Coastal Landforms (AQA GCSE Geography)

• Geology shapes the coastline over time, place and space
• A coastline made up of softer rocks such as sands and clays will be easily eroded by destructive waves to form low, flat landscapes such as bays and beaches
• Coastlines of more resistant, harder rock will take longer to erode and produce rugged landscapes such as headlands
• The differences between hard and soft rocks will also impact the shape and characteristics of cliffs

The Effects of Rock Type on the Coastline

• Geology, therefore, shapes the coastline vertically through the height and profile of a cliff and horizontally with bays and headlands

Headlands and bays

• Occur where there are alternating bands of hard and soft rocks run perpendicular to oncoming waves (discordant coastline)
• At first, the soft rock (e.g. clay) is eroded backward, forming an inlet
• As the inlet continues to erode it curves inwards, and a bay is formed, usually with a beach
• The hard rock (e.g. limestone) is left protruding out to sea as a headland

• A headland usually features:
  • Cliffs along its sides
  • Projects out to sea
  • Usually longer than it is wide
  • Geology is of resistant rock
• A bay usually has:
  • A wide, open entrance from the sea
  • A roughly, semi-circular shape extending into the coastline
  • Land that is lower than the headlands surrounding it
  • A bay may or may not have a beach

The formation of headlands and bays on discordant coastlines

Cliffs and wave-cut platforms

• Cliffs are shaped through erosion and weathering processes
• Soft rock erodes quickly and will form sloping cliff faces
• Steep cliffs are formed where there is hard rock facing the sea

• A wave-cut platform is a wide gently sloped surface found at the foot of a cliff:
  • As the sea attacks the base of a cliff between the high and low water mark, a wave-cut notch is formed
  • Abrasion, corrosion and hydraulic action further extend the notch back into the cliff
  • The undercutting of the cliff leads to instability and collapse of the cliff
  • The backwash of the waves, carries away the eroded material, leaving behind a wave-cut platform
  • The process repeats and the cliff continues to retreat, leading to a coastal retreat

The process of cliff retreat and wave-cut platform formation

Caves, arches, and stacks 

• These form in a headland because of wave action and sub-aerial weathering
• As waves approach the shore, their speed is reduced as they move along the sea floor
• This changes the angle of the waves, and they will turn so the crest becomes parallel to the coast - known as wave refraction
• This refraction concentrates erosive action on all sides of the headland
•  The erosional processes of hydraulic power, abrasion and some corrosion begin to attack any weaknesses in the headland
• As the crack begins to widen, abrasion will begin to wear away at the forming cave
• The cave will become larger and eventually breaks through the headland to form an arch
• The base of the arch continually becomes wider and thinner through erosion below and weathering from above
• Eventually, the roof of the arch collapses, leaving behind an isolated column of rock called a stack
• The stack is undercut at the base by wave action and sub-aerial weathering above, until it collapses to form a stump

The formation of a cave, arch, stack and stump

Beaches

• Beach formation usually occurs in the summer months when the weather is calmer
• Form in sheltered areas such as bays through deposition via constructive wave movement, where the swash is stronger than the backwash
• Blown sand can create sand dunes at the backshore of a beach
• When a constructive wave carries sediment up the beach, the largest material is deposited along the upper reach of the swash
• As the backwash moves back down the beach, it loses water and therefore energy as it travels due to the porosity of the sand
• Consequently, the deposition of sediment gets progressively smaller, and the beach is therefore, sorted by wave deposition, with the smallest mud particles settling in the low-energy environment offshore
• If a destructive wave forms due to a storm, then large shingle is thrown above the usual high tide level to form a ridge at the top of the beach called a berm

Sediment deposition

Sand dunes

• Sand dunes are a dynamic environment, with quick changes
• Sandy beaches usually have sand dunes at their rear, because of strong onshore winds transporting dried, exposed sand
• Sand grains are trapped and deposited against any obstacle (rubbish, rocks, driftwood etc) and begin to form embryo dunes
• Dune ridges move inland due to onshore winds pushing the seaward side to the leeward side
• It is the interaction of winds and vegetation that helps form sand dunes

Formation of a sand dune

• Windblown sand is deposited against an obstruction: Pebble or driftwood
• As more sand particles are caught, the dunes grow in size, forming rows at right angles to the prevailing wind
• Over time, the ridges of the dunes will be colonized and fixed by vegetation in a process called succession
• The first plants (pioneer species) have to deal with:
  • Salinity
  • Lack of moisture as sand drains quickly (highly permeable)
  • Wind
  • Temporary submergence by wind-blown sand
  • Rising sea levels

Coastal Dune Succession

• Embryo dunes
  • Wind-blown dried sand is trapped by debris and deposition begins
  • Pioneer species such as Lyme Grass and Sea Couch Grass begin to colonise
  • There is little soil content and high pH levels (alkaline)
  • Embryo dunes are very fragile and reach a maximum height of 1 metre
• Fore dunes
  • The embryo dunes bring some protection against the prevailing wind
  • This allows other species of plant to grow such as Marram Grass
  • Marram grass begins to stabilise the dune with its root system
  • These plants add organic matter to the dunes making the dunes more hospitable for plants that later grow
  • A microclimate forms in the dune slack
  • Maximum height is 5 metres
• Yellow dunes
  • These are initially yellow but darken as organic material adds humus to the soil
  • Marram grass still dominates the vegetation, but more delicate flowering plants and insects are found in the dune slacks
  • 20% of the dune is exposed, down from 80% 
  • Height does not exceed 8 metres
• Grey dunes 
  • Grey dunes are more stable, with less than 10% of exposed sand and have a good range of biodiversity
  • Soil acidity and water content increase as more humus is added
  • Shrubs and bushes begin to appear
  • Height is between 8 - 10 metres
• Mature dunes
  • As the name suggests, these are the oldest and most stable of the dunes
  • They are found several hundred metres or more from the shoreline
  • The soil can support a variety of flora and fauna such as oak trees and alders (climax vegetation)
  • This is the final stage in succession which is known as the climax community stage

Spits

• An extended stretch of sand or shingle that extends out to sea from the shore
• Spits occur when there is a change in the shape of the coastline
• Or the mouth of a river, which prevents a spit forming across the estuary
• A spit may or may not have a 'hooked' end, depending on opposing winds and currents
• A good example is Spurn Point, which stretches for three and half miles across the Humber Estuary in the northeast of England

Stages of formation

• Sediment is transported by the action of longshore drift
• Where the coastline changes direction, a shallow, sheltered area allows for deposition of sediment
• Due to increased friction, more deposition occurs 
• Eventually, a spit slowly builds up to sea level and extends in length
• If the wind changes direction, then the wave pattern alters and results in a hooked end
• The area behind the spit becomes sheltered
• Silts are deposited here to form salt marshes or mud flats

Spit formation

Bars

• When a spit grows across a bay, and joins two headlands together
• A bar of sand is formed (sandbar)
• Sandbars can also form offshore due to the action of breaking waves from a beach

Image showing how longshore drift contributes to spit and bar formation

• The geology of the Dorset coast is perfect for both erosional and depositional landforms
• It has bands of soft clay and harder limestone and chalk
• These rocks erode at different rates creating headlands, bays, arches, a long tombolo and more 

• Durdle Door is an example of an arch formation
  • Wave erosion opened a crack in the tough limestone headland
  • Further erosion led to a cave which developed into an arch in the headland
  • Softer rocks behind the limestone have been washed away leaving an eroding line of chalk cliffs
• Lulworth Cove is a small bay that was formed when a gap was eroded in the band of tough limestone 
  • Lying behind this limestone, is a band of soft clay, and this has been scooped out (eroded away) to form a bay 
  • The entrance to the cove is narrow because the harder band of limestone is more resistant to erosion

Map showing main features of the Dorset Coast, UK

• Swanage sits on two beach bays called Studland Bay and Swanage Bay
  • These are areas of soft sandstone and clay

  • Between the two bays is The Foreland, a headland of harder chalk

• Old Harry and his wife sit at the end of The Foreland
  • The chalk headland has eroded to form caves, arches and a stack (Old Harry)
  • Further erosion has resulted in a stump called Old Harry's Wife
• Chesil Beach is an 18-mile-long pebble tombolo 
  • Formed through the process of longshore drift, it joins the Isle of Portland to the mainland
  • There is a shallow lagoon behind the tombolo called The Fleet Lagoon