Geography · Year 12 · Coastal Landscapes and Systems · Autumn Term

Erosional Landforms: Cliffs, Arches, Stacks

Examine the formation and characteristics of major erosional coastal landforms.

• National Curriculum Attainment Targets
• A-Level: Geography - Coastal Landscapes and Change
• A-Level: Geography - Physical Systems and Processes

About This Topic

Erosional coastal landforms such as cliffs, arches, and stacks develop through wave action on headlands. Waves concentrate energy via refraction, delivering hydraulic action to enlarge cracks, abrasion to grind rock, and solution to dissolve material. This creates near-vertical cliffs undercut by wave-cut notches. Persistent erosion hollows out caves at cliff bases, and when caves from opposite sides join, a roofed arch forms above the sea.

Over time, arches collapse under gravity, leaving isolated stacks that further erode into stumps. Students trace this sequence and study wave-cut platforms, flat surfaces notched into bedrock during lower sea levels, now exposed as evidence of coastline change. This topic supports A-Level requirements in coastal systems, fostering analysis of process-form relationships and long-term landscape evolution.

Active learning excels here because students replicate erosion in controlled models, observe real-time changes, and connect diagrams to physical evidence. Collaborative sketching from field photos or virtual tours reinforces sequential understanding, while group discussions clarify complex interactions, making dynamic processes accessible and retained.

Key Questions

1. Analyze the sequence of events leading to the formation of a stack from a headland.
2. Differentiate between different types of caves and their erosional origins.
3. Explain how wave-cut platforms provide evidence of past sea levels.

Learning Objectives

• Analyze the sequential processes of erosion and deposition that lead to the formation of cliffs, wave-cut notches, caves, arches, stacks, and stumps.
• Compare the erosional characteristics of different coastal landforms, differentiating between caves, arches, and stacks based on their structural development.
• Explain how wave-cut platforms serve as geomorphological evidence for past sea-level changes and coastal retreat.
• Classify coastal landforms based on their dominant erosional processes and stage of development.

Before You Start

Introduction to Coastal Processes

Why: Students need a foundational understanding of wave types, energy, and basic erosional and depositional processes before examining specific landforms.

Rock Types and Weathering

Why: Knowledge of different rock resistances and weathering processes is essential for understanding how varied coastal geology influences erosion rates.

Key Vocabulary

Hydraulic Action	The force of moving water, particularly waves, compressing air in cracks in rocks, widening them and dislodging material.
Abrasion	The grinding and wearing away of rock surfaces by sediment particles carried by waves, similar to sandpapering.
Wave-Cut Notch	A groove or indentation at the base of a cliff, formed by wave erosion at the high tide line.
Wave-Cut Platform	A gently sloping, flat area of bedrock exposed at low tide, extending from the base of a cliff seaward, formed by wave erosion and subsequent sea-level lowering or cliff retreat.
Stack	An isolated pillar of rock standing in the sea, formed by the collapse of an arch.

Watch Out for These Misconceptions

Common Misconception

Waves erode cliffs uniformly across all coastlines.

What to Teach Instead

Wave energy focuses on headlands due to refraction, sparing bays. Tray modelling lets students see differential erosion firsthand, while group mapping reinforces spatial patterns over uniform ideas.

Common Misconception

Arches and stacks form quickly from single storms.

What to Teach Instead

These require prolonged, repeated wave action over centuries. Sequencing cards help students build timelines collaboratively, challenging instant-formation views through evidence-based discussion.

Common Misconception

Wave-cut platforms indicate current sea levels only.

What to Teach Instead

Platforms record past lower levels, now uplifted or exposed. Profile drawing activities prompt measurement and inference, with peer sharing correcting assumptions via comparative evidence.

Active Learning Ideas

Modelling: Headland Erosion Trays

Fill trays with layered sand and clay to form headlands. Direct water hoses or droppers to simulate waves, noting cliff retreat, cave formation, arch collapse, and stack isolation. Groups photograph each stage and annotate changes on worksheets.

45 min·Small Groups

Sequencing: Landform Evolution Cards

Distribute shuffled cards showing diagrammed stages from cliff to stump. Pairs arrange them chronologically, justify links with process explanations, then present to class for peer feedback.

30 min·Pairs

Profiling: Wave-Cut Platform Analysis

Provide cross-sectional photos of platforms from UK coasts. Individuals draw profiles, label erosion evidence, and infer past sea levels. Share in small groups to compare interpretations.

40 min·Individual

Concept Mapping: Virtual Jurassic Coast Tour

Use Google Earth to explore sites like Old Harry Rocks. Small groups identify and sketch cliffs, arches, stacks, adding labels for formation processes and sea level indicators.

35 min·Small Groups

Real-World Connections

• Coastal geomorphologists and environmental consultants assess the stability of coastal defenses, such as sea walls and groynes, by analyzing the erosional processes acting on cliffs and beaches in areas like the Jurassic Coast in Dorset.
• Tourism boards and local councils in coastal towns, such as Brighton or Scarborough, use their understanding of erosional landforms to manage visitor access, develop educational signage, and plan for the long-term impact of coastal erosion on infrastructure and natural attractions.

Assessment Ideas

Exit Ticket

Provide students with an image of a coastal headland. Ask them to label at least three erosional landforms visible or implied in the image and write one sentence explaining the primary erosional process responsible for one of the labeled features.

Quick Check

Present students with a series of diagrams illustrating the stages of arch formation and collapse. Ask them to number the diagrams in the correct sequence and write a brief description of the key erosional event occurring at each stage.

Discussion Prompt

Pose the question: 'How does the presence of a wave-cut platform provide evidence for changes in sea level or coastal position over time?' Facilitate a class discussion where students share their reasoning, referencing specific erosional processes and landform characteristics.

Frequently Asked Questions

How does a stack form from a coastal headland?

Waves erode headland weaknesses to form cliffs and caves. Joined caves create arches that collapse, isolating stacks. A-Level students analyze this via process diagrams, noting hydraulic action and abrasion rates vary by rock type and fetch, as seen in UK examples like Klefti Stack.

What evidence do wave-cut platforms provide about sea levels?

Platforms form at the active erosion zone during stable sea levels, then emerge if levels drop or land rises. Notched profiles on resistant rock, like at Lulworth Cove, show historical positions. Students measure gradients to quantify change, linking to isostatic rebound in Britain.

How can active learning help students understand erosional landforms?

Hands-on modelling with sand trays simulates wave processes, letting students witness cave and arch formation in real time. Collaborative sequencing and virtual mapping build process-sequence links. Field sketches from coastal videos connect theory to features, boosting retention through direct observation and discussion over passive reading.

What differentiates wave-eroded caves from other cave types?

Sea caves result from marine processes like hydraulic action at cliff bases, unlike limestone caves from dissolution or glacial caves from ice melt. A-Level analysis compares origins via rock type and location. Activities like labelling cross-sections clarify distinctions, preparing students for exam process questions.