Question

which geologic principle is used when a geologist observes an outcrop of rocks and determines that the bottom layer is the oldest?
○ inclusion
○ superposition
○ uniformitarianism
○ original horizontality

Explanation:

The principle of superposition states that in undisturbed sedimentary rock layers, the oldest layer is at the bottom and each successive layer is younger than the one below it. The principle of inclusion refers to inclusions (fragments) in a rock being older than the rock itself. Uniformitarianism is the idea that the same geological processes that operate today have operated throughout Earth's history. Original horizontality means that sedimentary layers are originally deposited horizontally. Since the question is about the bottom layer being the oldest, superposition is the relevant principle.

Answer:

superposition