Landforms of high-energy coasts

Overview

Blowhole and how does it form and how the geological structure of a cliff affects its profile.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Why do headlands and bays form along a coastline, and what kind of coast is needed?

   Model answer

   They form by differential erosion where bands of rock of different resistance meet the sea, most obviously on a discordant coast where the geology runs at an angle to the coastline so that hard and soft bands are exposed alternately along the shore. The less resistant rock, such as clay or shale, is eroded more rapidly and is cut back to form a bay, while the more resistant rock, such as limestone, chalk or granite, is left projecting seawards as a headland. Once the irregularity exists, wave refraction reinforces it in the short term by concentrating energy on the headland and encouraging deposition of a beach in the bay, though over the long term that same process tends to straighten the coast again.

2. 2

   Explain the formation of a cliff and a wave-cut platform as a sequence.

   Model answer

   Wave energy is concentrated at the base of a rocky shore between high and low water. Hydraulic action forces air and water into joints and abrasion grinds at the rock, cutting a wave-cut notch into the cliff foot at around the high water mark. As the notch is deepened, the rock above it is left unsupported and overhangs. Subaerial weathering and mass movement weaken the face from above, and eventually the overhang collapses under its own weight. The debris is broken up by attrition and removed by the waves, and the process begins again on the newly exposed face, so the cliff retreats landward while remaining steep. Left behind at the base is a gently seaward-sloping expanse of rock, the wave-cut or shore platform, marking the ground over which the cliff has retreated, typically exposed at low tide and often scarred with rock pools and abrasion grooves.

3. 3

   Why does a wave-cut platform stop growing beyond a certain width?

   Model answer

   Because the platform itself protects the cliff. As the platform widens, waves have to travel across a greater length of shallow water before they reach the cliff foot, and they lose an increasing proportion of their energy to friction over that distance. Beyond roughly half a kilometre so little energy remains that the notch is no longer cut effectively and retreat slows almost to a halt. The feature is therefore self-limiting, and this is another example of negative feedback at the coast.

4. 4

   Explain the sequence by which a crack in a headland becomes a cave and then an arch.

   Model answer

   Wave attack on a headland is concentrated at any line of weakness, such as a joint, fault or bedding plane. Hydraulic action forces water and compressed air into that weakness and abrasion widens it, so the crack enlarges into a cave, deepening along the line of weakness. The cave is enlarged by continued erosion and by the compression of air in its roof at each wave. If the headland is narrow and the cave cuts right through it, or if two caves developing on opposite sides of the headland meet back to back, the sea passes through the gap and an arch is created, with the roof of rock left standing above the opening. The arch is then widened at its base by marine erosion and weakened from above by weathering.

5. 5

   How does an arch become a stack and then a stump?

   Model answer

   Erosion continues to widen the base of the arch while weathering, rainwater and mass movement weaken the roof from above. Eventually the span becomes too wide and too weak to support its own weight, and the roof collapses. The seaward pillar is left detached from the headland as an isolated column of rock rising from the sea, which is a stack. The stack is then attacked from all sides by wave action, undercut at its base to form a notch, and progressively reduced in height until only a low remnant is left, usually visible at low tide and submerged at high tide, which is a stump. The whole sequence, from crack to cave to arch to stack to stump, is the classic evidence for a retreating headland.

6. 6

   Why do stacks and stumps eventually disappear?

   Model answer

   Because a stack is exposed on all sides and has no protection from a beach or from the headland behind it. Waves refract round it and attack from every direction, hydraulic action exploits its joints, and abrasion cuts a notch all the way round its base until the top collapses. Once reduced to a stump it lies mostly within the tidal zone, so it is subject to continual wave attack and eventually to complete removal, leaving the debris as part of the shore platform. Their disappearance is why the whole assemblage of headland features represents a stage in a retreat, not a permanent landscape.

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