Notes · GeologyUK · A-Levels

Geological map applications

This chapter develops the practical, examined skill of reading geological maps and cross-sections. It covers the conventions of dip, strike and the rule of Vs, the recognition of structures from outcrop patterns, the construction of cross-sections and the calculation of true thickness and dip, and the application of maps to engineering and extraction.

4 sections~15 min reading time3 competenciesLevel Standard 1 · Advanced 3

T·101010 / 13

Exam profile

AO1 · Describe the conventions of geological maps and cross-sectionsAO2 · Measure and calculate dip, true thickness and structure from maps and construct cross-sectionsAO3 · Interpret outcrop patterns to deduce sub-surface structure and history and to evaluate ground conditions

Operators:interpretdeducecalculateconstructexplainevaluate

basic level

AS-Level expects you to read dip and strike, recognise simple structures on a map and draw a simple cross-section.

higher level

The full A-Level requires you to interpret complex outcrop patterns, calculate true thickness and dip, and apply maps to engineering problems.

Depth

Text

Contents · 4 sections

Geological map applications

Reading a geological map: dip, strike and the rule of Vs#

AdvancedWJEC/Eduqas A level Geology (Component 1) — Geological map applications

Dip and strike

Fig. 1Strike is the horizontal direction on a bedding plane; dip is the angle of maximum slope, measured at right angles to the strike and pointing in the dip direction.

A geological map shows the distribution of rocks at the surface (the outcrop or, where covered by soil, the subcrop), and reading it means recovering the three-dimensional structure from the two-dimensional pattern. The orientation of a bed is described by its dip and strike. The strike is the compass direction of a horizontal line on the bedding plane; the dip is the angle of maximum slope of the bedding plane below horizontal, always measured at right angles to the strike, and it points in a definite direction (the dip direction). The dip and strike symbol on a map records both, and reading it is the first step to understanding the structure.

The pattern a bed makes on the map depends on its dip. Horizontal beds follow the topographic contours and outcrop in bands parallel to the contours, wider on gentle slopes; vertical beds run in dead-straight lines across the map regardless of the topography; and beds of intermediate dip make patterns in between. Steeply dipping beds make nearly straight outcrops, while gently dipping beds swing widely with the topography, so the width and shape of an outcrop band are themselves clues to the dip.

Where a dipping bed crosses a valley, its outcrop bends into a characteristic V, and the rule of Vs lets the dip be read from the map. A bed dipping downstream makes a V that points downstream (down the valley); a horizontal bed's outcrop simply follows the contours and Vs sharply upstream like the contours themselves; and a vertical bed does not V at all. The classic rule to remember is that an inclined bed's outcrop Vs in the direction of dip, so the shape of the V across a valley reveals which way the bed dips, a much-used map-reading technique.

Putting these together, a geologist reading an unfamiliar map first identifies the beds and their order, then reads the dip and strike symbols and the outcrop patterns (contour-parallel, straight, or V-ing) to establish how the beds are oriented, and so builds a mental three-dimensional model before drawing a cross-section. This disciplined reading — from outcrop pattern to structure to history — is the foundation of the map skills assessed in the practical component, and it draws directly on the structures of the deformation chapter.

Worked example

Reading dip from outcrop pattern

On a geological map, one bed runs in a perfectly straight line across hills and valleys alike, while a second bed swings in broad Vs across the valleys. Describe the dip of each bed and justify your reading.

The straight bed
A bed whose outcrop is a straight line unaffected by the topography must be vertical, because only a vertical plane cuts the surface in a straight line whatever the relief.
The V-ing bed
A bed whose outcrop swings in broad Vs across valleys is dipping (not vertical), and by the rule of Vs it dips in the direction the V points.
Conclude
The first bed is vertical; the second is inclined and dips in the direction of its V.

Result: The straight-line bed is vertical; the V-ing bed is inclined and dips in the direction its outcrop Vs.

Typical mistakes

Measuring dip in any direction; dip is the maximum slope and is always measured at right angles to the strike.
Misapplying the rule of Vs; an inclined bed's outcrop Vs in the direction of dip, whereas horizontal beds simply follow the contours.

Active revision

On a map, a sandstone outcrop makes a V pointing down a valley. State the dip direction of the sandstone and explain how you know.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — geological maps (British Geological Survey)

Structures and outcrop patterns#

AdvancedWJEC/Eduqas A level Geology (Component 1) — Geological map applications

Structures in cross-section

Fig. 2A cross-section read from a map: folded, faulted older beds are truncated by an unconformity and overlain by flat younger beds, recording a full history of events.

Once dip and strike are read, the larger structures reveal themselves in the outcrop pattern. Folds produce repeated, roughly parallel outcrop bands with a symmetry: an eroded anticline shows the oldest beds along its central axis with younger beds outward and dipping away on both sides, while an eroded syncline shows the youngest beds along its axis with older beds outward dipping inward. The map pattern of repeated beds mirrored about a central line is the signature of a fold, and reading the ages across it identifies which fold it is.

Faults appear on a map as lines along which the outcrop pattern is abruptly displaced, cut off or repeated. A fault may shift a marker bed sideways, bring different beds against one another, or cut out or repeat part of the succession; the sense and amount of displacement are read from how the beds are offset across the fault line. Because a fault is younger than the beds it displaces (cross-cutting), and may itself be cut by later features, faults are key to sequencing the events recorded on a map.

Unconformities have a distinctive map expression: the beds above an unconformity rest on, and cut across, the outcrop pattern of the older beds below, so an upper sequence with a simple pattern may lie directly across the truncated edges of folded or faulted older rocks. On the map, an unconformity often appears as a boundary above which the beds are undisturbed and below which they are tilted or folded and 'beheaded'. Recognising an unconformity is essential, because it marks a major gap in time and a change in the geological story.

The full skill is to read all these together and reconstruct the history: the order of the beds by superposition, the folds and faults and their relative ages by cross-cutting, and the unconformities marking the gaps, so that the map becomes a narrative of deposition, deformation, erosion and renewed deposition. This synthesis of outcrop pattern into structure and then into history is exactly the demanding AO3 task set in the examination, and it unites the dating principles, structures and stratigraphy of the whole course.

Worked example

Reconstructing a map's history

A cross-section shows tilted, folded beds cut by a fault, all truncated by a flat erosion surface overlain by horizontal beds. Place the events in order.

Oldest events
The folded beds were deposited first (superposition) and then folded by compression.
Faulting
The fault cuts the folded beds, so faulting came after the folding (cross-cutting).
Unconformity and cover
The erosion surface truncating the folded, faulted rocks is an unconformity (uplift and erosion), after which the horizontal beds were deposited on top.
Full order
Deposit and fold the older beds, fault them, uplift and erode (unconformity), then deposit the horizontal cover.

Result: Order: deposition and folding, then faulting, then uplift and erosion (unconformity), then deposition of the flat cover.

Typical mistakes

Reading repeated beds as a thicker succession rather than the two limbs of a fold; the mirror symmetry of ages reveals the fold.
Missing an unconformity where an upper sequence cuts across the outcrop of folded beds below.

Active revision

A map shows a set of beds repeated symmetrically either side of a central line of older rock, all cut off to the north by flat-lying younger beds. Interpret the structure and the northern boundary.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — interpreting maps (British Geological Survey)

Cross-sections and true thickness#

AdvancedWJEC/Eduqas A level Geology (Component 1) — Geological map applications

True thickness of a dipping bed

Fig. 3The true thickness of a dipping bed is measured at right angles to the bedding and equals the outcrop width times the sine of the dip; the outcrop width overstates the thickness.

A geological cross-section is a vertical slice through the ground, drawn from the map along a chosen line, that shows the structure at depth. It is constructed by projecting the surface outcrops down at the measured dip, honouring the ages and the structures read from the map, so that folds, faults and unconformities are shown in their true geometry. A well-drawn cross-section is the proof that the map has been correctly understood, and constructing one is a core practical skill; the vertical scale must equal the horizontal scale, or dips and shapes are distorted (vertical exaggeration).

A frequent and important calculation is the true thickness of a bed. The width of a bed's outcrop measured on the map (across the strike) is not its true thickness unless the bed is vertical; for a dipping bed the true thickness, measured at right angles to the bedding, is less than the outcrop width. On flat ground, the true thickness equals the horizontal outcrop width multiplied by the sine of the dip, so a gently dipping bed makes a wide outcrop for a modest true thickness. Calculating true thickness from outcrop width and dip is a standard quantitative task.

Dip itself can be calculated where the height of a bedding surface is known at different places, using the fact that dip is a gradient — the vertical drop of the surface divided by the horizontal distance, expressed as an angle. The three-point problem uses the heights of a bedding plane at three non-collinear points to find both the strike (the horizontal direction on the plane) and the dip, and is the standard method for recovering orientation from borehole or map data. These calculations turn map measurements into the numbers a cross-section needs.

The purpose of all this is to move confidently between the map, the cross-section and the numbers, so that a geologist can predict what lies beneath a point of interest — at what depth a coal seam or aquifer will be met, whether a structure continues at depth, how thick a resource is. This predictive power is why map interpretation is examined so heavily and why it underpins the engineering and extraction applications of the final section. Accuracy in the calculations, and consistency between the map, the section and the geology, are what the mark scheme rewards.

\text{true thickness} = w \times \sin(\text{dip})

True thickness of a dipping bed

On flat ground the true thickness of a bed (measured perpendicular to the bedding) equals its horizontal outcrop width w (measured across the strike) multiplied by the sine of the dip; only a vertical bed has true thickness equal to its outcrop width.

True thickness against dip

Fig. 4For a fixed outcrop width of 200 m, the true thickness = 200 sin(dip): small for gentle dips, rising to the full width only when the bed is vertical.

Worked example

Calculating true thickness

On flat ground a sandstone outcrops across a horizontal width of 200 m, measured at right angles to its strike, and dips at 30 degrees. Calculate the true thickness of the sandstone.

State the relationship
True thickness = outcrop width x sin(dip).
Substitute
True thickness = 200 x sin(30 degrees).
$t = 200 \times \sin(30^{\circ}) = 200 \times 0.5$
Evaluate
sin(30 degrees) = 0.5, so t = 200 x 0.5 = 100 m.
$t = 100\ \text{m}$
Comment
The true thickness (100 m) is half the outcrop width because the bed dips at 30 degrees; a steeper dip would give a thickness closer to the width.

Result: The true thickness is 100 m (200 m outcrop width times sin 30 degrees).

Typical mistakes

Taking the outcrop width as the true thickness; only a vertical bed has a true thickness equal to its outcrop width.
Exaggerating the vertical scale of a cross-section, which distorts the dips and the shapes of structures.

Active revision

A bed outcrops across a horizontal width of 150 m on flat ground and dips at 40 degrees. Calculate its true thickness.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — cross-sections (British Geological Survey)

Applied and engineering geology from maps#

StandardWJEC/Eduqas A level Geology (Component 1) — Geological map applications

Geology of a dam site

Fig. 5A dam site assessed from its geology: beds or joints dipping downstream, or permeable rocks, risk leakage and instability, so the sub-surface structure is read before design.

Geological maps and cross-sections are working tools of civil engineering, because the ground conditions determine whether a structure is safe and affordable. Before a dam, tunnel, road, bridge or large building is designed, the sub-surface geology is interpreted from maps, boreholes and cross-sections to identify the strength of the rocks, the orientation of bedding, joints and faults, the depth to firm rock and the position of the water table. Ground that looks the same at the surface can behave very differently depending on this hidden structure, so the geology is assessed first.

Different projects demand different ground. A dam and its reservoir need strong, watertight rock in the foundations and valley sides; permeable or soluble rocks (such as jointed limestone) risk leakage, and beds or faults dipping downstream risk sliding, so the geology of the site is critical. A tunnel needs to know the rocks it will pass through, the water it may meet, and any faults or weak beds along its line. Foundations for a building need rock or soil of sufficient bearing strength and must avoid compressible, unstable or swelling ground. In each case the map and cross-section predict the conditions before excavation begins.

The same structural features that interest a geologist are the ones that create engineering hazards. Bedding, joints and faults dipping out of a slope or a cutting are potential slide surfaces (as in the mass-movement chapter); faults may be zones of weak, shattered, water-bearing rock; soluble rocks may hide cavities; and the water table controls stability and how much water an excavation must handle. Reading these features from the map, and understanding how they affect a proposed structure, is applied structural geology of a very direct kind.

For extraction, maps and sections show where a resource reaches the surface or lies at workable depth, how thick and extensive it is, and what must be removed to reach it, so they guide whether to quarry, open-pit or mine, and how. Throughout, the geologist's role is to translate the map into a prediction of ground behaviour and resource potential, and to evaluate the suitability of a site — the balance of safety, cost and practicality — which is the applied, synoptic skill the specification most values in this final chapter.

Worked example

Assessing a site from the geology

An engineer proposes a dam across a valley floored by jointed limestone whose beds dip downstream. Using the geology, evaluate the site and suggest what to investigate.

Leakage risk
Jointed limestone is permeable and soluble, so water could leak through joints and dissolved cavities beneath and around the dam, losing the reservoir.
Stability risk
Beds dipping downstream provide surfaces along which the foundation or valley side could slide under the water load.
Investigate and conclude
Investigate the joint spacing, any cavities, the water table and the dip by boreholes and sections; a site on strong, watertight rock with beds dipping into the valley side would be safer.

Result: The site is poor: permeable, soluble limestone dipping downstream risks leakage and sliding; a strong, watertight foundation with favourable dip should be sought.

Typical mistakes

Judging ground from the surface alone; the sub-surface structure read from maps and boreholes controls how the ground behaves.
Ignoring the dip of beds and joints at a site; features dipping out of a slope or downstream of a dam are major hazards.

Active revision

Explain, using the geology, why a dam should not be built where jointed limestone beds dip downstream, and what site would be safer.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — engineering geology (British Geological Survey)

References & sources

Sources

WJEC / Eduqas

WJEC/Eduqas A level Geology specification

British Geological Survey

Geological map applications

4 sections~15 min reading time3 competenciesLevel Standard 1 · Advanced 3

Geological map applications

Key points

Reading dip from outcrop pattern

Key points

Reconstructing a map's history

Key points

Calculating true thickness

Key points

Assessing a site from the geology

Sources

Geological map applications

Key points

Reading dip from outcrop pattern

Key points

Reconstructing a map's history

Key points

Calculating true thickness

Key points

Assessing a site from the geology

Sources