2.3.2 Coastal Landforms

Erosional Landforms

Erosional Landforms

Cliffs and wave-cut platforms

• Cliffs are steep or sloping rocks, with varying profiles dependent on geology and topography

• The cliff face angle also depends on geology, but also wave attack at its base - low energy waves are less destructive than high energy ones

• Many cliffs have a 'knick-point' around the high-water mark, called the 'wave-cut notch', which is where the wave has undercut the rock

• Abrasion, corrosion and hydraulic action further extend the notch back into the cliff

• As undercutting continues, the cliff above becomes unsupported and unstable and eventually collapses

• 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

Headlands and bays

• Found in areas of alternating bands of resistant (hard) and less resistant (soft) rocks running perpendicular to oncoming waves (discordant coastline)

• Initially, less resistant rock (e.g. clay) is eroded back, forming a bay

• A bay is an inlet of the sea where the land curves inwards, usually with a beach.

• The more resistant 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

Caves, arches and stacks

• 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

• Any weaknesses in the headland are exploited by erosional processes of hydraulic action, abrasion and corrosion

• 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 stum

The formation of a cave, arch, stack and stump

Depositional Landforms

Depositional Landforms

Beach

• Form in sheltered areas such as bays

• Deposition occurs through constructive wave movement, where the swash is stronger than the backwash

• Beach formation usually occurs in the summer months when the weather is calmer

• Sometimes sand from offshore bars can blow onto the shore by strong winds

• Blown sand can create sand dunes at the backshore of a beach

Spit

• 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

Formation of a Spit

Bar

• 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

Lagoon

• A lagoon is where a small body of water is cut off from the sea 

• A lagoon may form behind a bar or tombolo

• Lagoons do not last forever and may fill with sediment and form new land

Tombolo  

• A tombolo is formed when a spit joins the mainland to an island

• Chesil Beach in Dorset is a tombolo, as the mainland is joined to the Isle of Portland 

Barrier island

• Barrier islands form parallel to the coast 

• The main difference between a bar and barrier island is that a bar joins two headlands, whereas a barrier island is open at one or both ends

Sand dunes

• Sand dunes are a dynamic environment, with changes occurring quickly 

• Sandy beaches are usually backed by sand dunes due to strong onshore winds which transports dried out, exposed sand

• Sand grains are trapped and deposited against an obstacle (rubbish, rocks, driftwood etc) to form 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 help 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