School of Earth and EnvironmentContents BGSIntroduction to Structural GeologyWorkbook 3 Geological MapsSchool of Earth and Environment2Contents BGSContentsIntroduction to geological maps 41. Outcrop patterns on geological maps72. Cross sections163. Structure contours 22Acknowledgements and references45School of Earth and Environment3ContentsHow to use this workbookThis workbook is intended as a refresher course for those who have some previous experience of geologicalmaps.Ifyouarecompletelynewto thesubjectitisworthhavingalookatsomeof thetextbooksinthebibliographyinconjunction with the workbook.School of Earth and Environment4ContentsIntroduction to geological mapsGeological maps represent the solid geology at theEarthssurfaceunconcealedbyvegetation, soil or buildings (fgure 1a).Differentrocktypesandformationsare illustratedbydifferentcoloursand/orsymbols. Otherfeaturessuchasfaults,mineralveins, coalseams,markerhorizonsandlandslipsare shown.Bedding and structural features such as cleavageandfoliationsareindicatedbystrike anddiporplungeandazimuthsymbols(fgure 1b).Thereareseveraldifferenttypesofgeological maps, the main ones being solid geology, which showbedrock,anddriftmaps,whichshow unconsolidated sediments such as river alluvium, peat and glacial deposits that lie above bedrock. Geologicalmapsusuallyincludestratigraphic columnsandoneormorecrosssection(fgure 1c).Stratigraphycolumnsshowthedifferent formations on the map in the order of deposition, with the oldest at the bottom and youngest at the top. Theyalsoshowtheirthicknessesandare usually drawn to scale.The cross sections show thesub-surface(andoftentheabove-surface) geology, as predicted from the features mapped at the surfaceMapscomeinavarietyofscalesdepending ontheamountofdetailneededfordifferent purposes.Inallcases,theywillshowagrid forlocation(inBritain,theNationalGridon land).Pointsonthemaparereferredtousing coordinates (eastings then northings) which are usually6or8fgurereferences. Thebasicgrid squarecovers100,000metres,withnorthings, forexample,givenas670000mN,givingan absolute location in metres. When working within amap,itisusualtogive3fgures(e.g.700N) or 4 fgures (e.g. 7000), depending on the scale ofthemap.Fourfguregridreferencesspecify locationstowithin10metres.Mapscoveringa larger area tend to use latitude and longitude in addition or instead. Maps for other countries will use other National Grids.This workbook looks frst at the outcrop patterns ongeologicalmaps,whichformduetothe interactionofthegeologywiththetopography. Athowfolds,faultsandunconformitiesform consistentoutcroppatternsandtheinformation thatcanbegainedaboutthesestructuresfrom a map.Itthenlooksathowtodrawcrosssections, including dealing with apparent dip.How to use structurecontourstoconstructcrosssections and to use structure contours to make accurate andquantitativepredictionsaboutsubsurface geology.Workingwithgeologicalmapsenhances theabilitytothinkin3Dandre-enforcesthe relationships, both temporal and spatial, between different geological units and structures.School of Earth and Environment5ContentsFigure 1a: Geological map of the Snowdon area, Wales, UK. ( British Geological Survey).School of Earth and Environment6ContentsFigure 1b (left): Key to the Snowdon map.Figure 1c (right): Cross section and stratigraphic columns on the Snowdon geological map ( British Geological Survey).School of Earth and Environment7ContentsWorksheet 1:Outcrop patterns on geological mapsIftheEarthssurfacewasfat,iftherewasno topography,thengeologicalmapswouldbe simple.Theywouldbeadirectrefectionof theunderlyinggeology.However,topography interactswiththegeologytoproducemore complex but predictable patterns. Horizontal and vertical strata:Horizontalandverticalstrataarethemost straightforward to interpret on a map.Horizontal outcropswillalwaysfollowthetopographic contours(fgure2a),whilstverticallayerswill always form straight lines (fgure 2b).Dipping strataDippinglayersinteractwiththetopographyin predictable ways.In valleys, beds will appear to vee either up or down the valley in the direction ofdip(fgure2cand2d).Thisisbecausethe valley side acts as an approximate cross section not always helpful on small scale maps but on large scale maps and in the feld this is a useful aid in interpretation.100m100m 200m300m 400m 200m300m400m100m100m 200m300m400m 200m300m400m100m100m 200m300m400m 200m300m400m100m100m 200m300m400m 200m300m400mFigure 2a: Horizontal beds Figure 2b: Vertical bedsFigure 2c: Beds dipping towards viewer Figure 2d: Beds dipping away from viewerSchool of Earth and Environment8ContentsEffects of dip of beds and width of outcropThewidthofoutcroponamapisaffectedby the dip of the beds and by the steepness of the topography.All the beds in fgure 3 have the same thickness. Figure 3a, 3b and 3c show the effect of dip.As the dip decreases from steeply dipping in Figure 3atoshallowlydippinginfgure3ctheoutcrop on the map becomes wider and wider.Infgure3dthebedshavethesamedip,but thetopographyvaries.Wherethetopography is steepest on the hillside the outcrop is narrow, where the topography is horizontal along the hill top the outcrop is wider.Figure 4 shows the Isle of Wight geological map. The Mesozoic sediments have been folded into a monocline with a steep dipping limb and a near horizontallimb.ArrowApointstotheshallow dippinglimb.Noticehowwidetheoutcropis here and how closely the geological boundaries follow the contours, indicative of beds with very lowdips.ArrowBpointstothesteepdipping limb.Noticethenarrowoutcroppatternand howthegeologicalboundariescutsacrossthe topography indicating the steeper dip.a bd cFigure 3: Effects of dip of beds and steepness of topography on width of outcropSchool of Earth and Environment9ContentsABFigure 4: Geological map of the Isle of Wight, southern England, UK.See text for details. British Geological Survey.School of Earth and Environment10ContentsFaults on mapsFaultscanoccuratanyanglewithrespectto beddingandsotheoutcroppatternsproduced are not unique to any one fault type.Figure 5a and 5b show two potential outcrop patterns for a normal fault.In a) the fault is parallel to the strike of the beds andinb)thefaultisperpendiculartothestrike ofthebeds.Figure5band5cshowhowthe sameoutcroppatterncanresultfromdifferent fault movement.Most geological maps include a symbol on the fault indicating the style of faulting.Figure6showsthegeologicalmapforKewick Thrustfaultsareshownwithaflledtriangleon theirhangingwall.Strike-slipfaultswithhalf arrowsindicatingthedirectionofmovement andnormalfaultsareshownwithatickonthe hangingwall.Foldaxialtracesarealsoshown on the map.a) b) c)Figure 5: Faults on geological maps.See text for details.School of Earth and Environment11ContentsNormal faultThrust faultStrike-slip faultAntiformSynformFigure 6: Geological map of Keswick, northwest England, showing different types of faults and folds.See text for details. British Geological Survey.School of Earth and Environment12ContentsFold on mapsFigure 7 shows the different outcrop patterns for folds.Thelimbsofthefoldsformarepeating patternoneithersideoftheaxialplane.For anticlines, the oldest rocks are in core of the fold andtherocksgetyoungerawayfromtheaxial trace(fgure7a).Forsynclines,theyoungest rocksareinthecoreandtherocksgetolder away from the axial trace (fgure 7b).Plunging folds form the same repeating pattern as non-plungingfolds,excepttheirlimbsconverge around the axial traces.The limbs of synclines openinthedirectiontheyplunge(fgure7c); whilst the limbs of anticlines close in the direction they plunge (fgure 7d).Figure 8 shows the Pemnbroke geologicalmap andcrosssection.Thisshowstherepeating pattern of geological units across the map typical of folding.Note the apparent closure of the fold ismostlikelyrelatedtotopographyasthedips do not change along the axial trace.See fgure 10 for an example of a plunging fold.a) b)c) d)Figure 7: Folds on geological maps.See text for details.School of Earth and Environment13ContentsFigure 8: Geological map and cross section of Pembroke, southwest Wales, UK.See text for details. British Geological Survey.School of Earth and Environment14ContentsUnconformitiesUnconformitiesrepresentabreakindeposition and a period of uplift and erosion. They can cover tens or hundreds of millions of years and tens or hundreds of metres of strata may be removed.Onlap: An unconformity where the younger series of rocks progressivelyonlaptheoldermembersofthe underlyingseries.Sothatyoungerbedsofthe seriesabovetheunconformityrestdirectlyon the beds beneath the unconformity.Also known as overlap.Overstep: Only the lowest bed of series above unconformity restsdirectlyonthebedsbeneaththe unconformity.Figure 10 is the geological map for Bristol.Here the Triassic (orange/brown) units unconformably overliethefoldedCarboniferous(blue)units. Thetwoclearestfoldsareanticlinesandare separatedbyanarrowsynclinelargelyhidden bytheTriassicsediments.Theupperanticline plunges to the east and the lower one to the west.a) b)Figure 9: a) An example of onlap.b) An example of overstep.School of Earth and Environment15ContentsFigure 10: Geological map of Bristol, southwest England, UK.See text for details. British Geological Survey.School of Earth and Environment16ContentsWorksheet 2: Cross sectionsA key goal of structural geologists is to understand thethreedimensionalgeometryoftherock layers. Ageologicalmapisatwodimensional representationofgeologicalfeaturesonthe Earthssurface.Inordertoprovideathird dimension, one or more slices through the Earth are needed.Although cross sections are usually vertical, there are instances where it is desirable toprojectgeologicstructuresontoadipping plane such as the profle plane of a plunging fold. Cross sections show thicknesses, dip directions, folds, faults, unconformities, sediment thickness changes, igneous intrusions etc.Todrawacrosssection,outcropdatafromthe Earths surface are projected into the subsurface geology.Sometimesthisinformationcanbe supplementedwithseismicorwelldata.Tobe able to draw a geologically realistic cross section an understanding of the geometry and kinematics of structures is needed.A cross section should be consistent with all the available data, although there are often several viable interpretations of the same data.Most cross sections are drawn to true scale, that is, where the horizontal scale is the same as the vertical scale.This means the true dip of the rock unitsareshown.Verticalexaggeration,where theverticalscaleisincreasedrelativetothe horizontal,issometimesusedtomakeacross sectionclearer.However,italsoincreasesthe dips of the rock units exaggerating the geological structures.Figure 11: Cross section through the Keswick map in fgure 6. British Geological Survey.School of Earth and Environment17ContentsHow to draw a cross section:Step 1: Determine the line along which to draw the section. Thelineofsectionshouldberepresentativeof thestudyarea,beperpendiculartothemajor structuralfeatureofthearea(e.g.largescale folds or faults), cross as many structural features as possible and run through areas with the most data readings.Step 2: Drawaxesofanappropriatescalewiththe topographic values. Unless there is a reason to do otherwise, draw a true-scale section.0m100m200m300m-100m-200m400m0m100m200m300m-100m-200m400m250200150100150200250300A A14141414N0m 500mMap 1: How to draw a cross sectionSchool of Earth and Environment18ContentsHow to draw a cross section: Step 3:Transferthetopographicinformationfromthe maptothesection.Projecttheheightofeach topographiccontour,whereitcrossestheline ofsection,ontothesectionanddrawinthe topography.0m100m200m300m-100m-200m400m0m100m200m300m-100m-200m400m250200150100150200250300A A14141414N0m 500mMap 1: How to draw a cross sectionSchool of Earth and Environment19ContentsHow to draw a cross section: Step 4:Transfer the lithological boundaries, faults etc on to the cross section in the same way.Step 5: Transferbeddingreadingsontothesection, correcting for apparent dip if necessary (see fgure 12).Plot the readings at the height at which they occur,sowhereareadingisextrapolatedfrom agreaterorlesserheightthanthetopography ofthecrosssectionplotitaboveorbelowthe topography as appropriate.0m100m200m300m-100m-200m400m0m100m200m300m-100m-200m400m250200150100150200250300A A14141414N0m 500m14141414Map 1: How to draw a cross sectionSchool of Earth and Environment20ContentsHow to draw a cross section: Step 6: Usingthebeddingreadingsasaguide,draw inthelithologicalboundariesbothaboveand belowthesurface.Geologyextendedabove the topography is shown by dashed lines. When drawingthesectionalwaysconsiderwhatis geologically reasonable behaviour for the layers e.g. sudden changes in a units thickness or dip should be justifable.Step 7: Stand back and admire your work.0m100m200m300m-100m-200m400m0m100m200m300m-100m-200m400m250200150100150200250300A A14141414N0m 500m14141414Map 1: How to draw a cross sectionSchool of Earth and Environment21ContentsApparent dip:Where a sequence of rocks are cut by a section whichisnotparalleltothemaximumdipofthe beds,thentheamountofdipwillappeartobe less.This is known as the apparent dip and it is always less than true maximum dipSotodrawanaccuratecrosssectionitis necessarytocalculatetheapparentdipofthe bedsthatdonotstrikeperpendiculartothe section.This is done using the equation: tan = tan cos whereistheanglebetweenthedipdirection and the cross section; is the angle of dip and is the apparent dip.As the section becomes more and more oblique to the maximum dip the apparent dip will become lessandlessuntil,wherethecrosssectionis parallel to strike the beds, it will be zero and the bedswillappearhorizontalregardlessoftheir true dip.ApparentdipCross section parallel to dip directionCrosssectionparallelto strikeTruedipBedding surfaceMap viewApparentdipMap viewLine of cross sectionparallel to dipLine of cross sectionat an angle to dipLine of cross sectionat an angle to dipLine of cross sectionperpendicular to dipFigure 12: Apparent dip illustrated as a block diagram and in map view.School of Earth and Environment22ContentsWorksheet 3: Structure contoursStructurecontoursconnectpointsofequal elevation(i.e.heightordepth)onageological surface(e.g.bedding)inthesameway topographiccontoursconnectpointsofequal heightabovesealevel.Structurecontours arealsoknownasstrikelinesbecausethey runparalleltostrike.Structurecontoursare commonly used to contour subsurface horizons ofinterest.Theycanbeusedinthefeldto predict geological boundaries and used on maps withnobeddingreadingstoaidcrosssection construction.Figure13ashowsabeddingplaneandheights above sea level.Figure 13b shows the structure contoursasprojectedontotheplane.Figure 13cshowshowthesestructurecontoursare projectedontothemapviewandfgure13d shows the map view.Aplanewithaconsistentstrikeanddiphas evenly spaced straight structure contours (fgure 13), whilst a folded plane or a plane with irregular dip will have folded or varyingly spaced structure contours (fgure 15). N400m300m200m100mN400m300m200m100mN400m300m200m100m400m300m200m100m400m300m200m100m400m300m200m100mNa) b)c) d)Figure 13:Structure contours on a simple bedding plane and the map view of the bedding plane.School of Earth and Environment23ContentsHow to draw structure contours:Whereatopographiccontourscutsthecontact betweentworocktypesitdefnestheheightof the structure contour for the contact at that point. Where a topographic contour cuts back and forth acrossacontactitgivesseveralpointsatthe same height.The red dots on map 2 are all at the same height: 250m - and so will all lie on the 250m structure contour for the boundary between the yellow and white units. 250200150100150200250300A AN0m 500mMap 2: How to draw structure contours.School of Earth and Environment24ContentsHow to draw structure contours:Assumingthatstrikedoesnotvaryalongthe boundarythenthestructurecontourswillbe straight lines, so the points can be joined to form the structure contour for 250m.Atanypointalongthestructurecontourthe boundary is at 250m.Where the topography is below 250m then boundary will be eroded away. Where the topography is greater than 250m the boundary will be underground.250200150100150200250300A AN0m 500m250mMap 2: How to draw structure contours.School of Earth and Environment25ContentsHow to draw structure contours:Theprocessisrepeatedfortheotherstructure contours.Assuming the boundary has a constant dipthenthestructurecontourswillbeevenly spaced, meaning the structure contours can be drawn in even where the bed does not outcrop.250200150100150200250300A AN0m 500m100m200m150m250m300m400m350m450mMap 2: How to draw structure contours.School of Earth and Environment26ContentsHow to draw structure contours:The structure contours can be used to construct the cross section for the boundary by projecting the heights of the structure contours, where they cross the line of section on the map, to the same height on the section itself.0m100m200m300m-100m-200m400m0m100m200m300m-100m-200m400m250200150100150200250300A AN0m 500m100m200m150m250m300m400m350m450mMap 2: How to draw structure contours.School of Earth and Environment27ContentsHow to draw structure contours:Thisprocessisthenrepeatedfortheother boundaries on the map.Notethatinthiscase,becausetheunitshave verticalthicknessesinmultiplesof50m,the samespacingasthestructurecontours,the structurecontoursforthedifferentboundaries overlie each other.0m100m200m300m-100m-200m400m0m100m200m300m-100m-200m400m250200150100150200250300A AN0m 500mMap 2: How to draw structure contours.School of Earth and Environment28ContentsHow to draw structure contours:Thecompletedcrosssectiondrawnusing structure contours.0m100m200m300m-100m-200m400m0m100m200m300m-100m-200m400m250200150100150200250300A AN0m 500mMap 2: How to draw structure contours.School of Earth and Environment29ContentsHow to measure strike and dip from structure contours.Strikeismeasuredclockwisefromnorth.In fgure 14 the structure contours run east - west and so have a strike of 090.Tomeasurediptrigonometryisused.The horizontaldistanceismeasuredfromthemap and the vertical distance is the difference between thevaluesofstructurecontours.Infgure14, thehorizontaldistanceis500mbetweenthe 400m and 100m structure contours.The vertical distance the difference between the two structure contours:300m.dip = tan-1(300/500) = 31N400m300m200m100m400m300m200m100m400m300m200m100mN100m300m500m500mFigure 14: a) Block model of structure contours on a bedding plane.b) Map view of structure contours.School of Earth and Environment30ContentsThree point problemsSofartocreatestructurecontourswehave needed at least two points on a boundary to be abletodrawinthecontour.Thethreepoint problem method allows structure contours to be drawn when only three heights/depths are known on a boundary.Map3showsthreesmalloutcropsofacoal seam.Outcrop A is at 150m, outcrop B at 200m and outcrop C at 250m.250200150100150200250300A AN0m 500mA.B.C.Map 3: Three point problem.School of Earth and Environment31ContentsThree point problemsTo fnd the structure contours for the coal seam a line is drawn between A and C.The mid point ofthislinecrossesthe200mstructurecontour. OutcropBalsoliesonthe200mstructure contour and therefore this structure contour can be drawn in on the map.250200150100150200250300A AN0m 500m200mA.B.C.Map 3: Three point problem.School of Earth and Environment32ContentsThree point problemsOutcropAliesonthe150mstructurecontour andoutcropConthe250mstructurecontour. Assumingthecoalseamisplanar,these structure contours can be drawn in parallel to the 200m structure contour.The rest of the structure contoursforthecoalseamcanbedrawninat the same spacing across the map.250200150100150200250300A AN0m 500m100m150m200m250m300m350m50mA.B.C.Map 3: Three point problem.School of Earth and Environment33ContentsThree point problemsTheoutcroppatternforthecoalseamcan beconstructedbyfndingwherethestructure contours and topographic contours of the same value cross. The coal seam can then be drawn in and a cross section drawn in the usual way.250200150100150200250300A AN0m 500m100m150m200m250m300m350m50mA.B.C.Map 3: Three point problem.School of Earth and Environment34ContentsStructure contours on a faulted surfaceStructurecontoursacrossfaultedsurfacescan bedrawninthesamewayasacrossbedding surface.Map4showsacoalseam(thinblack line)andadyke(thickdarkgreyline)cutbya fault (red line).250200150100150200250300A AN0m 500mMap 4: Structure contours on a faulted surface.School of Earth and Environment35ContentsStructure contours on a faulted surfaceStructurecontourscanbedrawnonthefault andthecoalseamintheusualmanner.They couldbedrawnonthedykebutasitisvertical the structure contours would all lie on top of each other.250200150100150200250300A A0m 500mNMap 4: Structure contours on a faulted surface.School of Earth and Environment36ContentsStructure contours on a faulted surfaceThecrosssectionisconstructedfromthe structure contours.0m100m200m300m-100m-200m400m0m100m200m300m-100m-200m400m250200150100150200250300A A0m 500mNMap 4: Structure contours on a faulted surface.School of Earth and Environment37ContentsStructure contours on a faulted surfaceProvidedthesectionistruescale,thethrow ofthefaultcanbemeasureddirectlyfromit. However,asingleboundarycannottellusthe displacementonthefault,informationfroma secondboundaryisneeded.Thedykeshows horizontal offset indicating the fault is an oblique sllip fault. 0m100m200m300m-100m-200m400m0m100m200m300m-100m-200m400m250200150100150200250300A A0m 500mNMap 4: Structure contours on a faulted surface.School of Earth and Environment38ContentsStructure contours on a faulted surfaceMap 5 shows another example of a faulted coal seam.Inthiscasethethrowonthefaultcan bedeterminedfromstructurecontoursalone. Notehowthevalueofthecoalseamstructure contourschangeby200mastheycrossthe fault.Thecoalseamtotheeastofthefaultis downthrownby200mrelativetothecoalseam on the west of the fault.From the map alone it is not possible to work out whether there has been any along strike displacement on the fault.250200150100150200250300A AN0m 500m300m250m200m150m100m50m300m350m250m200m150m100m50m0m-50mMap 5: Structure contours on a faulted surface.School of Earth and Environment39ContentsStructure contours on a folded surfaceSo far we have dealt with structure contours on a planar surface, that is one with a constant strike and dip.For surfaces which vary in strike and/or dip structure contours can still be constructed.Figure15showsafoldpairwithstructure contours.Asthestrikeofthebedsvaries aroundthefoldsodothestructurecontours. Figure 15b shows the map view of the fold.Note howwherethedipofthebedsaresteeperthe structure contours are closer together and where thedipofthebedsisshallowerthestructure contours are more widely spaced.100m200m300m400m500m600m700m100m200m300m400m500m600m700m800mN800m700m600m800m500m500m600m700m400m400m300m300m200m200m100m100mNa)b)Figure 15:a) Block model of a fold pair.b) Map view of the same fold pair.School of Earth and Environment40ContentsStructure contours on a folded surfaceStructurecontoursonafoldedsurfaceare constructedinthesamewayasforadipping surface.This frst example is for a simple, non-plunginguprightfold,wherethelimbshave consistent strikes and dips, making the process of drawing the structure contours and cross section the same as for a dipping bedding planes.250200150100150200250300A AN0m 500mMap 6: Structure contours on a folded surface.School of Earth and Environment41ContentsStructure contours on a folded surfaceThebedsmap6showtherepeatingoutcrop pattern of folded beds.The red dots show where the yellow/light grey boundary crosses the 250m topographiccontour.Therearetwowaysthe dotscouldbejoinedtoformstructurecontours -eithereast-westornorth-south.However, from the way the beds vee in the valley it can be deduced the axial planes of the folds run roughly north-south and so the structure contours for the limbs must also be north-south.250200150100150200250300A AN0m 500mMap 6: Structure contours on a folded surface.School of Earth and Environment42ContentsStructure contours on a folded surfaceThe structure contours for the rest of the boundary and the yellow/dark grey boundary can be drawn in in the usual way and projected on to the cross section as before.0m100m200m300m-100m-200m400m0m100m200m300m-100m-200m400m250200150100150200250300A AN0m 500mMap 6: Structure contours on a folded surface.School of Earth and Environment43ContentsStructure contours on a folded surfaceCompleted cross section.0m100m200m300m-100m-200m400m0m100m200m300m-100m-200m400m250200150100150200250300A AN0m 500mMap 6: Structure contours on a folded surface.School of Earth and Environment44ContentsStructure contours on a folded surfaceMap 7 shows a plunging syncline with structure contours.Thewhitestructurecontoursarefor theyellow/palegreyboundaryandtheorange structurecontoursarefortheyellow/darkgrey boundary.Thedashedlineistheaxialtrace. Theplungeofthesynclinecanbecalculated from the structure contours in the same way as dip is calculated using:plunge = tan-1(vertical/horizontal) wheretheverticalisthedifferenceinthevalue ofthestructurecontoursandthehorizontalis measured from the map.Azimuth is measured clockwise from north.250200150100150200250300N0m 500m100m100m250m250m150m150m200m200mMap 7: Structure contours on a plunging fold surface.School of Earth and Environment45ContentsAcknowledgments and referencesMap sources:GeologicalmapsareallcopyrightofNERC andBGSandhavebeenmadeavailablevia GeoScholar.Bibliography:Bennison,G.M.andMoseley,K.A.,2011.An introduction to geological structures and maps: a practical guide. Eighth edition, Hodder education. Thismoduleisbasedonthemapscomponent ofthefrstyearstratigraphicprinciplesand mapscourseoftheGeologicalSciences degreeprogrammeattheSchoolofEarthand Environment, the University of Leeds.Author: Dr Jacqueline Houghton, School of Earth and Environment, University of Leeds.