Coastal Landforms (AQA GCSE Geography)

Geological Structure & Rock Type

• 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

Erosional Landforms

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

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

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

Depositional Landforms

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

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

• 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

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

Case Study - The Dorset Coast

• 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

• 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