Coastal Physical Processes

2. Geological Structure and Rock Type

You can snap a dry twig easily, but try snapping a thick oak branch. The coast responds to wave energy in a very similar way depending on its lithology (rock type) and geological structure.

• Resistant rocks (like granite, limestone, and chalk) erode slowly, forming steep, high cliffs.
• Less resistant or unconsolidated rocks (like clay and sands) erode quickly, forming gentle slopes that are highly prone to mass movement.

Coastlines are classified by how their strata align with the sea:

• Discordant Coastline: Rock strata run perpendicular (at right angles) to the shore. This layout leads to the formation of headlands and bays due to differential erosion. For example, at Swanage Bay, the less resistant Wealden Clay forms the bay, while the resistant Chalk and Limestone form the protruding headlands.
• Concordant Coastline: Rock strata run parallel to the shore. This leads to the formation of coves.

Worked Example: The Formation of a Cove

1. Waves exploit a fault or joints in the highly resistant outer rock layer (e.g., Portland Limestone at Lulworth Cove).
2. Marine erosion breaches this outer resistant layer.
3. Waves reach the softer rock behind (e.g., Wealden Clay) and erode it rapidly.
4. Wave Refraction causes the waves to fan out once inside, creating a distinct horseshoe-shaped cove.

3. Coastal Erosion, Transport, and Deposition

A castle wall under constant siege is a great way to think about marine erosion; the sea uses four specific mechanisms to batter the cliffs.

• Hydraulic Action: Waves force water and air into cracks in the rock. As the wave retreats, the highly compressed air expands explosively, widening the crack.
• Abrasion: Also known as corrasion. The sea hurls stones and pebbles against the cliff face, acting like sandpaper to wear it away.
• Attrition: Rocks and pebbles carried by the sea collide with each other, gradually becoming smaller and rounder.
• Solution (Corrosion): Seawater chemically dissolves minerals in specific rocks, such as limestone or chalk.

Once eroded, the sea transports this material using four methods:

• Traction: Large boulders are rolled along the seabed.
• Saltation: Small pebbles are bounced along the seabed.
• Suspension: Fine silt and clay are carried within the water.
• Solution: Dissolved minerals are carried invisibly.

Material moves along the coast via Longshore Drift (LSD). The prevailing wind pushes the swash up the beach at an angle, while gravity pulls the backwash straight down at a $90^\circ$ angle. This creates a zig-zag movement of sediment. Deposition occurs when wave energy drops below the level of friction, such as in sheltered bays where constructive waves dominate.

The Cave-Arch-Stack-Stump Sequence:

1. A structural weakness (a joint or fault) is repeatedly attacked by hydraulic action and abrasion.
2. The crack widens into a cave.
3. The cave erodes completely through the headland to form an arch.
4. Sub-aerial processes weaken the arch roof from above, while the base is undercut by the sea.
5. The roof collapses due to gravity, leaving an isolated pillar called a stack.
6. The stack is further undercut at the base and collapses to form a stump.

4. Sub-aerial Processes

The sea isn't the only thing destroying the coast; attacks also come from the land and the sky above through sub-aerial processes.

Weathering is the breakdown of rock in-situ (without movement):

• Mechanical (Freeze-Thaw): Water enters cracks and freezes, expanding by approximately $9\%$. Repeated freezing and thawing shatters the rock.
• Mechanical (Salt Crystal Growth): Evaporating seawater leaves salt crystals behind, which grow and exert massive pressure on the rock.
• Chemical (Carbonation): Acidic rainwater (with a $pH \approx 5.6$) reacts with calcium carbonate in limestone to form soluble calcium bicarbonate.
• Biological: Plant roots widen cracks, and burrowing animals (like rabbits or piddocks) physically weaken the cliffs.

Mass Movement is the downhill movement of this weathered material under gravity:

• Rockfall: Fragments break from steep cliffs (often triggered by freeze-thaw) and collect as scree at the base.
• Slumping: Also known as rotational slip. This occurs on a concave or curved slip plane. It is highly common where permeable sand sits on top of impermeable clay; water saturates the sand, adding weight and lubricating the boundary until the cliff slumps.

5. Assessing the UK's Weather and Climate

Why do some stretches of the UK coast crumble by meters in a single week while others haven't changed in centuries?

• Prevailing Winds: In the UK, these blow from the South-West. They bring moist air across a massive fetch, generating high-energy destructive waves that accelerate erosion.
• Storms and Surges: High-frequency low-pressure systems (depressions) cause storm surges. A $1\text{ mb}$ drop in air pressure causes a $1\text{ cm}$ rise in sea level. During early 2022 storms, Spurn Head lost over 1 metre of coast in just three weeks.
• Rainfall: Prolonged heavy rain heavily saturates cliffs. This increases pore water pressure, greatly reducing internal friction and triggering slumping.

Evaluating Climate vs. Geology: To understand coastal retreat, you must compare the impact of climate against geological factors. The UK's weather and climate provide the energy for erosion—storms and prevailing winds dictate the power of the waves, while heavy rainfall triggers mass movement. However, the rate of retreat is ultimately controlled by geology. A severe winter storm will rapidly erode a coastline made of unconsolidated boulder clay (like Holderness), but the exact same storm will have almost no immediate impact on resistant granite (like Cornwall). Therefore, while weather accelerates the physical processes, geological structure and lithology are the most significant factors in determining a coastline's vulnerability.