3.1.2 Coastal Landforms

Question 1

What determines a coastal landform?

Answer 1

• Battering effect of waves and weathering
• Direction, strength and frequency of waves
• Nature of refraction
• Geology of coastline - rock type and structure important

Question 2

Where do headlands and bays form? How do rock types influence this?

Answer 2

Headlands and bays form along coasts with alternating rock and where the trend is at right angles to the coastline.
Hard rocks like limestone, granite and chalk withstand erosion compared to softer clays or sandstones as they are unconsolidated and absorb water easily so prone to mass movements. Granite is hard and impermeable.

This is called a discordant coastline, where rocks are 90 degrees to the coastline exposing multiple layers of different resistances. The softer rock erodes faster, forming a bay and leaving a headland exposed to marine processes. Once the bay is formed, it creates a balance in the rates of erosion as there is increased erosion at the headland but as it is refracted there is less in the bay.

Question 3

What is a concordant coastline?

Answer 3

A concordant coastline has multiple layers of rock parallel to the coastline and so only one layer of rock is actually exposed to the sea.

Question 4

What are the types of lines of weakness in rocks and what are the effects?

Answer 4

Lines of weakness cause instability as they are usually caused by weathering - dilation, pressure release, freeze/thaw, salt crystal as well as marine erosion processes

• Joints: vertical cracks
• Strata: any different layers in rock (conc/disc)
• Faults are any cracks in the rock
• Bedding planes are horizontal strata in sedimentary rock layers

Question 5

What are angles of dip?

Answer 5

-Angle of how bedding planes are laid down

Seaward dipping coasts are when rock slabs are sliding down the cliff, so dip seaward
Landward coasts dip inland with well developed joints at right angles to bedding planes, producing slide planes
Horizontal coasts are uniform horizontal layers, producing steep sided cliffs

Question 6

How do wave cut notches form?

Answer 6

The bay is sheltered and so there is less erosion as energy is lost to refraction - more is concentrated on the headland so wave refraction concentrates energy on and around the flanks of the headland, eroding lines of weakness known as geos - widened cracks or indents in the cliff face which can expand into wave cut notches.

Question 7

What creates cliff faces?

Answer 7

Like a waterfall, coastal and marine action undermines the rock, leading to slumping and rock falls. Rapid severe erosion create dent in the base of the cliff from long run wave attack, undercutting it and leaving a wave cut notch

Question 8

How do the features of a headland develop?

Answer 8

• Geos can expand into natural cavities in the rock face known as caves
• Common in areas with many lines of weakness which are exploited and expand.
• Inside cave hydraulic action, salt crystallisation, cavitation all erode away at the inside and enlarge it.
• Cracks are widened and deepened further into the body of the cliff
• This can create a tube upwards in the roof of the cave as air is forced upwards when water is forced into the cave, which can expand cracks upwards and erode weakness and create a blowhole
• If two caves meet either side of the cave expands large enough then the sides of the headland may force, creating an arch.
• Further erosion due to refraction and weathering of the joints of the arch can cause the roof of the arch to collapse under the weight and gravity. Marine erosion and weathering continue to be concentrated in the stack until it eventually collapses, leaving behind a stump

Question 9

How do wave cut platforms form?

Answer 9

Marine erosion at the base of a cliff forms a wave cut notch as before, when directly perpendicular to the coastline.

• This becomes too big as weathering weakens the upper parts, causing pressure release, salt crystallisation, chemical and mechanical exploits.
• Weathered rock above the notch falls under gravity, causing the cliff to retreat - similar to a gorge.
• Remaining rubble carried back out from backwards, creating a platform.
• The sea now attacks the cliff laterally instead of downwards, forming a flat rock platform at low tide
• This platform extends over time, forming an area of shallow water in front of it which waves travel over, making them break earlier.
• Over long periods of time the platform becomes deeply grooved as material erodes at the surface through abrasion.
• Wave cut platforms are found at the base of the cliff, are deeply grooved, gently sloping, inter tidal and have rocky platforms at the end.

Question 10

What are the 3 types of shore platform?

Answer 10

Sloping shore platforms:

• Mainly made of less resistant rock
• Gently descending erosion surfaces extending from base of a sea cliff to beneath sea level - no topographic break so smooth and flat
• Can collapse by slumping and sliding caused by rainwater

Sub-horizontal shore platforms

• Have a horizontal erosion surface in front of a sea cliff which terminates seaward by a marked scarp - tiny cliff
• Creates a wave cut platform, and a marked scarp where it ends
• Slumping is reworked by waves washed away and platform
• Often formed where there is moderate resistance such as limestone

Plunging cliffs:

• Plunge far below sea level as vertical face
• Erode by less than 0.01mm per year and collapse by rock fall occasionally but stay vertical as debris is swept away

Question 11

What factors influence cliff profiles?

Answer 11

-Hard rock produces vertical and steep faces - only MM is falls

Soft rock - gently sloping, stepped profiles. Rotational slumping common and develop mudflows at cliff base, usually fast retreat

Hard rock dipping towards sea - slopes towards sea and slabs of rock slide off

Hard rock dipping away from sea - almost vertical, extremely rugged and uneven, small rockfall but retreat is slow

Horizontal layers of soft rock - porous layer at top become heavy and unstable and lower layers form a slip plane lubricated by water, producing very unstable cliffs with slumping and flowing of material onto the beach.

Erosional history - sea level was higher in the past so cliff lines formed above current sea level and since sea level lower cliff line may have a degraded cliff line above it.

Question 12

What are depositional landforms?

Answer 12

Beaches are made of sand or shingle, forming between high and low water marks, inter tidal zone and spring tide when deposition exceeds erosion.

• Material fed by LSD along the coast, fine material worn down by attrition
• Further back, coarse material is found deposited which often has fallen from cliffs known as littoral deposits. Material may also come from rivers.

Question 13

What are the features of the backshore, foreshore, nearshore and offshore?

Answer 13

Backshore - storm beach made up of large sediment pushed up by storm waves. The breaking wave throws pebbles up the beach and backwash is too weak to pull it back down. This forms berms at the spring tide - ridges of shingle

Foreshore - made of sand. Low ridges formed parallel to the coastline formed by constructive waves, separated by runnels. On flatter areas there may be small scale sand ripples forming as the backwash returns to the sea

Nearshore - longshore bars formed due to the waves - ridges of sand that run parallel to coast, more big and permanent

Offshore - always underwater, longshore bars found here when the wave starts to touch the sea bed as the friction slows the waves sediment sweeps up and forms bars?

Question 14

What affects the beach?

Answer 14

• Wave energy - high wave energy produces sandy beaches. Shingle beaches found in low wave energy areas
• Wave type - destructive waves give stronger backwash so more material moves down the beach. Constructive waves have stronger swash so create shallower profiles

Longshore drift

Sediment size: shingle beaches generally steeper as backwash percolates easily through the shingle, reducing the energy of the backwash. Sand is less permeable so there is more backwash to drag material down the beach.

Question 15

How many beach profiles change in summer and winter?

Answer 15

Winter - more destructive waves throw coarse material up the beach, creating a storm beach and drag material off lower slopes, steepening the profile at the back but creating a flatter main section and offshore bars

In the summer the opposite happens as small waves build the beach up and make it more gentle

Question 16

What are the features of a sandy beach?

Answer 16

• Sand compact when wet so little percolation
• Most wave swash returns as backwash as little energy lost to friction
• Material not carried down by returning backwash as sand is cohesive
• Swash usually stronger
• Gentle beaches encourage steep waves, encourage backwash and gentle beaches

Runnels develop between troughs at low tides which are ridges running parallel to the water line and pushed up the beach

• Ripples form on the sand by the action of wave movement in the lower part of the beach which spread out producing ridges and intervening depressions called runnels
• The water caught in the runnels find its way back to sea by a drainage in channel

Question 17

How do offshore bars develop?

Answer 17

• Coarse sand combed from the beach is deposited where the waves first touch the seabed on the way in, so material deposited and pushed up when the offshore gradient is shallow creating a bar below the sea level, which can be seen at the lowest tides
• This creates a tidal lagoon between the beach and coastline and salt marshes often develop here, forming best when the tidal range is low and where the sea bed and adjacent coastline are gently sloping

Question 18

What are the features of shingle beaches?

Answer 18

Often have steeper profiles, often 10-20 degrees. The large spaces in the particles allow infiltration and percolation so little backwash returns and cannot impede the next swash.
-Backwash also does not drag material off the beach, leaving it steep - material is deposited and builds up, steepening the beach

Small tides are marked by ridges called berms, which show lower high tide marks as a result of constructive waves pushing material up the beach and depositing. They are generally created by smaller waves with less energy so the material is smaller. They may be pushed up further when the tide pushes it up at the top.

Storm beaches also form at the top of the beach as strong swash in storm conditions deposit the larger material, forming a storm beach of ridge

Question 19

What are cusps?

Answer 19

Cusps are formed on shingle beaches, when 2 waves break directly onto the beach in similar locations. They are semi circular depressions often formed when sand and shingle beaches meet alternating parallel to the sea. Swash breaks through the sandy layer and erodes the shingle with the backwash at it returns, creating semi circular depressions.

Once they develop, the sides of the cusp channel incoming swash into the centre and create a stronger backwash, increasing erosion in the centre and maintaining the semi-circular shape

Question 20

What are drift-aligned beaches?

Answer 20

Waves break at an angle to the coast, so swash occurs at an angle whereas backwash is perpendicular, causing LSD. They create spits, bars, tombolos and cuspate forelands

Question 21

What are swash aligned beaches?

Answer 21

Waves break in line with the coastline, so swash and backwash move material up and down the beach. They are smoothly curved, concave beaches. The beach face is orientated parallel to the fronts of the waves, producing bay head beaches, bay bars and barrier beaches.

Question 22

What are bar bays?

Answer 22

Ridges of material which occur when a spit extends across an inlet. Usually occurs when there is no river current to stop the spit growing, creating a lagoon between the bar and the coastline

This may also occur when an offshore bar migrates landwards due to the deposition from tides breaking. These form offshore as destructive waves comb out sediment and deposit it back out at sea or where incoming waves first touch the seabed causing deposition. The beaches offshore may be called barrier beaches

Question 23

What are spits?

Answer 23

On drift aligned beaches, waves meet the coast at an angle and swash moves material causing longshore drift. Prevailing winds and LSD push waves up and down the beach at angles, continuing until the mainland ends.

Here the water is deeper and slows the sediment down, causing deposition and creates an extension of the mainland attached to the coastline and out to sea, or into an estuary behind the spit. This deposition continues offshore as the mass slows down.

Recurved spits may form due to the shapes from tides or prevailing winds. This causes the deposition to curve at the top of the spits - also from the refraction of waves. As spits migrate they may force ridges of shingle and in the shelter of the spit mudflats and saltmarshes may development, increasing the size of the spit. A famous one is Spurn head

Tombolo occur when a beach extends outwards to join an offshore island. It also forms a lagoon behind it. A famous tombolo is Chesil Beach and the lagoon is called the fleet

Question 24

What are cuspate forelands?

Answer 24

Triangular shaped features due to meeting of spits from two directions. Wave refraction causes LSD to operate in both directions of an offshore island where waves usually approach at right angles, depositing in a triangular shape