Lithosphere / Rural Land / Exam Technique Supported Study (28.02.2011) Lithosphere / Rural Land / Exam Technique

Waves

Waves Waves are caused by the wind dragging on the surface of the water. The length of water the wind blows over is called its fetch.

When waves reach the coast (1) Out at sea, the waves roll. In a gale they can be over 30m high!

The sea kayak is 4m long

The sea kayak is 4m long

The sea kayak is 4m long

When waves reach the coast (2) They break in shallow water. The water that rushes up the sand is called the swash.

When waves reach the coast (3) The water rolling back into the sea is called the backwash. Backwash

The waves at work Waves have energy. That means they can work. They work non-stop, night and day shaping the coastline.

Erosion Waves erode in the same way as rivers

Human Activity The increase in pressure resulting from building on cliff tops and the removal of beach material which may otherwise have protected the base of the cliff both contribute to more rapid coastal erosion.

Deposition Waves continually carry material on and off the land. If they carry more on than off – a beach forms! Some beaches are made of sand. Some are made of shingle or small pebbles.

Coastal Landforms Old Harry Rocks in Dorset

This coastlines is made of different rock types, some hard, some soft This coastlines is made of different rock types, some hard, some soft. Once upon a time it was straight.

This coastline consists of parallel bands of hard and soft rock which were perpendicular to the sea. As a result of differential erosion, the bands of soft rock are eroded much more quickly than the bands of resistant rock to form bays and headlands. The headland is eroded back as a result of wave refraction. Caves, arches, stacks and stumps will form which will eventually be eroded away. The bays will advance as deposition will take place at their heads to form Beaches. Eventually, the coastline will become straight again and the whole process of differential erosion will start again and result in an irregular coastline.

Differential rates of erosion along a coast with alternate bands of hard and soft rocks results in an irregular coastline - Headland and Bay. As waves enter the shallow waters of the headland, the influence of the shallow waters causes the wave fronts to bent and get refracted. Due to the wave refraction, wave energy is more concentrated towards headlands and dissipated towards bays. Marine processes - hydraulic action and corrosion erode along lines of weakness (e.g. joints, faults and bedding planes).

Continuous erosion causes back-to-back caves to extend backward until the sea caves meet and a natural tunnel is formed. The top portion of the sea caves becomes an arch, linking the tip of the headland with the mainland. When the arch collapses, the sea ward pillar is left standing and becomes a stack. Continual wave erosion eventually reduces the stack into a stump.

How caves, arches and stacks form (1) The sea attacks cracks in the cliff at the headwall. The cracks grow larger – and form a cave.

How caves, arches and stacks form (2) The cave gets eroded all the way through. It turns into an arch. Then one day …

How caves, arches and stacks form (3) … the arch collapses, leaving a stack. In time, the waves erode the stack to a stump.

A general view of Old Harry Rocks from Ballard Down in late afternoon light at Easter. The progressive erosion of the headland can be seen, from the caves etched out at the waterline, to the enlarged hollows on the main stack, and eventually the detached, vertical, slim sea stack. Beyond the stacks is an area of wave-cut platform, exposed at this low tide.

Coves Lulwoth Cove, Dorset The river that runs into the Cove at one time reached the sea through a cut in the hard Portland Limestone barrier. This breach allowed the sea to erode the limestone. Once the sea reached the softer rocks behind, especially the Wealden Clay, rapid erosion took place. Once the sea reached the massive Chalk, erosion slowed, forming the perfect bay. To the west, the sea has been eroding the Portland Limestone at Stair Hole and the process of bay formation has just begun. Eventually Lulworth Cove and Stair Hole will combine into one larger cove, rather like Man O’ War Cove further along the coast. Lulwoth Cove, Dorset

Lulworth Cove Lulworth Cove Stair Hole Stair Hole

Cove formation (1) The Lulworth coast was once a solid cliff with a stream running down to the sea

Once breached, the softer cliffs behind were rapidly eroded Cove formation (2) Once breached, the softer cliffs behind were rapidly eroded

This process continues for many years Cove formation (3) This process continues for many years

Lulworth today

Lulworth tomorrow!

The next Durdle Door could well form between Lulworth Cove and Stair Hole

The future? Could Lulwoth become like Manowar Cove, Dorset Coast? Old Lulworth Cove? Old Stair Hole? Place of old arch? Sea Stump

Wave Cut Platforms

Destructive waves at an exposed coast erode a steep coastal slope through processes like hydraulic action and abrasion. The waves erode along lines of weakness in the rock face to form a notch. Continued erosion enlarges the notch and causes its roof to collapse, and a cliff is formed. Further undercutting at the base of the cliff results in an overhanging cliff which eventually collapses. As the steep cliff retreats landwards, a flat terrace at the foot of the cliff is exposed - wave-cut platform. The eroded materials which are transported away may be deposited in the sea to form an offshore terrace.

This view, from the north-east, shows the headland and, more notably, its wave-cut platform. Long hours about sea level has left this platform with a greenish tinge. My wife cynically says it's probably the fertilising influence of sewage from Swanage! (allegedly)

Coastal Spits

Coastal spits Spits are long, narrow accumulations of sand and/or shingle with one end joined to the mainland and the other projecting out into the sea. Sandy are formed by longshore drift.

Direction of longshore drift The end of spit is curved by the waves