COASTS

What processes make coasts change shape? Erosion attrition abrasion corrosion hydraulic action Deposition traction saltation suspension solution Transportation

Erosion 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!

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

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.

How 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-cut notch’ is made by the waves at high tide As it gets bigger, the overhanging rock will eventually tumble into the sea.

How cliffs are eroded Over time the cliff retreats leaving a wave-cut platform just below the surface, that shows only when the tide is out This is in Dorset near Purbeck at Kimmeridge Bay.

There are some special coastal features 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.

cave cliff crack

stack arch pillar

reef stump

original photo http://www.leler.com/hawaii/ Holei Sea Arch Can you remember the order? C P S R

Some examples What do all these show from Selwick?

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)

But beaches do not stay the same Most of the waves break on the beaches at an angle. Some of the waves are constructive, which means they bring material from another place and leave it. These waves have powerful swash waves – waves that drop anything they are carrying as they break These make beaches bigger.

Constructive Waves Spilling Breaker Powerful Swash Weak Backwash

But beaches do not stay the same Some of the waves are 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 waves – big waves that make more noise as they retreat, taking the sand and shingle with them. These make beaches smaller.

Destructive Waves Crashing breaker Weak swash Powerful Backwash

But beaches do not stay the same Some waves are a bit of both. This is called longshore drift

Longshore Drift Visit www.worldofteaching.com For 100’s of free powerpoints

Direction of swash Direction of swash

Direction of swash Direction of swash

Direction of swash Backwash Direction of swash Backwash

Direction of swash Backwash Direction of swash Longshore Drift

Build up of sediment against the groyne Direction of swash Longshore Drift Groyne traps sediment Build up of sediment against the groyne

And this happens if the long shore drift is allowed to carry on Deposits and seed are caught behind the shingle bar and a salt marsh begins to grow The coast changes direction This is called a spit But the long shore drift carries on But the load gets dropped as the longshore drift looses power

What can happen with longshore drift? As the River Humber joins the North sea

What can happen with longshore 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.

Homework for 2 weeks up to the end 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

Walton on the Naze Soft rock that is being eroded This tree’s roots’ soil was eroded away so it fell over Fine sand has been deposited

So what could you take pictures of to show beach features? Evidence of erosion (or not like cliffs ) Evidence of deposition Beaches – pebbles – cliffs – sand dunes – groynes – longshore drift - spits Remember Annotate = label AND explain