2 - How are coastal landforms developed?

Question 1

What are the key geomorphic processes?

Answer 1

Weathering
Mass movement
Wave processes
Fluvial processes
Aeolian processes

Question 2

What is weathering?

Answer 2

Uses heat energy to produce physically or chemically altered materials from surface or near surface rock, in coastal environments weathering can influence the formation of coastal landforms
Typically broken into physical, chemical and biological

Question 3

What is physical / mechanical weathering?

Answer 3

• Breakdown of rock is largely achieved by physical weathering processes that produce smaller fragments of the same rock
• No chemical alteration takes place
• By increasing the exposed surface area of the rock, physical weathering facilitates further weathering
• In many coastal landscapes, such as western Europe, the presence of the sea results in the moderation of temperatures, so air temperature rarely drops below 0 degrees, reducing the extend of fluctuations and rendering some of the physical weathering processes ineffective

Question 4

What are some examples of processes of physical weathering typical of coastal environments?

Answer 4

Freeze-thaw
Pressure release
Thermal expansion
Salt crystallisation

Question 5

What is freeze-thaw weathering?

Answer 5

Water enters cracks/ joins and expands by nearly 10% when it freezes, in confined spaces this exerts pressure on the rock causing it to split or pieces to break off, even in very resistant rocks

Question 6

What is pressure release?

Answer 6

When overlying rocks are removed by weathering and erosion, the underlying rock expands and fractures parallel to the surface as previously present pressure is released - this is significant in the exposure of sub-surface ricks such as granite
(also known as dilation)
(parallel fractures sometimes called pseudo-bedding planes)

Question 7

What is thermal expansion?

Answer 7

Rocks expand when heated and contract when cooled, therefore if they are subject to frequent cycles of temperature change then the outer layers may crack and flake off,, however experiments have cast doubts on its effectiveness unless water is present
(also known as insulation weathering)

Question 8

What is salt crystallisation?

Answer 8

Solutions of salt can seep into the pore spaces in porous rocks, here the salts precipitate, forming crystals - the growth of these crystals creates stress in the rock causing it to disintegrate
Sodium sulphate and sodium carbonate are particularly effective, expanding by about 300% in areas of temperature fluctuating around 26-28 degrees

Question 9

What is chemical weathering?

Answer 9

• Decay of rock is the result of chemical weathering, which involved chemical reactions beaten moisture, temperature and some minerals within the rock
• Chemical weathering may reduce the rock to its chemical constituents or alter its chemical and mineral composition
• Chemical weathering processes produce weak residues of different material that may then be easily removed by erosion or transportation processes
• Rate of most chemical reactions increases with temperature, Van’s-Hoff’s law states that a 10 degree increase in temperature leads to a 2.5 times increase in the rate of chemical reaction, so most chemical weathering processes occur at higher rates in tropical rather than temperate or polar regions
• Moist tropical environments experience the fastest rates of chemical weathering and cold, dry locations the slowest, however it is worth noting that carbonation can be more effective in low temperatures as CO2 is more soluble in cold water than in warm water
• Some weathering processes are especially important when rocks ate in contact with weakly acidic water, however unless affected by pollution, sea water is typically neutral to slightly alkaline
• One issue associated with climate change and increasing levels of atmospheric CO2 is that rainfall, and therefore sea water is becoming more acidic

Question 10

What are some examples of processes of physical weathering typical of coastal environments?

Answer 10

Oxidation
Carbonation
Solution
Hydrolysis
Hydration

Question 11

What is oxidation?

Answer 11

Some minerals in rocks react with oxygen, either in the air or in water - iron is especially susceptible to this process, it because soluble under extremely acidic conditions and the original structure is destroyed
It often attacks the iron-rich cements that bind sand gains together in sandstone

Question 12

What is carbonation?

Answer 12

Rainwater combines with dissolved carbon dioxide from the atmosphere to produce a weak carbonic acid, this reacts with calcium carbonate in rocks such as limestone to produce calcium bicarbonate, which is soluble
This process is reversible and precipitation of calcite happens during evaporation of calcium rich water in caves to form stalactites and stalagmites

Question 13

What is solution?

Answer 13

Some salts are soluble in water, other minerals such as iron are only soluble in very acidic water, with a pH of about 3 - any process by which a mineral dissolves in water is known as solution, although mineral specific processes, such as carbonation, can be identified

Question 14

What is hydrolysis?

Answer 14

Chemical reaction between rock minerals and water silicates combine with water to produce secondary minerals such as clays, eg. feldspar in granite reacts with hydrogen in water to produce kaolin

Question 15

What is hydration?

Answer 15

Water molecules added to rock minerals create new minerals of a larger volume, this happens when anhydride takes up water to form gypsum
Hydration causes surface flaking in many rocks, partly because some minerals also expand by about 0.5% during the chemical change because they absorb water

Question 16

What is biological weathering?

Answer 16

• May consist of physical actions such as the growth of plant roots or chemical processes such as as chelation by organic acids
• Although this arguably does not fit with the precise definition of weathering, biological processes are usually classed as a separate type of weathering

Question 17

What are some examples of processes of biological weathering typical of coastal environments?

Answer 17

Tree roots
Organic acids

Question 18

How do tree roots contribute to biological weathering?

Answer 18

Tree roots grow into cracks or joints in rocks and exert outward pressure, operates in a similar way, with similar effects to freeze-thaw
When trees topple, their roots can also exert leverage on rock and soil, bringing g them to the surface and exposing them to further weathering, burrowing animals may have a similar effect
This may be particularly significant on cliff tops and cliff faces

Question 19

How do organic acids contribute to biological weathering?

Answer 19

Organic acids produced during decomposition of litter cause soil water to become more acidic and react with some minerals in a process called chelation
Blue-green algae can have a weathering effect, producing a shiny film of iron and manganese oxides on rocks
On shore platforms, molluscs may excrete acids which produce small surface hollows in the rock

Question 20

What is mass movement?

Answer 20

This occurs when the forces acting on slope material, mainly the resultant force of gravity, exceed the forces trying to keep the material on the slope, predominantly friction
In coastal landscape systems, the most significant mass movement processes are those acting on cliffs, which lead to the addition of material to the sediment budget by transferring rocks and regolith down onto the shore below

Question 21

What are the main processes involved in mass movement?

Answer 21

Rock fall
Slides

Question 22

What is rock fall?

Answer 22

On cliffs of 40 degrees or more, especially if the cliff face is bare, rocks may become detached from the slope by physical weathering processes, these rocks then fall to the foot of the cliff under gravity
Wave processes usually remove this material, or it may accumulate as a relatively straight, lower angled scree slope

Question 23

What are slides?

Answer 23

May be linear, with movement along a straight line slip plane, such as a fault or a bedding plane between layers of rock or could be rotational, with movement taking place along a curved slip plane (rotational slides are also known as slumps)
In coastal landscape systems, slides often occur due to undercutting by wave erosion at the base of the cliff which removed support for the materials above
Slumps are common in weak rocks, such as clay, which also become heavier when wet, adding to the downslope force - a layer of sand above a layer of clay may particularly encourage this, as rainwater passes through the sand but cannot penetrate the impermeable clay below, this increasing pore pressure in the sands

Question 24

What are wave processes?

Answer 24

Waves are a source of energy in coastal landscapes system, and when they break on-shore, the energy can be expended through geomorphic processes to shape landforms, they can also supply material to the system in the form of sediment, which is either deposited in, or transported within, the coastal system

Question 25

What are the erosional wave processes?

Answer 25

Abrasion
Attrition
Hydraulic action
Pounding
Solution

Question 26

What is abrasion?

Answer 26

When waves armed with rock particles scour the coastline, rock rubbing against rock
(or corrasion)

Question 27

What is attrition?

Answer 27

Occurs when rock particles, transported by wave action, collide with each other and with coastal rocks and progressively become worn away, they become smoother and more rounded as well as smaller, eventually producing sand

Question 28

What is hydraulic action?

Answer 28

Occurs when waves break against a cliff face, and air and water trapped in cracks and crevices becomes compressed, as the wave recedes the pressure is released, the air and water suddenly expands and the crack is widened, the average pressure exerted by breaking Atlantic waves is 11,000kg per m^3

Question 29

What is pounding?

Answer 29

Occurs when the mass of a breaking wave exerts pressure on the rock causing it to weaken, forces of as much as 30 tonnes per m^2 can be exerted by high-energy waves

Question 30

What is solution?
(erosion)

Answer 30

Involves dissolving minerals like magnesium carbonate minerals in coastal rock, however as the pH of sea water is invariably around 7 or 8 this process is usually of limited significance unless the water is locally polluted and acidic, even then only coastal rocks containing significant amounts of soluble minerals are likely to be affected by this

Question 31

What are the transportational wave processes?

Answer 31

Solution
Suspension
Saltation
Traction

Question 32

What is solution?
(transportation)

Answer 32

Minerals that have been dissolved into the mass of moving water, this type of load is invisible and the minerals will remain in solution until water is evaporated and they precipitate out of solution

Question 33

What is suspension?

Answer 33

Small particles of sand, silt and clay can be carried by currents, this accounts for the brown or muddy appearance of some sea water, larger particles can also be carried in this way, perhaps during storm events

Question 34

What is saltation?

Answer 34

Series of irregular movements of material which is too heavy to be carried continuously in suspension, turbulent flow may enable sand-sized particles to be picked up and carried for a short distances only to drop back down again, similarly, other particles may be dislodged by the impact, allowing water to get beneath them and cause entrainment (to be picked up)

Question 35

What is traction?

Answer 35

The largest particles in the load may be pushed along the sea floor by the force of the flow, although this can be called rolling, again the movement is rarely continuous, large boulders may undertake a partial rotation before coming rest again

Question 36

What is longshore drift?

Answer 36

Once deposited on-shore, sediment may be moved along the shoreline by longshore drift, this occurs when waves approach the coast at an angle due to the direction of the dominant wind
When the waves have broken, the swash carries particles diagonally up the beach, under the influence of gravity the backwash moves them perpendicularly back down the beach - if this movement is repeated, the net result is a movement of material along the beach, this also leads to the attrition of beach sediment, so particles tend to become smaller and more rounded with increasing distance along the beach

Question 37

What is deposition?

Answer 37

Material is deposited when there is a loss of energy caused by a decrease in velocity and/or volume of water

Question 38

Where does deposition tend to take place in coastal landscape systems?

Answer 38

• Where the rate of sediment accumulation exceeds the rate of removal
• When waves slow down immediately after breaking
• At the top of the swash, where for a brief moment, the water is no longer moving
• During the backwash, when water percolates into the beach material
• In low-energy environments, such as those sheltered from winds and waves eg.estuaries

Question 39

In what order are particles deposited?

Answer 39

The velocity at which sediment particles are deposited is known as the settling velocity, the larger and heavier particles require more energy to transport them
As velocity decreases, the largest particles being carried are deposited first and so on, sequentially until the finest particles are deposited

Question 40

What are fluvial processes?

Answer 40

In coastal environments such as river mouths, fluvial processes often play an important part in the development of landforms, low-energy, estuarine environments have distinctive characteristics

Question 41

What is fluvial erosion?

Answer 41

Fluvial erosion in the upper catchment is the main source of a river’s sediment load, rivers use similar erosional processes to waves, with most channel erosion occurring during high-flow, high-energy events
Sediment is also derived from weathering and mass movement processes that result in material moving into river channels from the valley sides

Question 42

What is fluvial transportation?

Answer 42

Rivers also transport sediment by traction, suspension, saltation and solution - similar processes to those of waves

Question 43

What is fluvial deposition?

Answer 43

As rivers enter the sea, there is a noticeable reduction in their velocity as the flowing water moving through the channel enters the relatively static body of sea water, additionally, tides and currents may be moving in the opposite direction to the river flow, providing a major resistance to its forward movement - available energy is reduced and so some, or all, of the river’s sediment load is deposited
As the reduction in energy is progressive, deposition is sequential (largest particles deposited first, finest carried further out to sea)
Meeting of fresh water and salt water causes flocculation of clay particles, these fine, light materials clump together due to electrical charges between them in saline conditions, as a result they become heavier and sink to the sea bed

Question 44

What are aeolian processes?

Answer 44

Due to their exposes to open sea surfaces, coastal landscapes can be significantly influenced by winds, especially those blowing on-shore

Question 45

What is aeolian erosion?

Answer 45

Wind i able to puck up sand particles and move them by deflation, at speeds of 40km/hour, sand grains are moved by surface rolling and saltation
As grains of this size are relatively heavy compared to silt and slay particles, they are rarely carried in suspension - this restricts erosion by abrasion to a height of around 1m and has a limited effect in the erosion of rocky coastlines and cliffs
Erosive force increases exponential with increases in wind velocity, for instance a velocity increase from 2 to 4 metres per second causes an eight-fold increase in erosive capacity
Dry sand is much easier for wind to pick up than wet sand, as the moisture increases cohesion between particles, helping them to stick together
Attrition on land is particularly effective in wind as particles tend to be carried for much greater distances than in water, and the particles are not protected from collisions by the film of water around them

Question 46

What is fluvial transportation?

Answer 46

With the exception of solution, moving air is able to transport material using the same mechanisms as water moving in rivers and waves
Once particles have been entrained (picked up), they can be carried at velocities as low as 20km/hr
Saltating grains are typically 0.15-0.25mm in diameter, while those 0.26-2mm, which are too heavy to be saltwater move by surface creep (rolling), only the smallest grains (0.05-0.14mm) can be carried in suspension

Question 47

What is aeolian deposition?

Answer 47

Material carried by wind will be deposited when the wind speed falls, usually as a result of surface friction, in coastal areas this will occur inland where friction from vegetation and surface irregularities is much greater than on the open sea

Question 48

What are the main erosional landforms?

Answer 48

Cliffs and shore platforms
Bays and headlands
Goes and blowholes
Caves, arches, stacks and stumps

Question 49

How are cliffs and shore platforms formed?

Answer 49

• When destructive waves break repeatedly on relatively steeply sloping coastlines, undercutting can occur between the high and low tide levels where it forms a wave cut notch
• Continued undercutting weakens support for the rock strata above, which eventually collapses, producing a steep profile and a cliff
• The regular removal of debris at the foot of the cliff by wave action ensues that the cliff profile remains relatively steep and that the cliffs retreat inland parallel to the coast
• Cliff profiles vary depending upon their geology - horizontally bedded ad landward dipping rock strata support cliffs with a steep, near vertical profile, if the rock strata inclines landward, the profile tends to follow the angle of the dipping strata
• As the sequence of undercutting, collapse and retreat continues, the cliff becomes higher, at its base a gently sloping shore platform is cut into the solid rock - appears flat and even but are often dissected by abrasion due to the large amount of rock debris that is dragged across the surface by wave action
• Where rock debris is boulder-sized, it may be too large to be removed by the waves and will accumulate on the platform, eventually the platform will become so wide that it produces shallow water and small waves, even at high tide
• Friction from the platform slows down approaching waves sufficiently for them to break on the platform, rather than at the base of the cliff and so undercutting slows and eventually ceases - research suggests that shore platforms reach a maximum width of about 500m before this happens
• Although shore platforms are predominately formed by erosion, weathering processes are also important - solution, freeze-thaw and salt crystallisation may all take place (dependant on rock type and climatic conditions), marine organisms, especially algae can accelerate weathering when the platform is exposed at low tide
• At night, algae release CO2 as photosynthesis is not taking place, this mixes with sea water, making it more acidic, which results in higher rates of chemical weathering
• Shore platforms usually slope seawards at angles of between 0 and 3 degrees, as wave erosion can occur anywhere between the high and low tide levels, however as water levels are constant for longest at high and low tide, erosion is greatest at these points
• This explains the formation of a ramp at the high tide level and a small cliff at the low tide level
• These features develop best if the tidal range is less than 4m, if it is higher then erosion is spread over a wider area of the platform, the water is at its high and low tide positions for a shorter time, and so the platform tends to be more uniform and more steeply sloping

Question 50

How are bays and headlands formed?

Answer 50

• Typically form adjacent to each other, usually due to the presence of bands of rock of differing resistance to erosion
• If these rock outcrops lie perpendicular to the coastline, the weaker rocks are eroded more rapidly to form bays while the more resistant rocks remain between bays as headlands, this results in the formation of a discordant coastline
• The width of bays will be determined by the width of the band of weaker rock, bay depth will depend on the differential rates of erosion between the more resistant and weaker rocks
• Rocks lying parallel to the coastline produce a concordant coastline, if the most resistant rock lies on the seaward side, it protects any weaker rocks inland from erosion, the resultant coastline is quite straight and even, however even in this situation small bays or coves may occasionally be eroded at points of weakness such as fault lines

Question 51

What happens when waves approach an irregularly shaped coastline?

Answer 51

• Wave refraction takes place and they develop a configuration increasingly parallel to the coastline, this is particularly true on coastlines with bays and headlands
• A s each wave nears the coastline, it is slowed by friction in the shallower water off the headland, at the same time, the part of the wave crest in the deeper water approaching the bay moves faster as it is not being slowed by friction, this means that the wave bends or refracts around the headland and the orthogonals converge - therefore wave energy is focused on the headland and erosion is concentrated there
• In the bays, the orthogonals diverge and energy is dissipated, leading to deposition, as the waves break on the sides of the headland at an angle there is a longshore movement of eroded material into the bays, adding to the build-up of beach sediment

Question 52

How are geos and blowholes formed?

Answer 52

• Geos are narrow, steep-sided inlets, even on coastlines with resistant geology, there may be lines of weakness such as joints and faults
• These weak points are eroded more rapidly by wave action than the more resistant rock around them
• Hydraulic action may be particularly important in forcing air and water into the joints and weakening the rock strata
• Example: Huntsman’s Leap in Pembrokeshire, 35m deep and eroded along a large joint in the carboniferous limestone
• Sometimes geos initially form as tunnel-like caves running at right angles to the cliff line, which as they become enlarged by continuing erosion may suffer from roof collapse, creating a geo
• If part of the roof of a tunnel-like cave collapses along a master joint, it may form a vertical shaft that reaches the cliff top - this is a blowhole
• In storm conditions, large waves may force spray out of the blowhole as plumes of white, aerated water
• Both geos and blowholes may also be associated with mining shafts or the collapse of a cave roof, eg. in Trevone, Cornwall, which formed a 25m deep blowhole

Question 53

How are caves, arches, stacks and stumps formed?

Answer 53

• Due to wave refraction, energy is concentrated on the sides of headlands, any points of weakness such as faults or joints are exploited by erosion processes and a small cave may develop on one or even both sides
• Wave attack is concentrated between high and low tide levels and it is here that caves form, if a cave enlarges to such an extent that it extends through to the other side of the headland, an arch is formed
• Continued erosion widens the arch and weakens its support, aided by weathering processes, the arch may collapse, leaving an isolated stack separated from the headland
• Further erosion at the base of the stack may eventually cause further collapse, leaving a small, flat portion of the original stack as a stump
• Example: Old Harry Rocks near Swanage

Question 54

What are the main depositional landforms?

Answer 54

• Beaches
• Spits
• On-shore bars
• Tombolos
• Salt marshes
• Deltas

Question 55

How are beaches formed?

Answer 55

• Most common landform of deposition and represent the accumulation of material deposited between the lowest tides and the highest storm waves
• Beach material (sand, pebbles and cobbles) comes from cliff erosion (5%), offshore combed from the seabed often during periods of rising sea levels (5%) and rivers - remaining 90% is carried into the coastal system as suspended and bed load through river mouths

Question 56

What kind of beaches does sand produce?

Answer 56

• Beaches with a gentle gradient, usually less than 5 degrees, because its small particle size means that it becomes compact when wet, allowing little percolation during backwash
• As little energy is lost to friction and little volume is lost to percolation, material is carried back down the beach rather than being left at the top, resulting in a gentle gradient and the development of ridges and runnels parallel to the shore, these are occasionally breached by channels draining the water off the beach

Question 57

What kind of beaches does shingle produce?

Answer 57

• Shingle, a mix of pebbles and small to medium sized cobbles, produces steeper beaches because swash is stronger than backwash so there is a net movement of shingle on-shore
• Shingle may make up the upper part of the beach where rapid percolation due to larger air spaces means that little backwash occurs and so material is left at the top of the beach

Question 58

Effects of storms, berms and cusps

Answer 58

• Storm waves hurl pebbles and cobbles to the back of the beach, forming a storm beach or storm ridge
• Berms are smaller ridges that develop at the position of the mean high tide mark, again resulting from deposition at the top of the swash
• Cusps are small, semi-circular depressions, they are temporary features formed by a collection of waves reaching the same point and when swash and backwash have similar strength
• The sides of the cusp channel incoming swash into the centre of the depression and this produces a strong backwash, which drags material down the beach from the centre of the cusp, enlarging the depression - further down the beach, ripples may develop in the sand due to the orbital movement of water in waves

Question 59

How do beaches change?

Answer 59

Beaches are dynamic and their profiles change over time as wind strength and hence wave energy changes, beaches respond to these changes by developing an equilibrium profile, with a balance between erosion and deposition
High-energy, destructive waves remove sediment offshore and create flatter beach profiles, this results in shallower water, more friction and a reduction in wave energy
Low-energy, constructive waves transfer sediment in the opposite direction to form steeper profiles, this produces deeper water, less friction and an increase in wave energy

Question 60

How are spits formed?

Answer 60

• Spits are long, narrow beaches of sand or shingle that are attached to the land at one end and extend across a bay, estuary or indentation in a coastline
• Generally formed by longshore drift occurring in one dominant direction, which carries material to the end of the beach and then beyond into the open water
• As storms build up more and larger material they make the feature more permanent
• End of the spit often becomes recurved as a result of wave refraction around the end of the spit and potential presence of a secondary wind/ wave direction
• Over time spits may continue to grow and a number of recurves or hooked ends may develop
• If a spit forms across an estuary, its length may be limited by the actions of the river current
• In the sheltered area behind the spit, deposition will occur as wave energy is reduced, the silt and mud deposited build up and eventually salt-tolerant vegetation may colonise, leading to the formation of a salt marsh

Question 61

Example of a spit

Answer 61

Orford Ness - East Anglia
- Coastline is east-facing so is largely unaffected by Britain’s south-westerly prevailing winds
- North-easterly winds and waves are locally dominant, which has resulted in longshore drift from north to south
- A spit has formed across the estuary of the River Ore but the river current has prevented it reaching the land on the other side, instead the spout has continued to grow parallel to the coastline, diverting the river some 12km further south - historical maps provide evidence of this growth

Question 62

How are on-shore bars formed?

Answer 62

• Can develop if a spit continues to grow across an indentation, such as cove or bay, in the coastline until it joins onto the land at the other end
• This forms a lagoon of brackish water on the landward side
• Could also be formed by the on-shore movement of sediment during the post-glacial sea level rise that ended around 6000 years ago

Question 63

Example of a bar

Answer 63

• 100m wide bar at Slapton Sands in Devon may have been formed through a spit growing across an indentation, however there does not appear to be a significantly dominant direction of longshore drift on this east-facing coastline
• There is no obvious pattern to the distribution of sediment sizes along the 5km length of the bar between Torcross in the south and Strete in the north
• Conclusion is that this feature may, at least in part, have been formed by the on-shore movement of sediment, especially flint, during the post-glacial sea level rise that ended about 6000 years ago

Question 64

How are tombolos formed?

Answer 64

• Tombolos are beaches that connect the mainland to an offshore island
• Often formed from spits that have continued to grow seawards until they reach and join an island
• Can also be formed through on-shore movement of sediments during the post-glacial sea level rise that ended about 6000 years ago

Question 65

Examples of tombolos

Answer 65

• 30km long shingle beach at Chesil near Weymouth, Dorset
• Originally thought to be formed through spit that had grown seaward, however the on-shore movement of sediments is now thought to be the more likely cause, with it reaching its present position some 6000 years ago
• At its eastern end at Portland, the ridge of shingle is 13m high and composed of flinty pebbles
• At the western end, the ridge is only 7m high and composed of smaller, pea-sized shingle
• If longshore drift had been responsible, it would be expected that the sediment further east would be smaller
• It is more likely therefore, that the on-shore migration of shingle originally produced a uniform distribution. of sediment sizes
• Subsequently, strong longshore currents from the south-west have moved sediments of all sizes eastwards, while weaker longshore currents from the east have only been able to return the smaller particles westwards

Question 66

How are salt marshes formed?

Answer 66

• Features of low energy environments, such as estuaries and on the landward side of spits
• UK has 45,500 ha of salt marsh, mainly in eastern and northwest England
• Salt marshes are vegetated areas of deposited silts and clays, they are subjected twice daily to inundation and exposure as tides rise and fall
• Salt tolerant plant species such as eelgrass and spartina help trap sediment, gradually helping to increase the height of the marsh
• The stems and leaves of the plants act as baffles and trap sediment swept in by tidal currents while the roots stabilise the sediment
• The higher the marsh becomes, the shorter the period of daily submergence and the less saline the conditions, the low marsh on the seaward side, is characterised by high salinity, turbid water and long periods of submergence
• Few plant species can survive such conditions and so species diversity is poor
• Further inland, conditions are not as harsh, with salinity, turbidity and submergence periods all lower, this allowed a greater variety of species to survive, including sea aster, reeds and rushes
• Salt marshes have a shallow gradient which slopes seawards, a low cliff sometimes separates the Salt marsh from the unvegetated mudflats on the seaward side
• Even though the higher parts of the salt marsh are inundated less often, deposition rates are still quite high as at high water mark, low-energy slack water may be present for 2-3 hours
• The greater density of vegetation cover helps to trap and stabilise sediment
• Extensive networks of small, steep-sided channels, or creeks, drain the marsh at low tide and provide routes for water to enter the salt marsh as the ice rises
• Between the creeks, shallow depressions are often found, these trap water when the tide falls and these areas of saltwater, called salt pans, are often devoid of any vegetation
• The development of salt mashes depends on the rate of accumulation of sediment, with rates of 10 per year quite common
• Deposition of fine sediment occurs as rivers lose energy when they slow upon entering the sea, this is a key factor, but so too is flocculation
• Tiny clay particles carry an electrical charge and repel each other in fresh water, in salt water the particles are attracted to each other, combining together to form flocs, which being larger and heavier are unable to be carried in the river flow and so settle out of suspension, even at relatively high velocities

Question 67

What are deltas?

Answer 67

• Deltas are large areas of sedentary found at the mouths of many rivers, deltaic sediments are deposited by rivers and by tidal currents
• They form when rivers and tidal currents deposit sediment at a faster rate than waves and tides can remove it

Question 68

Where do deltas typically form?

Answer 68

• Rivers entering the sea are carrying large sediment loads
• A broad continental shelf margin exists at the river mouth to provide a platform for sediment accumulation
• Low-energy environments exist in the coastal area
• Tidal ranges are low

Question 69

What distinctive components make up the structure of deltas?

Answer 69

Upper delta plain: furthest inland, beyond the reach of tides and composed entirely of river deposits
Lower delta plain: in the inter-tidal zone, regularly submerged and composed of both river and marine deposits
Submerged delta plain:: lies below mean low water mark, is composed mainly of marine sediments and represents the seaward growth of the delta

Question 70

Structure continued

Answer 70

• Deltas are criss-crossed by a branching network of distributaries
• Overloaded with sediment, deposition in the channel forms bars which causes the channel to split into two - this produces two channels with reduced energy levels, and so more deposition and further dividing occurs
• Although these channels may be lined by levees on their banks, in times of flood these natural embankments are breaches and deposition of loves of sediment will then take place in the low-lying areas between the levees, called crevasse splays

Question 71

What are the three most common types of delta?

Answer 71

Cuspate: a pointed extension to the coastline occurs when sediment accumulates, but this is shaped by regular, gentle currents from opposite directions
Arcuate: sufficient sediment supply is available for the delta to grow seawards, but wave action is strong enough to smooth and trim its leading edge
Bird’s foot: distributaries build out from the coast in a branching pattern, with river sediment supply exceeding the rates of removal by waves and currents