In a cross-section where geologic units are parallel and not overturned, which principle allows you to infer that the rocks at the bottom are older than those above?

In a cross-section where geologic units are parallel and not overturned, which principle allows you to infer that the rocks at the bottom are older than those above?

• A. Law of Superposition
• B. Principle of Original Horizontality
• C. Principle of Fossil Succession
• D. Principle of Cross-Cutting Relationships

Answer: (A)

In an undeformed sequence of rocks that lie parallel and not overturned, the bottom layer formed first and the layers above it were deposited later. This is the Law of Superposition: in a stack of horizontally deposited sedimentary rocks, each higher bed is younger than the one beneath it. Because deposition proceeds from bottom to top over time, you can infer that the rocks at the bottom are older than those above. Original Horizontality explains why sediments settle flatly, but it doesn’t by itself establish the age order within a stack. Fossil Succession uses the evolution of life to correlate rocks across regions, which helps with relative dating over wider areas but isn’t the direct rule for a single, undisturbed vertical sequence. Cross-Cutting Relationships tell you that features like faults or intrusions are younger than the rocks they cut, which is useful for recognizing relative ages of features rather than ordering a intact bed stack by age.

In an undeformed sequence of rocks that lie parallel and not overturned, the bottom layer formed first and the layers above it were deposited later. This is the Law of Superposition: in a stack of horizontally deposited sedimentary rocks, each higher bed is younger than the one beneath it. Because deposition proceeds from bottom to top over time, you can infer that the rocks at the bottom are older than those above.

Original Horizontality explains why sediments settle flatly, but it doesn’t by itself establish the age order within a stack. Fossil Succession uses the evolution of life to correlate rocks across regions, which helps with relative dating over wider areas but isn’t the direct rule for a single, undisturbed vertical sequence. Cross-Cutting Relationships tell you that features like faults or intrusions are younger than the rocks they cut, which is useful for recognizing relative ages of features rather than ordering a intact bed stack by age.