Geological Structure and Coastal Landscapes

The shape and character of any coastline is fundamentally controlled by its geology — the type of rock, the arrangement of rock layers, and the structural features (folds, faults, joints, bedding planes) that determine how the rock responds to marine and sub-aerial processes. This lesson addresses Edexcel A-Level Geography Enquiry Question 1: Why are coastal landscapes different and what processes cause these differences? with a focus on the role of lithology and geological structure.

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Lithology: Rock Type and Resistance

Lithology refers to the physical and chemical characteristics of a rock — its mineral composition, grain size, cementation, porosity and permeability. These properties determine how resistant the rock is to erosion and weathering, and therefore how quickly the coastline retreats and what landforms develop.

Resistant vs Weak Rocks

Rock Type	Resistance	Characteristics	Coastal Expression
Granite	Very high	Crystalline igneous rock; very hard; low porosity; resistant to chemical weathering except carbonation of feldspars	Rugged cliffs, headlands (Land's End, Cornwall)
Carboniferous Limestone	High	Well-cemented; but soluble in weak acids (carbonation); jointed and bedded	Steep cliffs with caves, arches; karst features (Great Orme, North Wales)
Chalk	Moderate–High	Relatively soft calcium carbonate but pure and well-cemented; permeable through joints and bedding	Vertical white cliffs (Seven Sisters, Beachy Head, Old Harry Rocks)
Old Red Sandstone	Moderate	Cemented quartz grains; variable depending on cement type	Moderate cliffs and stacks (Orkney, north Scotland)
Shale / Mudstone	Low	Fine-grained, thinly bedded; prone to wetting and drying; absorbs water and expands	Rapid erosion, slumping (Holderness, Yorkshire)
Glacial till (boulder clay)	Very low	Unconsolidated mix of clay, sand, gravel and boulders; highly permeable; no cementation	Very rapid cliff recession, up to 2 m/year (Holderness)

Porosity and Permeability

These properties control how water interacts with rock, which directly affects weathering and slope stability:

• Porosity is the proportion of void space within the rock. Highly porous rocks absorb water, which adds weight, promotes chemical weathering and reduces shear strength.
• Permeability is the ability of water to pass through the rock. Primary permeability depends on interconnected pore spaces (e.g., sandstone). Secondary permeability depends on joints, faults and bedding planes (e.g., chalk, limestone).

Impermeable rocks such as granite and clay force water to flow over the surface as runoff, leading to surface erosion but relatively stable slopes. Permeable rocks allow water to infiltrate, which can lubricate failure planes within the rock mass and trigger landslides and slumps.

Exam Tip: When explaining differential erosion, always link rock type to specific properties (hardness, porosity, permeability, solubility, jointing). Avoid vague statements like "soft rock erodes faster" — explain why it erodes faster by referencing its physical properties.

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Concordant and Discordant Coastlines

The alignment of rock strata relative to the coastline is a fundamental control on coastal morphology.

Discordant Coastlines

On a discordant coastline, the rock strata run perpendicular (at right angles) to the coast. This means that bands of rock with different resistance are exposed to wave attack simultaneously. The weaker rocks are eroded more rapidly, forming bays, while the more resistant rocks project outward as headlands. This produces the classic headland-and-bay topography.

Case Study: The Dorset Coast (Swanage Bay to Studland)

The Dorset coast between Swanage and Studland is a textbook example of a discordant coastline. From east to west, the following sequence of rock strata meets the coast at right angles:

1. Chalk (resistant) — forms Ballard Point and the Foreland (Old Harry Rocks stacks)
2. Wealden clays and sands (weak) — eroded to form Swanage Bay
3. Purbeck and Portland limestone (very resistant) — forms Peveril Point and Durlston Head
4. Kimmeridge Clay (weak) — forms Kimmeridge Bay

The differential erosion of these alternating bands of resistant and weak rock has created a coast with projecting headlands and intervening bays. This pattern illustrates how lithology controls planform shape.

graph LR
    subgraph "Discordant Coast — Dorset (plan view)"
        A["Sea"] --- B["Chalk headland<br/>(Ballard Point)"]
        A --- C["Swanage Bay<br/>(weak clays)"]
        A --- D["Limestone headland<br/>(Peveril Point)"]
        A --- E["Kimmeridge Bay<br/>(weak clay)"]
    end
    style B fill:#c0c0c0,color:#000
    style C fill:#ffffcc,color:#000
    style D fill:#c0c0c0,color:#000
    style E fill:#ffffcc,color:#000

Concordant Coastlines

On a concordant coastline, the rock strata run parallel to the coast. This means that the outermost band of rock forms a relatively uniform coastline. If the outer band is resistant, it acts as a protective barrier; if it is breached, the sea can rapidly erode the weaker rock behind, forming a cove.

Case Study: Lulworth Cove, Dorset

Lulworth Cove is the most frequently cited example of a concordant coastline breach. The sequence of rock, from the sea inland, is:

1. Portland limestone (resistant outer barrier) — forms the narrow entrance to the cove
2. Purbeck limestone (moderately resistant) — partially eroded
3. Wealden clays and greensands (weak) — rapidly eroded to form the wide, circular cove
4. Chalk (resistant) — forms the steep back wall of the cove, preventing further inland erosion

The cove was created when the sea breached the resistant Portland limestone barrier (possibly through a fault or joint weakness). Once through, wave energy rapidly eroded the weak clays behind, scooping out the characteristic circular shape. Erosion then slowed as the waves reached the chalk ridge at the back.

Feature	Concordant Coast	Discordant Coast
Rock alignment	Parallel to coast	Perpendicular to coast
Typical planform	Relatively straight (unless breached)	Headlands and bays
Classic example	Lulworth Cove, Dorset	Swanage Bay, Dorset
Key process	Breach of outer resistant band leads to cove formation	Differential erosion of alternating rock types

The Dalmatian Coast