Last reviewed 16 Sept 2026 · 8 min read
Attitude of beds
Sedimentary beds are originally horizontal; tectonic forces tilt, fold and fracture them. Their orientation (attitude) is described by strike and dip.
| Term | Meaning |
|---|---|
| Strike | Direction of a horizontal line on an inclined bedding plane (bearing, e.g. N40°E) |
| Dip direction | Direction of maximum slope, perpendicular to strike |
| True dip (dip angle) | Angle between the bedding plane and the horizontal, measured perpendicular to strike — the maximum inclination |
| Apparent dip | Inclination measured in any vertical section not perpendicular to strike — always less than true dip |
- Measured with a clinometer compass (Brunton compass).
- Symbol on maps: a long line along strike with a short tick in the dip direction, with the dip angle written.
Apparent dip in a section making angle with strike:
( = true dip; gives true dip; gives zero dip along strike)
Outcrop width on level ground (measured perpendicular to strike) for true thickness :
Depth of a bed below a point at horizontal distance (perpendicular to strike, in the dip direction) from its outcrop on level ground:
Folds
Folds are bends or flexures in rock layers produced by compressive forces, usually in ductile conditions.
Parts of a fold
- Limbs — the two sides of a fold.
- Hinge (hinge line) — line of maximum curvature.
- Axial plane — plane dividing the fold as symmetrically as possible through the hinges.
- Crest and trough — highest and lowest points.
- Plunge — inclination of the hinge line from the horizontal.
Types of folds
| Fold | Description |
|---|---|
| Anticline | Convex upward (arch); limbs dip away from the axis; oldest beds in the core |
| Syncline | Concave upward (trough); limbs dip towards the axis; youngest beds in the core |
| Monocline | Single step-like bend in otherwise horizontal/gently dipping beds |
| Symmetrical | Vertical axial plane, limbs dip equally |
| Asymmetrical | Inclined axial plane, limbs dip unequally |
| Overturned | Both limbs dip in the same direction — one limb overturned |
| Recumbent | Axial plane nearly horizontal |
| Isoclinal | Limbs parallel to each other |
| Chevron | Sharp hinges, straight limbs |
| Plunging fold | Hinge line inclined — outcrop pattern closes (V/U shape) |
| Dome | Beds dip outward in all directions (circular anticline) — oldest in centre |
| Basin | Beds dip inward in all directions — youngest in centre |
| Anticlinorium / synclinorium | Large anticline/syncline made of smaller folds |
Recognition on maps: symmetric repetition of beds about an axis; in anticlines older beds at the centre, in synclines younger beds at the centre (after erosion).
Faults
A fault is a fracture along which there has been appreciable displacement of the rocks on either side.
Terminology
| Term | Meaning |
|---|---|
| Fault plane | Surface of displacement |
| Hanging wall | Block above an inclined fault plane |
| Footwall | Block below the fault plane |
| Throw | Vertical component of displacement |
| Heave | Horizontal component of displacement (perpendicular to strike) |
| Net slip | Actual total displacement along the fault plane |
| Dip of fault / hade | Dip angle; hade = angle with the vertical (90° − dip) |
| Fault zone | Zone of crushed rock — fault breccia, fault gouge (clay), mylonite |
| Slickensides | Polished, striated surfaces showing slip direction |
Types of faults
| Fault | Movement | Stress |
|---|---|---|
| Normal (gravity) fault | Hanging wall moves down relative to footwall — crust extends | Tension |
| Reverse fault | Hanging wall moves up — crust shortens | Compression |
| Thrust fault | Low-angle reverse fault (dip commonly less than 45°) | Compression |
| Strike-slip (wrench/transcurrent) fault | Horizontal movement parallel to strike — dextral (right-lateral) or sinistral (left-lateral) | Shear |
| Oblique-slip fault | Both dip-slip and strike-slip components | |
| Horst | Uplifted block between two normal faults | Tension |
| Graben (rift valley) | Down-dropped block between two normal faults | Tension |
| Step faults | Series of parallel normal faults |
Classification by relation to strata: strike faults, dip faults, oblique faults; by net effect: repetition or omission of beds on the surface.
Evidence of faulting
- Displacement of beds, dykes or veins; repetition or omission of strata.
- Slickensides, fault breccia, gouge, mylonite.
- Fault scarps, offset streams, aligned springs, abrupt changes in topography and vegetation.
- Sudden changes in rock type along a line on maps.