Hydraulic Action The force of waves hitting a cliff (or sea wall) compresses water and air into cracks and joints. This can be equivalent to 30,000kg per square metre. This increase in pressure may lead to cracks widening and pieces of rock breaking off.
Corrasion (abrasion) Rock fragments may be picked up by waves and thrown against the rock face of cliffs by subsequent waves. The effectiveness of the corrasion depends on the strength of the wave, the nature of its ‘load’ and the resistance of the rock in the cliff face. Corrasion is most effective at the base of cliffs
Attrition Rock fragments which have become detached by hydraulic action and corrasion are worn down into smaller and more rounded pieces. Currents and tidal movements cause the fragments to be swirled around and to grind against each other. This type of erosion produces pebble beaches.
Corrosion (solution) Salts and acids in sea water can react with rocks , slowly dissolving them away. The photo shows this process on a cliff of Portland limestone which has been chemically attacked by carbonic acid and other chemicals in sea water.
Caves Caves usually develop from widening and deepening of notches where there are weaknesses in the cliff face This large cave at Arbroath is fault guided. Can you see the fault?
Erosive waves may blast their way vertically through lines of weakness in the roofs of caves. This produces a blowhole on the cliff top. In stormy conditions sea spray may spout from blowholes. Blowholes
This huge blowhole is the Gaylet Pot near Auchmithie. The tractor on the skyline is at the top of the cliffs.
Arches These are formed by the wearing away of narrow headlands often by two back-to-back cave systems joining. Durdle Door The waves continue to erode at the foot of the arch widening it. Eventually the roof of the arch can no longer be supported and it will collapse.
Here a cave and arch are forming on the same headland at Auchmithie
Stacks and stumps Stacks often represent the seaward remnant of a collapsed arch. These tall, isolated pillars of rock such as ‘The Pinnacles’ on the Dorset coast, are reduced by further wave action to stumps.
The ‘Deil’s Heid’ stack at Arbroath is interesting because sea level has fallen since it was formed. There is very little erosion around its base so it is unlikely to become a stump for a very long time.
We can now imagine how the headland at Durdle Door might be evolving Durdle Door in the past………