How is structure read from the outcrop pattern on a geological map?
The interpretation of geological maps: reading dip and strike from outcrop patterns, the rule of Vs for outcrops crossing valleys, recognising horizontal, dipping, folded and faulted strata and unconformities, and using the pattern to deduce the underlying structure.
A focused WJEC and Eduqas A-Level Geology T2 answer on interpreting geological maps: reading dip and strike from outcrop patterns, applying the rule of Vs where outcrops cross valleys, and recognising horizontal, dipping, folded, faulted and unconformable strata to deduce the underlying structure.
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What this dot point is asking
T2 is the practical map skill that runs through the whole qualification and is tested directly in Component 1. This dot point covers reading structure from the map: dip and strike from outcrop patterns, the rule of Vs, and the diagnostic patterns of horizontal, dipping, folded, faulted and unconformable strata. It applies the structures of G2 to real map evidence.
The answer
Dip and strike
The rule of Vs
Where a dipping bed crosses a valley, its outcrop bends into a V, and the V is read for dip:
So the direction the V points gives the dip direction, and the sharpness of the V (and how closely the outcrop hugs the contours) indicates the dip angle.
Recognising structures on a map
- Horizontal strata: outcrop boundaries run parallel to the contours, capping hills and flooring valleys.
- Dipping strata: parallel outcrop bands that cross the contours, younging in the dip direction.
- Folds: outcrop bands repeat symmetrically about a fold axis; the same bed appears on both sides, with the oldest rocks in the core of an anticline and the youngest in a syncline.
- Faults: a sharp line offsetting, repeating or cutting out beds, with different rocks juxtaposed.
- Unconformity: a boundary that truncates the outcrops or structures of the older rocks beneath it, with the beds above resting on different older units.
Examples in context
British Geological Survey maps use exactly these patterns, and learning to read dip from outcrop in a valley is a core fieldwork skill. The flat-lying Carboniferous Limestone plateaux of the Pennines show horizontal strata with outcrops following the contours. Folded and faulted ground in Wales and the Lake District shows repeated, offset outcrop bands that record the Caledonian and Variscan structures studied in G2.
Try this
Q1. Define dip and strike. [2 marks]
- Cue. Strike is the compass direction of a horizontal line on a bedding plane; dip is the angle the bed makes with the horizontal, at right angles to strike, in the downslope direction.
Q2. State how the outcrop of a horizontal bed behaves relative to the topographic contours. [1 mark]
- Cue. It runs parallel to (follows) the contours, because it crops out at a constant height.
Q3. Explain how you would recognise a fault on a geological map. [2 marks]
- Cue. A sharp line along which a bed's outcrop is offset, repeated or cut out, with different rocks juxtaposed across it.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20196 marksExplain how the outcrop pattern of a dipping bed crossing a valley can be used to determine the direction and angle of dip.Show worked answer →
Apply the rule of Vs, because that is the method for reading dip from outcrop in a valley.
Where a dipping bed crosses a valley, its outcrop bends into a V. The rule of Vs states that the V points in the direction of dip, so the apex of the V shows which way the bed dips (for a bed dipping less steeply than the valley floor, the V points downstream and dips that way).
The angle of dip is judged from how far the outcrop is deflected. A bed dipping gently produces a long, sharp V that swings far up or down the valley; a steeply dipping bed produces a short, blunt V close to a straight line; a vertical bed runs straight across regardless of topography; and a horizontal bed follows the contours exactly.
So the direction the V points gives the dip direction, and the sharpness of the V (and how closely the outcrop follows the contours) gives a measure of the dip angle.
Markers reward the V pointing in the direction of dip, the link between a wide open V and gentle dip, a tight V or straight outcrop and steep or vertical dip, and outcrop following contours for horizontal beds.
WJEC Eduqas 20225 marksDescribe the outcrop patterns that would allow you to recognise (a) horizontal strata, (b) an unconformity and (c) a fault on a geological map.Show worked answer →
Take each feature and give its diagnostic map pattern, because three patterns are required.
Horizontal strata: the outcrop boundaries run parallel to the topographic contours, because a horizontal bed crops out at a constant height, so geological boundaries and contours coincide and outcrops cap hills and floor valleys.
An unconformity: a single geological boundary cuts across (truncates) the outcrops or structures of the older rocks beneath it, and the beds above the unconformity rest on different older units along its length, often with the lower beds tilted or folded and the upper beds more uniform.
A fault: a sharp, often straight line along which the outcrop of a bed is displaced, repeated or cut out, with different rocks juxtaposed across the line; marker beds are offset where the fault crosses them.
Markers reward outcrops parallel to contours for horizontal strata, a boundary truncating older structures for an unconformity, and a line offsetting or cutting out beds for a fault.
Related dot points
- The construction of a geological cross-section from a map, the projection of dipping beds, folds, faults and unconformities into the section, and the reconstruction of the full sequence of geological events of an area from the map and section.
A focused WJEC and Eduqas A-Level Geology T2 answer on constructing a geological cross-section from a map, projecting dipping beds, folds, faults and unconformities into the section, and reconstructing the full sequence of geological events of an area using superposition, cross-cutting relationships and unconformities.
- The geometry of folds (limbs, axial plane, hinge, fold axis, interlimb angle), the classification of folds (anticline, syncline, symmetrical, asymmetrical, overturned, recumbent, isoclinal) and the use of fold style to interpret the direction and intensity of compression.
A focused WJEC and Eduqas A-Level Geology G2 answer on fold geometry (limbs, hinge, axial plane, fold axis, interlimb angle), the classification of anticlines and synclines and of symmetrical, asymmetrical, overturned, recumbent and isoclinal folds, and how fold style is read to deduce the direction and intensity of compression.
- The classification of faults (normal, reverse, thrust, strike-slip) by the relative movement of the hanging wall and footwall and by the stress regime, the terminology of fault planes (dip, throw, heave, slickensides), and the recognition of faults in the field and on maps.
A focused WJEC and Eduqas A-Level Geology G2 answer on the classification of normal, reverse, thrust and strike-slip faults by hanging-wall and footwall movement and stress regime, the terms used to describe fault planes (dip, throw, heave, slickensides, fault breccia), and how faults are recognised in the field and interpreted on geological maps.
- The types of unconformity (angular, disconformity, nonconformity) and their significance, the structures of mountain belts (nappes, thrust stacks), and the use of cross-cutting relationships and superposition to reconstruct the sequence of tectonic events.
A focused WJEC and Eduqas A-Level Geology G2 answer on the three types of unconformity and what each records, the structures of orogenic belts such as nappes and thrust stacks, and how cross-cutting relationships, superposition and unconformities are combined to reconstruct the sequence of folding, faulting, intrusion, uplift and erosion in a region.
- The principles of relative dating (superposition, original horizontality, cross-cutting relationships, included fragments and unconformities) and how they are combined to establish the sequence of geological events.
A focused answer to WJEC and Eduqas A-Level Geology F3 on relative dating, covering the principles of superposition, original horizontality, cross-cutting relationships, included fragments and unconformities, and how they are combined to reconstruct the order of geological events in a sequence.
Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)