Coastal Landforms (DP IB Geography)

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

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

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

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

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

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