Which principle explains why older sedimentary rock layers are found beneath younger ones in an undeformed sequence?

• A. Law of Superposition
• B. Unconformity
• C. Catastrophism
• D. Intrusion

Answer: (A)

In an undeformed sedimentary sequence, layers form one on top of another over time, so each new layer sits above older ones. This stacking means the bottommost layer is the oldest and the topmost layer is the youngest, preserving a vertical timeline of deposition. The law of superposition is exactly what explains that pattern.

If you think about interruptions, like missing records due to erosion, that would create an unconformity and complicate the sequence, but it doesn’t establish the general, orderly bottom-to-top age relationship. Concepts like catastrophism describe sudden events shaping Earth’s features rather than the normal layering, and an intrusion involves younger rock cutting through older layers, which also doesn’t account for the standard bottom-to-top arrangement.