Notes · GeologyUK · A-Levels

Quaternary geology

This optional advanced theme examines the most recent slice of geological time, the Quaternary, in which repeated ice ages have shaped the landscape and the record of climate is exceptionally detailed. It covers the alternation of glacial and interglacial stages, the orbital (Milankovitch) controls on that alternation, the deposits and landforms left by ice, and the fossil and isotope proxies used to reconstruct Quaternary environmental change.

4 sections~14 min reading time3 competenciesLevel Standard 2 · Advanced 2

T·111111 / 13

Exam profile

AO1 · Describe the Quaternary climate record, its controls, deposits and proxy evidenceAO2 · Apply orbital-cycle and proxy concepts to interpret Quaternary evidenceAO3 · Analyse and evaluate proxy records of Quaternary environmental change

Operators:describeexplaininterpretdeduceanalyseevaluate

basic level

This is an optional advanced theme; a foundation grasp is the alternation of ice ages and the main glacial deposits.

higher level

The full theme requires you to link Milankovitch cycles to glacial-interglacial rhythms and to interpret proxy records critically.

Depth

Text

Contents · 4 sections

Quaternary geology

The Quaternary and glacial-interglacial cycles#

StandardWJEC/Eduqas A level Geology (Component 3, optional theme) — Quaternary geology

Glacial-interglacial cycles

Fig. 1Relative temperature through the late Quaternary (illustrative): repeated glacial (cold) and interglacial (warm) stages, with slow cooling and rapid warming on a roughly 100,000-year beat.

The Quaternary is the current geological period, beginning about 2.6 million years ago, and it is defined by the onset of repeated, large-scale glaciations in the northern hemisphere. It is not one continuous ice age but a series of cold glacial stages, in which great ice sheets advanced over northern continents, separated by warm interglacial stages like the one we live in now. This alternation of cold and warm climates, repeating many times, is the defining feature of the period and the reason its geology is dominated by the work of ice.

The evidence for these repeated glaciations is written across the landscape and in the sediments. Glacial deposits and landforms — till, moraines, erratics and striated rock surfaces — show that ice once covered regions now temperate; buried soils and interglacial deposits between glacial tills show the warm intervals; and the deep-sea and ice-core records preserve a continuous, finely resolved history of the climate oscillations. Because the Quaternary is so recent, its record is far more complete and detailed than that of any earlier period, making it a natural laboratory for studying climate change.

The glacial-interglacial cycles have a characteristic rhythm and shape. Over the last several hundred thousand years the dominant cycle has a period of roughly 100,000 years, and the cycles are saw-toothed: the climate cools slowly and irregularly into a glacial maximum over tens of thousands of years, then warms rapidly into an interglacial in only a few thousand (a rapid termination). Recognising this pattern — slow cooling, rapid warming, on a roughly 100,000-year beat — is important both for describing the record and for testing explanations of what drives it.

These cycles matter far beyond geology. They shaped the landscapes of northern Europe and North America, controlled the migrations and evolution of animals and early humans, and, because so much water was locked in ice sheets during glacials, lowered global sea level by over a hundred metres, exposing land bridges and changing coastlines. Understanding the Quaternary is also directly relevant to present-day climate change, because it reveals how the climate system behaves and how quickly it can switch state, which is why this theme connects the deep-time geology of the course to questions of immediate human concern.

Worked example

Interpreting a glacial record

A sequence of deposits shows glacial till, then a buried soil with temperate plant remains, then more glacial till. Interpret the climate history it records.

Read the tills
The two layers of glacial till indicate two separate periods when ice covered the area (glacial stages).
Read the buried soil
The buried soil with temperate plant remains between the tills indicates a warm interglacial stage when the ice had retreated and vegetation grew.
Assemble the history
The sequence records a glacial stage, then a warm interglacial, then a return to glacial conditions — the alternation typical of the Quaternary.

Result: The deposits record glacial, then interglacial, then glacial conditions, illustrating the Quaternary alternation of cold and warm stages.

Typical mistakes

Thinking the Quaternary was one long ice age; it is a series of glacials separated by warm interglacials, and we are in an interglacial now.
Describing the cycles as symmetrical; they are saw-toothed, with slow cooling and rapid warming.

Active revision

Describe the shape and rhythm of the late-Quaternary glacial-interglacial cycles and state one line of evidence that such cycles occurred.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — the Quaternary (British Geological Survey)

Controls on Quaternary climate: the Milankovitch cycles#

AdvancedWJEC/Eduqas A level Geology (Component 3, optional theme) — Quaternary geology

An orbital (Milankovitch) cycle

Fig. 2An orbital cycle idealised as a periodic forcing (here the ~100,000-year eccentricity beat); such astronomical periods appear in the Quaternary climate record.

The pacing of the glacial-interglacial cycles is explained by the Milankovitch theory, which holds that regular changes in the Earth's orbit and axis alter the amount and distribution of solar radiation reaching the Earth, and so trigger the advance and retreat of ice. The theory is astronomical: the changes are predictable consequences of the gravitational interactions of the Earth with the Sun, Moon and planets, and they combine to modulate, in particular, the summer sunshine at high northern latitudes, which controls whether winter snow survives the summer to build ice sheets.

There are three orbital cycles. Eccentricity is the slow change in the shape of the Earth's orbit from more circular to more elliptical and back, over a period of about 100,000 years; obliquity is the change in the tilt of the Earth's axis, between about 22 and 24.5 degrees, over about 41,000 years, which controls the strength of the seasons; and precession, the wobble of the axis (and the turning of the orbit), over about 21,000 to 23,000 years, changes which season occurs when the Earth is nearest the Sun. Each has its own period, and together they make a complex but predictable pattern of changing insolation.

The strongest support for the theory is that these astronomical periods are found in the climate record: the roughly 100,000-year, 41,000-year and 21,000-year rhythms of eccentricity, obliquity and precession all appear in the deep-sea and ice-core proxy records of the Quaternary. The dominance of the roughly 100,000-year cycle in the last several hundred thousand years matches the eccentricity period, and the presence of all three periods together is difficult to explain except by orbital forcing, which is why the Milankovitch theory is now widely accepted as the pacemaker of the ice ages.

The theory is nonetheless a subtle one and is not a complete explanation on its own. The orbital changes alter the distribution of sunshine far more than its total amount, so the climate response must be amplified by feedbacks within the Earth system — the growth of reflective ice and snow (which cools the planet further), changes in atmospheric carbon dioxide, and the response of the oceans — to turn a modest orbital nudge into a full glacial cycle. Evaluating how orbital forcing and internal feedbacks combine, and recognising that the 100,000-year dominance is still not fully understood, is exactly the critical AO3 thinking this advanced theme demands.

Worked example

Matching a rhythm to a cycle

A deep-sea proxy record of the last 500,000 years shows a strong repeating pattern with a period of about 100,000 years. Identify the orbital cycle responsible and explain the reasoning and its limitation.

Match the period
A period of about 100,000 years matches the eccentricity cycle (the change in the shape of the Earth's orbit).
State the support
Finding the eccentricity period in the climate record supports the Milankovitch theory that orbital changes pace the ice ages.
Note the limitation
Eccentricity changes total insolation only slightly, so its strong effect on climate implies powerful feedbacks (ice-albedo, carbon dioxide) amplify the orbital signal.

Result: The 100,000-year rhythm matches eccentricity, supporting orbital forcing, though feedbacks are needed to amplify so weak a forcing into full glacials.

Typical mistakes

Confusing the three cycles or their periods; eccentricity is about 100 kyr, obliquity about 41 kyr and precession about 21 kyr.
Treating orbital forcing as sufficient alone; it mainly redistributes sunshine and must be amplified by ice-albedo and carbon-dioxide feedbacks.

Active revision

Explain how the roughly 100,000-year rhythm of the recent glacial cycles supports the Milankovitch theory, and why feedbacks are invoked.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — climate change through time (British Geological Survey)

Quaternary deposits and landforms#

StandardWJEC/Eduqas A level Geology (Component 3, optional theme) — Quaternary geology

Glacial deposits and landforms

Fig. 3Quaternary deposits: ice-laid material is unsorted (till, moraines, erratics, striations); meltwater sorts its load (outwash); wind lays down loess.

The advance and retreat of ice left a distinctive suite of deposits and landforms that record the glaciations. The material deposited directly by ice is till (formerly 'boulder clay'), which is diagnostic because it is completely unsorted — a chaotic mixture of clay, sand and angular boulders all dumped together — since ice, unlike water or wind, does not sort its load by size. Ridges of till dumped at the ice margin form moraines, which mark the former positions of the ice front and so trace its advance and retreat.

Meltwater from the ice sorts and redeposits sediment, producing deposits quite different from till. Outwash (glaciofluvial sand and gravel), laid down by meltwater streams beyond the ice, is well sorted and bedded, because flowing water grades its load — the opposite of unsorted till. Distinguishing till from outwash is a key skill: unsorted, angular, mixed material is glacial (deposited by ice), while sorted, bedded, rounded material is glaciofluvial (deposited by meltwater), and the two together record the ice and its meltwater.

Several features record the direction and former extent of the ice. Erratics are boulders carried by ice and dropped far from their source, sometimes hundreds of kilometres, so matching an erratic to its distant parent rock shows the path the ice took. Striations are scratches gouged into bedrock by rock fragments frozen into the base of the moving ice, and their orientation records the direction of ice flow. Beyond the ice, cold, dry, windy conditions blew fine silt into thick blankets of loess. Together these deposits and landforms let a geologist map former ice sheets and their flow.

The value of all this is that the Quaternary landscape can be read, using uniformitarian comparison with modern glaciers, to reconstruct the ice ages in detail — where the ice sheets lay, which way they flowed, how far they reached and how they retreated. Because the deposits are young and unlithified they are also of great practical importance, forming much of the soil, aggregate and groundwater resource of formerly glaciated regions and posing engineering challenges (weak, variable ground). Reading glacial deposits is thus both a scientific and an applied skill within this theme.

Worked example

Distinguishing glacial deposits

In a former glaciated valley a geologist finds an unsorted mixture of clay, sand and angular boulders overlain by well-sorted, cross-bedded sands and gravels. Identify each deposit and reconstruct the sequence of events.

Identify the lower deposit
The unsorted mixture of clay, sand and angular boulders is till, deposited directly by ice.
Identify the upper deposit
The well-sorted, cross-bedded sand and gravel is outwash, deposited by meltwater streams that sorted the sediment.
Reconstruct
Ice occupied the valley and laid down the till; as the ice retreated, meltwater streams deposited sorted outwash on top.

Result: The lower deposit is ice-laid till, the upper is meltwater outwash; the sequence records glaciation then retreat with meltwater deposition.

Typical mistakes

Expecting glacial till to be sorted; ice does not sort its load, so till is a chaotic, unsorted mixture, unlike water-laid outwash.
Confusing erratics (transported boulders) with in-place bedrock; an erratic is identified by not matching the local rock.

Active revision

A deposit is a chaotic, unsorted mixture of clay and angular boulders; a nearby deposit is well-sorted, bedded sand and gravel. Identify each and the agent that laid it.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — glacial deposits (British Geological Survey)

Proxy evidence for environmental change#

AdvancedWJEC/Eduqas A level Geology (Component 3, optional theme) — Quaternary geology

Quaternary climate proxies

Fig. 4Quaternary proxies each reconstruct part of the past environment: pollen (vegetation/climate), beetles (temperature), foraminifera and oxygen isotopes (sea conditions and global ice volume).

Because the Quaternary is so recent, it preserves a wealth of biological and chemical proxies that record climate and environment in fine detail, often decade by decade. Fossil pollen preserved in lake and bog sediments is one of the most powerful: because different plants live in different climates and each has distinctive pollen, counting the pollen types through a sediment core reconstructs the changing vegetation, and hence the climate, as the ice advanced and retreated. Pollen diagrams are a standard tool of Quaternary science.

Other biological proxies add temperature and environmental detail. Fossil beetles (Coleoptera) are excellent thermometers because many species have narrow, well-known temperature tolerances and responded quickly to climate change, so a beetle assemblage gives a rapid, precise temperature signal. Fossil foraminifera and other microfossils in ocean sediments record sea-surface conditions through the species present and, crucially, through the chemistry of their shells, providing a continuous marine record to complement the terrestrial one.

The most quantitative proxy is the oxygen-isotope record. Sea water contains two stable oxygen isotopes, the common lighter oxygen-16 and the rarer heavier oxygen-18; during glacials the lighter isotope is preferentially evaporated and locked up in ice sheets, so the remaining ocean, and the calcite shells of foraminifera growing in it, become enriched in the heavier isotope. Measuring the oxygen-18 to oxygen-16 ratio in stacked foraminifera shells down a deep-sea core therefore gives a continuous proxy for global ice volume and temperature, and it is this record that reveals the orbital periods so clearly.

As always, the securest reconstructions combine several proxies and are read critically. Each proxy has strengths and limitations — pollen can be blown far from its source, beetles give temperature but not precipitation, isotopes integrate a global signal rather than a local one — so pollen, beetles, isotopes and the sediments themselves are cross-checked against one another. Interpreting a proxy record, understanding what each proxy does and does not record, and weighing them together, is the demanding analytical skill this final section develops, and it ties the Quaternary theme back to the climate-proxy reasoning of the past-life chapter.

Worked example

Interpreting an isotope proxy

In a deep-sea core, one interval shows foraminifera shells with a high oxygen-18 to oxygen-16 ratio. Interpret the climate at that time and state one check on the interpretation.

Recall the mechanism
During glacials, light oxygen-16 is preferentially locked into ice sheets, so the ocean and the foraminifera shells become enriched in heavy oxygen-18.
Interpret
A high oxygen-18 ratio therefore indicates a large global ice volume and a cold, glacial climate at that time.
Add a check
The interpretation can be cross-checked against pollen (cold-climate vegetation) or beetle (low-temperature) assemblages of the same age for consistency.

Result: High oxygen-18 indicates a glacial (large ice volume); cross-checking with pollen or beetle proxies tests the interpretation.

Typical mistakes

Assuming a single proxy gives the whole picture; each records a different aspect and has limitations, so proxies are combined.
Getting the isotope logic backwards; glacials lock light oxygen-16 in ice, so ocean shells become enriched in heavy oxygen-18.

Active revision

Explain how a rise in the oxygen-18 content of foraminifera shells down a deep-sea core would be interpreted, and why beetle or pollen data might be examined alongside it.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — Quaternary environments (British Geological Survey)

References & sources

Sources

WJEC / Eduqas

WJEC/Eduqas A level Geology specification

British Geological Survey

Quaternary geology

4 sections~14 min reading time3 competenciesLevel Standard 2 · Advanced 2

Quaternary geology

Key points

Interpreting a glacial record

Key points

Matching a rhythm to a cycle

Key points

Distinguishing glacial deposits

Key points

Interpreting an isotope proxy

Sources

Quaternary geology

Key points

Interpreting a glacial record

Key points

Matching a rhythm to a cycle

Key points

Distinguishing glacial deposits

Key points

Interpreting an isotope proxy

Sources