Which principle explains that in an undisturbed sequence, older rocks are at the bottom?

Which principle explains that in an undisturbed sequence, older rocks are at the bottom?

• A. Superposition
• B. Original Horizontality
• C. Uniformitarianism
• D. Cross-Cutting Relationships

Answer: (A)

The idea being tested is superposition. In an undisturbed sequence of sedimentary rock, each new layer is deposited on top of older material, so the bottom layer is the oldest and the layers above it are progressively younger. This allows scientists to determine relative ages just by looking at the vertical order of the rocks, assuming there hasn’t been any disturbance like tilting, folding, or erosion. Original Horizontality is about sediments originally being deposited in a nearly horizontal orientation, not about the age order of layers. Uniformitarianism is the principle that present-day processes explain the past but doesn’t specify how rock layers are arranged by age. Cross-Cutting Relationships state that features that cut through rocks (like faults or intrusions) are younger than the rocks they cut, which is a different concept from the vertical age sequence within undisturbed layers. So, the description aligns best with superposition.

The idea being tested is superposition. In an undisturbed sequence of sedimentary rock, each new layer is deposited on top of older material, so the bottom layer is the oldest and the layers above it are progressively younger. This allows scientists to determine relative ages just by looking at the vertical order of the rocks, assuming there hasn’t been any disturbance like tilting, folding, or erosion.

Original Horizontality is about sediments originally being deposited in a nearly horizontal orientation, not about the age order of layers. Uniformitarianism is the principle that present-day processes explain the past but doesn’t specify how rock layers are arranged by age. Cross-Cutting Relationships state that features that cut through rocks (like faults or intrusions) are younger than the rocks they cut, which is a different concept from the vertical age sequence within undisturbed layers.

So, the description aligns best with superposition.