Changing Coastal Environments » Cave, Arch, Stack and Stump Formation

What you'll learn this session

Study time: 30 minutes

The processes involved in coastal erosion
The step-by-step formation of caves, arches, stacks and stumps
Key factors that influence the rate of coastal landform development
Real-world examples of these coastal features from the UK and globally
How human activities can impact these coastal landforms

Introduction to Coastal Erosion Landforms

Coastlines are dynamic environments that constantly change due to the relentless power of waves, weather and geological processes. One of the most fascinating aspects of coastal geography is how distinctive landforms are created through erosion. The sequence of cave, arch, stack and stump formation represents a classic example of how coastlines evolve over time.

Key Definitions:

Cave: A hollow area formed when waves erode a weakness in a cliff face.
Arch: A curved rock formation with an opening beneath, created when a cave erodes through a headland.
Stack: An isolated pillar of rock standing in the sea, formed when an arch collapses.
Stump: A low, worn-down remnant of a stack that has been further eroded by wave action.
Headland: A piece of land that extends into the sea, often made of more resistant rock.

🌊 Coastal Erosion Processes

Before we dive into the formation of these landforms, it's important to understand the main processes that cause coastal erosion:

Hydraulic action: The force of waves compressing air in cracks, causing the rock to break apart
Abrasion: Waves throw sand, pebbles and rocks against the cliff, wearing it away like sandpaper
Attrition: Rocks and pebbles in the water collide and break into smaller, rounder pieces
Solution/Corrosion: Seawater dissolves certain rocks, particularly limestone

🧪 What Makes a Coast Vulnerable?

Some coastlines erode faster than others. The rate of erosion depends on:

Rock type: Soft rocks like clay erode faster than hard rocks like granite
Rock structure: Joints, faults and bedding planes create weaknesses
Wave energy: Powerful storm waves cause more erosion
Weather conditions: Freeze-thaw weathering can weaken cliff faces
Human activities: Coastal defences can change erosion patterns

The Formation Sequence: From Cave to Stump

The transformation from a solid headland to a stump is a fascinating process that can take thousands of years. Let's explore each stage in detail:

Stage 1: Cave Formation

It all begins with a weakness in the cliff face. This could be a fault line, a joint, or simply an area of softer rock.

💧 Initial Weakness

Waves focus their energy on weaknesses in the rock. Hydraulic action forces water and air into cracks, gradually widening them.

⚒️ Erosion Intensifies

As the crack widens, abrasion becomes more effective. Waves hurl pebbles and sand against the back of the developing cave, accelerating erosion.

🕳️ Cave Deepens

Over time, the cave gets deeper and wider as erosion continues. The roof may also be weakened by weathering processes like salt crystallisation.

Stage 2: Arch Formation

When a headland has caves on opposite sides, continued erosion can lead to the formation of an arch.

🏹 Cave Breakthrough

As caves on opposite sides of a headland grow deeper, they may eventually meet in the middle. The moment of breakthrough creates a small hole that will gradually enlarge.

🌉 Arch Development

The hole widens as waves continue to erode the opening. The arch is supported by the remaining rock above, which forms a natural bridge. The size and shape of the arch depend on the rock type and structure.

Stage 3: Stack Formation

Arches don't last forever. The constant battering of waves, combined with the weight of the rock above, eventually leads to collapse.

💥 Arch Collapse

Over time, the arch becomes increasingly unstable. Weathering weakens the roof and during storms or particularly high tides, the arch may suddenly collapse. This dramatic event leaves behind a column of rock isolated from the mainland.

🗿 Stack Characteristics

A stack is a free-standing pillar of rock in the sea, close to the shore. It's completely surrounded by water, especially at high tide. Stacks can be quite tall and often become home to seabirds like gulls and puffins.

Stage 4: Stump Formation

The final stage in this sequence is the formation of a stump, as the stack itself is gradually worn down.

🔨 Continuous Erosion

The stack continues to face the full force of the waves. Hydraulic action and abrasion attack the base of the stack, potentially creating a wave-cut notch. This undermines the structure, making it unstable.

🥥 Final Form

Eventually, the stack is reduced to a low, rounded stump that may only be visible at low tide. Some stumps may be completely submerged, forming a hazard for ships known as a "reef".

Case Study Focus: Old Harry Rocks, Dorset, UK

Old Harry Rocks on the Jurassic Coast in Dorset provides a perfect example of the cave-arch-stack-stump sequence. The white chalk stacks stand at the eastern end of the Jurassic Coast World Heritage Site.

Old Harry: The name of the remaining stack, standing about 18m high
Old Harry's Wife: Was once a second stack nearby, but collapsed in 1896, leaving only a stump
Geology: Formed from chalk, a relatively soft rock that erodes more easily than other types
Future predictions: Geologists estimate that Old Harry itself may collapse within the next few hundred years

This location shows multiple stages of the formation sequence in one place, making it an excellent study site for coastal geomorphology.

Global Examples of Coastal Erosion Landforms

These dramatic coastal features can be found worldwide, each with unique characteristics based on local geology and wave conditions:

🏴󠁧󠁢󠁳󠁣󠁴󠁿 Durdle Door, UK

A magnificent limestone arch on Dorset's Jurassic Coast. The arch formed in 140-million-year-old Portland stone and is one of the UK's most photographed landmarks.

🇦🇺 The Twelve Apostles, Australia

A collection of limestone stacks off the Great Ocean Road in Victoria. Despite the name, there were only ever nine stacks and due to ongoing erosion, only eight remain today.

🇮🇸 Dyrhólaey Arch, Iceland

A massive black basalt arch formed by volcanic activity and subsequent erosion. The arch is large enough for small boats to sail through during calm conditions.

Human Impact and Management

While these landforms are created by natural processes, human activities can influence their development and preservation:

🏗️ Human Influences

People can affect coastal erosion in several ways:

Sea defences: Can prevent erosion in protected areas but may increase it elsewhere
Tourism: Footfall erosion on coastal paths can destabilise cliffs
Climate change: Rising sea levels and more frequent storms accelerate erosion
Dredging: Removing material from the seabed can change wave patterns

🛡️ Conservation Approaches

Many iconic coastal features are now protected:

World Heritage status: Recognises the geological importance of sites
Restricted access: Limits visitor numbers to reduce erosion
Managed retreat: Accepting natural processes rather than fighting them
Education: Helping people understand the value of these landforms

Summary: The Coastal Erosion Cycle

The transformation from headland to stump represents a perfect example of how coastlines are constantly changing. While this process occurs over thousands of years, it demonstrates the powerful geological forces that shape our planet.

Remember that this sequence (cave → arch → stack → stump) is just one example of coastal erosion. Different rock types, wave conditions and geological structures can create variations in how these landforms develop. Understanding these processes helps us appreciate the dynamic nature of our coastlines and the need to manage them sustainably.

Exam Tip! ✏️

When answering questions about coastal landforms:

Always explain the specific erosion processes involved (hydraulic action, abrasion, etc.)
Use diagrams to show the stages of formation if possible
Mention the importance of rock type and structure
Include a named example to support your answer
Consider how human activities might affect these landforms

🧠 Test Your Knowledge!