Which principle is used to determine the relative order of rock layers in undisturbed sequences?

• A. Cross-cutting relationships
• B. Original horizontality
• C. Superposition
• D. Fossil succession

Answer: (C)

In undisturbed sequences of sedimentary rock, the relative ages of layers follow a simple rule: the oldest layer is at the bottom and younger layers accumulate on top. This is the principle of superposition, and it lets you read the stack to determine which layers formed earlier versus later without knowing their exact ages. For instance, a sandstone sitting atop shale is younger than the shale below it, because it was deposited later.

Other ideas describe different clues about timing but not the straightforward stacking order. Cross-cutting relationships are about features like faults or intrusions cutting through rocks, indicating those features are younger than the rocks they cut. Original horizontality explains why sediments start in flat layers and how tilting afterward can deform them, not how the layers themselves were laid down. Fossil succession uses the fossils within rocks to track relative ages across regions, aiding correlation rather than establishing the vertical order within a single undisturbed stack.