Reading a Roadcut: Relative Age With Faults, Dikes, and Fossils
During a field trip, students examine a 12-m roadcut with these observations from bottom to top: layer A, coarse sandstone with rounded pebbles; layer B, gray shale with marine shell fossils; layer C, limestone with coral fossils; a wavy eroded surface; layer D, flat-lying conglomerate. A vertical igneous dike cuts layers A, B, and C but ends beneath layer D. A fault offsets layers A and B by 1.5 m but does not offset layer C or layer D. The dike surface is fresh, while the fault surface is weathered. A loose piece of limestone resembling layer C is found in a stream bed 200 m away. The class has hand lenses, dilute acid, a geologic map showing no nearby volcanoes, and a camera. Students must construct a relative-age history and decide which relationships are certain and which are inferred.
1. Construct the most likely relative-age sequence for layer A, layer B, the fault, layer C, the dike, the erosion surface, and layer D. State which relationships are certain from cross-cutting and superposition and which are inferred.
Read the worked answer
Certain from superposition: A is below B, which is below C, which is below D, so the sedimentary order is A, B, C, D unless overturned, and there is no evidence of overturning. The fault offsets A and B but not C, so the fault is younger than B and older than C. The dike cuts A, B, and C and ends beneath D, so the dike is younger than C and older than D. The erosion surface lies between C and D, so it is younger than C and older than D. The uncertain relationship is dike versus erosion surface. If the dike is truncated by the erosion surface, the sequence is A, B, fault, C, dike, erosion, D. If the erosion surface predated the dike, the dike would cut it, which is not described. Weathering freshness alone is not a relative-age principle; it cannot override cross-cutting relationships.
2. Use the rock and fossil evidence to infer the depositional environment of layers A, B, and C. Explain why the dike cannot be assigned a relative age from fossils.
Read the worked answer
Layer A has rounded pebbles and coarse sand, indicating high-energy transport, such as a stream or beach. Layer B is shale with marine shells, indicating quiet marine water where fine mud settled and shells were preserved. Layer C is limestone with coral, indicating warm, shallow, clear marine water because corals need sunlight and relatively low sediment input. The dike is igneous, so it contains no fossils; its relative age comes from cross-cutting relationships, not fossils. It cuts A, B, and C but not D, so it is younger than C and older than D. Radiometric dating could give an absolute age if suitable minerals are present, but fossils cannot date an igneous intrusion.
3. The loose limestone piece is found 200 m downstream. Describe two observations or tests the students could use to test whether it came from layer C, and explain how each would strengthen or weaken the correlation.
Read the worked answer
First, compare fossil content: if the loose piece contains the same coral types and shell fragments as layer C, the correlation is strengthened; if it contains different fossils, it is weakened. Second, use dilute acid and hand-lens texture: both layer C and the loose piece should react similarly and show the same fine-grained limestone texture, color, and grain composition. A match strengthens the case; a mismatch weakens it. Students should also compare the piece with several samples from layer C rather than one, and check whether the stream flows past layer C upstream. No single observation proves the source because similar limestone could crop out elsewhere; the correlation is strongest when multiple independent properties match.