediment + organic matter accumulate in the still water → MUDFLATS and SALT MARSHES develop in this sheltered area. Wildlife (waders, breeding terns, seals) often colonise.

Named example — SPURN HEAD (East Yorkshire, UK). Spurn Head is a 5-km curved spit at the mouth of the Humber Estuary. It is formed by southward longshore drift along the Holderness coast — the rapidly eroding Holderness cliffs (~2 m/yr) supply boulder-clay sediment that is carried south and deposited at the bend where the coast turns west into the Humber Estuary. The spit shelters the estuary, the town of Spurn and Sunk Island salt marshes behind it. The spit has been reshaped by major storms (a major breach in 2013) and is currently managed as a dynamic natural feature.

Named coastline: Dorset Jurassic Coast (UK), a UNESCO World Heritage Site.

The Dorset coast displays both erosional and depositional landforms produced by the interplay of marine processes acting on alternating hard and soft rocks.

Erosional landforms. At Swanage, bands of hard chalk and soft Wealden clay run perpendicular to the coast (discordant). Differential erosion produces:

• Old Harry Rocks — a chalk headland with sea STACKS (Old Harry) and STUMPS (Old Harry's wife). These formed when a CAVE in the chalk became an ARCH, the arch collapsed leaving a STACK, and continued erosion reduced the stack to a STUMP.
• Studland Bay — soft Wealden clay retreated faster, forming a wide curved bay with a sandy beach.

At Lulworth (concordant), the outer Portland limestone band was breached, allowing waves to erode the soft Wealden clay behind into the circular Lulworth Cove — a near-perfect example of concordant-coast erosion.

Depositional landforms.

• Studland Bay has a SAND BEACH and well-developed SAND DUNES (with marram grass), built by constructive waves + longshore-drift deposition.
• Chesil Beach (just west of Lulworth) is one of the world's largest TOMBOLO BARS — a 29-km long ridge of pebbles connecting the mainland to the Isle of Portland. Constructive waves and a swash-aligned beach process built it over thousands of years; pebbles are graded from west (smaller) to east (larger) — a unique distribution.
• Slapton Sands (Devon, nearby) is a classic example of a BAR enclosing a freshwater lagoon (Slapton Ley).

Working together. Erosional landforms (Old Harry, Durdle Door, Lulworth Cove) SUPPLY sediment to the coastal system through cliff retreat. Longshore drift then carries this sediment along the coast to depositional landforms (Chesil Beach, Studland Bay beaches). The two process families are INTERDEPENDENT: erosion produces the sediment that depositional landforms need.

The Dorset Jurassic Coast is UNESCO-listed precisely because it shows this whole sequence in compact form — a "geological textbook in 3D".

The erosional landforms of a coastline are not independent — they form in a SEQUENCE that traces the long-term retreat of a headland under wave attack.

Stage 1 — discordant coast establishes headland. On a discordant coast, rock bands run perpendicular to the sea. Soft rock is eroded faster → forms a BAY. Hard rock RESISTS → projects as a HEADLAND. The starting condition for the rest of the sequence requires a resistant rock headland with lines of weakness (joints, faults).

Stage 2 — cliff develops on the headland. As waves attack the headland, hydraulic action + abrasion cut a wave-cut NOTCH at the high-water mark on its exposed sides. Undercutting weakens the rock above. Eventually, the rock collapses → the cliff face retreats landward → a CLIFF is established. This cliff is the foundation of subsequent landform development.

Stage 3 — wave-cut platform develops. As the cliff retreats, the rock at the old cliff base remains as a flat, gently-sloping platform exposed at low tide → WAVE-CUT PLATFORM. The platform widens as cliff retreat continues. Eventually, very wide platforms cause waves to lose energy before reaching the cliff, slowing further retreat.

Stage 4 — caves form on the cliff face. Lines of weakness in the cliff face (joints, faults) are exploited by hydraulic action + abrasion. The waves enlarge these into CAVES extending into the headland.

Stage 5 — arch forms. Continued erosion can extend a cave THROUGH the headland to the other side — forming an ARCH. The arch is supported above by the cap rock. Durdle Door (Dorset) is a famous limestone arch formed this way.

Stage 6 — stack forms. Weathering attacks the top of the arch (freeze-thaw, salt-crystal growth); continued wave erosion undermines its base; eventually the arch ROOF COLLAPSES → leaving an isolated pillar of rock offshore = STACK. Old Harry (Dorset) is a chalk stack created by collapse of a former arch.

Stage 7 — stump. Continued wave erosion + weathering + mass movement reduce the stack to a low, often-submerged remnant = STUMP. Old Harry's wife (Dorset) is the textbook example, visible mostly at low tide.

Stage 8 — eventual disappearance. Given enough time, even the stump is reduced to a wave-cut platform extension. The cliff continues retreating landward in the same direction.

Synthesis — single system. All eight landforms form part of a single evolutionary SEQUENCE driven by:

• ROCK TYPE (hard rock with lines of weakness).
• MARINE PROCESSES (hydraulic action, abrasion, solution).
• WEATHERING (freeze-thaw, salt-crystal growth).
• MASS MOVEMENT (rockfalls, sliding).
• TIME (thousands to tens of thousands of years).

The Dorset Jurassic Coast contains examples of EVERY stage simultaneously — different headlands are at different points in the sequence. Old Harry was once an arch; today its 'wife' is the stump that follows. Lulworth Cove shows what happens when concordant rock structure is breached instead. The whole coast is a 'time-lapse' record of cliff retreat.

This sequence is a key Pearson 4GE1 testable framework — examiners value candidates who can trace it stage by stage with named examples.

Erosional and depositional coastal landforms can seem like opposites — one destroys material, the other builds it up. But examined more closely, they are inseparable parts of a single coastal SYSTEM where sediment flows continuously from one to the other.

The case for them being products of the same system.

1) Erosion supplies sediment to deposition. Cliffs being eroded (by hydraulic action + abrasion) release rock fragments. These are reduced by attrition during transport, becoming sand and pebbles. Longshore drift then carries this sediment along the coast to depositional landforms (beaches, spits, bars). Without erosion, depositional landforms would starve of sediment. The Holderness coast (UK) loses ~2 m/year to erosion; this sediment is transported south by longshore drift to build SPURN HEAD (5 km spit at the Humber Estuary). Without Holderness erosion, Spurn Head would not exist.

2) Erosional and depositional landforms COEXIST on the same coast. The Dorset Jurassic Coast shows both simultaneously — Old Harry Rocks (erosional stacks) sits alongside Studland Bay (depositional sandy beach + dunes); Chesil Beach (depositional 29-km bar) sits along the same Jurassic coast as Durdle Door (erosional arch). The two landform families are intermingled.

3) Wave processes work both ways. Hydraulic action (erosional) and longshore drift (depositional transport) are driven by the SAME waves. Constructive waves deposit; destructive waves erode — but on the same beach the balance varies seasonally. A beach is BOTH a depositional landform (built by sediment) AND part of the cliff defense system (it absorbs wave energy, slowing cliff retreat).

4) Geology connects them. The same rock types that erode to produce sediment also determine which depositional landforms can form. Holderness boulder clay → fine silt → deposited in Spurn Head + downstream salt marshes. Cornwall granite → coarse fragments → deposited as sandy/pebbly beaches at the bottom of cliffs.

5) Time scales overlap. Both erosional and depositional landforms develop over similar time scales (centuries to millennia). The same wind regime that drives longshore drift also drives wave attack on cliffs.

The case for them being distinct phenomena.

1) They are driven by DIFFERENT energy balances. Erosion happens where wave energy EXCEEDS what is needed to remove material; deposition happens where energy is INSUFFICIENT to keep material moving. The threshold is different.

2) They produce visually different landforms. Cliffs vs beaches are obviously different in appearance.

3) Different management responses. Erosion threatens infrastructure (Holderness cliffs); deposition tends to be valuable (Brighton beach economy).

4) Local sediment budgets can be CLOSED. A sheltered bay can have its own beach independent of cliff erosion elsewhere — fed mostly by river sediment or in-bay weathering.

The fundamental truth — coastal SYSTEM.

The coast is best understood as a SEDIMENT BUDGET:

• Inputs: cliff erosion, river sediment, biogenic sources (shell fragments, coral).
• Stores: beaches, dunes, spits, bars, mudflats.
• Transfers: longshore drift, waves.
• Outputs: offshore deposition, deep ocean.

If inputs > outputs, the coast is BUILDING UP. If inputs < outputs, the coast is RETREATING. Most natural coasts are roughly in balance; many human-modified coasts (with sediment-trapping groynes, dam-blocked rivers, or dredged beaches) are now in DEFICIT.

Climate change perspective. Sea-level rise is shifting the balance: it increases erosional pressure on cliffs while drowning depositional landforms (mangroves, salt marshes, low beaches). The coast as a system is being squeezed.

Judgement. Erosional and depositional landforms ARE products of the same coastal system. They share sediment, drive each other, coexist in space, and respond to the same wave + climate forcings. The Dorset Jurassic Coast, Holderness + Spurn Head, and almost every natural coastline shows this. They look different — but the boundary between them is one of energy and sediment supply, not of fundamental nature. The most effective coastal management treats the coast as ONE system and manages the SEDIMENT BUDGET across all landforms rather than addressing erosion and deposition separately. This is the basis of modern Shoreline Management Plans (spec 2.3c).