Coastal Landforms (SL IB Geography)

Headlands and bays

• Headlands and bays form at discordant coastlines:
  • Different types of rock alternate along the cliff face
  • The rock runs perpendicular to incoming waves
• Softer rocks, like clays, erode more easily than harder rocks, like chalk 
• Bays form where softer rock is prevalent due to wave erosion:
  • Bays are curved, partly enclosed inlets of water
  • Bays often contain beaches
• Harder, more resistant rocks leave headlands sticking out at sea

Formation of a bay and headland  

Formation of a bay and headland 

Cliffs and wave-cut platforms

• Cliffs are vast walls of rock found on the coastline
• Constructive waves and softer sediment help form gentle, sloping cliffs
• Destructive waves and resistant rock help form steep cliffs
• Wave erosion processes, like hydraulic action and abrasion, wear away at the base of the cliff 
• Wave refraction causes the highest wave energy to focus on the headland
• Waves break near the bottom of the cliff, so wave power is stronger
• Repeated erosion at the base of the cliff forms a wave-cut notch
• This notch widens with further erosion
• Sub-aerial weathering weakens the top of the cliff
• These combined processes cause the top of the cliff to collapse:
  • Over time, this process repeats, causing the cliff to retreat
• Backwash brings the cliff material outwards towards the sea, forming a wave-cut platform

Formation of a wave-cut platform

Formation of a wave-cut platform

Cave, arches, stacks and stumps

• Wave and sub-aerial processes wear away at headlands, causing weaknesses
• These weaknesses expand into cracks, eventually growing into larger gaps or caves
• Continuous erosion and weathering from both sides of the headland will form an arch
• With wave erosion at the base and weathering at the top, the arch will give way and collapse
• This leaves behind a long, vertical piece of rock out at sea, called a stack
• Erosion undercuts the base of a stack. Weathering continues from above, until the stack collapses, leaving behind a stump

Formation of a cave, arch stack and stump

The formation process of a cave, arch stack and stump

Beaches

• Beaches are the buffer point between the ocean and the land
• Constructive waves have a stronger swash, moving more sediment up the coastline:
  • Lots of larger sediment builds up at the back of the beach
• Constructive waves have a weaker backwash, so deposit sediment more easily:
  • The water also percolates into deposited sediment, so less water returns with the backwash
• Constructive waves occur more in summer, so beaches build up during this season (beach accretion)
• Destructive waves take away sediment from the beach, typically during the winter (beach excavation)
• Other landforms can form on beaches:
  • Ridges – raised areas along the beach
  • Runnels – dips along the beach 
  • Berms – ridges that represent the high tide mark
  • Cusps – arc-shaped indents or hollows on the beach
• Sediment moves along the beach through longshore (littoral) drift, controlled by prevailing winds
• The differing angles of prevailing winds can produce different types of beaches:
  • Swash aligned – waves hit parallel to the shore and there is less longshore drift. Beaches are large and wide, with landforms like berms and sand dunes
  • Drift aligned – more influenced by longshore drift, moving sediment along the coast. Produces more narrow beaches, and other depositional landforms like spits and bars

Cross section of a beach profile

 

Cross section of a beach profile

Spits

• A spit is an elongation of beach sediment, connected to the mainland and jutting out to sea
• Longshore drift transports and deposits sediment as it moves along the coastline
• Waves deposit sediment if the coastline changes, e.g. changes direction or reaches a river:
  • Waves lose energy due to friction or counter-currents
• This material builds up over time, extending outwards into the sea
• Depending on the direction of the prevailing wind, the spit can curve inwards at the end
• Salt marshes can develop behind the spit where there is protection from wind and waves

Formation of a spit

Formation of a spit

Bars, tombolos, lagoons and barrier islands 

• Bars are spits that join two separate headlands together
• They are more visible during low tide
• During high tide, bars can make the water shallower, so the wave breaks sooner
• This can cause a lagoon to form in a bay behind the bar
• If a spit joins a mainland with an island, this forms a tombolo
• Barrier islands form parallel to the coast. They are areas with lots of built-up sediment
• Barrier islands can contain beaches, sand dunes and even forests
  

Diagrams showing different landforms of deposition

Diagrams showing different landforms of depositio