Durdle Door, Dorset
Coastal landscapes are formed by a combination of erosion, transportation and deposition processes.
The force of the sea changes the coastal landscape. Waves get their energy from the wind.
The size of the wave is determined by:
- the speed of the wind
- the length of time the wind has been blowing
- the distance of sea it has travelled over (the fetch)
The stronger the wave, the more erosion it will cause.
The four processes involved in erosion are:
Hydraulic Action is the sheer force of waves crashing against the shore and cliffs. The power of the waves forces air into cracks, compresses it and blows the rock apart as the pressure is released.
Attrition happens when rocks and pebbles carried by the waves smash into each other, wearing each other away and gradually becoming smaller, rounder and smoother.
Abrasion (also called corrasion) is the process of rocks and pebbles carried by the waves wearing away rocks as they are thrown against cliffs.
Solution (also called corrosion) is when chemicals in the seawater dissolve minerals in the rocks, causing them to break up.
Features of coastal erosion
Main Features of coastal erosion
How did the original headland shape become eroded to the present coastal landscape?
A number of stages are involved:
- All rocks have lines of weakness. The sea and its waves use hydraulic action, abrasion, attrition and solution to erode along any lines of weakness. Undercutting takes place all around the headland.
- These lines of weakness get enlarged and develop into small sea caves.
- The caves are deepened and widened on both sides of the headland until eventually the sea cuts through the headland, forming an arch.
- The rock at the top of the arch becomes unsupported as the arch is enlarged, eventually collapsing to form a stack.
- The stack gets eroded until only a stump remains.
- Over time the stump will disappear.
- As the headland retreats under this erosion, the gently sloping land at the foot of the retreating cliff is called a wave-cut platform.
Coastal transportation and deposition
Transportation of particles along a coastline is influenced by the strength of waves and the angle at which the waves strike the shoreline. This is determined by the direction from which the prevailing wind blows.
- A pebble or sand particle moves from point A to B, carried by the swash up the beach, the angle determined by the wave and wind direction.
- It is then pulled down the beach from B to C, carried by gravity and the wave’s backwash.
- This process is repeated over and over again and the particle moves along the shoreline. This process is called longshore drift.
- When large numbers of sand particles or pebbles are moved along a coastline in this way, a depositional feature called a spit is formed.
Features of Coastal Deposition
Depositional features produced by longshore drift include spits, bars and tombolos. The main features of coastal deposition are shown on the diagram below.
The diagram shows the building of a spit by longshore drift across the mouth of a river. Sandspits often have a curved or hooked end as a secondary wind and wave direction curves the end of the spit as waves strike from this second and different direction. A series of such hooks can develop over time. The spit creates an area of calmer water, sheltered by the spit. A lagoon, salt marsh and finally dry land can develop in this sheltered area.
Landscapes of glacial erosion on an OS map
Landscapes of glacial erosion are found in the North and West of Britain including the North-west Highlands, the Cairngorms, the Lake District (Cumbria) and Snowdonia (part of the Welsh Cambrian Mountains). You should know how to describe and explain features of glacial erosion and be able to recognise them on an OS map.
Corrie and tarn
A corrie is an armchair shaped hollow, high on a mountain with steep back and side walls. After glaciation, the hollow may be filled by a small lake or tarn.
Snow gathers in mountain hollows, especially north facing hollows, where there is more shade. This snow builds up and compacts to ice (neve). The action of gravity means the ice moves down the hill. As it goes, it sticks to back walls and plucks rock from the surface. Rocks on the backwalls are loosened by freeze-thaw action. A gap between the wall and the ice develops, called abergschrund. Ice moving with loose rock acts like sandpaper and deepens the hollow by abrasion. Most erosion is where the weight of the ice is the heaviest. Stones frozen in the base of the ice grind or abrade the corrie base, deepening it. Ice in a corrie has a rotational movement which means that the front of the corrie is less eroded, and a lip forms. The glacier retreats and melts, often leaving a Tarn/glacial lake in the base of the corrie.
An arête is a narrow knife-edged ridge where two corries have eroded back to back. That is, when the back walls of a corrie have been eroded back so far that only a narrow ridge separates them.
Pyramidal peaks or horns have a sharp summit and steep slopes on at least three sides. A pyramidal peak may form where three or more corries erode back so far that they produce aretes with a pyramidal peak in between.
U-shaped valleys have steep sides and a wide, flat floor. They are usually straight and deep.
U-shaped valleys are formed in river valleys which, during the ice age, have been filled by a large glacier. These glaciers have deepened, straightened and widened the valley by plucking and abrasion.
Hanging valleys and truncated spurs
A hanging valley is a smaller side valley left ‘hanging’ above the main u-shaped valley. A waterfall can often be seen. During glaciation the smaller side valley contains less ice than the main glacial valley, which is why it is not as deeply eroded.
Truncated spurs are rounded areas of land which have been cut off. They are often rounded at the top but steep at the bottom. They are formed when glaciers move through the main valley and cut off spurs.
Ribbon lakes and misfit rivers
A ribbon lake is a large, narrow lake occupying a u-shaped valley. It forms in a hollow when a glacier has more deeply eroded less resistant rock or it may fill up a valley behind a wall of moraine across the valley.
Misfit rivers meander through the flat, wide U-shaped floor. They did not erode the valley, as they formed in the valley after glaciation had carved out the U-shaped valley.
Landscapes of glacial deposition
Eratic boulder, Yosemite National Park, USA
Around 10,000 years ago as the ice age advance began to melt, glacial deposits or drift were left behind. These glacial deposits were of two kinds:
- Till – mixed or unstratified materials directly deposited by ice. Examples of till deposits include drumlins, moraines and erratics.
- Fluvioglacial – layered or stratified materials deposited in layers by meltwater. When ice is melting, materials are sorted in the water. Examples of fluvioglacial deposits include eskers, kames or kame terraces.
Drumlins are oval hills which form in groups called swarms. The unsorted till appears moulded by ice to form a blunt end with a more streamlined, gentler lee slope.
Moraines are mounds of poorly sorted till where rock debris has been dumped by melting ice or pushed by moving ice. The different types of moraine include terminal moraine, which marks the end of an ice sheet or valley glacier, and lateral moraine, which forms at the edge of a glacier at the valley side.
Erratics are boulders carried by ice, often for many kilometres, and deposited in areas of completely different rock type.
Outwash plains are areas of sorted sand and gravel deposited at the mouth of meltwater rivers which were often braided. Kettle holes may be found if a block of ‘dead ice’ is partially buried by fluvio-glacial deposits. When the ice melts a ‘hole’ is left which may fill with water to form a kettle-hole lake.
Eskers are long, winding ridges of layered sand and gravel similar to railway embankments. They are formed inside the ice, in tunnels in which meltwater streams flowed.
Kame terraces are gentler slopes of layered sand and gravel at the side of valleys. They were formed at the edge of the glacier and valley side where meltwater forms on the surface.
Different mass movements occur on slopes under different conditions. We’ll look at four types; rockfall, mudflow, landslip and soil creep.
Rockfall is the rapid, free-fall of rock from a steep cliff face. Rock fragments fall from the face of the cliff because of the action of gravity. This is made worse by freeze-thaw action loosening the rock. Bare, well-jointed rock is very vulnerable to rockfall – water enters the joint, freezes and expands, cracking the rock. A scree slope of fallen rock is formed at the bottom of the cliff.
Mudflow occurs on steep slopes over 10°. It’s a rapid sudden movement which occurs after periods of heavy rain. When there is not enough vegetation to hold the soil in place, saturated soil flows over impermeable sub soil, causing great devastation and endangering lives.
Landslips or landslumps are occasional, rapid movements of a mass of earth or rock sliding along a concave plane. They can occur after periods of heavy rain, when the water saturates overlying rock, making it heavy and liable to slide. Undercutting of a steep slope by river or sea erosion weakens the rock above, also making a slump likely.
Soil creep is a very slow movement, occuring on very gentle slopes because of the way soil particles repeatedly expand and contract in wet and dry periods. When wet, soil particles increase in size and weight, and expand at right angles. When the soil dries out, it contracts vertically. As a result, the soil slowly moves downslope.