Reading a Cliff Without a Diagram
A coastal cliff exposes five units, described from the bottom upwards; no unit has been overturned, so lower means older. Unit 1 is a limestone packed with coral fossils. Unit 2 is a mudstone containing well-preserved fern leaves. Unit 3 is a conglomerate made of rounded pebbles. A vertical sheet of igneous rock cuts through units 1, 2 and 3 but stops at the base of unit 4. A fault displaces units 1, 2 and 3 and also displaces the igneous sheet, but the fault does not enter unit 4. Unit 4 is a sandstone containing rounded fragments of the same igneous rock as the sheet. Unit 5 is a loose soil containing fragments of pottery.
1. Arrange the following events in order from oldest to youngest: intrusion of the igneous sheet, deposition of units 1–3, faulting, deposition of unit 4, formation of the soil. Justify each step using a geological principle.
Read the worked answer
Oldest → youngest: deposition of units 1, 2, 3 (in that order) → intrusion of the igneous sheet → faulting → deposition of unit 4 → formation of the soil.
- Units 1–3 are layered and undisturbed, so superposition makes 1 the oldest and 3 the youngest of them.
- The igneous sheet cuts across 1–3, so it must be younger than all of them (cross-cutting relationship).
- The fault displaces the sheet as well as the layers, so the fault is younger than the sheet.
- Unit 4 is not faulted and contains fragments of the igneous rock, so it formed after both the sheet and the faulting; it lies on an eroded surface, an unconformity.
- Unit 5 is loose soil with pottery, a human artefact, so it is the youngest.
2. Describe how the environment at this place changed between the deposition of unit 1 and the deposition of unit 3, using the rock types and fossils as evidence.
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Unit 1 is a limestone with coral: coral builds reefs only in warm, clear, shallow seawater, so the area was under a shallow sea.
Unit 2 is a mudstone with fern leaves. Mud settles only in quiet water, and ferns are land plants, so their leaves must have been washed into a quiet lake or floodplain. The sea had withdrawn.
Unit 3 is a conglomerate of rounded pebbles, which requires fast-flowing water or strong wave action to round and carry stones of that size.
So the sequence records a shallowing and withdrawal of the sea followed by high-energy river or beach conditions - a regression, with the energy of the water increasing from unit 2 to unit 3.
3. A visitor says: "The igneous sheet must be the oldest rock here, because igneous rock comes up from deep below." Evaluate this claim, and state what unit 4's pebbles prove.
Read the worked answer
The claim is wrong. Origin at depth says nothing about age; what matters is the relationship between the sheet and the rocks around it. The sheet cuts across units 1, 2 and 3, so it must have been intruded after they were deposited - it is younger than all three.
The pebbles in unit 4 are fragments of the same igneous rock, broken off and rounded before being buried. A rock cannot supply fragments to a sediment until it already exists and has been exposed and eroded, so unit 4 is younger than the igneous sheet. Together the two observations place the sheet between unit 3 and unit 4, not at the beginning of the story.