WJEC A-Level Geology F3 Time and Change: a deep dive on relative dating, fossils and biostratigraphy, radiometric dating and the geological timescale

What F3 actually demands

F3 is about geological time and how we read it. Examiners want fluent application of the relative dating principles to order a cross-section, secure knowledge of how fossils form and how zone fossils correlate rocks, confident half-life calculations for radiometric ages, and a working grasp of the timescale and the events that divide it. The dating questions are highly examined, so the reasoning must be exact.

This guide ties the four topics of the unit together. Each has a matching dot-point page with practice questions; this overview connects them.

Reading the order of events

Relative dating uses superposition (lower is older), original horizontality, cross-cutting relationships (a feature that cuts another is younger), inclusions (fragments predate their host) and unconformities (buried erosion surfaces recording uplift, erosion and a time gap). Combined, these let you order any cross-section from oldest to youngest, after first checking way-up.

Fossils and correlation

Fossils form where there are hard parts, rapid burial and anoxic conditions, by modes such as permineralisation, mould and cast, replacement and carbonisation. Faunal succession (fossils follow a fixed order) underpins biostratigraphy, and zone fossils, with their short vertical and wide horizontal ranges, correlate and relatively date strata between areas.

Numerical ages and the timescale

Radiometric dating measures the decay of a radioactive parent to a daughter, using half-life and the parent-to-daughter ratio to give an age in years, with the isotope system chosen to suit the age and material. The geological timescale is a hierarchy of eons, eras, periods and epochs, built relatively and calibrated radiometrically, with mass extinctions defining its major boundaries.

Check your knowledge

Attempt these under timed conditions, then check against the solutions.

1. State the principle of cross-cutting relationships. (1 mark)
2. List the events recorded by an angular unconformity, in order. (2 marks)
3. Why must a good zone fossil have a wide horizontal range? (2 marks)
4. A mineral has a parent-to-daughter ratio of 1:3; the half-life is 600 million years. Calculate its age. (2 marks)
5. List the divisions of the timescale from largest to smallest. (1 mark)
6. Name the event that defines the Palaeozoic-Mesozoic boundary. (1 mark)