Coastal Landforms: Erosion & Deposition
Why Coastlines Look the Way They Do
UK coasts are not built randomly. They are the product of two things working together — the underlying geological structure and the four wave-erosion processes from the previous lesson. Where bands of hard and soft rock meet the sea at right angles (a discordant coast), the soft bands erode faster, leaving the hard bands sticking out as headlands. Swanage Bay in Dorset is the classic example. Where the rock runs parallel to the sea (a concordant coast), the coast looks straight and uniform.
Erosion Landforms
Once a discordant coast forms headlands and bays, erosion attacks the headlands themselves. The sequence of landforms is one of the most-tested topics in the course, so learn it as a chain.
1. Crack. Hydraulic power and abrasion exploit a vertical weakness or joint in the headland. Over decades the crack widens.
2. Cave. Continued erosion enlarges the crack into a cave at the base of the cliff, undercutting the rock above.
3. Arch. When two caves meet through the headland, or a cave breaks through the back of the headland, an arch is formed. Durdle Door in Dorset is a textbook example.
4. Stack. Erosion thins the arch's roof until it collapses, leaving an isolated pillar of rock standing offshore. Old Harry Rocks at the eastern end of Studland Bay show this stage.
5. Stump. Wave action eventually wears the stack down to a low rump visible only at low tide.
Cliffs and Wave-Cut Platforms
Cliffs retreat by a slightly different process. Waves attack the base of the cliff, cutting a wave-cut notch. With the base undermined, the rock above eventually collapses under gravity. The cliff line steps backwards, and the flat surface left at the base of each cliff retreat becomes a wave-cut platform. The platforms at Hunstanton in Norfolk and at Robin Hood's Bay in North Yorkshire show neat striations of bedded rock.
The erosion sequence on a headland is: crack → cave → arch → stack → stump. UK examples: Durdle Door (arch) and Old Harry Rocks (stack), both Dorset.
Deposition Landforms: Beaches
When waves no longer have the energy to carry sediment, they drop it — usually inside a sheltered bay between two headlands. The result is a beach. Sandy beaches form where wave energy is low (e.g. Bournemouth, with constructive waves and fine river-supplied sand). Pebble beaches form where wave energy is higher (e.g. Brighton, where larger material is dropped first). Beaches are dynamic — they are the same beach in summer and winter, but their profile changes dramatically with the season.
Sand Dunes
Where there is plenty of sand, a wide flat beach exposed at low tide and a steady onshore wind, sand can be blown landwards and trapped by vegetation to form sand dunes. The youngest dunes (embryo dunes) sit closest to the sea; older, more vegetated dunes (yellow dunes, then grey dunes) form further inland. Studland Bay in Dorset and Formby in Lancashire are good UK examples.
Spits and Bars
When longshore drift carries sediment along a coast and the coast suddenly changes direction (for example at a river mouth), the sediment keeps moving in its original direction. It builds out into the sea as a long, narrow ridge of sand and shingle — a spit. The end of the spit is often curved (a recurved end), shaped by waves arriving from a different direction during storms. Behind the spit, sheltered water collects mud and silt and a salt marsh develops.
Spurn Head, at the southern end of the Holderness coast, is the textbook UK example. Sediment eroded from cliffs further north is carried south by longshore drift and dropped where the Humber Estuary opens. Spurn extends nearly 5 km into the estuary; salt marsh has developed in its lee. If a spit grows right across a bay or river mouth and joins the land on the other side, it is called a bar. Slapton Sands in Devon and Chesil Beach in Dorset are bars.
Spurn Head is a 5 km spit at the south end of the Holderness coast, formed by longshore drift carrying eroded cliff material southwards. A salt marsh has developed in the sheltered water behind it.
Putting It Together: A UK Coastline
The Dorset coast in southern England is the classic UK case study. The discordant section between Studland and Swanage produces headlands (Old Harry Rocks) and bays (Studland, Swanage). Further west, hard bands run parallel to the sea (concordant), interrupted only where the sea has broken through to form the circular cove at Lulworth. Further west still, the long shingle ridge of Chesil Beach — a 29 km bar — protects the lagoon (the Fleet) behind it. Erosion landforms (Durdle Door arch) and deposition landforms (Chesil Beach bar) sit within walking distance of each other, all on the same stretch of coast.
In Dorset the resistant rock is Portland limestone and chalk; the weak rock is Wealden clay and Bagshot sand. The chalk forms vertical cliffs (Old Harry Rocks) because it is hard but jointed; the clay erodes back into bays (Swanage Bay) because it is much softer. Where the chalk runs along the coast (further west towards Lulworth), it forms long straight cliffs broken only where the sea has punched through to the softer rock behind. Geology, in short, is the master variable: it sets the speed, shape and direction of every landform you see.