Question 12 of 21: Question 4, Part 3: Geologic Cross-Section – Relative Age

Question 4, Part 3: Geologic Cross-Section – Relative Age (4 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. Horizontal sedimentary beds, top to bottom: x, p, c, z, and an unlabelled basal bed, all cut by two crosscutting light-toned bands: a wider feature "m" and a narrower feature "h"; the figure shows "h" cutting directly across "m" near the middle of the section.

Find. (a) The youngest feature in the section; (b) the oldest feature.

Approach. Apply the Law of Superposition to the horizontal beds (in an undisturbed sequence, the lowest bed is oldest) and the Principle of Cross-Cutting Relationships to the two intrusive/structural features m and h (whichever cuts across the other is younger).

(a) Youngest – feature "h". Both m and h crosscut all five horizontal sedimentary beds, so both post-date the entire layered sequence; but "h" is drawn cutting directly ACROSS "m", and by the cross-cutting relationships principle, a feature that cuts across another must be younger than the feature it cuts – so h is younger than m, and (being younger than everything else in the section) h is the overall youngest feature.

(b) Oldest – the basal sedimentary bed. By the Law of Superposition, in an undisturbed, non-overturned sequence of horizontal strata the lowest bed was deposited first and is therefore the oldest; here that is the bed below "z" (illegible label, but unambiguously the lowest/oldest of the layered sequence, and older than both crosscutting features m and h since those cut through it).