Coastal Erosion Landscapes

Understanding the formation of Coastal Features

Coastal Landscapes are shaped by the following Coastal Processes:

Erosion
Transportation
Deposition

Each of these processes plays a key role in shaping our coastal areas in one way or the other and as such, they have various coastal landforms associated with them.

Erosion

Starting with Coastal Erosion, we will begin to look at the various landforms shaped by this process. As you know, erosion is the wearing away of rocks. Coastal erosion is caused by four main processes. These processes are as follows:

Hydraulic action - This is where the sheer force of the water erodes the rock. As strong waves pound the base of the cliffs forcing water into cracks, the air compresses, and as it leaves the pressure is suddenly released, which breaks down the rock from inside. This enables a dislodging of rocks with some of the rock coming off in large chunks.
Abrasion/Corrasion - Pebbles, sand, and shingles are picked up by the sea and flung against the cliff. As these materials are pushed up and down the cliff, it creates a sandpapering or grating effect which causes the cliffside to erode away. This occurs more in storm conditions.
Attrition - This is where the material being carried by the sea are worn down as pebbles and rocks collide with each other. This will wear away sharp edges over time to create rounded and smooth pebbles. The rocks may collide with such force that they can break into much smaller pieces.
Solution - This is where the chemicals in the water react with the minerals in the rock to dissolve them. This is particularly effective in certain rock types, e.g. limestone and chalk.

Coastal erosion is most effective when the waves are powerful and contain a lot of energy. The rate of erosion is higher when the waves reaching the coastline have a large fetch so the waves have time to build up, where there are strong winds (like sides of the islands that face the prevailing winds), when the beach is not very wide so there is less material to absorb the wave's energy, where the rocks are soft so they are less resistant to erosion, and where the rocks have lots of joints, so they are weaker. These waves need to break at or close to the foot of a cliff if they are to carry out erosion.

coastal Landforms created by erosion

Coastal erosion helps to shape the following coastal landforms:

Wave-cut Platform

At the base of cliffs, through continuous undercutting caused by repeated erosion (hydraulic action and corrosion), a wave-cut notch forms. As the wave-cut notch becomes enlarged, it creates an overhand of rocks above it. Overtime, the overhang of rocks collapses (this is usually due to a mix of lack of support and a period of stormy weather). The fallen rocks form a pile at the foot of the cliff which is then eroded away by the sea. Through abrasion, the area left behind is smoothened and extends the underlying wave-cut platform. This process repeats itself causing the cliff face to retreat and the wave-cut platform to become more extensive.

Diagram showing the formation of a wave-cut platform (Taken from CSEC Study Guide pg 81)

Example of a Wave-cut Platform

Headlands and Bays

Headlands and bays are common place coastal features in the Caribbean and elsewhere in the world. Some of our best examples of headlands and bays can be found at Pearn's Point, Jennings Ext., Antigua. When you travel over the hill on the other side of Hermitage bay you should be able to see them.

Headland - A section of rocky coastline that protrudes into the sea.
Bay - a pronounced indentation in the coastline usually found between two headlands.

Headlands form when waves crash into a coastline area with layers of hard and soft rock. Through wave action, the soft rock is eroded away faster than the hard rock causing differential erosion. The indentation formed by this erosion becomes a bay.

Diagram showing the progression of a coastline into Headlands and Bay

Caves, Arches and Stacks

Exposed headlands become the point of further erosion that leads to the formation of sea caves, arches and stacks. Through the power of powerful destructive waves, waves crash into weaknesses in the cliffs, such as joints (vertical cracks) and faults be exploited by the processes of hydraulic action and corrosion to form sea caves on opposite sides of a headland.

Sea cave at Pearn's Point, Antigua.

Over time the sea caves becomes enlarged and eventually join to form an arch. Weathering and erosion processes enlarge the arch thus causing the roof to gradually become thinner.

Hell's Gate Island, Antigua and Barbuda

Eventually, the roof of the arch collapses under its own weight (remember it is always getting thinner which means it loses support), leaving behind an isolated pillar of rock called a stack. This means that some time in the future, we may no longer see this arch at Hell's Gate Island, Antigua. Be sure to check it out before its gone.

Gradually, the stack is eroded until all that is left is a low rock outcrop exposed only at low tide. This is called a stump.

Stump left behind at Pearn's Point, Antigua.

Here's a Diagram to assist with the long explanation you just read. This will help to put things into perspective.

Diagram showing the gradual formation of a stump from a sea cave (Taken from CSEC Geography Study Guide pg 82)

BRAIN TEASER:

Looking at the image below, can you figure out if Devil's Bridge, Antigua should be considered an arch? Why or why not?

Devil's Bridge, Antigua and Barbuda

Move on to Coastal Deposition Landscapes to learn about transportation and Coastal Deposition next.

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