What does G2 of WJEC A-Level Geology cover?

G2, Rock Deformation, is the second topic of the A2 section Interpreting the Geological Record. It covers the mechanics of deformation: stress and strain and the difference between compressional, tensional and shear stress; elastic, brittle and ductile behaviour and the factors that control it; the geometry and classification of folds; the four types of fault and the stress that drives each; unconformities and orogenic structures such as nappes and thrusts; and the use of dating principles to reconstruct the sequence of tectonic events.

What controls whether a rock folds or faults?

Whether a rock deforms ductilely (folding) or brittly (faulting) depends on five factors. Higher temperature, deeper burial and higher confining pressure favour ductile flow; lower temperature and pressure near the surface favour brittle fracture. A slow strain rate favours flow, a fast one favours fracture. Competent rocks such as sandstone and limestone tend to fracture, while incompetent rocks such as shale and salt flow. High pore-fluid pressure promotes brittle failure by lowering the effective stress.

How do you tell an anticline from a syncline?

Use the dip of the beds and, more reliably, the age of the rocks in the core. In an anticline the limbs dip away from the axis and the oldest rocks lie in the core; in a syncline the limbs dip towards the axis and the youngest rocks lie in the core. The age test is decisive even where erosion has flattened the surface, because it does not depend on the present topography or on whether the fold makes a hill or a valley.

How are the four fault types distinguished?

By the relative movement of the hanging wall and footwall and by the stress regime. A normal fault drops the hanging wall under tensional (extensional) stress; a reverse fault pushes the hanging wall up on a steep plane under compression; a thrust is a low-angle reverse fault that stacks older rocks over younger; a strike-slip (tear) fault slides blocks horizontally past each other under shear stress. Throw is the vertical displacement and heave the horizontal.

What does an unconformity tell you?

An unconformity is a surface of erosion or non-deposition marking a gap in the record. An angular unconformity (younger horizontal beds on older tilted or folded beds) records deposition, deformation, uplift, erosion and renewed deposition, so it is strong evidence of mountain building between the two sets of beds. A disconformity records an erosion gap between parallel beds, and a nonconformity records sediments laid on eroded igneous or metamorphic basement.

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WJEC A-Level Geology G2 Rock Deformation: a deep dive on stress, folds, faults and structural history

A deep-dive WJEC and Eduqas A-Level Geology guide to G2, Rock Deformation. Covers stress and strain and the controls on brittle versus ductile behaviour, the geometry and classification of folds, the four fault types and their stress regimes, unconformities and orogenic structures, and how to reconstruct a sequence of tectonic events, with exam-style worked questions.

Generated by Claude Opus 4.815 min readWJECUpdated 2026-06-16

Reviewed by: AI editorial process; not yet individually human-reviewed

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What G2 actually demands
Stress and behaviour
Folds and faults
Structures and history
Check your knowledge

What G2 actually demands

G2 is the structural geology of the course. Examiners want you to move from cause (stress) to effect (folds and faults) to interpretation (the history a structure records). The most valued skill is reconstruction: given a folded, faulted, intruded and unconformity-bearing cross-section, put the events in the right order using the dating principles. That is the bridge to the map and cross-section work in T2.

Each part of the topic has a dot-point page with worked questions; this overview connects them.

Stress and behaviour

Stress is the applied force per unit area; strain is the deformation that results. Compression shortens rocks (folds, reverse and thrust faults), tension stretches them (normal faults), and shear slides blocks past each other (strike-slip faults). A rock responds elastically (recoverable, the source of earthquake energy), brittly (fracturing) or ductilely (flowing), controlled by temperature, confining pressure, strain rate, rock competence and fluid pressure.

Folds and faults

Folds are the ductile response: described by limbs, hinge, axial plane, fold axis and interlimb angle, and classified as anticlines and synclines and by symmetry. Axial-plane orientation gives the direction of compression and interlimb angle gives its intensity. Faults are the brittle response: normal, reverse, thrust and strike-slip, classified by hanging-wall movement and stress, recognised by offset beds on maps and by breccia, gouge and slickensides in the field.

Structures and history

Unconformities record gaps and the deformation, uplift and erosion behind them. Orogenic belts stack nappes and thrusts. The sequence of events is reconstructed by superposition, cross-cutting relationships, inclusions and unconformities, which is the heart of the cross-section question.

Check your knowledge

Attempt these under timed conditions, then check against the solutions.

Define stress and strain. (2 marks)
State the structure produced by tensional, compressional and shear stress. (3 marks)
Explain how the age of the rocks in the core distinguishes an anticline from a syncline. (2 marks)
State the relative movement and stress regime of a normal fault and a reverse fault. (2 marks)
Name the three types of unconformity. (3 marks)
A fault cuts a fold. State which is younger and the principle used. (1 mark)

Sources & how we know this

WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)