How is geological time divided, and how do we correlate rocks between distant areas?
The geological record: the hierarchy of the geological time scale (eon, era, period, epoch) and the major divisions (Precambrian and the Phanerozoic eras); correlation of strata by lithostratigraphy (matching rock units) and biostratigraphy (matching fossils and biozones); the use of marker horizons such as volcanic ash bands; the distinction between rock units (systems) and time units (periods).
A focused answer to the OCR H414 dot point on the geological time scale and correlation. Covers the eon, era, period and epoch hierarchy and the major divisions, correlation by lithostratigraphy and biostratigraphy, the use of marker horizons such as ash bands, and the distinction between rock units and time units.
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What this dot point is asking
OCR wants you to describe the hierarchy of the geological time scale and its major divisions, to explain correlation by lithostratigraphy and biostratigraphy, to explain the value of marker horizons such as volcanic ash bands, and to distinguish rock units (systems) from time units (periods).
The answer
The geological time scale
Geological time is divided into a nested hierarchy, from longest to shortest:
- Eon (the longest, for example the Phanerozoic).
- Era (for example Palaeozoic, Mesozoic, Cenozoic).
- Period (for example Jurassic, Cretaceous).
- Epoch (a subdivision of a period).
The two biggest divisions are the long Precambrian (most of Earth's history, with few fossils) and the Phanerozoic eon, split into the Palaeozoic, Mesozoic and Cenozoic eras, each ended by a major change in life (often a mass extinction). The periods you should recognise include the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian (Palaeozoic), Triassic, Jurassic, Cretaceous (Mesozoic), and the Paleogene and Neogene (Cenozoic).
Rock units versus time units
A subtle but examinable distinction:
Correlation
Correlation matches rocks of the same age between separate areas, using two main methods:
- Lithostratigraphy. Matching rock units by their physical properties (lithology, colour, grain size, sequence). Useful where a distinctive unit can be traced laterally, and it needs no fossils. Its weakness is that the same rock type can form at different times in different places.
- Biostratigraphy. Matching strata by their fossil content (zone fossils and biozones). Because a species existed at the same time everywhere, this correlates widely separated areas and across different rock types, and it gives a relative age. It is the stronger method for long-distance and time correlation.
Marker horizons
A marker horizon is a thin, distinctive, widespread bed used as a reference line. The best example is a volcanic ash band: it is deposited from a single eruption over a geologically instantaneous time, so it represents the same moment everywhere, is widespread and recognisable, and can even be radiometrically dated to give an absolute tie point. Matching an ash band between areas correlates the sequences precisely at that level.
Examples in context
Example 1. Ammonite biozones in the Jurassic. The rapidly evolving ammonites divide the Jurassic into fine biozones, letting geologists correlate marine strata across Europe far more precisely than rock type alone allows.
Example 2. Bentonite (ash) bands as time lines. Altered volcanic ash bands (bentonites) form thin, datable marker horizons that tie sequences together at a single instant and provide absolute ages within a relative framework.
Try this
Q1. List the geological time hierarchy from longest to shortest. [2 marks]
- Cue. Eon, era, period, epoch.
Q2. State the difference between a period and a system. [2 marks]
- Cue. A period is an interval of time; a system is the body of rock that formed during that period.
Q3. Explain why biostratigraphy can correlate areas with different rock types. [2 marks]
- Cue. It uses fossils, and a species existed at the same time everywhere, so the same zone fossil shows the beds are the same age regardless of the rock type.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H414/01 20204 marksExplain the difference between lithostratigraphic and biostratigraphic correlation, and give one advantage of each.Show worked answer →
Define each method, then give an advantage.
Lithostratigraphic correlation matches rock units between areas using their physical characteristics: rock type (lithology), colour, grain size and sequence of beds. Advantage: it is straightforward where a distinctive rock unit (for example a thick limestone) can be traced laterally, and it needs no fossils.
Biostratigraphic correlation matches strata using their fossil content, especially zone fossils and biozones. Advantage: because fossil species existed at the same time everywhere, it allows correlation between widely separated areas even where the rock types differ, and it gives a relative age.
The key contrast. Lithostratigraphy matches what the rock is; biostratigraphy matches when it formed (using fossils), so biostratigraphy is better for long-distance and time correlation.
Markers reward a clear definition of each and a valid advantage (lithostratigraphy needs no fossils; biostratigraphy correlates across different rock types and over distance).
OCR H414/02 20194 marksA volcanic ash band is found within sedimentary sequences in two separate areas tens of kilometres apart. Explain why such a marker horizon is especially useful for correlation.Show worked answer →
Explain why an ash band is an ideal time marker.
- It is instantaneous
- A volcanic ash band is deposited from a single eruption over a very short time (geologically instantaneous), so it represents the same moment in time everywhere it is found.
- It is widespread and distinctive
- Ash falls over a large area and forms a thin, recognisable band of a distinctive composition, so it can be identified and matched between separate areas.
- Why this helps correlation
- Because the ash band is the same age everywhere, matching it between the two areas correlates the sequences precisely at that level: the beds immediately below the ash in one area are the same age as those below it in the other. It can also be radiometrically dated, giving an absolute tie point.
Markers reward the ash band being effectively instantaneous (a true time line), widespread and distinctive, and therefore an excellent correlation marker.
Related dot points
- Relative dating: the principles used to order geological events (superposition, original horizontality, cross-cutting relationships, included fragments and faunal succession); the recognition of way-up evidence; the application of these principles to construct the geological history of a cross-section, including faults, intrusions and unconformities.
A focused answer to the OCR H414 dot point on relative dating. Covers superposition, original horizontality, cross-cutting relationships, included fragments and faunal succession, way-up evidence, and how to apply these principles to reconstruct the geological history of a cross-section with faults, intrusions and unconformities.
- Fossils: the conditions that favour preservation (rapid burial, anoxia, hard parts, fine sediment); the modes of preservation (moulds and casts, permineralisation, carbonisation, and preservation in amber or ice); the properties of a good index (zone) fossil (abundant, widespread, easily recognised, short stratigraphic range); the distinction between body and trace fossils.
A focused answer to the OCR H414 dot point on fossils. Covers the conditions favouring preservation, the modes of preservation (moulds and casts, permineralisation, carbonisation, amber and ice), the properties of a good index or zone fossil, and the distinction between body and trace fossils.
- Evolution and the fossil record: evidence for evolution from the fossil record (morphological change through time, transitional forms, adaptive radiation); the models of evolutionary change (gradualism versus punctuated equilibrium); mass extinctions and their causes and effects (for example the end-Permian and end-Cretaceous events); the incompleteness and biases of the fossil record.
A focused answer to the OCR H414 dot point on evolution. Covers the fossil evidence for evolution (morphological change, transitional forms, adaptive radiation), the gradualism and punctuated equilibrium models, mass extinctions (the end-Permian and end-Cretaceous events) and their causes and effects, and the incompleteness and biases of the fossil record.
- Radiometric dating: radioactive decay of unstable parent isotopes to stable daughter isotopes; the concept of half-life as a constant; the use of parent-to-daughter ratios to calculate absolute ages; the main isotopic systems (uranium-lead, potassium-argon and carbon-14) and their suitable age ranges; the assumptions and limitations of radiometric dating; the combination of absolute and relative dating.
A focused answer to the OCR H414 dot point on radiometric dating. Covers radioactive decay of parent to daughter isotopes, half-life as a constant, calculating absolute ages from parent-to-daughter ratios, the uranium-lead, potassium-argon and carbon-14 systems and their ranges, the assumptions and limitations, and combining absolute with relative dating.
- Basin analysis: the definition of a sedimentary basin and the mechanisms of subsidence (thermal subsidence after lithospheric stretching, flexural loading and sediment loading); the concept of accommodation space and its control by subsidence and sea-level change; the main basin types (rift, passive-margin and foreland); the use of vertical facies successions and burial-history curves to reconstruct basin evolution.
A focused answer to the OCR H414 dot point on basin analysis. Covers the definition of a sedimentary basin, mechanisms of subsidence (thermal, flexural and sediment loading), accommodation space and its control by subsidence and sea level, the rift, passive-margin and foreland basin types, and the use of facies successions and burial-history curves to reconstruct basin evolution.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)