Barcelona Field Studies Centre


Abrasion: wearing away of cliffs by sediment flung by breaking waves. See Corrasion.

Angle of Dip: the angle of the bedding planes which affects the shape of the cliff. If the rock layers are horizontal, small overhangs of more resistant rock form in the cliff face. If the bedding planes dip towards the sea, a gently sloping cliff face is formed with few overhangs. If the bedding planes dip away from the sea, a steep cliff face with many large overhangs of more resistant rock is formed.

Arch: wave-eroded passage through a small headland. This begins as a cave formed in the headland, which is gradually widened and deepened until it cuts through.

Aspect: the direction a cliff faces; facing the direction of the prevailing wind can lead to high rates of cliff erosion.

Attrition: erosion caused when rocks and boulders transported by waves bump into each other and break up into smaller pieces.

Backwash: the return of water to the sea after waves break on a beach.

Bar: where a spit grows across a bay. A bar can eventually enclose the bay to create a lagoon.

Bays: found between headlands where there are alternating outcrops of resistant (harder) rock and less resistant (softer) rock. Waves erode the areas of softer rock more rapidly to form bays. The more resistant, harder rock forms the headlands that protrude out to sea.

Beach: the temporary deposition of sand and shingle along the coastline. Without its beach a coast is vulnerable to erosion, e.g. the cliffs at Barton on Sea were easily eroded following the construction of a groyne updrift at Bournemouth.

Beach Depletion: the loss of beach material e.g. by offshore dredging of shingle banks.

Bedding Plane: a line in rocks separating two different layers: one usually more resistant to erosion, one usually weaker. The layers, deposited horizontally millions of years ago as sediment on the sea bed, have often been tilted through earth movements (tectonics), creating an angle of dip.

Beach Replenishment: the addition of new material to a beach naturally, through the action of longshore drift or artificially, through the dumping of large amounts of material.

Biological Weathering: the breakdown of rock through the action of plants and animals.

Breakwaters: offshore coastal defence structures built of stone parallel to the coastline; they help absorb the energy of breaking waves. Deposition occurs in the calmer water created behind the breakwater.

Cave: found in coasts formed of resistant rock. Corrasion, Corrosion and Hydraulic action widen any weakness within the rock e.g. joint, bedding plane or fault, to form a cave.

Chemical Weathering: the decomposition (or rotting) of rock caused by a chemical change within that rock; sea water causes chemical weathering of cliffs.

Clay Cliffs: clay is a soft, impermeable rock which soaks up water to become saturated. When this happens the clay becomes unstable and begins to slump. Clay cliffs have gentle slope angles.

Cliffs: hard, resistant rocks form steep cliffs; soft rocks such as clay create low, gentle cliffs.

Cliff Collapse: steep cliffs made of hard, resistant rock, fall down when there is a loss of supporting rock underneath caused by wave attack.

Cliff Drainage: steel barriers and drains put into a cliff to intercept the water movement through the cliff which causes mass movement.

Constructive Waves: found on low-angled beaches and mainly responsible for coastal deposition. They are gently breaking, with a much stronger swash than backwash.

Corrasion: wearing away of cliffs by sediment flung by breaking waves. See Abrasion.

Destructive Waves: found on steep beaches, are steeply breaking and mainly responsible for coastal erosion. Their backwash is much stronger than their swash.

Dredging: excavating sand and shingle from the sea bed; this can contribute to coastal erosion.

Erosion: the wearing away of the land by rivers, ice sheets, waves and wind.

Estuary:  the tidal mouth of a river where it meets the sea; wide banks of deposited mud are exposed at low-tide.

Exfoliation: a form physical weathering that occurs in very warm climates when a rock is repeatedly heated and cooled.

Fault: a large crack in the rock caused by earthquake movements.

Fetch: the maximum distance of water over which winds can blow. In the case of south-west England the maximum fetch is from the south-west (5000 miles). This also coincides with the direction of the prevailing wind and leads to large storm waves attacking Barton on Sea, particularly in Winter.

Fiord (or Fjord): a long, narrow, steep-sided inlet formed by glaciers and later drowned by a rise is sea level. Fjords are often over 3 kilometres deep.

Freeze-Thaw Weathering: also called frost-shattering as it occurs in cold climates when temperatures are often around freezing point and where exposed rocks contain many cracks. Water enters the cracks during the warmer day and freezes during the colder night. As the water turns into ice it expands and exerts pressure on the surrounding rock, causing pieces to break off.

Gabions: steel wire mesh filled with boulders used in coastal defences.

Geological Structure: see bedding planes, angle of dip and fault.

Groyne: a wooden barrier built out into the sea to stop the longshore drift of sand and shingle, and so cause the beach to grow. It is used to build beaches to protect against cliff erosion and provide an important tourist amenity. However, by trapping sediment it deprives another area, down-drift, of new beach material (beach replenishment). See Rock Strongpoints.

Headlands: areas of land protruding out to sea formed of resistant (harder) rock. They help protect the bay which forms between them from wave attack.

Human Factors: see Coastal Erosion Factors and Coastal Deposition Factors.

Hydraulic Action: the process by which breaking waves compress pockets of air in cracks in a cliff. The pressure may cause the crack to widen, breaking off rock.

Impermeable Rock: a rock that will not allow water to pass through it e.g. clay.

Joints: small cracks in the layers of rock created during earth movements.

Lagoon: a former bay cut off from the sea by a bar.

Land Reclamation: areas of land that were once below the sea; the sea has either been blocked off by dykes and the sea water pumped out (e.g. Dutch Polders), or material has been dumped into the sea to raise the level of the seabed until it becomes dry land.

Longshore Drift: waves approaching the coast at an angle result in the gradual zig-zag movement of beach materials along the coast.

Managed Retreat: allowing cliff erosion to occur as nature taking its course: erosion in some areas, deposition in others. Benefits include less money spent and the creation of natural environments.

Mass Movement: the downhill movement of weathered material under the force of gravity. The speed can vary considerably, from soil creep, where the movement is barely noticeable, to slumps, slides and mudflows, where the movement becomes increasingly more rapid.

Mud Flows or Slides: occur after periods of heavy rain when loose surface material becomes saturated and the extra weight causes the material to become unstable and move rapidly downhill in an almost fluid state.

Notch: an undercut part of the cliff base where wave attack concentrates erosion. See Wave Attack Zone.

Off-shore: out at sea, away from the land.

Onion Weathering: see Exfoliation.

Permeable Rock: allows water to percolate or pass through it e.g. limestone, sandstone and chalk.

Physical Weathering: the disintegration of rock into smaller pieces without any chemical change in the rock; this is most likely in areas of bare rock where there is no vegetation to protect the rock from extremes of weather e.g. freeze-thaw and exfoliation (or onion weathering).

Polders: areas of reclaimed land that were once part of the sea bed in the Netherlands. See lsselmeer Polders.

Prevailing Wind: the direction from which the wind usually blows.

Raised Beach: beach left stranded high on a cliff face after a fall in sea level.

Revetments: wooden, steel, or concrete fence-like structures that allow sea water and sediment to pass through, but the structures absorb wave energy. A beach can build up behind the revetment and provide further protection for the cliff. These are used as part of coastal defences.

Ria: a river valley drowned by a rise in sea level. It provides an excellent, natural, sheltered harbour.

Rip-Rap: large boulders dumped on the beach as part as part of coastal defences.

Rock Strongpoints: rocks dumped into sea to form a narrow artificial headland; these have replaced wooden groynes at Barton on Sea. Their aim is to control longshore drift of sediment in a similar way to wooden groynes and have proved to be more effective as they have a stronger structure to resist storm waves.

Saturation: loose surface material after heavy rain can become saturated and therefore unstable due to the extra weight, leading to mud slides. Where permeable sand rock overlays impermeable clay (e.g. the cliffs at Barton on Sea), the sand can become saturated and slump or slide along a shear plane.

Sea Level Changes: changes in the level of the sea against the land are caused by either the building up of melting of polar ice caps, or by rising and falling land levels.

Sea Defences: measures taken to defend the coast from erosion, cliff collapse and flooding.

Sea Walls: aim to prevent erosion of the coast by providing a barrier which reflects wave energy.

Sediment: material originating from rock weathering and erosion. Shingle and sand are examples found along the coast.

Sediment Cell: Sediment moved along the coast by longshore drift appears to form part of a circular cell which leads to it eventually returning updrift. Dredging of offshore shingle banks can therefore contribute to beach depletion.

Shear Plane: a bedding plane or dividing line between a permeable rock, e.g. sand, and an impermeable rock, e.g. clay. This can become saturated after prolonged heavy rain and provides a line over which part of the cliff can shear (break) away. See Slumping.

Slides: saturated weathered material moving down a slope under the influence of gravity. See Mud Slides.

Slumping: involves a whole segment of the cliff moving down-slope along a saturated shear-plane.

Soil Creep: the slowest of downhill movements, occurring on very gentle and well-vegetated slopes. Although material may move by less than 1 cm a year, its results can be seen in step-like terracettes on hillsides.

Spit: a long, narrow accumulation of sand and shingle formed by longshore drift and deposited where the coastline abruptly changes direction. One end of the spit is connected to the land and the other end projects out to the sea, often with a curved (hooked) end.

Stack: rock left standing out at sea after wave erosion has separated it from the mainland. This is the next stage from an arch. Waves will continue to erode the foot of the arch until its roof becomes too heavy to be supported. When the roof collapses, it will leave part of the former cliff isolated.

Storm Surge: a rapid rise in sea level caused by storms forcing water into a narrowing sea area. Low air pressure at the centre of the storm also causes sea levels to rise.

Stump: formed by continuing wave action attacking a stack until it collapses.

Swash: forward movement of a wave up a beach.

Terracettes: step-like features on banks and hillsides, the result of soil creep.

Terracing: the construction of horizontal steps down the cliff face; once these become vegetated, they help stabilise the cliff and prevent erosion.

Time: an important factor in coastal erosion and deposition.

Tombolo: a spit joining an island to the mainland.

Tourist Developments: resorts such as Barton on Sea wish to build their beaches to attract tourists who are an important source of income to the area. Cliff-top hotels, however, can actually contribute to erosion, creating an impermeable zone that increases saturation in the surrounding cliff area. Tourists walking on the cliff face also contribute to erosion by destroying vegetation.

Updrift: areas that provide a supply of material for deposition by longshore drift further along the coast. Updrift areas along the south coast of England are to the west.

Vegetation: a ground cover of bushes and grass on a cliff face helps prevent cliff erosion; their roots hold and trap (stabilise) soil and prevent it being lost by mass movement.

Waves: caused by the transfer of energy from the wind blowing over the surface of the sea. The largest waves are formed when winds are very strong, blow for lengthy periods and cross large expanses of water. See Fetch and Prevailing Wind.

Wave Attack Zone: the area between low and high tide where wave erosion is most effective.

Wave Cut Platform: a gently sloping, rocky platform found at the foot of an eroding cliff and exposed at low tide.

Wave Erosion: the power of the wave is generated by the fetch. Waves erode cliffs by abrasion/corrasion and hydraulic pressure.

Weathering: the break-down of rock by physical or chemical processes.