Geological structures: the response of rocks to stress (folds and faults); fold elements and types (anticline and syncline, limb, hinge and axial plane); fault types and the stress regime they record (normal from tension, reverse and thrust from compression, strike-slip from shear); joints; dip and strike; the recognition and significance of unconformities (angular unconformity, disconformity and nonconformity).

What this dot point is asking

OCR wants you to describe how rocks respond to stress by folding and faulting, to name fold elements and types, to classify faults and state the stress each records, to describe joints, to use dip and strike, and to recognise and interpret unconformities (angular unconformity, disconformity and nonconformity).

The answer

Folds

When rocks are compressed and behave ductilely, they fold. The elements you must label are the limbs (the sides), the hinge (the line of maximum curvature) and the axial plane (the plane that bisects the fold).

• Anticline. An up-arched fold; the oldest rocks are in the core (centre).
• Syncline. A down-warped fold; the youngest rocks are in the core.

The which-is-oldest rule is the examiner's favourite test, because it lets you read a fold on a map.

Faults

When rocks are stressed and behave brittlely, they fracture and slip along faults. The fault type records the stress regime:

• Normal fault. The hanging wall moves down relative to the footwall; produced by tension (extension), which lengthens the crust (rifts, constructive margins).
• Reverse fault. The hanging wall moves up relative to the footwall; produced by compression, which shortens the crust. A low-angle reverse fault is a thrust fault (important in fold-and-thrust belts).
• Strike-slip (tear) fault. Horizontal movement; produced by shear, at conservative margins (for example the San Andreas).

Joints and dip and strike

• Joints are fractures with no displacement (unlike faults). They form sets from cooling, unloading or tectonic stress and strongly affect permeability and weathering.
• Dip is the angle of maximum slope of a bed below horizontal, in a stated direction; strike is the horizontal line on the bed, at right angles to the dip. Together they define the orientation of a tilted bed (for example "dipping $30^{\circ}$ to the south-east").

Unconformities

An unconformity is a surface representing a gap in the geological record (a period of erosion or non-deposition). Three types:

• Angular unconformity. Older beds are tilted or folded and eroded, then overlain by younger, more horizontal beds at a different angle.
• Disconformity. Beds above and below are parallel, but an erosion surface (a time gap) separates them.
• Nonconformity. Sedimentary rocks overlie eroded igneous or metamorphic basement.

An angular unconformity records a full sequence: deposition, folding or tilting, uplift, erosion, then renewed deposition.

Examples in context

Example 1. Thrust faults in the Alps. Continental collision produced compression that shortened the crust along low-angle thrust faults, stacking older rocks over younger ones, a structure that records the convergent stress regime.

Example 2. Hutton's unconformity. A classic angular unconformity, where steeply tilted older strata are truncated and overlain by gently dipping younger beds, records a full cycle of deposition, deformation, uplift, erosion and renewed deposition, and was historic evidence for the immensity of geological time.

Try this

Q1. State which rocks (oldest or youngest) lie in the core of an anticline. [1 mark]

• Cue. The oldest rocks.

Q2. State the type of stress that produces a normal fault and describe the movement of the hanging wall. [2 marks]

• Cue. Tension (extension); the hanging wall moves down relative to the footwall.

Q3. Name the type of unconformity where horizontal sediments rest on eroded granite. [1 mark]

• Cue. A nonconformity.