Reconstruct relative geological time
Use superposition, cross-cutting and missing records without inventing an absolute age.
In this guide
A sequence is different from a date
Relative dating asks which event happened before another. It can organize deposition, deformation, intrusion and erosion without assigning an age in years. Absolute or numerical dating asks a different question and requires suitable measurements and assumptions. A correct relative sequence does not become a numerical age merely because the story is detailed. Start by naming events rather than only naming rock units. “Layer A was deposited,” “layers were tilted,” and “an erosion surface formed” are events. An intrusion is also an event that affected pre-existing material. Turning a sketch into an event list helps reveal which relationships are actually visible and which are only possible interpretations.
Three relationships to use carefully
In an undeformed or correctly interpreted sedimentary succession, lower layers were deposited before overlying layers. This is the principle of superposition. A feature that cuts another geological feature is younger than the feature it cuts. An included fragment must have existed before the rock containing that fragment formed. These statements require attention to context. Beds can be overturned or faulted, and a diagram may not show all of the relevant contacts. An unconformity represents a gap in the geological record associated with nondeposition, erosion or both. It is not necessarily a single short event, and the visible surface does not by itself tell you how many years are missing.
Worked example: deposition, intrusion and a fault
In a hypothetical section, sedimentary layers A, B and C occur from bottom to top and are not overturned. A narrow igneous intrusion cuts all three layers. A fault then offsets both the layers and the intrusion. The supported order is deposition of A, then B, then C, then intrusion, then fault displacement. The intrusion must be younger than C because it cuts C, not merely because it looks darker or narrower. The fault displacement is younger than the intrusion because the intrusion is displaced. Nothing in these relationships supplies the duration between events. A sequence of five events could span widely different intervals depending on evidence not provided in the sketch.
Worked example: an angular unconformity
Imagine older sedimentary beds that were deposited, tilted, and eroded. A later horizontal layer rests on the eroded ends of those tilted beds. The sequence requires deposition of the older beds before tilting, tilting before the erosion surface that truncates them, and erosion before deposition of the younger horizontal layer. The change in orientation across the contact is why the relationship is described as an angular unconformity. The younger layer need not share the tilt of the older beds because it formed after that deformation. However, the sketch alone cannot establish the exact number of missing formations or the duration of the hiatus. Those claims would require additional field or dating evidence.
Avoid a visually appealing but unsupported story
A common mistake is to assign a younger age to whichever rock lies geographically higher or appears lighter in color. Relative dating relies on geological relationships, not those visual shortcuts. An intrusion can rise through older rocks, and a fault can place older material above younger material. Another error is assuming every contact is depositional. A fault contact, intrusive contact and erosional surface imply different histories. When a question supplies only part of a section, state the minimum sequence required by the observed relationships. Multiple broader histories can fit the same limited evidence, so do not add certainty that the problem has not earned.
Try it yourself
Three non-overturned layers are arranged D below E below F. An intrusion cuts D and E but stops at an erosion surface beneath F, and F lies across that surface. What can be concluded if the diagram clearly shows the intrusion was truncated by that erosion?
Reveal the worked answer
D was deposited before E. The intrusion is younger than D and E, the erosion that truncates it is younger than the intrusion, and F was deposited after that erosion. Thus the intrusion is older than F in this stated relationship. Its failure to cut F alone would be weaker evidence; the explicitly described truncation and overlying deposit establish the order. In the linked geology practice, write each observed contact beside the event-order statement it supports. This turns a memorized rule into a defensible reconstruction.
D was deposited before E. The intrusion is younger than D and E, the erosion that truncates it is younger than the intrusion, and F was deposited after that erosion. Thus the intrusion is older than F in this stated relationship. Its failure to cut F alone would be weaker evidence; the explicitly described truncation and overlying deposit establish the order. In the linked geology practice, write each observed contact beside the event-order statement it supports. This turns a memorized rule into a defensible reconstruction.
Put one idea into practice.
Try a short general geology session, then use the explanations to choose what to review next.
Start a short sessionSources & further reading
BCcampus, Steven Earle - Relative dating methodsSomething unclear or incorrect? Suggest a correction to this guide.
Original FreeReviewer.ph guidance, informed by the linked sources. Exam requirements and procedures should always be checked with the examining body.