Physical geography · GCSE Geography
Cave, arch, stack and stump
GCSE Geography cave–arch–stack–stump sequence at Old Harry Rocks: joints, hydraulic action and abrasion, then weathering and collapse, with every process named in order.
Crack, cave, through-blow, collapsed roof, lonely stack, tide-covered stump. Old Harry is the chalk cartoon you must be able to annotate in sixty seconds.
The important bits
What you need to know
- 1
The sequence happens on a headland, usually of jointed rock (chalk at Old Harry, Studland, Dorset). Waves attack lines of weakness. Headlands stick out because they are more resistant than neighbouring bays; they still lose.
- 2
Hydraulic action: air and water compressed in cracks, then released, widening joints. Abrasion (corrasion): pebbles used as sandpaper. Attrition: rocks smash each other in the surf. Solution (corrosion): chalk and limestone dissolve. Name the job, not just “erosion”.
- 3
A crack becomes a cave as the processes concentrate on the weakness. Caves often sit at the base where wave energy is highest, above a developing wave-cut notch.
- 4
If caves meet from opposite sides of a narrow headland, or one cave breaks through, an arch forms. Old Harry’s family of chalk stacks and stumps sits in this sequence; Durdle Door is a named arch on the same coast.
- 5
Weathering (freeze–thaw, salt, biological) and gravity weaken the arch roof. When it collapses, a stack stands detached from the cliff. Old Harry is the named stack.
- 6
Basal erosion and weathering reduce the stack to a stump, often covered at high tide. The cliff line has retreated; a wave-cut platform may fringe the whole story.
- 7
Do not insert longshore drift or spit formation in the middle. This is erosional headland evolution, not a depositional beach landform.
- 8
Exam habit: write the sequence as numbered processes with Old Harry at the stack stage. A 4-mark question is often won by order plus two named processes plus the place.
Quotations worth analysing
Short evidence. Real method.
“Hydraulic action and abrasion widen a crack into a cave.”
Two named processes beat “the sea bashes the rocks”. Air in joints, pebbles as tools.
“When the roof of an arch collapses, a stack is left.”
Collapse is weathering plus gravity, not a third type of wave. Then the stack becomes a stump.
“Old Harry Rocks are chalk stacks on the Dorset coast.”
Chalk is well jointed and soluble enough for a textbook sequence. Yorkshire till does not make Old Harry.
Go deeper
The sequence with no missing rung
Examiners punish skipped steps. Start with a joint or fault on a headland. Hydraulic action and abrasion open a cave. Breakthrough makes an arch. Roof collapse — weathering and mass movement, not a magical extra wave type — leaves a stack. The stack is undercut and weathers to a stump. A wave-cut platform is the pavement left as the cliff retreats; it can sit beside the whole sequence. If the paper asks for formation of a stack, you still need the cave and arch; a stack does not grow from a beach. Practise a side-view sketch: cliff, notch, cave, arch, stack, stump, platform, with arrows for wave attack. Sixty seconds of drawing saves a 4-marker. Put Old Harry on the stack, not on a spit.
Go deeper
Name the processes or lose the marks
“Erosion” is a category, not a mechanism. Hydraulic action is compression of air in cracks. Abrasion is the grinding load. Attrition rounds the ammunition but does not cut the cliff directly. Solution matters on chalk: weak carbonic acid in seawater and rain attacks calcium carbonate. Weathering attacks the arch roof between tides. Students who write four synonyms for “wearing away” still miss the process marks. Match process to stage: waves at the base for cave and notch; weathering and gravity for roof collapse; waves plus weathering for stump. That mapping is what a teacher means by “processes named”. Old Harry’s chalk makes solution and joints easy to justify — use the rock type as evidence, not as a caption.
Go deeper
Headland, not Holderness till
The sequence needs a sticking-out, jointed headland. Old Harry sits at the end of the Foreland chalk. Holderness till slumps in slabs; it does not reliably pose as a neat arch. Mixing case studies is the classic coast fail: Spurn is a spit from LSD; Old Harry is erosional sculpture. Durdle Door is an arch you can name if the question wants a living arch rather than a stack. Swanage Bay next door shows how bays erode faster in clay, leaving chalk headlands — the discordant-coast frame that explains why the sequence is happening here at all. If you have eight minutes, give discordant geology in one sentence, then the crack-to-stump cartoon. Geology sets the stage; waves write the play.
See the idea in action
Question: explain the formation of a stack (4–6 marks). Headland of jointed chalk (Old Harry). Hydraulic action and abrasion widen a crack to a cave, then an arch. Weathering and collapse of the roof isolate a stack. Further basal attack leaves a stump; platform as the cliff retreats. Do not mention groynes. A labelled sequence sketch plus those sentences is full marks territory.
Exam technique
Turn knowledge into marks
Memorise the order crack–cave–arch–stack–stump and park Old Harry on it. If you cannot name hydraulic action and abrasion, you are describing a holiday photo.
Common mistakes
Do not give these marks away
- 01
Skipping cave or arch, or turning a stack into a spit with longshore drift.
- 02
Writing “erosion” without hydraulic action, abrasion, attrition or solution.
- 03
Putting Old Harry on Holderness, or Holderness till in Dorset.
What is the correct order in the Old Harry sequence?
ASpit, bar, tombolo, stack
BCrack and cave, arch, stack, stump
COxbow, levee, floodplain, stack
DGroyne, sea wall, managed retreat, stump
Show the answer
Crack and cave, arch, stack, stump. Waves exploit a weakness on a headland, break through to an arch, the roof collapses to a stack, and the stack weathers to a stump. Depositional landforms belong in a different answer.
Quick questions
If this is the bit you searched
Where are Old Harry Rocks?
Chalk stacks (and related stumps) on the Dorset / Isle of Purbeck coast near Studland, a standard AQA erosional landform example.
What is the difference between a stack and a stump?
A stack still stands above typical high water as a pillar. A stump is the eroded remnant, often covered at high tide.
Which erosion processes form a cave?
Hydraulic action and abrasion on a joint or fault, helped by solution on chalk. Attrition prepares the pebbles that do the abrading.
Is Durdle Door the same as Old Harry?
Same coast family, different stage: Durdle Door is a famous arch; Old Harry is taught as stacks. You can use both if you keep the stages straight.