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 stump
The formation of a cave, arch, stack and stump
Exam Tip
Make sure that you can draw and annotate the formation of this feature as it is a popular question in the exams.
Remember that attrition is not part of the formation of this feature; attrition is the knocking together of rocks to smooth and round them.
Corrosion is an active part of the formation of these features, as all salt water is slightly acidic and most rock contains some soluble minerals that will react with the salt water.
Sub-aerial weathering (from above) also contributes to the collapse of the arch and stack.