Reading a Cliff Face Without a Diagram
A new road cuts through a hillside and exposes a cliff. A geologist records the layers from bottom to top:
- 12 m of sandstone with fish fossils and ripple marks.
- 4 m of shale with fern fossils and mud cracks.
- 6 m of limestone with clam fossils.
- A thin layer of volcanic ash.
- Loose gravel with no fossils.
A band of dark, fine-grained igneous rock runs vertically through layers 1, 2, and 3, but it stops at the bottom of layer 4. None of the layers has been overturned, and there are no faults. A classmate says, "The igneous band has to be the oldest rock here, because igneous rock comes from deep inside the Earth."
1. Determine whether the igneous band is older or younger than layers 1–3 and than layer 4. Explain your reasoning.
Read the worked answer
The band is younger than layers 1, 2, and 3 and older than layer 4.
Reasoning uses two rules together:
- Cross-cutting relationships: a feature that cuts across existing layers must have formed after them. The band slices through the sandstone, shale, and limestone, so those three layers were already there when the molten rock pushed up. The magma then cooled and hardened in place.
- Superposition: in an undisturbed sequence, lower layers are older. The ash layer 4 sits above the limestone and is not cut by the band, so the ash was deposited after the intrusion cooled.
The order is therefore: layers 1 → 2 → 3 → igneous intrusion → ash layer 4 → gravel.
2. Evaluate the classmate's claim that the igneous band must be the oldest rock because igneous rock "comes from deep inside the Earth."
Read the worked answer
The claim is wrong. Where magma originates says nothing about when it arrived at this spot. The magma came from depth, but it pushed into rock that already existed, so it must be younger than the rock it cuts.
Compare it with the reverse case: if the igneous rock were a flow lying on top of the sedimentary layers, superposition would also make it younger. Either way, the deep source is irrelevant to age. Cross-cutting relationships and position in the sequence decide relative age.
3. Layer 1 has fish fossils and ripple marks; layer 2 has fern fossils and mud cracks. What does this change suggest about the area over time?
Read the worked answer
Layer 1 formed under water. Fish live in water, and ripple marks are formed by moving water working on sand, so the sandstone was once loose sediment in a lake or sea.
Layer 2 records a shift toward a wet-then-dry surface. Mud cracks form when fine mud is exposed to air and dries out, which means the sediment was at the surface and periodically dried - the water had become shallow or had pulled back. Fern fossils show that land plants were growing nearby and their remains were buried in the same deposit.
The sequence, then, is water → exposure and drying → shallow marine conditions (clam-bearing limestone) → volcanic ash fall. That is a changing environment, not one that stayed the same the whole time.