Coastal Landforms (Cambridge (CIE) IGCSE Geography)

Erosional Landforms

Cliffs and wave-cut platforms

• Cliffs are sloping or steep rocks with different profiles depending on the topography and geology

• Geology and wave attack at the base of the cliff also affect the angle of the cliff face; low-energy waves cause less damage 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

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

• The more resistant rock (e.g. limestone) is left protruding out to sea as a headland

• A headland:

  • projects out to sea

  • has cliffs along its sides

  • is usually longer than it is wide

  • has a geology 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

• Wave speed reduces as they near the shoreline

• The angle of the waves changes and the crest becomes parallel to the coast, known as wave refraction

• Because of this refraction, the headland's erosive effect is concentrated on all sides of the headland

• Hydraulic action, abrasion, and corrosion take advantage of any weaknesses in the headland

• As a crack begins to widen, erosion will act to form a cave

• The cave will become larger and eventually break through the headland to form an arch

• Due to weathering from above and erosion below, the arch's foundation continues getting thinner and wider

• 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

Formation of a cave, arch, stack and stump

Depositional Landforms

Beach

• Form in sheltered places like bays

• Constructive wave action causes deposition, with the swash being stronger than the backwash

• Typically, beach formation takes place during the calmer summer months

• Strong winds can occasionally carry sand from offshore bars onto the shore

• Blown sand near a beach's backshore can form sand dunes

Spit

• A spit is an expanse of sand or shingle that stretches from the shore out to sea

• Spits occur when:

  • There is a change in the shape of the coastline

  • A river mouth stops a spit forming across the estuary

• A spit may or may not have a 'hooked' end, depending on the winds and currents

• Spurn Point, which stretches three and a half miles over the Humber Estuary in north eastern England, is a good example

Stages of formation:

• The process that moves sediment is longshore drift

• Where the coastline changes direction, a shallow, sheltered area allows sediment to be deposited

• More deposition happens as a result of higher friction

• 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

Bar

• A bar is 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

Types of coastal landforms 

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 transport 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, such as a pebble or driftwood

• As more sand particles are caught, the dunes grow in size, forming rows at right angles to the prevailing wind

• In a process known as succession, vegetation will eventually colonise and fix the ridges of the dunes

• The first plants (pioneer species) have to cope with:

  • salinity

  • lack of moisture as sand drains quickly (highly permeable)

  • wind

  • temporary submergence by wind-blown sand

  • rising sea levels

Formation of a coastal sand dune by succession

• Embryo dunes

  • Deposition starts when debris or rubbish traps wind-blown dried sand

  • 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 give 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 and 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