GCSE Geography | Landforms of Erosion: Caves, Arches, Stacks and Stumps (Coastal Landscapes 5)

Processes of erosion have led to the formation of distinctive coastal features - we call these erosional landforms.

Headlands jut out into the sea as they are made from resistant rock with erodes more slowly than the surrounding rocks. However, they are hit by destructive waves because of wave refraction to they are constantly being re-shaped be erosion. This creates landforms such as caves, sea arches, stacks and stumps.

How do caves, arches, stacks and stumps form?

This sequence starts with hydraulic action forcing water into a crack which puts pressure on it causing it to grow larger. The crack will grow into a notch, which will then form into a cave due to the destructive waves that are converging on the headland. Two caves can form on either side of the headland due to wave refraction - over time these caves will get larger through abrasion, and the sea may break through the back of them, turning them into a sea arch. Further hydraulic action and abrasion will widen the base of the arch. The top of the arch will be weakened by freeze-thaw weathering, making it less stable so it will eventually collapse, leaving a stack, which is a detached pillar of rock. Erosion will cause notches to form at the base of the stack which makes it unstable, which along with weathering processes, will make the stack topple into the sea, leaving a stump which is usually only visible at low tide.

What are these landforms like?

Caves

• Tallest and widest at the entrance - sometimes several metres wide and tall, and they taper off at the back
• Can vary in size and some caves will be very small so you can't actually enter them, whereas others will be big enough for small boats to sail into (and often do as part of a tour of distinctive stretches of coastline)
• May have a blow hole in its roof near the back, where pressure from waves may push water out of the top

Sea arches

• Water goes right through the gap from one side of the headland to the other
  
• The top of the arch is unsupported so not very stable
• Wide base due to erosion - also wave-cut notches at both sides
• Made of hard rock such as chalk, limestone or granite
• Usually very large - very distinctive landforms, e.g. Durdle Door (along the Jurassic Coast in Dorset)

Stacks

• Tall pillars of rock, detached from the headland (after an arch collapse)
• Made of hard rock such as chalk, limestone or granite
• Can be several metres high - although a group of stacks together can vary significantly in size
• Have wave-cut notches at base - which make them unstable over time
• Very distinctive landforms, e.g. Old Harry (the eastern most point of the Jurassic Coast in Dorset)
• Sometimes pinnacle shaped, e.g. Needles (off the coast of the Isle of Wight)

Stumps

• Bases of collapsed stacks
• Often only visible at low tide
• E.g. Old Harry's Wife (lies just beyond Old Harry Rocks)