Geological and environmental field interpretation: maps, cross-sections and data
Map interpretation is where marks are most easily won and lost, because answers are numerical and checkable. The PAU expects scale arithmetic, contour reading, construction of a topographic profile, and reading geological maps and cross-sections.
Topographic maps
- Scale: 1:25,000 means 1 cm = 250 m. Always convert units before computing.
- Contour lines: equidistance is the vertical spacing; steeper slopes give closer contours.
- Gradient (%) = (vertical difference / horizontal distance) × 100.
- Profiles: plot every contour crossing at its true altitude and join smoothly; vertical exaggeration = vertical scale / horizontal scale.
Geological maps and sections
- Strike and dip symbols give bed attitude; dip direction is perpendicular to strike.
- Rule of V's: beds dipping downstream form V shapes pointing downstream; beds dipping upstream point upstream, against the valley slope, revealing the dip.
- Outcrop width depends on bed thickness, dip and topography.
- Faults: recognised by offset marker beds, repetition or omission of strata, and drag folds. Normal faults omit section; reverse faults repeat it.
- Folds: anticlines have the oldest beds in the core, synclines the youngest; plunging folds give converging outcrop traces.
- Borehole logs and geophysics extend interpretation below the surface.
Worked example
On a 1:50,000 map, two points are 6 cm apart and lie at 400 m and 700 m. Reasoning: the real distance is 6 × 50,000 cm = 300,000 cm = 3 km; the gradient is 300 m / 3,000 m = 0.10, that is 10 % or about 5.7°.
Traps that cost marks
- Forgetting to convert centimetres to metres or kilometres.
- Measuring a straight line when the route follows a valley.
- Reversing the sense of a fault, or calling an anticline a syncline because the outcrop closes.
- Quoting vertical exaggeration without units, or ignoring that contours never cross.
- Confusing apparent dip on a section with true dip in the field.