Which geologic law explains the relative ages of rocks when the units are roughly parallel and not overturned?

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

Answer: (A)

The key idea is that any feature that cuts across rocks is younger than the rocks it disrupts. When rock units are roughly parallel and not overturned, you can look for intrusions, faults, or erosion surfaces that cross through them. A dike or sill that intrudes the parallel beds must have formed after those beds were laid down, so it’s younger than them. A fault that displaces the beds is younger than the beds it offsets. By identifying which features cut or displace the parallel units, you establish a relative age order among the rocks: the disrupted rocks are older, and the cutting feature is younger. This is why the cross-cutting principle is the best fit for determining ages when the units are parallel and undisturbed.

Original horizontality would tell you the beds originally formed flat but not how their ages relate to one another once features cut through; lateral continuity helps with correlating identical beds across distances, not directly with the age of the beds themselves; fossil succession relies on fossils to rank rocks in time, which isn’t indicated in the scenario.