How does weathering and mass movement shape the coastline?

Swanage Coastline

This stretch of coastline is located in Dorset on the South coast of England. It is known for being part of the Jurassic coast, characterised by its many fossils along the coastline.

In general, weathering processes are responsible for weakening the structure of rocks along the coast, which in turn can amplify the effects of other processes, specifically erosional processes. For example, the mechanical/physical weathering processes of freeze-thaw and wetting and drying cause some rock to loosen from a cliff face. Following this, it becomes easier for erosional processes like hydraulic action and abrasion to make this loosened rock come away from the cliff. As all these combined processes happen more and more over time, it can lead to quite an extent of cliff retreat, but this varies with rock type. Along the Swanage coastline, this has particular effect because of the differing rock types along the coast. Softer rocks, like the clay and sands, are more easily weathered and eroded than the harder chalk and limestone, so with the combined processes occurring along this particular stretch of coastline, the clays and sands retreat more quickly than the chalk and limestone, meaning the shape of the coastline becomes made up of headlands (hard rock) and bays (soft rock).

This idea also supports the development of the features at Old Harry Rocks. This is a headland reaching out into the sea, as well as a stack (Old Harry) and a stump (Old Harry’s wife). This part of the coastline is made up of chalk (Ballard Point). Weathering processes, particularly carbonation, causes the chalk to weaken a lot. The carbonation process does this by involving a reaction. Rainwater becomes slightly acidic as carbonic acid because carbon dioxide dissolves in it. When it comes into contact with the chalk, the carbonic acid reacts with the calcium carbonate in the rock and produces calcium bicarbonate, which dissolves easily in water, meaning the chalk becomes a lot weaker. Erosional processes then exploit the weaknesses created by the weathering processes, to produce the landforms. In the case of Old Harry Rocks, the stack was once attached to the headland by a natural arch. Carbonation would have weakened the top of the arch, making it easier for it then to break away becoming a stack.

Further west to Old Harry is Lulworth Cove, a location of fascinating geography and geology. It is a small inlet of water by the village of West Lulworth, which probably began forming at the end of the last Ice Age. The theory is that a stream ran over the top layer of Portland limestone, eventually breaching it – this may have happened as a result of weathering to some extent, like the hydration process whereby water is added to rock minerals causing expansion and stress, making it weaker and more susceptible to other types of weathering. The following layers of rock, Purbeck beds and Wealden clays, were also broken through by the stream, eventually forming what can now be seen as the cove. Weathering and erosional processes are responsible for forming the entrance to the cove, by creating a narrow gap in the limestone bands. As well as processes such as hydration, carbonation may have been involved as well, as this works well on limestone. The carbonic acid reacts with the calcium carbonate, forming calcium bicarbonate, leading to weaknesses developing in the rock, which can then be exploited by further weathering or erosional processes.

Holderness Coastline

This stretch of coast is located in the North of England, running between Spurn Point (by Humber Estuary) and Flamborough Head, a total of around 61km. It is known for being one of the fastest retreating coastlines in Europe, as a result of the great erosion, weathering and mass movement which works on the coastline.

Geologically, there are bands of rock perpendicular to the sea (making it a stretch of discordant coastline), with chalk in the North, around Flamborough Head, then clay/till south of that. The clay is weaker than the chalk, so is more prone to succumbing to erosional and weathering processes, which can then lead to mass movement. Mappleton is a town along the Holderness coastline, which has experienced a significant extent of mass movement. When there is lots of precipitation, hydration, a type of chemical weathering can occur. Essentially, this is just the addition of water to the rock. The clay minerals absorb the water, causing expansion of the rock, resulting in stress. It weakens the rock, so it may become more susceptible to other forms of chemical weathering. In addition, when this stretch of clay is succumbed to a great amount of water, it leads to slumping, which is the main form of mass movement experienced at Mappleton. The water saturates the clay, so it becomes very lubricated and prone to slipping. The overlying weight from rock above forces down on the clay so it starts moving seawards.

At Flamborough Head there are coastal features on the headland such as caves and arches. These partially form as a result of weathering processes. The headland is made up of chalk, a relatively resistant rock. However, weathering processes still weaken it to some extent. Carbonation, a chemical weathering process, is particularly prominent in chalk. Rainwater has some dissolved carbon dioxide in, so becomes mildly acidic as carbonic acid. Chalk contains calcium carbonate, which reacts strongly with carbonic acid, so when rain reaches the chalk, they react to make calcium bicarbonate which dissolves quickly in water. This weakens the chalk significantly, so when erosional processes act on the headland, like hydraulic action and abrasion, it takes less to then form the caves and arches, as the rock has already been weakened by the weathering processes.

Along the Holderness bolder clay cliffs, the action of processes like hydration significantly weakens the rock, meaning sediment is more likely to come away from the cliff (it is amplified further after erosional processes). Wetting and drying is another physical weathering process which can contribute to this process. It is particularly active on clay, and involves the expansion of the rock when it is wet, and contraction when it dries. The constant changes cause cracks and weaknesses to develop, making it more prone to other kinds of weathering, like freeze-thaw, as well as erosional processes. This loosened sediment can then be transported up the beach in the longshore drift process. Over many years, this has led to the formation of Spurn Head, a large recurved spit. If weathering processes on the clay, like hydration and wetting and drying, the weakening of the rock would be more limited, so sediment transportation may also be limited due to slightly less sediment being removed from the cliffs.