In horizontal sedimentary rock layers, the oldest is at the bottom and each higher layer is younger; which principle explains this?

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

Answer: (C)

In undisturbed, horizontally deposited sedimentary layers, each new layer forms on top of the older ones, so the oldest material is at the bottom and the younger material sits above. This pattern is known as the Law of Superposition. It works because deposition builds up sequentially over time, and gravity tends to spread sediments out evenly in horizontal layers, preserving their order unless something disturbs them. The other processes wouldn’t explain why the bottom layer is oldest and the top layers are progressively younger: faulting would offset and misplace layers, an unconformity would reveal a gap in time, and intrusion cuts through existing rocks rather than forming as part of the original deposition sequence.