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.

What this dot point is asking

Eduqas wants you to reconstruct the geological history of an area from a cross-section, using the principles of superposition, original horizontality, cross-cutting relationships and included fragments. You need to deduce the relative order of deposition, deformation, intrusion, erosion (unconformities) and faulting, and to name the principle that justifies each step. This is the headline interpretive skill of Component 2.

The answer

The principles of relative dating

You cannot put exact ages on the rocks in a cross-section, but you can put them in order using a small set of logical principles:

• Superposition. In an undisturbed sequence, younger beds lie on top of older ones. So you read deposition from the bottom up.
• Original horizontality. Sediments are laid down in roughly horizontal layers. So if beds are now tilted or folded, the tilting or folding happened after they were deposited.
• Cross-cutting relationships. A feature that cuts across another rock is younger than the rock it cuts. A fault, a dyke or any intrusion must be younger than the beds it cuts through.
• Included fragments (inclusions). A rock that contains fragments of another rock is younger than the rock those fragments came from (the fragments had to exist first). A conglomerate with granite pebbles is younger than the granite.

Putting events in order

To read a cross-section, work through it systematically:

1. Deposition. Order the sedimentary beds by superposition (bottom oldest, top youngest).
2. Deformation. If beds are folded, tilted or faulted, that deformation came after they were deposited (original horizontality and cross-cutting).
3. Intrusion. Place each dyke, sill or pluton by what it cuts (cross-cutting): it is younger than the rocks it cuts and older than any rock that cuts it or is deposited on top of it.
4. Erosion (unconformities). An unconformity records uplift and erosion between the rocks below and above it. Insert this gap at the right point.
5. Faulting. A fault is younger than every bed it offsets, so a fault cutting the whole sequence is usually one of the latest events.

The result is a relative sequence: a list of events from oldest to youngest, even though no actual ages are given.

A worked logic

Suppose a dyke cuts beds A, B and C but not bed D, which lies on top. The dyke is younger than A, B and C (cross-cutting) but older than D (D was deposited on it, superposition). So the dyke was intruded after C and before D. Reasoning like this, step by step, builds the whole history.

Examples in context

Example 1. A dyke that dates a sequence. A basalt dyke cutting a folded sandstone but sealed beneath an unconformity must have been intruded after the folding and before the overlying beds, pinning its place in the history.

Example 2. Granite pebbles in a younger grit. A coarse grit full of rounded granite pebbles records the erosion of an older granite, so the grit is unambiguously the younger of the two by the inclusion principle.

Try this

Q1. State the principle of superposition. [1 mark]

• Cue. In an undisturbed sequence, younger beds lie on top of older beds.

Q2. A dyke cuts through a limestone. Which is younger, and which principle tells you? [2 marks]

• Cue. The dyke is younger, by the cross-cutting principle (a feature that cuts another is younger than it).

Q3. Explain why a conglomerate containing pebbles of slate must be younger than the slate. [2 marks]

• Cue. By included fragments: the slate had to form first, then be eroded into pebbles, which were deposited and cemented into the conglomerate, so the conglomerate is younger.