How do dip and strike describe the orientation of a tilted bed?
Dip is the angle a bed makes with the horizontal, measured in the direction of steepest slope; strike is the compass direction of a horizontal line on the bed, at right angles to the dip; dip and strike are measured with a compass-clinometer and recorded with the dip and strike symbol on geological maps, and the apparent dip seen in a cross-section can differ from the true dip.
A focused answer to the Eduqas GCSE Geology statement on dip and strike. Covers the definitions of dip (angle of steepest slope from horizontal) and strike (horizontal direction at right angles to dip), how they are measured and shown by the map symbol, the link to outcrop width, and how apparent dip differs from true dip.
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What this dot point is asking
Eduqas wants you to define dip (the angle a bed makes with the horizontal, measured down the steepest slope) and strike (the horizontal direction on the bed, at right angles to the dip), to know how they are measured and shown by the dip and strike symbol on a geological map, and to understand that the apparent dip seen in a cross-section can be less than the true dip. Dip and strike are the language for describing tilted beds and underpin the map work in Component 2.
The answer
Dip: the angle of steepest slope
When beds are tilted by Earth movements, the dip describes how steeply they slope. Dip is the angle the bedding makes with the horizontal, measured down the line of steepest slope (the direction water would run). It is recorded as an amount (for example ) and a direction (for example towards the south-east). A horizontal bed has a dip of ; a vertical bed dips at .
Strike: the horizontal direction
The strike is the compass bearing of a horizontal line drawn on the surface of the tilted bed. Because that line is level, the strike has no angle of slope, only a direction. Crucially, the strike is always at right angles (90 degrees) to the dip direction. Picture a tilted table top: water runs off in the dip direction, and the level edge across the slope is the strike, perpendicular to it.
Measuring and recording them
Geologists measure dip and strike with a compass-clinometer: the compass gives the strike direction and the dip direction; the clinometer measures the dip angle. On a geological map they are shown by the dip and strike symbol, a long line (the strike) with a short tick (the dip direction) and a number (the dip angle) beside it. Reading these symbols tells you instantly how the beds are oriented underground.
True dip versus apparent dip
The true dip is the steepest possible angle of the bed, measured exactly down the dip direction. If you cut a cross-section in any other direction, the bed appears to slope more gently: this gentler angle is the apparent dip. The apparent dip is always less than or equal to the true dip:
- It equals the true dip only when the section runs exactly down the dip direction.
- It falls to zero when the section runs along the strike (the bed looks horizontal).
So when you read a slope off a cross-section, remember it may be an apparent dip, smaller than the real steepness of the bed.
Dip and outcrop width
Dip also controls how wide a bed's outcrop is on the map. A gently dipping bed crosses a lot of ground, giving a wide outcrop; a steeply dipping bed of the same thickness crosses little ground, giving a narrow outcrop. On flat ground, a horizontal bed's outcrop follows the land, while a vertical bed makes a straight, narrow strip. This link between dip and outcrop width is a favourite map-interpretation point.
Examples in context
Example 1. Reading a dipping coastline. Where coastal beds dip gently inland, they form long ramps into the sea and wide outcrops on the map; steeply dipping beds form narrow, near-vertical strips.
Example 2. The clinometer in the field. A geologist lays the compass-clinometer flat on a bedding plane to read the strike, then turns it down the steepest slope to read the dip, the two measurements that fix the bed's orientation.
Try this
Q1. Define strike and state its angular relationship to the dip direction. [2 marks]
- Cue. Strike is the compass direction of a horizontal line on the bed; it is at right angles (90 degrees) to the dip direction.
Q2. A bed dips at , but a cross-section shows an apparent dip of . What does this tell you about the direction of the section? [2 marks]
- Cue. Apparent dip equals true dip only when the section runs exactly down the dip direction, so the section is cut in the dip direction.
Q3. State how the outcrop width of a steeply dipping bed compares with that of a gently dipping bed of the same thickness. [1 mark]
- Cue. The steeply dipping bed has a narrower outcrop; the gently dipping bed has a wider one.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20194 marksDefine dip and strike, and explain how they are related to each other on a tilted bed.Show worked answer →
The mark is for accurate definitions and the right-angle relationship.
- Dip
- Dip is the angle that a tilted bed makes with the horizontal, measured down the line of steepest slope. It is given as an angle (for example 30 degrees) and a direction (for example to the south-east).
- Strike
- Strike is the compass direction of a horizontal line drawn on the surface of the bed. It has no angle of slope because it is horizontal.
- Their relationship
- The strike is always at right angles (90 degrees) to the direction of dip. If you imagine water running down the bed, it flows in the dip direction; the strike runs across the slope, level, at 90 degrees to that flow.
Markers reward dip as the angle of steepest slope from horizontal, strike as the horizontal direction on the bed, and the statement that strike is perpendicular to dip.
Eduqas 20224 marksA bed dips at 20 degrees. On a cross-section drawn in a direction that is not exactly down the dip, the bed appears to dip at only 14 degrees. Calculate the difference between the true and apparent dip, and explain why an apparent dip is always less than or equal to the true dip.Show worked answer →
A short calculation plus an explanation.
The difference. True dip is 20 degrees and apparent dip is 14 degrees, so the difference is .
Why apparent dip is smaller. The true dip is the steepest possible angle of the bed, measured exactly down the line of steepest slope. Any cross-section drawn in a different direction crosses the bed at an angle to that steepest line, so the slope it shows is gentler. The apparent dip therefore equals the true dip only when the section runs exactly down the dip direction, and is less than the true dip in every other direction (it falls to zero along the strike).
Top answers give the 6 degree difference and explain that an oblique section shows a gentler slope, so apparent dip is at most the true dip.
Related dot points
- Rocks deform when stressed: compression produces folds (anticlines arch upwards, synclines sag downwards) and reverse faults, while tension produces normal faults; the type and orientation of folds and faults are evidence of the direction of past Earth movements and are shown on geological maps and cross-sections.
A focused answer to the Eduqas GCSE Geology statement on folds and faults. Covers how compression produces folds (anticlines and synclines) and reverse faults, how tension produces normal faults, the parts of a fold and fault, and how these structures record the direction of past Earth movements.
- Joints are fractures with no movement, formed by cooling, drying or pressure release; an unconformity is a buried erosion surface separating older rocks below from younger rocks above, recording a gap in time during which deposition stopped and erosion occurred; unconformities and joints are interpreted from cross-sections to reconstruct geological history.
A focused answer to the Eduqas GCSE Geology statement on joints and unconformities. Covers how joints form (cooling, drying, pressure release) with no movement, what an unconformity is and the sequence of events it records (deposition, uplift, erosion, renewed deposition), and how to read these from cross-sections.
- Geological history is reconstructed from a cross-section using the principles of superposition (younger beds lie above older), original horizontality, cross-cutting relationships (a fault or intrusion is younger than the rocks it cuts) and included fragments; the order of deposition, deformation, intrusion, erosion (unconformities) and faulting is deduced to give a relative sequence of events.
A focused answer to the Eduqas GCSE Geology statement on reading cross-sections. Covers the principles of superposition, original horizontality, cross-cutting relationships and included fragments, and how to combine them to deduce the relative order of deposition, intrusion, deformation, erosion and faulting in an area.
- A simplified geological map shows the distribution of rock units at the surface using colours and a key, with a scale, a north arrow and grid lines; features are located using grid references (four-figure for a square, six-figure for a precise point), and the map is read together with topography to identify the rock units present, the order of the beds, and structures such as folds and faults shown by the outcrop pattern.
A focused answer to the Eduqas GCSE Geology statement on geological maps. Covers what a simplified geological map shows (rock units, key, scale, north arrow, grid), how to give four-figure and six-figure grid references, and how the outcrop pattern reveals the rock units, the order of beds and structures.
- Fieldwork involves recording observations systematically: making annotated field sketches, recording rock type, colour, grain size, texture, structures and fossils, measuring features such as dip and bed thickness, and identifying hand specimens of minerals and rocks using their physical properties; observations must be objective, located on a map or grid reference, and recorded safely and accurately so they can be interpreted later.
A focused answer to the Eduqas GCSE Geology statement on field observation. Covers recording observations systematically (annotated field sketches, rock type, grain size, texture, structures, fossils), measuring features in the field, identifying hand specimens by physical properties, and recording objectively, located and safely.
Sources & how we know this
- WJEC Eduqas GCSE (9-1) Geology specification (teaching from 2017) — WJEC Eduqas (2017)