INTRODUCING GEOLOGICAL MAPS*

HORIZONTAL BEDS ON MAPS

HORIZONTAL BEDS ON MAPSTrue thickness only seen in vertical cliffs/quarry faces

Truethicknessof bed invertical section

HORIZONTAL BEDS ON MAPS

HORIZONTAL BEDS ON MAPSOutcrop width will be apparent thicknessOutcrop width will increase with decreasing angle of slopeIn a valley, outcrop pattern will V upstream The oldest beds will outcrop at the bottom of valleys, the youngest on top of hills

HORIZONTAL BEDS ON MAPS

HORIZONTAL BEDS ON MAPSDraw simple annotated diagrams to illustrate how you would recognize horizontal beds on geological mapsRemember to highlight the importance of the relationship between outcrop pattern & contours!

HORIZONTAL BEDS ON MAPSTime for your first Geological mapwork problem!1ABennison map 1 p6/7Complete map using outcrop pattern1BBolton map1 p5/6Complete map using borehole dataYou will need to draw a stratigraphic log first

The BGS Moreton-in-Marsh sheet

"Where geological boundaries between strata are parallel to the topographic contours, the strata must be horizontal. "Outcrops of horizontal or gently inclined strata usually form prominent V-shaped patterns that point upstream in valleys. "For horizontal or gently dipping strata of constant thickness, narrow strip-like outcrops occur on steep slopes and wide outcrops occur on gentle slopes". "An outlier is an outcrop of rocks separated from the main outcrop by the process of erosion and surrounded entirely by older strata (hill) An inlier is an outcrop of rock that has been exposed by erosion of younger strata in such a way that it is entirely surrounded by the younger strata remaining" (valley)

Inliers and OutliersInlier-an area of older rock completely surrounded by younger rocksOutlier-an area of younger rock completely surrounded by older rocks

Inliers & Outliers

Draw simple annotated diagrams to illustrate the difference between inliers & outliers

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Strike and Dip

DIP AND STRIKEDip = Angle between horizontal and the maximum slope of a bedding planeTo measure dip angleClinometer To measure dip / strike directionCompass

Dipping 60 EastStrike North-South

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Strike and dip

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True and Apparent Dip'True Dip is the greatest' - anything less is Apparent DipTrue dip = parallel to the gable end & at 90 to the ridge. Strike direction = parallel to the ridge.

Apparent DipA dip amount which is less than the true dip. Cliff sections commonly show apparent dip. The direction of dip and strike will also be incorrect if apparent dip is measured.Always assess outcrops in the field carefully and ensure that the true dip is recorded.Beds apparently horizontalTrue dip 45

STRIKEDirection of a horizontal line on a dipping bedding planeie at 90 to dip direction, measured in degreesStrike lines are structural contoursOutcrop patterns tend to follow strike lines at 90' to dipNb Always record strike direction clockwise from dip direction (right hand rule Thumb points in dip direction heel of hand in strike direction)

Dipping 60 EastStrike North

Dip and Strike 3Summerleaze Beach, BudeStrike direction East-WestBeds dip North at 15Beds dip South at 58

Recording Dip & StrikeStrike and dip symbols are drawn on a geologic map for each outcrop with dipping or tilted bedsThe long line of the symbol is aligned with the compass direction of the strikeThe small tick, which is always drawn perpendicular to the strike line, is put on one side or the other, depending on which of the two directions the beds actually dip

The angle of dip is given as a number next to the symbol on the mapBeds with vertical dip require a unique symbol

Recording Dip & StrikeGeologic Cross SectionA geologic cross section represents a vertical slice through a portion of EarthOn a geologic map, cross sections are constructed by projecting the dip of rock units into the subsurface

Dip & StrikeDip =Instrument used to measure angle of dip=Strike =Instrument used to measure strike & dip direction=True dip=Only seen when looking in ______ directionApparent Dip=Will be horizontal if looking in ___ directionCopy the diagram

*What is the relationship between strike direction & outcrop boundaries on geological maps?

Outcrop PatternsGeological maps showa) Topographyb) GeologyWhere you can see rocks = ExposureWhere you would see rocks if superficial (unconsolidated) deposits were removed = OutcropIntersection of ground surface and geological boundaries = Outcrop PatternDepends on:

Thickness of rock unitsOrientation (dip) of geological surfaceShape of ground surface (Topography)

Outcrop PatternsExposure=Outcrop=Outcrop Pattern=

1 Thickness of rock unitsThick rock unit = Wide OutcropThin rock unit = Narrow outcrop

1 Thickness of rock unitsThickness measured at 90' to surface = True thicknessEg Horizontal bed outcrop in vertical cliff/quarry faceor the outcrop of a Vertical bed

Apparent thickness =Thickness measured vertically through layer, regardless of dipV = Vertical thicknessD = Angle of dipW = Width of outcrop

True thickness in the field:T = Vx cos DTrue thickness on a map:T = Wx sin DBed thickness can also be measured from the cross sections and the geological column BUT only if drawn with equal vertical and horizontal scales

2 Orientation (dip) of geological surfaceVertical beds: outcrop width = true thickness (a)Steeper the dip: narrower the outcrop (b)Gentler the dip: wider the outcrop (c and d)

abcdd

3 Slope of ground surfaceThe shape of the land (topography) has a major influence on outcrop patternSlope parallel to dip = wide outcrops (b & d)Slope at right angles to dip = narrower outcrops (a,c,e,f)However, luckily for you, AS Geology exam maps usually assume a horizontal lands surface to keep things simple!abcdef

Outcrop PatternsDraw simple sketches to explain how the following affect outcrop patterns:

Thickness of rock unitsOrientation (dip) of geological surfaceSlope of ground surface (Topography)

The Principle of SuperpositionIf one bed lies on top of another, the one on top is the younger, assuming they have not been overturnedCheck for way-up criteria such as graded bedding or mudcracksBeds get younger in direction of dip

Dipping beds and Outcrop PatternsBeds dip N (upstream) V-pattern points upstream in the valleyBeds dip S (downstream) V-pattern points downstream in the valleyThe outcrop pattern of dipping beds cuts across the contours on maps (shown as dashed lines)Outcrop pattern V points in direction of dipBut beware of horizontal beds!

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V-rule model

Relationship between dip and outcrop patternInfluence of slopeGentle slope- wide outcropSteep slope- narrow outcropHorizontal beds:Horizontal land- theoretically infinite widthOutcrop pattern will be parallel to topographic contoursIn a valleyOutcrop 'V' points upstreamDipping bedsGentle dip- wide outcropSteep dip - narrow outcropIn a valley- outcrop 'V' points in direction of dipVertical bedsStraight narrow outcrop (width = true thickness)Outcrop pattern has no relationship to contours

Outcrop PatternsDraw simple sketches to illustrate:How the relative age of beds can be established from the dip direction.How the dip direction can be determined in a valleyExplain how you could distinguish between the V shaped outcrop patterns in a valley caused by horizontal & dipping bedsHow would you identify vertical beds on a geological map?

A clinometer can also be used to calculate height by taking the angle reading from the clinometer and using the formula;Height = H + D x Tan(A)H=Height of Clinometer to floor baseD=Distance of Clinometer to objectA=Angle taken from Clinometer Reading

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