What are the two 'who is older' rules in relative dating?

An included fragment is older than the rock that contains it, because it had to exist before it could be incorporated (for example xenoliths in a granite, or pebbles in a conglomerate). A cutting feature is younger than what it cuts, because the rock had to be there first to be cut (for example a dyke or a fault that cuts a sequence of beds). Students often swap these, so fix them firmly: inclusion older, cutter younger.

What makes a good index (zone) fossil?

Four properties: abundant (so it is likely to be found), geographically widespread (so it allows correlation between distant areas), easily recognisable (so it is not misidentified), and a short stratigraphic range (so it existed for only a short time). The short range is the most important, because the shorter the time a species lived, the more precisely its presence pins down the age of the rock. Graptolites, ammonites and trilobites are classic examples.

How do I calculate a radiometric age from a parent-to-daughter ratio?

Three steps. First convert the ratio to the fraction of parent remaining: a ratio of 1 parent to 3 daughter means 1 out of 4 original atoms remains, so the fraction is one quarter. Second, work out how many half-lives that is: one quarter is two half-lives (one half then one quarter). Third, multiply the number of half-lives by the half-life. The common trap is treating the ratio itself as the fraction remaining, so always convert to the fraction of the original parent first.

Why must you match the isotopic system to the age of the rock?

Each isotope has a characteristic half-life suited to a particular age range. Carbon-14 has a short half-life (about 5730 years) and works only on recent organic material up to roughly 60,000 years. Potassium-argon has an intermediate half-life for dating igneous rocks. Uranium-lead has very long half-lives for the oldest rocks and the age of the Earth. Using carbon-14 on an ancient granite fails because almost all the carbon-14 has decayed and a granite has no organic carbon anyway.

What is the single most common mistake in this module?

Two: confusing a parent-to-daughter ratio with the fraction of parent remaining in dating calculations, and forgetting that the fossil record is incomplete and biased. A ratio of 1 to 3 is one quarter remaining (two half-lives), not three. And the record over-represents hard-bodied, marine, widespread organisms, which is partly why some evolutionary transitions look abrupt and why punctuated equilibrium can appear in the data.

EnglandGeology

OCR A-Level Geology: geological time and the fossil record overview

A deep-dive OCR A-Level Geology guide to geological time and the fossil record. Covers relative dating and stratigraphic principles, fossils and preservation, the geological time scale and correlation, evolution and mass extinctions, and radiometric dating with half-life calculations, with the exam patterns OCR repeats.

Generated by Claude Opus 4.819 min readOCR-H414-Module-4Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this topic actually demands
Relative dating and stratigraphic principles
Fossils, preservation and index fossils
The geological time scale and correlation
Evolution and the fossil record
Radiometric dating and half-life
How this topic is examined
Check your knowledge

What this topic actually demands

Geological time and the fossil record is where OCR Geology learns to read the history of the Earth from the rocks. The topic runs from ordering events relatively, through how fossils form and how they date and correlate strata, to evolution and mass extinctions, and finally to measuring absolute ages by radioactive decay. The examiners test two linked skills: reasoning out a sequence of events, and the quantitative skill of radiometric dating.

This guide walks through the five clusters in a sensible build order, then sets out the exam patterns OCR repeats. Each cluster has a matching dot-point page with practice questions; this overview ties them together.

Relative dating and stratigraphic principles

Relative dating orders events without absolute ages, using superposition (oldest at the base), original horizontality (tilting came later), cross-cutting relationships (a cutting feature is younger than what it cuts), included fragments (an inclusion is older than its host) and faunal succession (fossils in a fixed order). In folded or overturned rocks, way-up evidence (graded bedding, ripple marks) finds the original top. To reconstruct a cross-section, order the beds by superposition, add deformation events and unconformities, then place faults and intrusions as younger than everything they cut. This is the backbone of the Paper 3 cross-section question.

Fossils, preservation and index fossils

Fossilisation is rare because most remains are destroyed first; it is favoured by rapid burial, low oxygen, hard parts and fine sediment. The modes of preservation are moulds and casts, permineralisation (petrification), carbonisation, and preservation in amber or ice. A good index (zone) fossil is abundant, widespread, easily recognised and, above all, short-ranging, so it dates rocks precisely. Distinguish body fossils (the organism's remains) from trace fossils (footprints, burrows), which record behaviour and, being rarely transported, indicate the local environment.

The geological time scale and correlation

Geological time is a hierarchy of eon, era, period and epoch, with the long Precambrian and the Phanerozoic eon (Palaeozoic, Mesozoic, Cenozoic eras) as the major divisions. Keep the distinction between a period (an interval of time) and a system (the rock formed then). Correlation matches rocks of the same age between areas by lithostratigraphy (rock type, no fossils needed but it can mislead because the same rock forms at different times) or biostratigraphy (zone fossils, correlating over distance and across rock types). A volcanic ash band is an ideal marker horizon because it is effectively instantaneous, widespread and datable.

Evolution and the fossil record

The fossil record is evidence for evolution: morphological change through time, transitional forms (for example Archaeopteryx) and adaptive radiation. Two models describe the rate: gradualism (slow, continuous, smooth transitions) and punctuated equilibrium (long stasis interrupted by rapid bursts, few intermediates). Mass extinctions (for example the end-Permian and end-Cretaceous) remove many species suddenly worldwide, empty ecological niches and trigger adaptive radiation of survivors, marking era boundaries. The record is incomplete and biased toward hard-bodied, marine, widespread organisms.

Radiometric dating and half-life

Radiometric dating measures absolute ages from the decay of an unstable parent to a stable daughter at a constant rate, set by the half-life. An age comes from the parent-to-daughter ratio: convert it to the fraction of parent remaining, count the half-lives, and multiply by the half-life. Match the isotope to the age: uranium-lead for the oldest rocks, potassium-argon for igneous rocks, carbon-14 for recent organic material. The method assumes a closed system, and combining absolute dates (from lavas and ash bands) with relative dating calibrates the time scale in years.

How this topic is examined

A typical OCR profile for geological time and the fossil record:

Cross-section reconstruction (Paper 3). Ordering events using the relative-dating principles, including faults, intrusions and unconformities, naming the principle at each step.
Fossil and correlation questions (Papers 1 and 2). Judging a zone fossil, describing preservation modes, and choosing or justifying a correlation method (including ash-band markers).
Calculation questions (Paper 1). Radiometric ages from parent-to-daughter ratios and half-lives, and explaining why a given isotope is or is not suitable.
Level-of-response extended answers (Papers 1 and 2). Comparing gradualism and punctuated equilibrium, and discussing mass extinctions and their aftermath, are predictable extended questions.

Check your knowledge

A mix of recall and application questions covering the whole topic. Attempt them under timed conditions, then check against the solutions.

State the principle of superposition and the principle of cross-cutting relationships. (2 marks)
State the four properties of a good index (zone) fossil. (4 marks)
Describe how a mould and a cast form. (2 marks)
Explain the difference between lithostratigraphic and biostratigraphic correlation. (2 marks)
A mineral has a parent-to-daughter ratio of 1 to 3; the half-life is 700 Ma. Calculate its age. (3 marks)
Explain why carbon-14 cannot date a 400-million-year-old granite. (2 marks)
Compare gradualism and punctuated equilibrium. (4 marks)
Explain how a mass extinction can lead to the adaptive radiation of surviving groups. (3 marks)

Solutions

Q1: Superposition: in an undeformed sequence each bed is younger than the one below and older than the one above. Cross-cutting relationships: any feature that cuts across a rock is younger than the rock it cuts.
Q2: Abundant; geographically widespread; easily recognisable; short stratigraphic range.
Q3: The buried shell dissolves, leaving a hollow cavity that preserves its shape (a mould). If that cavity is later filled by minerals or sediment, the infill takes the shape of the shell (a cast).
Q4: Lithostratigraphy matches rock units by physical properties (lithology, colour, grain size); biostratigraphy matches strata by fossil content (zone fossils). Biostratigraphy correlates over distance and across different rock types and gives a relative age.
Q5: A ratio of $1$ parent to $3$ daughter means $\frac{1}{4}$ of the parent remains, which is $2$ half-lives. Age $= 2 \times 700 = 1400\ \mathrm{Ma}$ ( $1.4$ billion years).
Q6: Carbon-14 has a short half-life (about $5730$ years) and is useful only up to about $60\,000$ years, so at $400$ million years essentially all the carbon-14 has decayed and cannot be measured; a granite also has no organic carbon.
Q7: Gradualism: slow, continuous, small changes over long periods, giving a smooth series of transitional forms. Punctuated equilibrium: long periods of stasis interrupted by short bursts of rapid change, giving few intermediates and abrupt appearances. The contrast is continuous change versus stasis plus sudden jumps.
Q8: The extinction removes dominant groups and empties their ecological niches, reducing competition for food and space. Surviving groups then evolve rapidly, with populations adapting to the different vacant niches (adaptive radiation), producing many new species, for example mammals after the dinosaurs.