Evidence from the Geological Record
In 1815, William Smith completed the first geological map of England and Wales, establishing the principle of faunal succession — the observation that each rock stratum contains a unique assemblage of fossils that identifies it, regardless of location. His map identified 48 distinct formations and allowed geologists worldwide to match strata by their fossil content. Smith's insight created the framework that all palaeontology and ecology uses today: the rock record is a time-ordered archive of past ecosystems, readable once you know the language of stratigraphy.
A palaeontologist digs up a fossil bone and announces it is "about 12,000 years old". Nobody was there when the animal died.
Before reading: how do you think a scientist can work out the age of a fossil? Can you think of more than one method, and would they all give an exact number of years?
Know
- How fossils form and what the geological record is
- The law of superposition and the role of index fossils
- The difference between relative and absolute (radiometric) dating
- How half-life underpins radiometric dating
Understand
- How fossils in strata provide evidence of past environments and change
- Why different dating technologies suit different samples and ages
- Why the fossil record is strong but incomplete evidence
Can Do
- Interpret a sequence of rock strata to order events in time
- Select an appropriate dating method for a given sample
- Perform a simple half-life calculation to estimate age
Core Content
Layers of rock as a record of past life and environments
In 1815, William Smith completed his geological map of England and Wales — the first of its kind — and demonstrated the principle of faunal succession: each rock stratum contains a unique fossil assemblage that identifies it regardless of where in the country the rock was found. His 48 identified formations could be matched across the country by their fossils. Smith's key insight was that sedimentary rock builds up in layers (strata), trapping organism remains as they die and settle — so the rock record is a time-ordered diary of past ecosystems, with older organisms in lower layers and younger organisms above.
A fossil is the preserved remains, impression or trace of a once-living organism. Most fossils form in sedimentary rock: an organism is buried in sediment (mud, sand), and over long periods the sediment compacts into rock while minerals replace the remains.
Because new sediment is deposited on top of old, the rock forms strata (layers). The law of superposition states that in undisturbed strata, lower layers are older and upper layers are younger — so the vertical order of fossils reflects their order in time.
Fossils lower in undisturbed strata are older — the record reads from bottom (oldest) to top (youngest)
Fossils also reveal the past environment: finding marine fossils (e.g. shellfish) in high mountain strata shows the area was once under the sea and was later uplifted — evidence that environments and ecosystems change over time.
Pause — copy the highlighted points into your book before the check below.
In undisturbed rock layers, the principle that lower layers are older than those above is the law of _____.
Putting events in order — without a clock
We just saw that strata record events in order. That raises a question: how do scientists use that order, and match rock layers between different sites? This card answers it → relative dating and index fossils.
Relative dating establishes the order of events — which is older and which is younger — without giving a number of years.
Relative dating uses the law of superposition to order strata and the fossils within them. It tells us the sequence of events (older vs younger) but not an age in years.
Index fossils make relative dating powerful across locations. An index fossil is from a species that was geographically widespread but existed for only a short period — so its presence pins a rock layer to a narrow time range and lets layers at different sites be correlated (matched in time).
Add the relative-dating and index-fossil points to your notes before the check below.
Which feature makes a species a good index fossil?
Reading a Rock Strata Sequence
Pattern — Interpret & Order
A cliff face shows five undisturbed layers. From bottom to top they are: Layer E (trilobite fossils), Layer D (fern fossils), Layer C (no fossils), Layer B (ammonite fossils), Layer A (mammal fossils). In your book:
- List the layers from oldest to youngest, and state the principle you used.
- Which organisms appeared earliest in this record, and which most recently?
- Layer C has no fossils. Suggest two reasons a layer might contain no fossils.
- If ammonites are a known index fossil for a specific period, explain how Layer B could be used to date a layer at a different site.
Putting a number of years on the past
We just saw that relative dating only gives order. That raises a question: how do scientists get an actual age in years? This card answers it → radiometric dating, using radioactive decay and half-life.
Radioactive isotopes decay at a fixed, known rate — so measuring how much is left acts like a built-in clock.
Radioactive isotopes (the "parent") decay into stable "daughter" products at a constant rate, measured by the half-life — the time for half the parent atoms in a sample to decay. Radiometric dating measures the ratio of parent to daughter and, using the known half-life, calculates a numerical age.
Each half-life, the amount of parent isotope halves — a fixed clock that gives a numerical age
Different isotopes suit different ages and materials:
- Carbon-14 (half-life ~5730 years): organic material (bone, wood, charcoal) up to ~50,000 years old.
- Potassium-argon / uranium-lead (half-lives of millions–billions of years): very old volcanic and igneous rock, used to date the strata around fossils.
Choose the dating method to match the sample: carbon-14 for recent organic remains; potassium-argon or uranium-lead for ancient rock. Relative and absolute dating are used together to build the geological timescale.
Add the half-life definition and method-selection points to your notes before the check below.
Radiometric dating gives a numerical age in years, whereas relative dating only gives an order.
Carbon-14 dating is suitable for recent organic material, not for rock millions of years old.
After one half-life, none of the original radioactive parent isotope remains.
Half-Life Calculations
Pattern — Structured Calculation
Carbon-14 has a half-life of approximately 5730 years. Use this to answer the following in your book:
- A wooden tool contains 50% of its original carbon-14. How old is it? Show your reasoning.
- A bone contains 12.5% of its original carbon-14. How many half-lives have passed, and what is its approximate age?
- A geologist wants to date a 200-million-year-old basalt rock. Explain why carbon-14 dating would be unsuitable and name a more appropriate method.
- Radiometric ages are usually written as, for example, "11,460 ± 200 years". Explain what the "± 200" represents and why no radiometric date is perfectly exact.
The geological record
- Fossils form mainly in sedimentary rock; strata = layers over time.
- Law of superposition: lower = older, upper = younger.
- Fossils reveal past environments (e.g. marine fossils on mountains = uplift).
Relative dating
- Gives order (older/younger), not a number of years.
- Index fossils = widespread + short-lived → date & correlate strata between sites.
Absolute (radiometric) dating
- Uses radioactive decay; half-life = time for half the parent to decay.
- C-14 (~5730 yr) for recent organics; K-Ar / U-Pb for ancient rock.
- Gives a numerical age (with uncertainty); used with relative dating.
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence — that tells the system what to drill next.
UnderstandBand 3(3 marks) 1. Explain how the law of superposition and index fossils are used in relative dating.
1 mark: law of superposition (lower = older) · 1 mark: index fossil defined · 1 mark: how index fossils correlate strata between sites
ApplyBand 4(4 marks) 2. A fossil bone contains 25% of its original carbon-14 (half-life 5730 years). Calculate its approximate age, showing your reasoning, and explain why a different method would be needed to date the 60-million-year-old rock layer it was found in.
1 mark: 25% = 2 half-lives · 1 mark: age ≈ 11,460 years · 1 mark: C-14 unsuitable for very old rock (too few atoms left / beyond range) · 1 mark: names suitable method (e.g. K-Ar, U-Pb)
EvaluateBand 5(4 marks) 3. Evaluate the usefulness of the fossil record as evidence for past ecosystems and change over time, referring to both its strengths and its limitations.
Up to 2 marks: strengths (ordered record, past environments, dating) · up to 2 marks: limitations (incomplete, biased toward hard parts, gaps) + judgement
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Answers
Q1 (3 marks): The law of superposition states that in undisturbed sedimentary strata, lower layers were deposited first and are therefore older than the layers above them — so fossils can be placed in relative order of age by their position. An index fossil is a fossil of a species that was geographically widespread but existed for only a short geological period. Because such a fossil marks a narrow window of time, finding the same index fossil in rock layers at different locations shows those layers formed at about the same time — allowing strata across different sites (even different continents) to be correlated and placed on a shared timescale.
Q2 (4 marks): 25% of the original carbon-14 remaining means two half-lives have passed (100% → 50% → 25%). The age is therefore approximately 2 × 5730 = 11,460 years. Carbon-14 cannot be used to date the 60-million-year-old rock layer because carbon-14 has a short half-life (~5730 years); after about 50,000 years so little remains that it can no longer be measured reliably, and rock is not organic. A method with a much longer half-life and suited to rock — such as potassium-argon or uranium-lead dating — would be used to date the surrounding volcanic/igneous rock.
Q3 (4 marks): The fossil record is a powerful source of evidence: combined with the law of superposition it provides a time-ordered sequence of the organisms that lived in the past; fossils reveal past environments (e.g. marine fossils in mountain strata indicate former seas and later uplift); and, dated radiometrically, it allows extinction events and ecosystem change to be placed in absolute time. However, it is incomplete and biased: fossilisation is rare and requires specific conditions (rapid burial, hard body parts), so soft-bodied organisms and many environments are under-represented; many fossils have been destroyed by erosion or metamorphism, or remain undiscovered; and gaps exist in the record. Overall the fossil record is strong but partial evidence — extremely useful for reconstructing past ecosystems and change, provided its limitations are kept in mind.
Timed questions on the geological record, relative dating and radiometric dating. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
⚔ Enter the arenaWilliam Smith's 1815 principle of faunal succession established that fossils give the order of events — which stratum is older and which is younger — but not the number of years. His method identified 48 formations across England and Wales purely by their fossil assemblages, without any knowledge of absolute age. To put actual years on those formations requires radiometric dating: measuring the ratio of remaining parent to daughter isotopes against the known half-life of the decay system.
The 1815 geological map gave relative dating. Carbon-14 (half-life 5,730 years), potassium-argon (half-life 1.25 billion years), and uranium-lead (half-life 4.47 billion years) gave absolute dating. Both tools together — order and age — allow the complete reading of the rock record diary that Smith first opened.