Chapter 25 Mapping Geological Planes Lines and Folds

Year 2 · Structural Geology · G221-L1

Learn strike, dip, geological map symbols, fold geometry and stereographic plotting through an original map interpretation exercise.

Fold geometry and shortening in a schematic layered succession. Original Geology Hub teaching diagram.

This independent lesson follows topics in the supplied University of Peshawar BS Geology curriculum labelled session 2004–05 onward. It is a learning resource, not an official university course or a statement of current degree requirements.

Learning objectives

Record the orientation of a plane and a line unambiguously.
Distinguish fold geometry from interpretations of relative age.
Connect an outcrop observation to a map and a stereonet.

From an outcrop to a map

A geological map combines a topographic base with interpreted rock-unit boundaries, structural observations, a scale, a north reference, a legend and cross-section lines. A contact drawn through covered ground is an inference. Show it differently from a contact directly observed at an exposure. Location accuracy, vegetation and the chosen map scale determine how confidently a line can be positioned.

Strike is the azimuth of a horizontal line within an inclined plane. Dip is the greatest downward inclination within that plane, measured perpendicular to strike. State the dip direction as well as the angle: a plane striking north–south can dip either east or west. A line is described by its trend and plunge. A bedding plane and a mineral stretching lineation therefore require different symbols and measurements.

Describing folds

A fold has limbs, a hinge region and an axial surface connecting hinges in successive layers. An antiform arches upward; a synform curves downward. Anticline and syncline additionally describe the arrangement of older and younger beds. In an overturned sequence, shape alone cannot establish which name is correct. Search for younging evidence such as graded bedding or cross-bed truncations before interpreting stratigraphic order.

Classify folds using measurable properties: interlimb angle, axial-surface orientation, hinge plunge and the relation of thickness between hinge and limbs. A small tight fold and a broad open fold can record very different local strain while belonging to the same regional deformation. Draw the exposure before choosing a genetic explanation. A sketch should show scale, viewing direction, lithology and uncertainty.

Using projections

A stereonet represents three-dimensional orientations on a two-dimensional circle. A plane plots as a great circle, while its perpendicular pole plots as a point. A line plots as a point. Equal-area nets preserve area, helping compare populations of orientations; they do not preserve every angle measured on the paper. Specify lower- or upper-hemisphere convention and keep it consistent throughout the project. Clusters of bedding poles suggest a common orientation, but scattered poles can arise from folding, multiple domains or poor measurements.

Contours connect surface and subsurface geometry

A topographic contour joins points at equal land-surface elevation. A structure contour joins points of equal elevation on a geological surface, such as a bed boundary. The two are different datasets. If a planar bed has parallel, evenly spaced structure contours, their direction gives strike and their spacing constrains dip. Where a structure contour and a topographic contour of the same elevation intersect, the bed can meet the ground surface. This relationship explains why a planar contact can weave across a valley without being folded.

Calculate an apparent dip

True dip is measured down the steepest slope of a plane. A vertical section parallel to strike shows zero apparent dip for that ideal plane, while one perpendicular to strike shows true dip. For an intermediate section, tan δa = tan δ × sin α, where δ is true dip and α is the angle between section direction and strike. For δ = 30° and α = 30°, tan δa = 0.577 × 0.5, so δa is about 16.1°. The smaller angle reflects the section direction, not a change in the bed. The calculation assumes a planar surface and equal horizontal and vertical scales.

Make fold descriptions measurable

Fold wavelength is the distance between equivalent neighbouring hinges, and amplitude expresses displacement from a reference surface under a stated convention. Interlimb angle compares the limbs; a smaller angle indicates a tighter fold. Record whether hinges are straight or curved and whether the axial surface is upright, inclined or near-horizontal. Thickness may remain approximately constant measured normal to bedding in one fold style but vary between hinges and limbs in another. A fold classified from one photograph may change appearance in a different section, so state the viewing direction and the scale of the structure.

Choose the appropriate projection

An orthographic construction uses perpendicular projections to calculate distances or relationships between objects in plan and section. A stereographic construction represents directions on a hemisphere and is suited to orientation questions. To compare two bedding attitudes, poles on a stereonet are efficient; to position a bed beneath a mapped hill, an orthographic section with elevations is more useful. Neither projection supplies missing observations. As a check, redraw the same north-dipping plane in plan, in a north–south section and as a stereonet great circle. The views should express the same orientation even though their shapes differ. Inconsistent dip directions reveal a convention or transfer error that should be resolved before interpretation.

Worked activity

In an invented upright sequence, beds on the west limb dip 35° west and those on the east limb dip 40° east. Older limestone occupies the centre and younger shale occurs outside. Draw a west–east section and identify the fold.

Solution: the limbs dip away from the centre, producing an antiform. The older central unit makes it an anticline. The different limb dips indicate asymmetry. Without the age evidence, only antiform would be justified. Add a horizontal scale and avoid inventing a fold depth unsupported by observations.

Self-check

Can strike alone specify a plane?
Answer
No. Dip angle and dip direction are also needed.
Why use a pole to a plane?
Answer
It condenses a plane orientation into one point, making populations easier to compare.
Does a map contact have equal certainty everywhere?
Answer
No. Exposure and location quality vary; the map should communicate that uncertainty.

Chapter 25 multiple choice questions

Choose one best answer for each question. Open each answer panel after making your choice.

A geologist records bedding as north–south strike and 30° dip. Which additional observation resolves the remaining ambiguity?
1. Whether the bed dips east or west
2. The colour of the bedding surface
3. The map’s publication year
4. The length of the exposed bed
Answer and explanation
A. Whether the bed dips east or west A north–south plane may dip either east or west. Dip direction completes the orientation.
An upward-arching fold has younger beds in its centre because the sequence is overturned. Which purely geometric term remains appropriate?
1. Anticline
2. Antiform
3. Synform
4. Normal fault
Answer and explanation
B. Antiform Antiform describes the upward-arching shape without an age claim. Anticline requires older beds in the core.
Which pair of measurements describes a mineral stretching lineation?
1. Strike and dip direction only
2. Interlimb angle and fold wavelength
3. Trend and plunge
4. Latitude and bed thickness
Answer and explanation
C. Trend and plunge A linear feature is oriented by trend and plunge. Strike and dip describe a plane.
A bedding contact is visible at two stations but hidden by vegetation between them. How should its intervening map trace be represented?
1. As directly observed throughout
2. As a measured fault without further evidence
3. As an unbroken topographic contour
4. As an inferred contact with its uncertainty explained
Answer and explanation
D. As an inferred contact with its uncertainty explained The exposed endpoints constrain an interpretation, but they do not make the covered segment directly observed.
What does a cluster of bedding poles on a consistently constructed stereonet most directly suggest?
1. A group of bedding planes with similar orientations
2. A unique deformation age
3. An identical lithology at every station
4. A measured value of tectonic stress
Answer and explanation
A. A group of bedding planes with similar orientations A pole represents the orientation of a plane. Clustering therefore first describes orientation similarity rather than age, composition or stress.

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