Fieldwork and maps: the recording of field observations (field sketches, measurements and logged sections); the interpretation of geological maps (outcrop patterns, the rule of Vs and the relationship between topography and dip); the construction of a geological cross-section from a map; the recognition of structures (folds, faults and unconformities) on maps and cross-sections.

What this dot point is asking

OCR wants you to describe how field observations are recorded (sketches, measurements, logs), to interpret geological maps (outcrop patterns, the rule of Vs, and the relationship between topography and dip), to construct a geological cross-section from a map, and to recognise folds, faults and unconformities on maps and sections.

The answer

Recording field observations

Fieldwork is the foundation of the Practical Endorsement and is examined on paper. You record:

• Field sketches, annotated to show the key features (beds, structures, contacts).
• Measurements, especially dip and strike with a compass-clinometer.
• Logged sections, recording lithology, grain size, bed thickness, structures and fossils up a succession.

Good records are clearly annotated, to scale where possible, and note the location.

Interpreting outcrop patterns

A geological map shows where each rock unit reaches the surface (its outcrop). The pattern of an outcrop reveals the structure:

• Horizontal beds follow the contours (their outcrop runs parallel to the topographic contours).
• Vertical beds run in straight lines across the map, ignoring the topography.
• Inclined (dipping) beds cut across the contours, and where they cross a valley they bend into a V (the rule of Vs).

So the outcrop pattern, combined with the topography, lets you deduce the dip direction without a stated dip arrow.

Constructing a cross-section

To draw a cross-section along a line on the map:

1. Draw the topographic profile by plotting the height where each contour crosses the line (same horizontal and vertical scale to avoid exaggeration).
2. Project the geological boundaries down from where they cross the line.
3. Draw each boundary into the subsurface at its dip, keeping bed thicknesses consistent.
4. Complete the structure, showing folds (matching dips), faults (offsetting beds) and unconformities (an erosion surface truncating older beds), and label the units in order.

Recognising structures on maps

• Folds. Repeated, mirror-image outcrop patterns; the oldest beds in the core mark an anticline, the youngest a syncline.
• Faults. A line on the map that offsets (displaces) the outcrops of the beds it cuts.
• Unconformities. A boundary where younger beds rest on the eroded, often differently oriented, edges of older beds, cutting across them.

Examples in context

Example 1. V-ing outcrops in a dissected landscape. On a map of a hilly area, dipping beds V sharply where they cross the valleys, and reading the direction of the V gives the dip direction, which is essential for drawing the cross-section.

Example 2. A faulted, folded sequence. A map showing repeated outcrops (a fold) cut by a line that offsets them (a fault) is a classic Paper 3 exercise: identify the fold from the symmetry and ages, the fault from the offset, and reconstruct the order of events.

Try this

Q1. State what the outcrop of a horizontal bed does relative to the topographic contours. [1 mark]

• Cue. It follows (runs parallel to) the contours, staying at a constant height.

Q2. Using the rule of Vs, state the dip direction if a bed's outcrop Vs downstream where it crosses a valley. [1 mark]

• Cue. The bed dips downstream (the V points in the direction of dip).

Q3. Describe the first two steps in constructing a cross-section from a map. [2 marks]

• Cue. Draw the topographic profile along the line (plotting heights where contours cross, with no vertical exaggeration), then project the geological boundaries down onto the profile where they cross the line.