Detailed explanation on how Arches, Caves, Stacks and Stumps
When the sea attacks the headland, erosion in joints creaks caves. Wave refraction, type and resistance of rock depends on how long the erosion actually takes, and how much of the headland is eroded.
As erosion continues, the headland is eroded right through to form a natural arch. Once again the resistance of rock is once again an important factor.
The roof of the arch eventually falls down as it becomes unstable, as it has no support. It gradually wears away until it the roof can’t hold itself up anymore. It then collapses into the sea. This creates a stack.
A stump is created when a stack is eroded. The sea cuts into the stack making the top unstable and causes it to fall into the sea, leaving a stump. Also a wave cut platform is left from the erosion.
(handfast Point)
Types of erosion that erode the coast
Hydraulic Action
- Wave crashes against cliffs
- Air compressed into cracks/joints
- Pressure put on rock
- Waves falls back
- Pressure released
- Process repeated
- Block of rock collapses from cliff
Corrasion/Abrasion
- Waves pick up sand and pebbles
- Thrown against base of cliff
- Process repeated by next wave
- Rock from cliffs worn away
Attrition
- Blocks of rock fall off the cliff
- Crashed against other pebbles and boulders
- Pieces of rock broken off
- Rocks became smaller
- Eventually rocks become sand
Corrosion
- Acid in sea water
- In contact with cliffs
- Rock slowly dissolves
Wave Refraction, Headlands and Bays
The rock out of which the coastline is made is important in shaping the landscape. The rate of erosion will be slow if the rock is hard (resistant). Joints in the rock are enlarged by corrasion and hydraulic action and means that rocks are eroded more easily.
A Headland is an area of land adjacent to water on three sides. A bay is the reverse, an area of water bordering land on three sides. Headlands and bays are usually, but not always, found together on the same stretch of coastline. Headlands and bays form on concordant coastlines, where bands of rock of alternating resistance run perpendicular to the coast. Bays form where weak (less resistant) rocks (such as sands and clays) are eroded, leaving bands of stronger (more resistant) rocks (such as chalk, limestone, granite) forming a headland, or peninsula. Wave refraction occurs on headlands concentrating wave energy on them so many other landforms, such as caves, natural arches and stacks, form on headlands. Wave refraction disperses wave energy through the bay, and along with the sheltering effect of the headlands this protects bays from storms. This effect means that the waves reaching the shore in a bay are usually constructive waves, and because of this most bays feature a beach. A bay may be only metres across, or it could be hundreds of kilometres across
Headlands and bays may also form in lakes. At a Headland the waves are destructive (backwash more powerful than swash). At a bay the waves are constructive (swash more powerful than backwash) (more info on pg 10)
Rock Types
The main rock types along the Dorset coastline are Chalk, which is fairly resistant (hard), sands and clays which is not resistant (soft) and limestone which is resistant (hard)
The coastline consisted of parallel bands of hard and soft rock, which were perpendicular to the sea.
As a result of differential erosion, the bands of soft rock were eroded much more quickly than the bands of resistant rock to form bays and headlands.
The headlands will be eroded back as a result of wave refraction. , , and will form which will eventually be eroded away. The bays will advance as deposition will take place at their heads to form Bay-head Beaches.
Eventually, the coastline will become straight again and the whole process of differential erosion will resume again to result in an irregular coastline.
Cliffs
Cliffs are common features on upland coasts. Two reasons:
- The coast is constantly beings undercut by wave action
- The rocks eroded from cliffs are quickly removed by waves.
This means where the land meets the sea, there is always a steep coastal slope or cliff.
The speed of cliff retreat depends on rock strength. Thus cliffs made from weaker rocks like shales of Lyme Bay may retreat rapidly.
Lyme Bay.
Wave erosion concentrated at the high water mark, gradually cuts a notch at the base of the cliffs. This undermining causes cliffs to collapse and gradually retreat inland
(see pictures on next page)
Waves
There are three factors that affect wave height and strength.
- Wind Speed
- Wind duration
- Fetch
Wind Speed- When it’s very windy the waves crash against the coastline- however the wind doesn’t have to be strong to generate large waves.
Wind Duration- This is the length of time it takes for the waves to travel after a blow of wind. For instance, if the wind was giving one powerful but short blow the waves wouldn’t travel that far. In other cases, like the wind was blowing long and steady then the waves could travel further.
Fetch- The distance of how far the waves have to travel. Where the fetch is long, very large waves can occur.
Project By : Erin Woodland
Class: Geography 10GSCE
Teacher: Mr Battle
