Geological maps and cross-sections: reading outcrop patterns, reading dip from outcrop width and topography, and the younging direction; constructing a cross-section from a map; deducing the geological history (the order of events) using superposition, cross-cutting relationships, unconformities and included fragments; the difference between simplified map extracts (Component 1) and real published map extracts (Component 3); and three-point problems in outline.

What this dot point is asking

Eduqas wants you to read a geological map (its outcrop patterns, the dip from outcrop width and topography, and the younging direction), to construct a cross-section from a map, and to deduce the geological history (the order of events) using the four classic principles: superposition, cross-cutting relationships, unconformities and included fragments. You must know the difference between the simplified map extracts used in Component 1 and the real published extracts used in Component 3, and understand three-point problems in outline. This dot point ties the whole module together: it is where stress, folds, faults and unconformities are read off a real map.

The answer

Reading outcrop patterns

A geological map shows where each rock unit reaches the surface (its outcrop), and the pattern reveals the structure:

• Horizontal beds follow the contours (their outcrop runs parallel to the topographic contours, staying at one height).
• Vertical beds run in straight lines across the map, ignoring the topography.
• Inclined (dipping) beds cut across the contours and V where they cross a valley (the rule of Vs, the V pointing down-dip).
• Folds give repeated, mirror-image outcrop patterns; faults offset the outcrops along a line.

Reading dip from outcrop width and topography

The width of an outcrop, read together with the topography, gives the dip:

• A wide outcrop (on given ground) means a gentle (shallow) dip, because a gently dipping bed intersects the surface over a broad band.
• A narrow outcrop means a steep dip.
• The direction of dip comes from the rule of Vs (the V points down-dip) or from a dip arrow on the map.

Outcrop width therefore reads dip qualitatively on a map, just as it lets you calculate true thickness in the field.

The younging direction

The younging direction is the direction in which the beds get younger. By the principle of superposition, in an undisturbed sequence the beds young upwards, so on a tilted sequence they young in the direction of dip towards the top of the pile. You confirm it with way-up (geopetal) evidence (graded bedding, cross-bedding, ripple marks, desiccation cracks, fossils in life position), which is essential where folding may have overturned the beds and the simple "younger on top" rule could be reversed.

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, using the same horizontal and vertical scale to avoid exaggeration.
2. Project the geological boundaries (and any fault) down onto the profile 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 the dips on either side), faults (offsetting the beds by the throw) and unconformities (an erosion surface truncating older beds), and label the units in order.

Deducing the order of events

You reconstruct the geological history with four principles, building the sequence oldest first:

• Superposition. In an undisturbed sequence the lowest bed is the oldest.
• Cross-cutting relationships. A structure (an intrusion, a fault) is younger than anything it cuts.
• Unconformities. A gap in the record; the deformation below it pre-dates the renewed deposition above it.
• Included fragments. A clast is older than the rock that contains it (so a conglomerate's pebbles pre-date the conglomerate).

Simplified versus real map extracts, and three-point problems

Component 1 uses simplified map extracts (clean, schematic, designed to test the principles), while Component 3 uses real published map extracts (genuine survey maps with the full complexity of topography, many units and natural outcrop patterns). The skills are the same, but the real extracts demand more careful reading of contours and outcrop shapes.

A three-point problem finds the dip and strike of a planar bed from the heights of three points on its surface (for example three boreholes). In outline: the line joining points of equal height is a structure contour defining the strike; the line of greatest slope at right angles gives the dip direction; and the height difference over horizontal distance gives the dip angle. The key idea is that three known heights fix a plane's orientation.

Examples in context

Example 1. A faulted, folded sequence under an unconformity. A real Component 3 extract showing repeated outcrops (a fold) cut by a line that offsets them (a fault) and overlain by flat younger beds (an unconformity) is the classic exercise: identify the fold from the symmetry and ages, the fault from the offset, the unconformity from the truncation, then order the events with the four principles.

Example 2. An intrusion dated by what it cuts and what cuts it. A granite that cuts folded sediments but is itself truncated by an unconformity is bracketed in time: it is younger than the folding (cross-cutting) but older than the cover beds resting on its eroded top, a relative date read entirely from the map.

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. On given topography, what does a wide outcrop of a bed indicate about its dip, and what does a narrow outcrop indicate? [2 marks]

• Cue. A wide outcrop indicates a gentle (shallow) dip; a narrow outcrop indicates a steep dip.

Q3. Name the principle that tells you an intrusion is younger than the rocks it cuts, and the principle that tells you a pebble is older than the conglomerate containing it. [2 marks]

• Cue. Cross-cutting relationships (the intrusion is younger than what it cuts); the principle of included fragments (the pebble is older than its host rock).