IntroductionThe geology of the rocks forming the coast is one of a number of factors(see Fig.1) which influence coastal landforms. Structure is defined as ‘theway the rocks are disposed or geologically arranged’ whereas lithology isthe ‘make-up’ of each individual rock type. Both structure and lithologyplay a very important role in influencing coastal landforms. Structure canprovide, as a result of folding and faulting, a range of rock types withlithologies of differential resistance to subaerial and marine processes.The combined impacts of structure and lithology affect:

A. the coastline in plan (as on a map) B. the coastline in profile (leading to a variety of cliffs and wave

cut platforms)C. the distribution of micro-features (caves, arches, stacks etc).

The purpose of this Factsheet is to increase the confidence andunderstanding of AS/A2 geography students who are not studyinggeology, so they avoid simplistic explanations of coastal land forms.

Importance of structure and lithologyA number of factors combine:

1. Hardness of rock types. As a result of heating and compression duringtheir formation, as a general rule igneous and metamorphic rocks are harderand therefore more resistant to erosion, forming many high cliffs in NorthWest Britain. In contrast, many of the rocks forming the coastlines ofSouthern and Eastern Britain are ‘soft rocks’ – unconsolidated sands andclays of tertiary age, as well as deposits of glacial boulder clay and gravels.Other factors being equal, these rocks tend to be very easily eroded –especially if the bases of the cliffs are poorly protected by beaches – at a rateof 3m – 6m per year. An extreme example of erosion rates of 30m per yearhas been recorded in the ash from the Krakatoa eruption in coastal Sumatra.Indirectly the higher cliffs are harder to erode because the collapse ofthe cliff face as a result of recession provides greater amounts ofdebris to be transported away before new erosion can occur.

Fig. 1 Factors influencing coastal landforms

2. Permeability occurs as a result of the incidence of pores (for examplein an open-textured sandstone), or as a result of fissures, cracks andjoints (for example in chalk or limestone). As any surface water seepsthrough the cliffs, it increases resistance to subaerial processes soadding strength to some relatively soft rocks. This explains why chalkinvariably forms relatively high, near-vertical cliffs, and supportsarches and stacks. Where permeable rocks such as chalk are underlainby impermeable clays (for example at Folkestone where the chalk isunderlain by gault clays) a zone of lubrication occurs which can leadto cambering and extensive mass movement (see Fig. 6 profile 1).

3. Physical make-up of rocks, i.e. the amount of joints, bedding planesand faults, has an impact on the rates of weathering (both freeze-thawand chemical). Where joints and bedding planes occur at a high densitythis weakens the rock and makes it subject to increased subaerial andmarine erosion. Faults or isolated master joints can be exploited by thesea to form a range of micro-features as at Tintagel in North Cornwall(see Fig. 2). Both folds and faults can become shattered zones, and beeasily exploited by the sea to form inlets and bays.

Fig. 2 The fault at Tintagel, North Cornwall, which forms ashatter zone of weakness.

G FApril 2002 Number 129

eo actsheet

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The impact of structure and lithology on coastal landforms

GEOLOGYStructure and lithology (rock type)

BIOTIC FACTORSImpact of vegetation, coral reefs etc

TECTONICSCoastal uplift

Volcanic activity

MARINE FACTORSWavesWinds Tides

Salt spray Currents

SUBAERIAL FACTORSTemperatures

Weather - rain, snow, frost, winds, sun CLIMATIC FACTORSWinds – generate waves and currentsWeather – affects weathering of cliffs,sources of beach materialClimate changeGlaciation – changes in sea level eustatic/isostatic

GEOMORPHIC FACTORSRivers

GlaciersMass movement

HUMAN FACTORSPollution

Conservation managementBuildingsRecreation

COASTALLANDFORMS

The Island

BarrasNose

TunnelBlackarock

Formation of seacaves at weak zone

Tunnels are formed along zoneof weakness as a result of

hydraulic action and abrasionFault zone

Merlin’sCave

SEA

0 metres 400N

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4. Chemical composition – some rocks, such as quartzite or mostsandstones, are made almost completely from silica which is chemicallyinert. The very low rate of chemical weathering adds resistance to rocks.Other rocks are more prone to rapid chemical weathering because of theirchemical composition. Iron compounds oxidise in some sandstones, andfeldspars are altered into clay minerals by hydrolysis in rocks such asgranite. These ‘rotted’ zones increase vulnerability to both subaerial andmarine erosion. The chemical decomposition of limestone bycarbonation happens even more rapidly under the influence of saltwater,leading to accelerated disintegration of some wave cut platforms. Theimpact of saltwater causes rocks such as basalt to weather 14 times morerapidly than under fresh water conditions.

Usually, therefore, certain aspects of lithology and structure combine tomake neighbouring rocks more or less subject to weathering and erosionand therefore they erode at faster or slower rates relative to each other.

This process of differential erosion is the key to understandingcoastal erosional landforms.

(A) The coastline in planStructure is the key to explaining the pattern of headlands, bays, islandsand inlets found along a coastline. Indirectly it is the type of structurewhich determines whether there is a variety of rocks at the coast, and thejuxtaposition of rocks of varying lithologies. Fig. 3b is an upgradedversion of a simple GCSE style diagram which shows how the trend ofthe geology (the general direction of the long axis of any folds) can leadto the development of concordant and discordant coastlines.

In concordant coastlines the trend lies parallel to the shore, as in SouthDorset. Initially this presents minimal variety of rocks for the sea toexploit. Differential erosion usually results, as in (A), from theexploitation of a master joint or a fault line which acts as a weakness forthe sea to develop into an incipient (beginning) core such as Stair Holewest of Lulworth Cove in Dorset. Once marine action has breached therelatively resistant wall of limestone it reaches the relatively weakwealden shales which are easy to erode into an elongated cove such asLulworth as in (B). Erosion inland is prevented and slowed down by thecomparatively resistant chalk. Over time neighbouring coves can coalesceby erosion to form a wide bay such as Mupe Bay as in (C). Residual partsof the original limestone wall form stacks such as Mupe Rocks.

The Dalmatian coast of Croatia (formerly Yugoslavia) is a ‘drowned’concordantcoast typified by numerous elongated islands parallel to the shoreline.

In discordant coastlines the structural trend is found at approximately rightangles to the shoreline as in East Dorset. Marine erosion leads to headland andbay formation as in (E). Swanage Bay, formed from less resistant wealdenshales is differentially eroded. It is flanked by two headlands of relativelyresistant chalk (Studland Point) and limestone (Durleston Headland).

Fig 3a Map of the coastline of Dorset.

Exam Hint: Avoid basic GCSE diagrams and always upgradethem by supplying details of the geology and located examplesof features, as in Figs 3a and 3b.

Fig. 3b Differential erosion on concordant and discordant coasts.

Key to the geology:

Eocene:

Sands

Cretaceaous:

Chalk

Wealden clays

Jurassic:Portland limestone

Kimmeridge

R = Resistant

W = Weak

PooleHarbour

St Alban’s Head

SwanageBay

Lulworth Cove

Stair Hole

ManowarRocks

Durdle Door

0 km 1

0 km 5Durleston Head

OldHarryRocks

W

R

W

R

Concordant coast

Discordant coast

Weaknesses exploited Cove beginning to form

Coves formed as at Lulworth Cove

Incipient coreguided by amaster joint

Uniformwall of

resistantrock

Erosion W-Eon weaker rock

- elongatedshape

Coalescence of coves

Possible arch,e.g. Durdle

Door

A

B

C

Islands detached - residualstacks and stumps, e.g

Manowar Bay and Mupe Bay

D

E

Wave attack - concentrated on areas of

weaker rock

Wave refraction

SWANAGE BAY

DURLESTON HEADHeadland

STUDLAND HEADHeadland

Sea stacks, e.g. Old Harry Rocks

Key:

Resistant Weak

(B) The coastline in section - cliffs and shore platformsFig. 5 shows how as a result of cliff recession a shore platform (sometimesknown as a wave cut platform) develops, until it reaches such a width that itis no longer reached by any marine action other than storm waves associatedwith the highest spring tides. Both the form of the cliff profile and the shoreplatform are strongly influenced by geological structure and lithology.

Fig. 5 Hard rock sequence of Cornwall, S. Gower.

As can be seen from Fig. 6, which shows a variety of contrasting cliff profiles,there are a number of important controls on cliff form. Hard rocks such asgranite (see profile 4) will erode slowly to produce high, steep cliffs, whilesofter rocks such as glacial boulder clay will be weathered more quickly withmass movement processes such as slumping dominant, resulting in less steepslopes. Rock types also influence the nature of subaerial processes, for examplein profile 2 the boulder clay has been stripped back, to expose the sandstone.Joints and bedding planes can determine the cliff form and the amount ofmovement of material to the cliff foot. For instance in profile 5, the seaward dipof the rocks leads to slabs of rock sliding into the sea to give a very low angleprofile. In profile 1, as a result of the underlying soft clay, a combination of azone of lubrication and wave attack has led to land slipping and the cliff is nowactually temporarily protected by the recent mass movement. As this ‘toearmour’ is removed by wave action further renewed cliff collapse could takeplace. In profile 3, a massive rock with widely spaced master joints, chemicalweathering and hydraulic pressure from the waves have combined to producean array of micro-features (stacks and arches). Differential erosion is veryapparent in profile 2 where the coal seam contributes to rapid undercutting ofthe cliffs by the formation of a deep wave cut notch.

Fig. 6 Variations in cliff profile.

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Case Study: the Pembrokeshire CoastThe Pembrokeshire coast clearly shows the importance of structure,rock type and differential erosion. The main trend shown by the rocksin Pembrokeshire is West-East as a result of the Armorican (Hercynian)earth movements some 270 million years ago. The coastline in planvaries with the relationship between the line of the coast and thegeological outcrops and structures. Concordant coastlines are found onthe north side of St David’s Head, where the igneous rocks form aresistant wall and in the South in Castle Martin, where the jointing andfaulting in the carboniferous limestone has led to the development ofstacks and arches. Elsewhere the discordant coast has exposed anenormous variety of rock type at a meso scale. St Bride’s Bay has beendifferentially eroded from the weaker coal measures, between theresistant igneous rocks. At a micro scale there are enormous variations,for example within St Bride’s Bay. The relief and structure of thecoastline locally results in variations in wave type, exposure and fetch.Geological weaknesses such as joints, bedding planes, fault lines areeroded more quickly, to form caves. Narrow bays, such as at Druids’Haven, result from structural weaknesses where the rocks have beenstretched and compressed by folding.

Fig. 4 The Pembrokeshire Coast

Exam Hint: Obtain a geological map for any coastline youhave been studying and start to match up the headlands andbays and features such as caves to structure, varying rocktype and the process of differential erosion.

Key:Pre-Cambrian:

Igneous rocks(intrusive &extrusive)

Cambrian:Sedimentary rocks

Ordovician:Igneous rocks(intrusive &extrusive)Sedimentary rocks

Silurian:Igneous rocks

Sedimentary rocks

DevonianOld red sandstone

CarboniferousMain limestone &lower limestone shalesMillstone gritCoal measures

N

0 km 10

Millford Haven

St. Ann’sHead

Skomer Island

Castle Martin

RamseyIsland

St. David’sHead

St. BridesBay

Discordant

Concordant

Concordant

a b c dHigh tideLow tide

Wave Cut Platform

Original land profile

Notch

Possible zone ofaccumulation

a Steep cliffresulting from

vigorous wave attack

b & c Cliff lesssteep because

fewer waves reachcliff base to cause

undercutting

d Cliff ‘degraded’-very little erosion bywaves, weatheringand rivulets shape

cliff face

1. South facing

2. East facing

3. South facing

4. West facing

5. North East facing

Chalk

Soft claye.g. Folkestone, Kent

Glacial boulder clay

Sandstone

Coal seamSandstone

e.g. Newbiggin, Northumberland

Massive limestone with master joints

and bedding planes

e.g. Castle Martin, Pembrokeshire

Hard igneous rock

Fault line

e.g. Ramsey Island, Pembrokeshire

Sedimentary rocks dipping

towards the sea

e.g. Hawick Haven near Boulmer,Northumberland

20m

100m

20m

30m

40m

Deep water

Thus there is a direct relationship between rock type, erosion rate and theresulting cliff morphology. Note however that as in profile 4 other factors canplay a part here; because of the deep water off-shore, waves are not directlyeroding the coast, but are reflected back (known as the clapotis effect). Otherimportant influences include aspect of coast in relation to the dominant windsand the fetch (the distance of open sea over which waves can be generated).

A shore platform is a relatively flat, gently sloping (usually range 1° -5°) expanse of rock found at the foot of a cliff and extending out to sea.It is marked as ‘flat rocks’ on the OS map. In Great Britain these shoreplatforms are inter tidal, i.e. between high and low water mark, and areformed as a result of cliff recession (see Fig. 5). As the cliff recedes, theplatform is progressively lowered by weathering and marine erosion(abrasion). A number of processes contribute to its formation:• salt weathering (alternate wetting and drying leads to salt

crystallisation) • mechanical wave erosion aided by pebbles (abrasion)• bio erosion by boring organisms• chemical weathering – especially important for platforms made of limestone• the type of wave reaching the shore.

Structure and lithology again influence the effectiveness of the processesoperating on the platforms. Generally the widest flattest platforms occuron near horizontal, unresistant rocks. Where the rocks are steeply dippingand resistant, platforms are narrow and ridged. The platforms also showthe impacts very clearly of differential erosion which leads to variationsin micro-relief, caused by joints, bedding planes and even igneousintrusions such as dykes. Limestone platforms are often ‘pitted’ as aresult of limestone carbonation and even exhibit limestone pavementswith clints and grykes. It is lithology and structure which determine thetype of material available for abrasion at the cliff foot, for example veryfine grained shales produce negligible supplies of pebbles.

Fig. 7 ‘Cliff architecture’ with fault-controlled micro-features.

Note: Sometimes, as on the Gower Coast, you will find extremelyextensive shore platforms with several levels, which reflect the impact ofchanging sea levels. You can actually find evidence of raised beachdeposits cemented on to the higher parts of the current wave cutplatforms with pebbles and fossil limpets embedded in them.

(C) The formation of micro-featuresAn analysis of the distribution of micro-features, suggests markedconcentrations within certain rock types in particular locations, for examplethe Old Red Sandstone found in Caithness and Orkney (NE Scotland).

For micro-features to form the following conditions are required:• Sufficient thickness of a uniform rock type which is relatively resistant

to erosion and has the internal strength to support tunnels and caves.• A massive rock is ideal in terms of lithology as it has well spaced joints and

bedding planes which can be exploited by the hydraulic action of the sea.• In some areas, as in Orkney, the development of a system of cross

faulting is the main control for the distribution of the various micro-features – known as cliff architecture.

As can be seen from Fig. 7 there are two likely sequences of development.Initially sea caves develop, their distribution invariably controlled bygeological weaknesses (as can be seen on Flamborough Head).

(1) In some cases the impact of air and water forced into the caves will leadto the development of vertical shafts and tunnels upwards to the groundsurface to form a blowhole (or gloup). Air and water will be forcedthrough the blowhole, at certain states of the tides and during particularwind directions, with an explosive force, by breaking waves. Thiscauses large pressure changes in the cave and further erosion. Theblowhole roof may collapse to form a geo or inlet. Alternatively thegeo may be fault guided – differential erosion exploits the weakness ofthe fault or shatter zone to form a very long narrow gulley.

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The impact of structure and lithology on coastal landforms Geo Factsheetwww.curriculumpress.co.uk

Exercise: Cliff form greatly influences the rate of erosion. Workin groups to try to develop a sequence based on the likely ratesof erosion of the cliffed coasts shown. Justify your sequence.

Source: C.R. Warn

Exam Hint: Using Fig. 7 as a framework, select an area such asGower, S Pembrokeshire, Kent, Dorset or Northumberland anddevelop a case study of micro-features with named examplesand descriptions of all features.

Stack

Blow hole

Notch

Low tide

Cave

Fault

Fault

Fault

Fault

ArchSEA

Keyhole

Geo

50m

Stump

(2) Differential erosion may result in caves adjacent to each other, perhaps oneither side of a headland, joining together to form an arch (natural arch).Fig. 8 shows how the shape of these arches is very much structurallycontrolled. Most arches have a comparatively short time span perhaps of100 years. In many cases you can obtain old postcards, showing formerarches (e.g. on 14 January 1990 one arch of the coastal formation calledLondon Bridge collapsed as a result of severe storms on the coast ofVictoria, Australia). The collapsed arch can lead to development of a seastack (but not all sea stacks were initially arches). A very impressiveexample is the Old Man of Hoy in Orkney, which actually has a shapelike a face, and is a challenge for mountaineers. Frequently stacks areeroded away to form stumps. Some stacks such as Charley’s Garden atSeaton Sluice on the North East coast have now vanished into oblivion.

Fig. 8 Structural and lithological control of micro features in Gower.

In conclusion, therefore, the combined impacts of structure and lithologyplay a major role in determining both major and minor landforms at thecoast, especially where the coasts are composed on ‘hard rocks’.

Further researchLandform Systems, Bishop and Prosser 2nd Edition Collins provides up-to-date information suitable for A2 standard.The Geographical Association Classic Landforms of East Dorset Coast,Gower Coast, North Devon Coast, South Devon Coast, Sussex Coast,West Dorset Coast are all useful.The Dorset Coast published by Dorset Geological Society (2000).

Websiteswww.users.globalnet.co.uk/draynes/recintrowww.angliacampus.com/public/sec/geog/coastinwww.learn.co.uk/glearning/secondary/topical/KS3/coastalerosion

Practice exam questionDescribe and suggest reasons for the coastal features shown on Fig. 9.

20 marks

Fig. 9 Coastal features.

This exam question was developed from an idea from Ken Allott, a geology teacher at King

Edward VI School, Morpath, Northumberland.

Guidelines• As this is an A2 type question you must devise a structure before you

begin to ensure a logical sequence. Use a pencil to annotate all thekey features shown on the diagram and then try to develop an orderas suggested below.

• Always use geographical terms – try to include structural trend,concordant, discordant, fault guided, incipient cove, wave refraction,differential erosion, massive rock, master joints, wave cut platform,lithology etc in your account.

• Check through that you have used all the evidence provided on theresource.

• Quote examples of similar features you have studied to support youranswer.

AcknowledgementsThis Factsheet was written and researched by Sue Warn.Geo Press. Unit 305B, The Big Peg, 120 Vyse Street, Birmingham B18 6NFGeopress Factsheets may be copied free of charge by teaching staff or students,provided that their school is a registered subscriber.No part of these Factsheets may be reproduced, stored in a retrieval system, ortransmitted, in any other form or by any other means, without the prior permissionof the publisher. ISSN 1351-5136

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The impact of structure and lithology on coastal landforms Geo Factsheetwww.curriculumpress.co.uk

10m

5m

Three Cliffs Bay

Devil’s Bridge, Worms Head

Fault

Shatter zone

Erosion of master‘bedding plane’

Sandybeach

Rocks

Stack

Headland

Stack

NaturalarchCave

Cave

Cave

Cove

Geo

Clif

fsC

liffs

Shale

Low

waterm

ark

High

water mark

Sanddunes

Dolge

rite Dyk

e

Faul

t

Cliffs

0 km 1 N

Key:Flat rocksexposed atlow tideAngle of dipSand dunesFelsite (anintermediateigneous rock)BouldersJointedlimestoneShaleMassivesandstone

Clays and

mudstones

Closely bedded

clays & mudstones

Felsite sill

Bedded shale

� Major features:• fault separates: East coast - concordant

N East coast - discordant• impact of fault itself – geo.

� Features of concordant coast: • cove sequence, • impact of limestone lithology (joints).

� Features of discordant coast: • headland and bay sequences• wave cut platform• impact of dolerite dyke• beach and link to sand dunes (reasons for formation).