Geological Structure and Erosional Landforms

Several factors influence this resistance. Hard minerals like quartz make a rock physically tougher, while porous rocks that allow water to pass through (permeable) may suffer less surface runoff but more chemical weathering. Crucially, the presence of joints, faults, and bedding planes creates weak points that water can exploit.

When rocks of differing hardness are exposed to the same wave energy, differential erosion occurs. Destructive waves use powerful marine processes like hydraulic action (the explosive force of trapped air and water in cracks) and abrasion (flinging pebbles against the rock) to attack the softer material much faster than the hard rock. This unequal rate of retreat is the primary mechanism that shapes irregular coastlines. Note that some rocks like limestone also suffer from solution, where seawater chemically dissolves calcium carbonate minerals.

The Role of Geological Structure: Discordant and Concordant Coasts

If you slice a layered cake from the top down, you expose all the different fillings at once, but if you slice along the side, you only see the outer layer. The geological structure of a coast describes how layers of rock (strata) are arranged relative to the sea, which dictates the exact types of landforms that will develop.

A discordant coastline develops in areas where alternating layers of tough and weak rock meet the ocean at exactly $90^\circ$, running perpendicular to the shoreline. This alignment is the perfect recipe for forming a headland and a bay.

1. Initial Attack: Destructive waves continuously crash against a coastline featuring alternating bands of resistant (e.g., limestone) and less resistant (e.g., clay) rock running perpendicular to the sea.
2. Differential Erosion: The sea rapidly erodes the softer clay using hydraulic action and abrasion.
3. Formation of Inlets: The soft rock retreats significantly inland, creating a semi-circular inlet known as a bay.
4. Protrusion: The more resistant limestone bands erode much slower, leaving steep-sided ridges that protrude out into the sea as headlands.
5. Wave Refraction: As incoming waves encounter this newly irregular shape, they slow down and bend towards the protruding rocks in a phenomenon called wave refraction. This concentrates erosive energy onto the headlands while dissipating energy in the sheltered bays, leading to the deposition of sand and shingle beaches.

In contrast, a concordant coastline occurs where bands of differing rock types run parallel to the shore. This alignment typically creates a smooth, uniform cliff face because the sea only interacts with the tough outer layer of rock.

• If the outer layer is a tough, resistant rock, the coast remains relatively straight.
• However, if waves manage to breach a fault or weakness in this tough outer barrier, they can quickly access the softer rock hidden behind it.
• The sea then rapidly erodes the softer material in all directions (lateral erosion), hollowing out a horseshoe-shaped cove with a narrow entrance.

The Dorset Coast in the UK perfectly illustrates these structures. Along its discordant sections, the rapidly eroding soft clays and sands have formed Swanage Bay, which is neatly sandwiched between two protruding hard rock headlands: Ballard Point (made of chalk) and Durlston Head (made of limestone). On the same stretch of coastline, Lulworth Cove serves as a classic example of a concordant feature, where the sea has breached a tough barrier of Portland Limestone to hollow out the weaker Wealden Clay behind it.

How Structure and Lithology Shape Cliff Profiles

Why do some cliffs stand vertically like a brick wall, while others slope gently towards the beach? A cliff profile is the side-on, cross-sectional view of a cliff face. A cliff's overall shape is fundamentally controlled by a combination of its lithology and its dip, which is the angle at which the rock strata are tilted.

Bedding planes are the natural layers formed within sedimentary rocks. The exact angle at which these planes tilt heavily influences how the cliff will ultimately collapse:

• Horizontal dip: The strata lie entirely flat, creating highly unstable, near-vertical cliff faces (around $90^\circ$). Marine erosion heavily undercuts the base, while sub-aerial weathering attacks specific notches high up in the layers.
• Seaward dip (low angle, $<45^\circ$): The strata slope gently in the direction of the ocean. This creates a steep cliff face with dangerous rocky overhangs that are extremely unstable and likely to experience major collapse when the sea undercuts their base.
• Seaward dip (high angle, $>45^\circ$): The rock layers tilt steeply towards the water, creating a much gentler, sloping cliff face. These structures are extremely vulnerable to rock slides because entire slabs of rock can easily slip down the angled bedding planes via mass movement.
• Landward dip: The rock layers tilt backwards, pointing inland away from the ocean. Because gravity actively pulls any loose blocks back into the bulk of the cliff, these faces form very steep (typically $70^\circ$ to $80^\circ$) and structurally secure profiles that strongly resist mass movement, even though they still suffer from marine erosion at their base.

The physical hardness (lithology) also fundamentally changes the profile. Hard rock cliffs form towering, bare, and steep faces with large boulders scattered at their base. Soft rock cliffs form lower, gentler slopes (around $30^\circ$ to $45^\circ$) that are much smoother and frequently show signs of slumping (a rotational type of mass movement), leaving mud and sand at the foot of the cliff.

The Step-by-Step Mechanism of Cliff Retreat

1. Undercutting: Destructive waves concentrate their hydraulic action and abrasion at the high-tide mark, gouging out a hollow at the base of the cliff called a wave-cut notch.
2. Instability: As the notch grows deeper over time, the weight of the unsupported overhanging rock becomes too great to support itself. The rock's specific structural weaknesses (like its dip, faults, and joints) dictate exactly how it will fail.
3. Collapse: The cliff face eventually succumbs to gravity and collapses downwards via mass movement (such as a rockfall or slump).
4. Retreat: The cliff line recedes further inland, leaving behind a flat rocky expanse known as a wave-cut platform.