Discordant and concordant coastlines

Coastal landscapes vary based on the arrangement of rock types relative to the shoreline. This arrangement influences how erosion shapes the coast, leading to distinct features like headlands, bays, and coves.

Discordant coastlines and headlands and bays

A discordant coastline occurs where bands of alternating hard rock and soft rock run at right angles to the shoreline. Hard rock resists erosion more than soft rock, which is less resistant and erodes more easily.

This setup leads to the formation of headlands and bays:

• Waves erode the less resistant soft rock more quickly, creating indented bays with gentler slopes.
• The more resistant hard rock erodes more slowly, protruding into the sea as headlands.

Once formed, headlands experience faster erosion due to greater exposure to waves and a process called wave refraction, where waves bend and concentrate energy on the headland sides.

Headlands also face higher wave heights compared to bays:

• As waves approach a headland, they slow down, causing water to build up behind the wave crest and increasing wave height, which boosts erosive power.
• This results in steep cliff faces on headlands, along with features such as arches and caves.
• In contrast, the curved shape of bays spreads out wave energy, reducing wave height and leading to less erosion over time.

As a result, the differences between headlands and bays may lessen gradually, as headlands erode faster than bays.

Concordant coastlines and coves

A concordant coastline features bands of rock running parallel to the shoreline, often with layers of varying resistance.

This parallel arrangement can lead to the formation of coves:

1. Waves first breach the outer layer of more resistant rock, such as limestone, exposing the less resistant inner rock, such as shale.
2. The softer inner rock erodes at a faster rate, widening the opening into a cove.
3. Erosion slows when waves reach the next layer of harder rock, limiting the cove's inland expansion.

Coves typically have a rounded shape with a beach at the back, formed by deposited sediments.

Dalmatian and haff coastlines

Certain coastal features arise from specific geological and depositional processes, often influenced by changes in sea level or sediment buildup.

Dalmatian coastlines

Dalmatian coastlines develop in areas where valleys run parallel to the coast and become flooded due to rising sea levels.

Formation of Dalmatian coastlines:

• These valleys form through tectonic folding, which creates anticlines (upward-sloping rock layers) and synclines (downward-sloping rock layers).
• As sea levels rise, the valleys flood, leaving elongated islands parallel to the shoreline.

This results in a coastline with numerous parallel islands, separated by narrow channels of water.

Haff coastlines

Haff coastlines form where deposits of sand and other materials build up parallel to the shoreline on offshore bars. Lagoons, which are shallow bodies of water, develop in the spaces between the bars and the shoreline.

This type of coastline is characterised by long, straight features with enclosed water bodies.

Geological structures influencing coastal morphology

The shape and features of coastlines, known as coastal morphology, are shaped by underlying geological structures. These include rock types, layers, and deformations caused by natural processes.

Rock strata and resistance

• Most coastlines consist of different rock types arranged in strata, which are horizontal layers formed over time.

• Hard, resistant rocks like granite or basalt erode slowly and form prominent features.

• Soft, less resistant rocks like clay erode quickly, leading to recessed areas.

Deformation through folding, dipping, and faulting

Tectonic activity causes strata to deform in several ways.

Types of deformation:

• Folding - Bends and crumples the layers.
• Dipping - Creates an angle of inclination from the horizontal.
• Faulting - Cracks or weaknesses where strata shift out of alignment, making them more prone to erosion as water can flow between layers.

These changes expose strata to varying pressure, influencing erosion rates.

Joints and bedding planes

During formation or drying processes, rocks develop splits.

Types of splits:

• Joints - Divide rocks vertically into blocks.
• Bedding planes - Separate rocks horizontally into layers.

How geological structure affects cliff profiles

Cliff profiles refer to the shape and slope of coastal cliffs, determined by rock resistance, coastal energy levels, and the orientation of rock strata relative to the sea.

High-energy coastlines with strong waves lead to more rapid erosion, while low-energy areas erode more slowly.

Cliff profiles based on strata dip

Sedimentary rocks are initially deposited horizontally, but tectonic folding and faulting alter their dip.

This creates different profiles:

• Horizontal dip - Waves erode softer rocks within the strata, forming notches at the base. If a notch grows too large, it destabilises the overlying rock, leading to a rockfall.
• Landward dip (away from the sea) - Strata angle downwards inland, preventing undercutting by waves and resulting in very stable cliffs with minimal erosion.
• Seaward dip (towards the sea) - Strata angle towards the water, allowing waves to undercut resistant rock and cause mass movement.
  • Gentle seaward dip leads to large wedge-shaped sections breaking away.
  • Steep seaward dip causes large slabs to slide down along the dip angle.

Micro-features in cliff profiles

Cliff profiles may include small-scale erosional or weathering features, such as notches or small caves. Their positions depend on fractures like joints or bedding planes within the rock face and strata.

The example of the Glamorgan Heritage Coast

The Glamorgan Heritage Coast provides a real-world illustration of how geological structure and coastal processes interact to shape landscapes.

Key features of the Glamorgan Heritage Coast

• This 14-mile stretch of high-energy coastline in South Wales faces the Bristol Channel, which channels large, destructive waves from the south-west.

• It features a discordant coastline with high vertical cliffs made of sedimentary rocks, including Carboniferous limestone, sandstone, shale, and mudstone.

• Headlands form at locations like Witches Point and Nash Point, while sandy bays occur near Dunraven and Merthyr Mawr.

• Extensive wave-cut platforms, exposed areas of original limestone strata, are visible, especially around Nash Point, where softer shale layers have been eroded away.

• The coastline is prone to cliff falls due to its eroded nature.

• The Southerndown Coast section is designated as a Site of Special Scientific Interest.