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COASTSCOASTS

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What processes make coasts change What processes make coasts change shape?shape?

• Erosion attrition abrasion corrosion hydraulic action

• Deposition traction saltation suspension solution

• Transportation

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ErosionErosion

• How easily a coast is eroded depends on how hard the rock is

• Capes and Bay!• There is an animation of this on the blog!

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Studland Bay – and you can see in Studland Bay – and you can see in the distance some rocks we will be the distance some rocks we will be looking at later – those are chalk looking at later – those are chalk

cliffs with a soft sandstonecliffs with a soft sandstone

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Erosion: which is which?

The sea picks up stones and throws them against the cliff wearing it away.

The waves force air into cracks in the rock and the pressure causes the rocks to split and break up.

Boulders already eroded from the cliffs are broken down into smaller and more rounded particles.

Rocks are dissolved by the chemical content of the water.

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How cliffs are erodedHow cliffs are eroded

• The tougher rocks of the headlands are gradually eroded away by attrition, hydraulic action (and sometimes corrosion, e.g. limestone or chalk) and gradually a small ‘Wave-Wave-cut notchcut notch’ is made by the waves at high tide

• As it gets bigger, the overhanging rock will eventually tumble into the sea.

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How cliffs are erodedHow cliffs are eroded

• Over time the cliff retreats leaving a wave-cut wave-cut platformplatform just below the surface, that shows only when the tide is out

• This is in Dorset near Purbeck at Kimmeridge Bay.

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There are some special coastal There are some special coastal featuresfeatures

• These form where chalk or limestone form the hard band of rock.

• Here the erosion is help by corrosion. If the water is slightly acid, then the calcium carbonate that makes up these rocks will dissolve slowly.

• The rocks were laid down millions of years ago in a warm sea and are made mostly of the shells of molluscs.

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cliffcliff

craccrackk

cavcavee

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archarch

pillarpillar

stacstackk

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stumstumpp

reefreef

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original photooriginal photo http://www.leler.com/hawaii/http://www.leler.com/hawaii/

Holei Sea ArchHolei Sea Arch • Can you remember the

order?• C• C• P• S• S• R

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Some examplesSome examplesWhat do all these show from What do all these show from

Selwick?Selwick?

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What happens next?What happens next?

• Once the rock has been eroded away from the cliff by abrasion and corrosion and hydraulic action

• The pieces will be further broken down by attrition, becoming smoother and smaller over time

• Some of it will be transported away and deposited somewhere else

• Broken rock ends up on beaches as sand or shingle (round pebbles)

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But beaches do not stay the sameBut beaches do not stay the same

• Most of the waves break on the beaches at an angle.

• Some of the waves are constructive,constructive, which means they bring material from another place and leave it.

• These waves have powerful swash swash waveswaves – waves that drop anything they are carrying as they break

• These make beaches bigger.

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Constructive Waves

Spilling Breaker

Powerful SwashWeak Backwash

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But beaches do not stay the sameBut beaches do not stay the same

• Some of the waves are destructive,destructive, which means that they break on the beach and as the water runs back, it drags part of the beach away with it.

• These waves have powerful backwash backwash waveswaves – big waves that make more noise as they retreat, taking the sand and shingle with them.

• These make beaches smaller.

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Destructive Waves

Crashing breaker

Weak swash

Powerful Backwash

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But beaches do not stay the sameBut beaches do not stay the same

• Some waves are a bit of both. • This is called longshore drift

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Longshore DriftLongshore Drift

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Direction of swash

Direction of swash

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Direction of swash

Direction of swash

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Direction of swash

Bac

kwas

h

Direction of swash

Bac

kwas

h

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Direction of swash

Bac

kwas

h

Longshore Drift

Direction of swash

Longshore Drift

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Direction of swash

Longshore Drift

Groyne traps sediment

Build up of sedimentagainst the groyne

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And this happens if the long shore And this happens if the long shore drift is allowed to carry ondrift is allowed to carry on

The coast changes direction

But the long shore drift carries on

But the load gets dropped as the longshore drift

looses power

Deposits and seed are caught behind the shingle bar and a

salt marsh begins to grow

This is called a

spit

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What can happen with longshore What can happen with longshore drift?drift?

• As the River Humber joins the North sea

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What can happen with longshore What can happen with longshore drift?drift?

• The sand and shingle are dragged down the coast from the NW.

• The blue line shows a shingle bank

• It continued across the gap between the main land and the island to form a tombolo. tombolo.

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Homework for 2 weeks up to the Homework for 2 weeks up to the end of half termend of half term

• If possible (if not we need to talk).• To visit the coast and to gather

photographic evidence or you can sketch it and scan it if you like.

• Now I want the evidence ‘annotated’, which means not just labels but explanations too.

• See over for an example

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Walton on the NazeWalton on the Naze

Soft rock that is being

eroded

Fine sand has been deposited

This tree’s roots’ soil

was eroded away so it fell over

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So what could you take So what could you take pictures of to show beach pictures of to show beach

features?features?Evidence of erosion (or not like cliffs )

Evidence of depositionBeaches – pebbles – cliffs – sand dunes

– groynes – longshore drift - spits Remember

Annotate = label ANDAND explain