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Geomorphic Processes

  • Weathering is when rock is broken down
    • Mechanical weathering is the breakdown of rock without changing its chemical composition
      • Freeze-thaw weathering occurs when rocks are porous (contain holes) or permeable (allow water to pass through)
      • Water enters cracks in the rock
      • When temperatures drop, the water freezes and expands causing the crack to widen
      • The ice melts and makes its way deeper into the cracks
      • The process repeats until the rock splits entirely
    • Biological weathering is the breakdown of rock by living things
      • Plants and animals influence rocks
      • Roots burrow into the rock until it breaks away
      • Plants can get into small cracks in the rock
      • As the roots grow, the cracks become larger
      • This causes small pieces of rock to break away
    • Chemical weathering is the breakdown of rock by changing its chemical composition
      • Carbonation weathering is a type of chemical weathering that occurs in warm and wet conditions
      • Rainwater has carbon dioxide dissolved in it which makes it a weak carbonic acid
      • This reacts with the rock (e.g. limestone) that contains calcium carbonate
      • Forms calcium bicarbonate which is soluble in water and therefore deepens the cracks
  • Mass movement is the downhill movement of sediment that moves because the force of gravity is stronger than the force supporting the mass
    • Mass movement causes coasts to retreat rapidly
    • The process is more likely to occur when permeable rock is saturated because it acts as a lubricant and makes it heavier
    • Undercutting a slope by erosion and weathering of rock increase the chance of mass movement
    • There are slides – material shifts down a slope in a straight line
    • There are slumps – material shifts down a slope with a rotation
  • There are four processes of erosion that occur
    • Hydraulic action occurs when the force of the water goes against rock; the water compresses the air in the cracks; this puts pressure on the rocks; repeated compression widens the cracks and causes bits of rock to break off
    • Abrasion occurs when eroded particles that are already in the water scrape and rub against rock, removing small pieces and wearing them away
    • Attrition occurs when eroded rock pieces that are already in the water smash into each other and break into smaller fragments; the edges of the rocks get rounded off as they rub together
    • Solution occurs when dissolved carbon dioxide makes river and sea water slightly acidic; the acid reacts chemically with some rocks and dissolves them
  • Eroded material can be transported in four different ways
    • Solution occurs when minerals in rocks like chalk and limestone are dissolved in sea water and then carried in solution (the load is not visible)
    • Suspension occurs when small particles such as silts and clays are suspended in the flow of the water (the water may look cloudy)
    • Saltation occurs when pebble-sized particles are bounced along the sea floor or river bed by the force of water
    • Traction occurs when pebbles and larger material are rolled along the sea floor or river bed by the force of water
  • Deposition is when material being carried by the water is dropped; it occurs when the water carrying sediment loses velocity, so it can’t carry so much sediment
    • Coastal deposition occurs with constructive waves because they deposit more material than they erode
      • The amount of material that’s deposited increases when there’s lots of erosion elsewhere on the coast or when there’s lots of transportation of material into the area
    • River deposition occurs when the volume of the water in the river falls, when the amount of eroded material in the water increases, where the water is shallower, or when the river reaches the sea or lake at its mouth

Coastal Landforms

  • Headlands and bays form when there are alternating bands of resistant and less resistant rock along a coast
    • The less resistant rock is eroded to form bays with gently sloping sides
    • The resistant rock is left jutting out, forming a headland with steep sides
  • Caves, arches, stacks and stumps are formed on a headland
    • Waves erode the base of the cliff through hydraulic action and abrasion
    • This leaves notches, which progress to form caves
    • The cave becomes larger and breaks through the headland to form an arch
    • The base of the arch becomes wider through further erosion and the roof is weathered, until it is too heavy and collapses, leaving a stack
  • Beaches are found on coasts between the high watermark and the low watermark
  • They’re formed by constructive waves depositing materials such as sand and shingle
    • Sandy beaches are flat and wide; sand particles are small, and the weak backwash can move them back down the beach, creating a long, gentle slope
    • Shingle beaches are steep and narrow; shingle particles are large, and the weak backwash can’t move them back down the beach, so the shingle particles build up and create a steep slope
  • Longshore drift is the movement of sediment across a coastline by waves
    • Swash, in the direction of the prevailing wind, carries material up the beach at an angle
    • Backwash carries material straight back down
    • Overtime, material zigzags along the coast
  • A spit is formed where longshore drift continues across a river mouth
    • Waves and wind push material up the river, curving the end of the spit
  • The sheltered area behind the spit is protected from waves so lots of material accumulates in the area, meaning plants can grow there
    • This sheltered area can become a mud flat or a salt marsh
  • Sometimes a spit can grow across a bay, and joins two headlands together, forming a bar
    • Shallow lakes can be trapped behind them, and these are known as lagoons
    • Lagoons do not last forever and may be filled up with sediment
  • Sometimes, a spit can reach an island and connect it to the mainland, forming a tombolo

River Landforms

  • Waterfalls and gorges are found in the upper course of a river
    • Waterfalls form when a river flows from a hard rock onto a softer rock
    • Erosion of the soft rock by the waterfall undercuts the hard rock above
    • Hydraulic action by the fast-flowing water in the plunge pool weakens the soft rock
    • Without the support of underlying rock, the ledge of hard rock collapses
    • Abrasion by the broken lumps of rock in the plunge pool continues to erode the soft rock
    • The waterfall slowly retreats upstream, creating a gorge
  • V-shaped valleys are formed by vertical erosion in the upper course of a river
    • In the upper course of a river, fast-flowing water, following heavy rain and high turbulence causes loose rough particles and boulders to be transported by the river and scraped along the river bed
    • This causes downwards erosion of the river channel by the process of abrasion
    • The valley sides are exposed to weathering; the weathered material that falls down the valley sides into the river channel causes further erosion by abrasion
    • The river doesn’t usually have enough energy to erode laterally, so vertical erosion of the river bed is dominant, which deepens the river valley, creating a steep-sided V-shape
  • Floodplains and levees are found in the lower course of a river
    • As the river nears its moth, it widens further, and its valley becomes flatter and wider
    • The river may flood, depositing fine silt, or alluvium, on the valley floor to form a floodplain
    • Material deposited close to the river raises the height of the river banks to form levees
      • The heaviest material is deposited closest to the river channel, because it gets dropped first when the river slows down
      • Overtime, the deposited material builds up, creating levees along the edges of the channel
    • Levees raise the height of the river bank, and the river returns to its normal level
  • Rivers develop meanders and ox-bow lakes in the middle and lower courses of a river
    • Rivers naturally bend due to the way water flows in a corkscrew fashion
    • The current is faster on the outside of the bend because that’s where the river channel is deeper so there’s less friction to slow the water down
      • More erosion takes place on the outside of the bend, forming river cliffs
    • The current is slower on the inside of the bend because the river channel is shallow so there’s more friction to slow the water down
      • Eroded material is deposited on the inside of the bend, forming slip-off slopes
    • Erosion causes the outside bends to get closer until there’s only a small bit of land left between the bends, called the neck
    • The river breaks through the neck, usually during a flood, and the river flows along the new, shorter course
    • Deposition eventually cuts off what was the meander, forming an ox-bow lake

Case Study: Coastal Landscape, Dorset Coastline

  • Dorset is in the south of England, its coastline has many examples of erosional and depositional landforms
  • The Foreland is a headland made from chalk in between two bands of softer rock (Studland Bay and Swanage Bay)
    • An arch at the end of the Foreland collapsed to form a stack; Old Harry and a stump called Old Harry’s Wife
    • The headland, stack and stump are being eroded by hydraulic action and abrasion, and weathered by salt and carbonation weathering, and by the vegetation growing through cracks
  • Lulworth Cove is a small bay formed after a gap was eroded in a band of limestone
    • Behind the limestone is a band of clay; the clay is softer, so it has been eroded and transported away forming the bay
    • The limestone cliffs forming the back wall of the bay are vulnerable to mass movement, and sometimes experience small slides and slumps
  • Durdle Door is an arch formed on a hard limestone headland
    • Erosion by waves opened a crack in the headland, which became a cave and the developed into an arch
    • The arch is gradually broken down by mechanical, chemical and biological weathering
  • The cliffs backing Swanage Bay are made of clay
    • Towards the northern end of the bay, the cliffs are covered in vegetation, stabilising them and protecting them from mechanical and chemical weathering
    • Elsewhere, the cliffs are not stabilised by vegetation, so wet weather weakens them and can cause slumps
    • Longshore drift carries material from the south to the north of the beach in the bay
    • Overall, erosion is the dominant process in the bay
  • Chesil Beach is a tombolo that joins the Isle of Portland to the mainland
    • It has been formed by longshore drift
    • Behind Chesil beach is a shallow lagoon called The Fleet Lagoon
  • The coastline is made from bands of hard rock and soft rock; the rocks have been eroded at different rates, creating different landforms
    • Soft rock like sandstone or clay are easily eroded by hydraulic action and abrasion
    • The harder chalk and limestone cliffs are weathered and eroded more slowly, meaning that they stick out into the sea as exposed headlands
      • Chalk and limestone are vulnerable to erosion by solution, where the water chemically reacts with the rock, causing it to dissolve
    • Weathering tends to happen gradually and cause minor changes
    • Erosion can happen more suddenly on a much larger scale
      • A single storm can cause substantial amounts of erosion along a big stretch of coast
  • The Dorset Coast has warm, dry summers (21°C) and mild, wet winters (3°C)
    • Salt weathering is the dominant form of mechanical weathering, particularly in summer
      • The warm temperatures cause sea water to evaporate from rocks quickly, leaving a build-up of salt crystals in tiny cracks in the rock
    • The mild winters mean that freeze-thaw weathering is less common because it’s not usually cold enough for ice to form
  • The Dorset coast’s location means that it’s exposed to prevailing winds from the south west
    • These prevailing winds can bring storms to the UK from the Atlantic Ocean
    • Storms bring high energy, destructive waves which increase erosion of the cliffs
    • Hydraulic action and abrasion both increase during a storm and erode the base of the cliffs
    • This makes the cliffs unstable, making mass movement more likely to happen
  • The Dorset coast receives relatively low amounts of rainfall annually but can experience very wet winters, with rainfall heaviest during storm periods
    • Soils and rocks become heavier when they are saturated, which can make them prone to mass movement
    • In January 2016, intense rainfall combined with high-energy waves during Storm Frank to cause the collapse of cliffs between Burton Bradstock and West Bay
  • Coastlines like the Dorset Coast need to be managed to balance the geomorphic process against the need to protect people and infrastructure
  • Two types of coastal management strategies are hard engineering and soft engineering
    • Hard engineering works against natural processes to protect the coastline, it is often more expensive and requires maintenance
    • Soft engineering works with nature to protect the coastline, it is often less expensive and doesn’t require much maintenance as it doesn’t depend on building structures
  • A sea wall is built along the sea front to protect cliffs and buildings
    • There are concrete sea walls in places along most of Swanage beach
    • Sea walls are beneficial as they reflect the waves back out to sea, preventing erosion and flooding of the coast
    • However, they can create a strong backwash which removes sediment from the beach and the area under the wall can get eroded
    • The prevention of erosion of the cliffs can be bad for the coastline as there’s no new material being eroded to replenish the beach, so the beach level will gradually lower
    • These are very expensive and do not look natural or blend in with the coast
  • Groynes are barriers built at right angles to the beach to prevent the transport of materials by longshore drift
    • In 2005-6, timber groynes were put in place along Swanage beach
    • They’ve helped to stop the loss of beach material
    • However, by stopping beach material from moving along the coast, they’ve starved the areas further down of sediment, making narrower beaches which don’t protect the coast, leading to erosion there
    • These are not too expensive however they are ugly and interfere with the scenery
  • Beach replenishment makes the beach wider by importing more sand
    • In 2005-6, sand and shingle dredged from the sea bed at Poole Harbour were added to the upper parts of Swanage Beach
    • This has created wider beaches, which slow the waves and help protect cliffs and coastal properties from erosion
    • However, it cost £5 million to replenish the beach and needs to be repeated every 20 years
  • Human activity on the land also affects geomorphic processes; these include industry and tourism
    • A lot of quarrying has taken place along the coast because limestone is a valuable building stone
      • There are several quarries on the Isle of Portland and to the west of Chesil Beach
      • Up until the 1960s, gravel was moved from Chesil Beach for use in the construction industry; materials were removed from the beach quicker than it was replenished by the sea, damaging the landform
    • The Dorset coastline attracts large numbers of tourists each year
      • Coastal footpaths run along the cliff tops and are gradually worn down as people repeatedly walk on them
      • Vegetation along the cliff top may be trampled on and worn away by repeated use of the footpaths; this can expose the underlying rock and soil to weathering and erosion by wind and rain

Case Study: River Basin, Eden Basin

  • The Eden Basin is in north-west England, between the mountains of the Lake District and Pennines
    • The River Eden’s source is in the Pennine hills in south Cumbria; it flows north-west through Appleby-in-Westmorland and Carlisle; its mouth is in the Solway Firth at the Scottish border
    • The river basin is a largely rural area, with many scenic landscapes that are popular with tourists; there are a variety of river landforms
  • Hell Gill Force is a waterfall at the source of the River Eden
    • It has formed where there is a change in the rock type from hard limestone to softer sandstone
    • The water has eroded the sandstone forming a step in the river channel
    • Below the waterfall there is a steep-sided gorge, left behind as the waterfall has retreated up the valley
  • Many streams flow down the steep slopes of the hillsides at the edge of the basin from about 600m above sea level
    • Weathering by freeze-thaw, transportation by traction, and erosion by abrasion have carved out steep-sided V-shaped valleys, such as in the north-east Lake District
  • Carlisle is built on the floodplain of the River Eden
    • Here, the land is low-lying and flatter
    • As meanders have migrated across the valley floor, the floodplain has become wider
    • Sediment has also been deposited when the river has flooded, building up the floodplain
  • As more tributaries join the River Eden, the river gets bigger; giving it more power to erode the river channel sideways
    • In the lower course, the river valley becomes wider and flatter, and meanders form on the valley floor, such as in Salkeld
    • As these meanders have grown, some have been cut off to form ox-bow lakes, such as where Briggle Beck joins the Eden near Salkeld
  • The harder rocks around the edge of the Eden basin have remained as high ground as they are resistant to erosion, however exposed limestone is open to carbonation weathering
    • Igneous rocks, such as those found in the west of the Eden basin, tend to be impermeable
      • Because water can’t soak into the ground, high rainfall causes surface streams to form, which have a lot of power to erode vertically, creating steep-sided V-shaped valleys
    • Through the middle and lower courses of the Eden, the river valley is made up of sandstone (soft rock)
      • The river’s increasing volume and energy in its lower course mean that there’s lots of lateral erosion of the sandstone
      • This widens the river channel and forms meanders and steep river cliffs
  • Cumbria is on the west coast of the UK, facing the prevailing south-westerly winds
    • As a result, Cumbria’s climate is mild and wet,
    • The area generally has cool summers and mild winters
  • Cumbria is one of the wettest parts of the UK, often experiencing periods of intense rainfall
    • Many of the UK’s highest rainfall records were recorded in Cumbria
  • Despite the generally mild winters, temperatures can be much colder on higher ground, such as the land around the source of the River Eden
    • In winter, this higher ground can regularly freeze
    • During these cold periods, freeze-thaw weathering can slowly break up the exposed rock of the valley sides in the upper course of the river
    • If the valley sides are weakened, sudden mass movement, such as landslides, become more likely
    • Material from landslides is added to the river’s load, increasing the erosive power of the river through abrasion
  • During periods of intense rainfall, the ground can become saturated
    • This makes it heavier and less stable; this can cause the river banks to slide or slump into the river channel
    • Heavy rain can flow quickly over the surface and into the River Eden and its tributaries; this can cause the volume of the water in the river to rapidly increase
    • The high volume of water can increase transportation of material by the river, which can cause more erosion by abrasion (particularly in the upper course of the river)
  • The rivers in the Eden basin have been managed to meet the needs of people in the area; the strategies have affected the geomorphic processes in the river basin; these include flood walls and embankments, reservoirs, tree planting and channel management
  • 10km of raised flood defences have been built along the River Eden and Caldew in Carlisle
    • These are designed to contain water within the river channel, so that the floodplain can be built on
    • They interrupt the natural processes of the river and can prevent the natural formation of meanders and the deposition of sediment on the floodplain
  • Castle Carrock beck has been damned to create a reservoir
    • Reservoirs limit the natural flow of the water downstream
    • Material carried by the river is deposited in the reservoir and not built up along the river’s natural course
    • This can increase erosion downstream, and reduce the natural build-up of the floodplain in the lower course of the river
  • Near Dalston, the landscape has been changed by the planting of 1000 trees to reduce flooding and to reduce erosion by stabilising the soil
    • Trees intercept rainfall and reduce surface runoff
    • This prevents rapid increase in the volume of water in the river because it takes longer for water to reach the river channel
    • As a result, the river will have less energy, reducing lateral and vertical erosion, meaning meanders and ox-bow lakes may be less likely to form
  • In the past, the river landscape in the Eden basin was changed by channel straightening
    • Many sections of the river were diverted into artificial channels to try and reduce flooding
    • Channel straightening makes the water flow more quickly than it naturally would, which can increase erosion and decrease deposition
    • In the artificial channel, conditions aren’t right for meanders to form as they usually would; so, the natural river landscape is changed
    • More recently, some areas of the Eden basin have been restored to their original state by having artificial meanders put in
  • This happened on the River Lyvennet, to the south-west of Appleby
    • These meanders slow the river’s flow, increasing deposition
    • This can encourage the development of more meanders, and allows the natural build-up of the floodplain
  • Human activity on the land also affects geomorphic processes; these include deforestation and farming
    • Natural woodland and heathland have been cleared from many upland areas in the Eden basin
      • This increases surface runoff when it rains, and means that more water ends up in river channels more quickly
      • This increase in volume gives rivers more energy for erosion, and can cause sliding and slumping of the river banks
    • Some upland areas have been drained of moisture to make them more suitable for farming
      • This reduces the stability of the soil, meaning that more of it is washed into the river channel by rain
      • The increased load of the river increases deposition downstream, changing the floodplain landscape from its natural state