Which geologic law explains the relative ages of rocks with respect to one another in a map or cross section in which all the geologic units are roughly parallel and are not overturned?

Which geologic law explains the relative ages of rocks with respect to one another in a map or cross section in which all the geologic units are roughly parallel and are not overturned?

• A. Law of Superposition
• B. Principle of Original Horizontality
• C. Principle of Faunal Succession
• D. Principle of Uniformitarianism

Answer: (A)

In undisturbed, stacked sedimentary rocks that are roughly parallel and not overturned, the order of layers directly records the passage of time. The bottom layer formed first and is the oldest; each successive layer records deposition after the previous one, so the top layer is the youngest. This straightforward vertical-to-temporal relationship is what the Law of Superposition captures, making it the best way to determine relative ages on a map or cross section where the beds stay in place. Original Horizontality explains that sediments are deposited near horizontally, which helps interpret why layers appear flat, but it doesn’t itself establish the age order of the beds. Faunal Succession uses fossils to correlate layers across regions, which is helpful but requires fossil content and isn’t the direct rule for ordering non-fossil beds. Uniformitarianism is a broad principle about processes shaping Earth’s history, not the specific method for ranking the ages of stacked layers.

In undisturbed, stacked sedimentary rocks that are roughly parallel and not overturned, the order of layers directly records the passage of time. The bottom layer formed first and is the oldest; each successive layer records deposition after the previous one, so the top layer is the youngest. This straightforward vertical-to-temporal relationship is what the Law of Superposition captures, making it the best way to determine relative ages on a map or cross section where the beds stay in place.

Original Horizontality explains that sediments are deposited near horizontally, which helps interpret why layers appear flat, but it doesn’t itself establish the age order of the beds. Faunal Succession uses fossils to correlate layers across regions, which is helpful but requires fossil content and isn’t the direct rule for ordering non-fossil beds. Uniformitarianism is a broad principle about processes shaping Earth’s history, not the specific method for ranking the ages of stacked layers.