Scarborough Sixth Form CollegeA2 Geology

Module GL4E4 – Geological Map Interpretation

Name: _________________________________________

Key Idea 1Outcrop patterns on geological maps can be used to identify and interpret structural elements

Key Idea 2Geological maps contain information relevant to a wide range of geological applications

This unit is examined as part of the GL4 exam in June. You will have to answer questions that require you to use a geological map to discover or assess:

1. The attitude of beds and folds2. The types and dips of fault 3. The types and nature of intrusions4. Unconformities5. Relative ages of the features6. The geological implications of construction or land use projects (landfill,

quarrying, extraction of water or fossil fuels and mining) and the geohazards associated with them (see GL3 module from AS level).

To do this you will need to develop your mapwork skills.

Skill Check

SkillI am

Confident UnsureDescribing outcrop patternsOutcrop width and dip of bedTrue & apparent dipRelative dating of unitsIdentify conformable and unconformable sequencesIdentify unconformitiesIdentify folds – including plunging foldsDescribe folds using maps & cross sectionsIdentify & describe faultsMeasure offset of faultsIdentify types of intrusionsIdentify metamorphic sequencesInterpreting environments of deposition from rock typesAssess potential sites for a specific use – mining, waste disposal Identify possible geological hazardsInterpret subsurface geology in terms of groundwater, radon, fossil fuels and geothermal energyIdentify environmental issues in relation to extraction of resources

Past Examination Questions

The papers should be completed in this order as the skills tested become more complex.

Order Year Map location Applied question topicMark obtained

and grade

1 2005 CastletonOpening a road on Mam Tor landslide – monitoring and stabilising

2 2002 Cheddar Quarries and landfill sites

3 2006 Alloa Quarries and landfill site

4 2008 Matlock Radon and reservoir

5 2007 Worcester Springs and tunnel construction

6 2003 Hawes Reservoir site suitability

7 2004 StokeDeveloping housing on a previously mined area

8 2009 Monmouth Mining hazards

9 2010New

CumnockCoal

The aim of this booklet is to help you to learn ‘the rules’ or instructions that will allow you to interpret any geological map.

1.0 Essential things to know about a ‘real’ geological map

What is on a typical map?This will depend on what map the examiner’s give you. However, there are certain things that are usually there. Scale Key Cross section

1.1 ScaleIt is always important that you know what scale the map and even the cross section is in. Therefore before you start any map exercise look at the scale.

The scale for a map can appear in two ways: as a ‘ruler’ – you can directly measure things and you should already be familiar

with this method. as a ratio number - the two scales that are often used on the maps are 1:25,000

and 1:50,000. This means that 1cm on the map represents 25000cm on the ground. In the table below say what each scale represents in metres and kilometres.

scale 1:25000 1:50000In metresIn kilometres

WARNING: on cross sections the horizontal and vertical scales may be different and many beds are shown as varying in thickness.

1.2 KeyThe key on the map can be divided into several different sections and can be displayed in numerous ways. The purpose of the key is to tell you what type of rock is present as well as giving an indication of its age. Again it is important that you look at the key before you start any map.

1.3 Lithology (rock type)Lithology can be divided into three groups. It is important to remember that drift, sedimentary and metamorphic rocks are displayed using the law of superposition – oldest at the bottom of the list.

1.31 DriftDrift deposits are thin, unconsolidated Pleistocene or Recent sediments. They may be shown on the map as black symbols over the bedrock colours, or as blocks of colour themselves if it is a drift map. They are shown in order of age but not to scale as their thickness will vary considerably from place to place.

Common symbols for drift deposits are listed below.

PEAT – post glacial

ALLUVIUM – post-glacial, deposited and confined to modern rivers or

past channels

GLACIOFLUVIAL deposits – sediment associated with glacial meltwater

TILL – glacial boulder clay

HEAD – sediment produced by solifluction processes

Another symbol that you can come across is:

This indicates what the solid geology (i.e. rock) is below the drift deposits. The boundary

between the two is known as the rockhead.

1.32 Solid geology

Sedimentary and metamorphic rocks are shown in order of age and usually to scale.

The lettering on a sedimentary rock follows the following conventions:a, b, c …….a older than b etca1, a2-3 …… a1 older than a2-3 etca4ii, a4iii ……fewer ticks mean older beds

There are many other symbols but these will be used to identify the specific bed rather than show its age. If in doubt look at the key or generalised vertical section.

IgneousIgneous rocks are displayed in rock types. Don’t worry if you come across rock types that you are not familiar with. They are displayed neither in age order nor to scale.

1.4 Symbols usedThis can vary from map to map however there are the standard ones which you should definitely learn.

1.5 Grid ReferencesOS grid references are used. In the exam you are likely to see either 4 or 6-figure numbers depending on what they want you to look at or do.

A 4-figure reference gives the bottom left hand corner of the square in question. Remember the rule ‘Go along the bottom before going up the stairs’ to read off a grid reference.

Most maps will not display a north arrow as we use the convention that north is towards the top of the map, following the north gridlines. This therefore allows you to orientate a dip direction for beds or faults as well as commenting on the trend or strike of a feature.

1.6 Apparent and true dipYou should recognise the difference between true and apparent dip and how they relate to strike.

horizontal strata vertical

strata

30

inclined strata

65overturned

strata

boundary between two units

landslide

Coal seam

Cleavage plane

backfill

fault – tick on downthrown

side

Using the above diagram write in the table below your own definition for:

Strike

True dip

Dip direction

Apparent dip

1.61 Measuring the dip of planar featuresYou could be asked to interpret which way a bed, fault or in deed any planar structure is dipping using the map and/or a photograph from a particular view point.

To do this, remember that “the dip is the angle of dangle” in other words the angle down from the horizontal. Measure the angle by turning your protractor upside down. This is illustrated in the diagram below.

1.62 Visualising how a feature would look from different anglesIf you are asked to compare what is on the map to what it would look like orientated from different direction you will have to imagine what you would expect to see. This can be difficult but be patient as it can be done.

One way is to use your hands as a guide, orientating them according to the dip arrows on the map and then trying to look at it from different angles. You could even sketch small diagrams to help you.

A key phrase that you will have to use is that the view or the rock ‘shows apparent dip’.

1.63 Describing the outcrop pattern for a unitA very common and easy question involves you describing the outcrop pattern for a particular unit. To do this successfully you need to comment on various characteristics such as:

1.7 Bedding and contour relationship

If bedding planes/ outcrop pattern is parallel to contours = horizontal beddingIf outcrop pattern cuts across the contours at a steep angle = inclined bedding

This will be very useful when describing the outcrop pattern or working out whether the rock unit is dipping.

1.8 Bedding and topography

1. ‘V-ing‘ up valley = dipping upstream into valley

2. ‘V-ing’ down valley = dipping downstream out of valley

3. The less the ‘v-ing’ of the outcrop pattern the more steeply inclined the unit is.

1.9 Outcrop width and dip

The direction in the outcrop pattern is striking in (trend of the outcrop) The shape of the outcrop – is it continuous or discontinuous, linear or

curved Where the outcrop occurs - is it limited to a particular area of the map e.g.

just in the northern part of the map The average thickness or dimensions of the outcrop (scaled as on the

ground not the map) or does it vary, particularly if a fold is involved?

2.0 UNCONFORMITY

These are formed by the following sequence of events: A sequence of beds is deposited The beds are folded or tilted and raised above sea level The beds are eroded in part Conditions change, such as a rise in sea level, and eventually a second

sequence of beds are deposited.

An unconformity can be considered to be a plane and should be taken to act like a bed of rock. Rock sequences above the plane of unconformity are said to be younger than the unconformity and their outcrop patterns should follow the unconformity. Anything older than the unconformity have their boundaries cut by the unconformity (principle of cross-cutting relationships). Usually the bed above this unconformity is likely to be a conglomerate called a basal conglomerate and it represents the products that were produced by the erosion of the underlying rock during the formation of the unconformity.

So to spot an unconformity on a map you are looking for beds that are adjacent but with different dips (angles and/or directions) and beds to be missing from the sequence. There are many minor unconformities (slight breaks in deposition called discontinuities) but exam questions will concentrate on the major ones shown on the key as thus:

Identify using the principle of cross-cutting relationships:

3.0 FAULTS

Faults cause people the most trouble. The examiners do not want to trip you up. They want you to identify and describe faults. If you learn the ‘rules of thumb’ you will be able to do this with ease.

Faults have three key features:1. A fault plane dips and the rules for recognising the dips are the same as those

for any geological boundary.2. There is a downthrown and upthrown side or the fault shows lateral

displacement (in which case it will offset vertical boundaries). The

Thin outcrop width = steeply inclined

Thick outcrop width = shallowly dipping

look at the outcrop pattern and identify any areas where the boundaries, strike is disrupted by younger rock units (this is the easiest one to identify)

gap in the ages of the deposits – use map and stratigraphical column

change in dip and dip direction (this can be very subtle) usually less steep in young rocks

downthrown side is indicated on a map by a tick mark. Rule: the younger bed is on the downthrown side.

3. Faults with vertical displacement can be normal or reverse.

The three key features are in each statement. If you are told or can work out 2 of them then the other one follows.

Run through these examples:

1) Mark on the downthrown side

If the fault dips to the W is it normal or reverseIf the fault dips to the E is it normal or reverse

2) What type of fault?Hint: use the V-rule.

3) What is the type of fault?

Common questions include describing the strike of a fault and the direction it has been downthrown in.

Features for movementCommon questions for faults with lateral displacements involve you determining the sense of displacement and giving a direction.

Types of fault

Step faults

NORMAL faults DIP TOWARDS the DOWNTHROWN sideREVERSE faults DIP AWAY from the DOWNTHROWN side.

Look at the key to see symbol for a fault – see if the fault has been mineralised

Look at tick on fault this denotes the downthrown side (if there is any). Very straight lines, no bends, cutting across contours = vertical or

near vertical fault Strike is an azimuth – this means you should say for example NE-SW Downthrown direction is towards something – for example E or N

Look for the offset of vertical beds or intrusions – tells you direction and amount of movement.

Describe lateral displacements in terms of sinistral or dextral.

Graben

4.0 FOLDS

Identified using:

Classified or described using:

Another common and easy question involves you describing the folding pattern on the map. To do this successfully you need to comment on the following things:

You do not need to comment on everything just what is relevant and appropriate for the number of marks available. Two marks for two features remember to say what your reasoning is.

Trend of the fold Synform or antiform based on the direction of dip either side of the fold axis Anticline or syncline based on whether the oldest or youngest rocks are in the

core of the fold Asymmetric or symmetrical fold based on either different outcrop thickness for

the same unit either side of the fold axis or different dip amounts either side of the fold axis

Evidence of plunge from the closure of the beds

repetition of beds changes in dip direction

Diverging dip direction - antiform

Oldest rocks at centre – anticlineYoungest rocks at centre – syncline

Plunging folds

Identified from:

Tip – if you are unsure about the outcrop pattern of a plunging fold get a piece of scrap paper fold length ways and tear the paper in half at an angle.

The question usually asked about plunging folds is to what is the direction in which they are plunging. To do this it is important that you find out what type of fold that is present. This will allow you using the rules mentioned above to determine the direction of plunge.

Plunge is always written as one direction e.g. towards the south-east.

Plunging synclines-limbs/beds appear to diverge in direction of plunge

Plunging anticlines-limbs/beds appear to converge in direction of plunge

‘swing’ of strike direction or dip direction. outcrop pattern will also bend towards the fold

axis.

5.0 IDENTIFICATION OF INTRUSIONS

It is relatively easy to distinguish between sills, dykes and plutons (usually large with a metamorphic aureole made of coarse grained rock). We have already covered the field evidence that you can use to classify them. More irregular intrusions are volcanic necks (usually small and made of a mixture of broken pieces of country rock, lava and ash called a breccia or an agglomerate).

5.1 Dips of boundaries and metamorphic aureoleA common question for metamorphic aureoles requires you to explain why its thickness can vary around the same pluton. It is important to consider that the aureole is just another layer and its outcrop width reflects the dip of the aureole (see outcrop width and dip diagram). It can also be influenced by topography or that there is a bit of unexposed intrusion just near the surface as illustrated in the diagram.

6.0 Relative agesThe key gives the relative ages of sedimentary beds so questions usually refer to faults, igneous intrusions and unconformities. Cross-cutting is the key dating techniques but you need to get the terminology right.

Dykes – discordant to bedding, cutting across topographical contours, straight, various thicknesses, sometimes occur in swarms, can be mineralised

Sills – can be wobbly in outcrop pattern, concordant to bedding, either follow or cut contours depending on the dip of the intrusion

Pluton – discordant, large body, coarse-grained rock Lava flow – concordant to bedding, fine grained (look at key), limited extent Vent agglomerate – small outcrop, can be circular in shape, it is a breccia that

is volcanic (explosive) in origin (the reason why it is brecciated)

You could be asked to describe the map evidence for the relative age of a unit to another unit or structure. Beware you could be told the relative ages or you could be asked to work it out.

It is usual for this question to appear in table format and this has in the past proved to be confusing. Take your time to understand the table before proceeding. However, the examiners have also used diagrams and the ages from the stratigraphic column.

Depending on what is appropriate to the question consider:

A fault cuts across and displaces anything older An unconformity cuts across and covers anything older An igneous rock cuts across and intrudes older beds if it is discordant (dyke,

pluton, vent agglomerate) whereas a sill will intrude between 2 older beds but this will not be obvious from the map unless it transgresses

The cross-cutting relationships and the law of superposition Age of rock in the centre of fold from the generalised stratigraphic column That intrusions will follow path of least resistance if they are intruding a fault. If

the fault is younger you could expect to see bits of the rock either side of the fault

The law of included fragments Ages of the units and missing ones from a normal sequence Sedimentary structures that can be used as way-up indicators

7.0 COMMON IDEAS TO THE ‘APPLIED’ BITS OF MAPWORK

This is really a revision of the material that you learn as part of the GL3 module. Topics could include:

1. Mining – either coal, quarry or mineral veins and the problems associated with it

2. Road – the building or reopening an existing road or even a tunnel3. Reservoir – the position of a potential reservoir,4. landfill site –the position of a new landfill site using an existing or proposed

quarry

The questions tend to involve you assessing and/or evaluating the evidence for or against a proposal. If you do not assess or evaluate you will automatically loose 1 mark. So make sure that you have both positives and negatives and say what your decision is.

The following points are by no means an exhaustive list but they should help to jog your memory.

Mining Problems associated with mining – subsidence due to collapse of tunnels or settling of made ground (backfilled land), slope failure, ground and surface water pollution, gas escape and explosions, fault movement due to loading.

Road and tunnelsProblems associated with roads or tunnels – permeability, fault reactivation, rock strength (if support is required or easier for tunnelling), structure (angle of repose for rocks)

Stabilisation techniques – (will depend on hard or soft sediment) drainage, vegetation, loading the toe/wall, terracing, gabions, reprofiling of the slope, bolts, piles, lining with concrete if in a tunnel.

Monitoring techniques – peg, satellite, surveying, mapping, groundwater pressure, creep measurements, strain measurements

ReservoirSuitability – permeability, porosity, strength of rock, faults, dip of rock, mineral veins that could contaminate the water supply, leakage, silting up, water supply, grouting LandfillSuitability - Rock type, permeability (pores and joints), bedding, leachate, gas, tectonic structure, liner, proximity to groundwater levels, mined ground, drift deposits.

Location of resources such as water or fossil fuels and geothermal energy Suitability – permeability, porosity, faults, heat source, traps such as a fold, steepness of beds.