Erosional Landforms (Caves, Arches, Stacks, Stumps)

Formation of Caves

Caves form along coastlines where there are weaknesses in the rock, such as joints or faults. These weaknesses allow waves to attack the rock more easily. The main process involved is hydraulic action, where the force of waves compresses air in cracks, causing the rock to break apart over time.

As waves repeatedly hit the rock, these cracks enlarge and deepen, eventually forming a hollow cave. This process is most effective on softer rocks or where the rock has many natural weaknesses. Other processes like abrasion and solution can also contribute to cave formation.

For example, at the Jurassic Coast in Dorset, caves have formed in limestone cliffs where wave energy exploits joints in the rock.

Learning example: If a wave hits a crack in the rock with great force, the trapped air compresses and expands, gradually breaking the rock apart. Over years, this enlarges the crack into a cave.

Development of Arches

An arch forms when a cave is eroded all the way through a headland. Waves continue to erode the cave from both sides until the two caves meet, creating a hole through the rock.

This process takes a long time and requires constant wave energy to enlarge the opening. The arch is a natural bridge of rock with water flowing beneath it.

For example, Durdle Door on the Dorset coast is a famous limestone arch formed by this process.

Learning example: Imagine a headland with caves on either side. Over time, the caves grow larger due to wave erosion until they join, forming an arch.

Formation of Stacks

A stack forms when an arch becomes unstable and collapses. The collapse leaves an isolated pillar of rock standing away from the coast.

Stacks are exposed to the sea and continue to be eroded by waves, especially at their base. Over time, they become thinner and weaker.

For example, Old Harry Rocks on the Dorset coast are stacks formed from chalk cliffs.

Learning example: When the roof of an arch falls due to erosion and gravity, the remaining rock column is called a stack.

Creation of Stumps

A stump is formed when a stack erodes further at its base and collapses. Stumps are often partially submerged at high tide, making them less visible.

The process of erosion continues with hydraulic action and abrasion, gradually wearing down the stack until only a low stump remains.

For example, some stumps can be seen near Old Harry Rocks, where the sea has worn down the stacks.

Learning example: A stack 9 metres tall erodes 1 metre at its base every 3 years. After 18 years, it will have lost $\frac{18}{3} \times 1 = 6\,\mathrm{m}$, leaving a stump 3 metres tall.