Water Landforms and Water Landforms and PeoplePeople

Starter: How did this feature Starter: How did this feature form?form?

Marine Erosion Marine Erosion ProcessesProcesses

VV ‘05

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.

Hydraulic Hydraulic ActionAction

Corrasion (abrasion)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

AttritionAttritionRock 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)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.

Rates of erosion depend on Rates of erosion depend on many factors:many factors:

• Waves – strength, frequency, heightWaves – strength, frequency, height

• Weather – frequency of storm conditionsWeather – frequency of storm conditions

• Geology of the coastline :Geology of the coastline :

type of rocktype of rock

degree of resistancedegree of resistance

stratificationstratification

stabilitystability

Coastlines of Coastlines of ErosionErosion

1. Headlands and bays

2. Cliffs and wave cut platforms

3. Headland erosion and the formation of caves, blowholes, arches, stacks and

stumps

Headlands and baysHeadlands and bays

• These are most likely to be found in areas where alternating resistant and less resistant rock meets the coast at right angles

• Erosion erodes the softer rocks more quickly forming bays

• Headlands are formed of rocks which are more resistant to erosion

Softer rock

Harder rock

Harder rock

chalk

clay

limestone

Swanage Bay

An example of headlands An example of headlands and bays on the Dorset and bays on the Dorset coastline…coastline…

Headland and bay near Headland and bay near ArbroathArbroath

The waves attack the base of the cliff through the processes of abrasion, corrosion, hydraulic action and attrition.

Over time the cliff will be undercut and a wave-cut notch is formed.

Eventually the cliff becomes unstable and collapses. Further cliff retreat will form a wave-cut platform.

The The formation of formation of

cliffs and cliffs and wave cut wave cut platformsplatforms

Wave cut notch - Wave cut notch - AuchmithieAuchmithie

Wave cut Wave cut notch and notch and platform - platform - ArbroathArbroath

Headland Headland erosionerosion

Caves, arches, stacks and Caves, arches, stacks and stumpsstumps

CavesCavesCaves usually develop from Caves usually develop from widening and deepening of widening and deepening of notches where there are notches where there are weaknesses in the cliff faceweaknesses in the cliff face

This large cave at This large cave at Arbroath is fault guided. Arbroath is fault guided.

Can you see the fault?Can you see the fault?

Erosive waves may blast Erosive waves may blast their way vertically their way vertically through lines of weakness through lines of weakness in the roofs of caves.in the roofs of caves.

This produces a blowhole This produces a blowhole on the cliff top.on the cliff top.

In stormy conditions sea In stormy conditions sea spray may spout from spray may spout from blowholes.blowholes.

BlowholesBlowholes

This huge blowhole is the Gaylet Pot near Auchmithie. This huge blowhole is the Gaylet Pot near Auchmithie. The tractor on the skyline is at the top of the cliffs.The tractor on the skyline is at the top of the cliffs.

ArchesArches

These are formed These are formed by the wearing by the wearing away of narrow away of narrow headlands often headlands often by two back-to-by two back-to-back cave back cave systems joining.systems joining.

Durdle Door

The waves continue to erode at the foot of the arch widening The waves continue to erode at the foot of the arch widening it. Eventually the roof of the arch can no longer be it. Eventually the roof of the arch can no longer be supported and it will collapse.supported and it will collapse.

Here a cave and arch are forming on the same Here a cave and arch are forming on the same headland at Auchmithieheadland at Auchmithie

Stacks and Stacks and stumpsstumpsStacks often represent the Stacks often represent the seaward remnant of a collapsed seaward remnant of a collapsed arch. These tall, isolated pillars arch. These tall, isolated pillars of rock such as ‘The Pinnacles’ of rock such as ‘The Pinnacles’ on the Dorset coast, are reduced on the Dorset coast, are reduced by further wave action to by further wave action to stumps.stumps.

The ‘Deil’s Heid’ The ‘Deil’s Heid’ stack at Arbroath is stack at Arbroath is interesting because interesting because sea level has fallen sea level has fallen since it was formed.since it was formed.

There is very little There is very little erosion around its erosion around its base so it is unlikely base so it is unlikely to become a stump to become a stump for a very long time.for a very long time.

We can now imagine We can now imagine how the headland at how the headland at Durdle Door might be Durdle Door might be evolvingevolving

Durdle Door Durdle Door in the in the past………past………

DurdleDurdle DoorDoorAs it is at As it is at present present ………………

Durdle DoorDurdle Door

sometime in sometime in the future?the future?

Old Harry Rocks in Dorset which show many of Old Harry Rocks in Dorset which show many of the features of headland erosionthe features of headland erosion