Reading a Rock Layer Puzzle
Students examine a cliff with four horizontal sedimentary layers, top to bottom: Layer A (sandstone with ripple marks), Layer B (shale with fish fossils), Layer C (limestone with shell fossils), Layer D (conglomerate with rounded pebbles). A vertical crack cuts through all four layers and is filled with a different rock, Layer E. A student says, "Layer D is on the bottom, so it must be the youngest because it formed last as things settled." Another says, "The crack must be the oldest feature because it's at the very bottom of the cliff."
1. Use the principle of superposition to correct the first student's claim about which layer is youngest, and explain the reasoning.
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In undisturbed sedimentary layers, the oldest is at the bottom and the youngest at the top-this is superposition. So Layer D (bottom) is the oldest, not the youngest, and Layer A (top) is the youngest of the sedimentary layers. The student has the order backwards. Reasoning: sediments are deposited in sequence, with new material settling on top of older material, so lower layers were laid down first. The ripple marks in A and rounded pebbles in D are clues about environment (moving water), not age order. So the correct sequence from oldest to youngest among A–D is D, C, B, A.
2. Evaluate the second student's claim that the crack (Layer E) is the oldest feature, and determine where Layer E fits in the age sequence.
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The second claim is wrong. A feature that cuts across existing layers must be younger than the layers it cuts-this is the principle of cross-cutting relationships. Layer E fills a crack that passes through A, B, C, and D, so the crack formed after all four layers existed. Therefore Layer E is younger than A–D, not the oldest. Its position 'at the bottom' just means the crack extends downward; it doesn't make it old. Putting it together: sedimentary layers D (oldest) → C → B → A, and then the crack and its filling (E) formed after A, making E the youngest feature. The two students made opposite errors by ignoring superposition and cross-cutting relationships.