Notes · GeologyUK · A-Levels

Geological evolution of Britain

This optional advanced theme reads the geological history of Britain from its rocks. It covers Britain's assembly from separate crustal terranes as it drifted across the globe, the Caledonian orogeny that closed the Iapetus Ocean, the Variscan orogeny and the Coal Measures, and the Mesozoic-Cenozoic cover rocks and the distant effects of the Alpine orogeny, illustrating how plate tectonics, stratigraphy and climate proxies combine into a national history.

4 sections~14 min reading time3 competenciesLevel Standard 4

T·121212 / 13

Exam profile

AO1 · Describe the main orogenies and the outline stratigraphy of BritainAO2 · Apply plate-tectonic and stratigraphic principles to Britain's historyAO3 · Interpret and evaluate the evidence for Britain's changing environments and tectonic history

Operators:describeexplaininterpretdeduceanalyseevaluate

basic level

This is an optional advanced theme; a foundation grasp is the names and order of the three main orogenies and the broad British succession.

higher level

The full theme requires you to reconstruct Britain's drift and changing environments from its rocks and to link them to plate-tectonic events.

Depth

Text

Contents · 4 sections

Geological evolution of Britain

Britain's drift and terrane assembly#

StandardWJEC/Eduqas A level Geology (Component 3, optional theme) — Geological evolution of Britain

Britain's tectonic milestones

Fig. 1The tectonic milestones of Britain's history: the Iapetus Ocean, then its closure in the Caledonian orogeny, the Variscan orogeny, and the distant Alpine orogeny.

Britain has not always existed as a single piece of crust in its present position; it was assembled from separate fragments, or terranes, that were brought together by plate movements, and it has drifted across the globe from far south of the Equator to its present temperate latitude. This journey is recorded directly in its rocks, because the sediments and fossils laid down at each stage carry the fingerprint of the climate and latitude at the time — desert sandstones from arid belts, coals from the tropics, chalk from warm shallow seas — so the rocks of Britain are a logbook of a long voyage.

In the early Palaeozoic the crust that would become Britain was split between two continents separated by a wide ocean, the Iapetus Ocean. The rocks now forming Scotland and north-west Ireland lay on the margin of the continent Laurentia (with North America), while those forming England and Wales lay on a separate microcontinent, Avalonia, on the other side of the ocean. The differences in the early Palaeozoic rocks and fossils on either side of the Iapetus suture, which runs across northern Britain, are the evidence that Scotland and England had genuinely separate origins before they were joined.

The assembly of Britain happened as this ocean closed and the terranes collided during a series of mountain-building events (orogenies). Three orogenies dominate the story: the Caledonian, which closed the Iapetus Ocean and welded the northern and southern halves together in the early Palaeozoic; the Variscan (Hercynian), which deformed southern Britain in the late Palaeozoic; and the far-off Alpine, which gently folded southern England in the Cenozoic. Each orogeny left its structures and its mountains, and between them Britain accumulated the cover of sediments that records its drift.

Reading this history uses every principle of the course: relative and absolute dating to order the events, stratigraphy and correlation to trace the succession, structures to identify the orogenies, and climate proxies to reconstruct the changing environment. The result is a coherent narrative in which plate tectonics moves Britain across climate belts while orogenies periodically crumple and raise it, and the sediments deposited in between record both the journey and the mountain-building. This synoptic reading of a national geology is the essence of the theme.

Worked example

Evidence for separate origins

Early Palaeozoic rocks in Scotland and in England contain different fossil faunas either side of a line (the Iapetus suture) across northern Britain. Explain what this indicates.

Interpret the difference
Different contemporaneous faunas indicate the two areas were separated by a barrier to migration when the rocks formed.
Identify the barrier
The barrier was the wide Iapetus Ocean, with Scotland on Laurentia and England on Avalonia on opposite shores.
Conclude
The suture marks where the ocean finally closed, joining two areas that had genuinely separate origins.

Result: The contrasting faunas show Scotland and England lay on opposite sides of the Iapetus Ocean and were only later joined along the suture.

Typical mistakes

Assuming Britain has always been one landmass; Scotland and England had separate origins on opposite sides of the Iapetus Ocean.
Placing the orogenies in the wrong order; the sequence is Caledonian, then Variscan, then Alpine.

Active revision

Explain what the differences between the early Palaeozoic rocks of Scotland and of England and Wales tell us about Britain's origins.

Active recall

Recall the key points — then reveal.

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

The Caledonian orogeny#

StandardWJEC/Eduqas A level Geology (Component 3, optional theme) — Geological evolution of Britain

Closure of the Iapetus Ocean

Fig. 2The Caledonian orogeny: the Iapetus Ocean closes by subduction until Laurentia (Scotland) and Avalonia (England) collide, raising the Caledonian mountains.

The Caledonian orogeny was the mountain-building event that closed the Iapetus Ocean and welded the two halves of Britain together, reaching its climax around the end of the Silurian and into the early Devonian. As the ocean closed by subduction, its floor was consumed, its sediments were scraped up and deformed, and finally the two continental margins collided, crumpling the intervening rocks into a great mountain chain — the Caledonides — whose eroded roots form the Scottish Highlands, the Lake District, Wales and much of Ireland and Scandinavia.

The rocks record the ocean's history in order. Through the Cambrian, Ordovician and Silurian, thick marine sediments (including the graptolitic mudstones met as zone fossils) accumulated in and around the Iapetus Ocean; volcanic rocks record the subduction that consumed its floor (as in the Lake District and Snowdonia); and the intense folding, faulting, cleavage and regional metamorphism of these Lower Palaeozoic rocks record the final collision. Granite intrusions, generated by the melting associated with the collision, were emplaced into the deforming crust.

The immediate aftermath is written in the Old Red Sandstone. Once the ocean had closed and the mountains had risen, Britain lay within a large continent in an arid belt south of the Equator, and rivers draining the new Caledonian mountains spread continental red sandstones and conglomerates — the Old Red Sandstone — across the Devonian landscape. The red colour (oxidised iron) and the desert and river deposits record a hot, dry climate, and the coarse sediments record the erosion of the freshly raised mountains, so the Old Red Sandstone is the molasse (erosional debris) of the Caledonian orogeny.

The Caledonian orogeny is therefore a complete case study in reading plate tectonics from the rock record: the marine sediments and volcanics record an ocean and its subduction, the deformation and metamorphism record a continental collision, and the overlying Old Red Sandstone records the erosion of the resulting mountains under a known climate and latitude. Recognising this sequence, and dating it by its position between the marine Silurian below and the continental Devonian above, exercises the stratigraphic and structural reasoning of the whole course on a real, mapped example.

Worked example

Reading the Caledonian record

In Wales, folded and cleaved marine Silurian mudstones are overlain unconformably by red continental sandstones of Devonian age. Reconstruct the events.

The marine phase
The Silurian marine mudstones were deposited in the Iapetus Ocean before it closed.
The orogeny
Their intense folding and cleavage, and the unconformity above them, record the Caledonian collision that closed the ocean, raised mountains, and was followed by uplift and erosion.
The aftermath
The overlying red continental Old Red Sandstone records rivers and deserts draining the new mountains in an arid climate south of the Equator.

Result: The sequence records marine deposition, then the Caledonian collision (folding, cleavage, unconformity), then continental Old Red Sandstone shed from the new mountains.

Typical mistakes

Treating the Old Red Sandstone as marine; it is a continental (river and desert) deposit recording an arid climate after the ocean closed.
Placing the Caledonian orogeny in the wrong interval; its climax is around the Silurian-Devonian boundary.

Active revision

Explain how a sequence of marine Silurian mudstones, intensely folded and overlain unconformably by continental Old Red Sandstone, records the Caledonian orogeny.

Active recall

Recall the key points — then reveal.

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

The Variscan orogeny and the Coal Measures#

StandardWJEC/Eduqas A level Geology (Component 3, optional theme) — Geological evolution of Britain

The Carboniferous succession

Fig. 3The Carboniferous of Britain records a tropical world: shallow-sea limestone at the base, deltaic gritstone above, and swamp-forest Coal Measures at the top.

Through the Carboniferous, Britain lay on or near the Equator, and its rocks record a warm, tropical world in a clear upward succession. At the base, the Carboniferous Limestone was deposited in warm, clear, shallow tropical seas, full of corals and other shelly fossils. Above it, the Millstone Grit records the advance of great river deltas bringing sand from the north into the sea. Above that, the Coal Measures record the coastal swamp forests that grew on the delta tops, whose accumulated plant matter became the coal seams. This upward change from limestone to gritstone to coal is a classic reconstructable succession.

The Coal Measures are of enormous historical and economic importance, because their coal fuelled the Industrial Revolution and shaped the geography of industrial Britain. The coals formed from the lush vegetation of equatorial swamps, and the repeated alternation of coal, mudstone, sandstone and marine bands (cyclothems) records the repeated advance and retreat of the sea over the low-lying delta, each cycle a swamp drowned and re-established. The plant fossils and the coal itself confirm the hot, humid, equatorial climate of Carboniferous Britain.

The Carboniferous ended with the Variscan (or Hercynian) orogeny, a mountain-building event, driven by the assembly of the supercontinent Pangaea, that deformed southern Britain in the late Carboniferous and Permian. It folded and faulted the rocks of south-west England and south Wales, emplaced the granites of Cornwall and Devon (with their associated tin and copper mineralisation), and structured the coalfields. Its effects were strongest in the south, so that southern Britain bears the Variscan imprint while the north preserves the older Caledonian grain.

After the Variscan orogeny, Britain drifted into the arid subtropics of Pangaea, and the Permian and Triassic are recorded by the New Red Sandstone — desert dune sandstones, river deposits and evaporites laid down under a hot, dry climate, the counterpart of the earlier Old Red Sandstone but following the Variscan rather than the Caledonian mountains. Reading the Carboniferous succession and the Variscan structures, and the desert deposits that followed, again exercises the whole toolkit of the course on the well-documented geology of Britain.

Worked example

Reconstructing the Carboniferous

A Carboniferous sequence passes upward from coral-rich limestone, through cross-bedded sandstone, into coal seams with fossil trees. Reconstruct the environmental history.

The limestone
Coral-rich limestone indicates a warm, clear, shallow tropical sea.
The sandstone
Cross-bedded sandstone above indicates advancing river deltas bringing sand into the sea.
The coal
Coal with fossil trees indicates swamp forests on the delta tops, in a hot, humid, equatorial climate.
Synthesis
The sequence records tropical Britain shallowing from a clear sea, through deltas, to coastal swamp forests.

Result: The succession records a shallowing from a warm tropical sea (limestone), through deltas (gritstone), to equatorial swamp forests (coal).

Typical mistakes

Getting the Carboniferous succession upside down; it passes upward from shallow-sea limestone, through deltaic gritstone, to swamp Coal Measures.
Attributing the deformation of northern Scotland to the Variscan; the Variscan mainly affected southern Britain, the north being Caledonian.

Active revision

Explain how the upward change from Carboniferous Limestone to Coal Measures records a changing environment in tropical Britain.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — Carboniferous and coal (British Geological Survey)

The Mesozoic-Cenozoic cover and the Alpine orogeny#

StandardWJEC/Eduqas A level Geology (Component 3, optional theme) — Geological evolution of Britain

Britain's journey through the climate belts

Fig. 4Britain's drift recorded in its rocks: Devonian and Permo-Triassic deserts, Carboniferous tropical swamps, the Cretaceous Chalk sea, and Quaternary ice.

Through the Mesozoic, as Britain drifted northward out of the deserts and the sea returned, a cover of largely marine sediments was laid down across southern and eastern Britain, recording a warm, often shallow-sea world. The Jurassic is represented by alternating clays and limestones rich in ammonites and marine reptiles, deposited in shelf seas; the Cretaceous famously ends in the Chalk, a remarkably pure limestone of microscopic plankton (coccoliths) laid down slowly over a wide, warm, clear sea that covered much of Britain. These cover rocks lie relatively undisturbed on the deformed Palaeozoic basement.

The Cenozoic brought renewed activity. As the North Atlantic Ocean began to open, igneous activity affected north-west Britain in the early Paleogene, producing the basalt lava flows and central volcanoes preserved in Skye, Mull, Antrim and the Giant's Causeway — the British part of a great volcanic province marking the birth of the ocean. This igneous episode is a reminder that plate tectonics continued to shape Britain long after the great orogenies, and it links the national story to the opening of an ocean still widening today.

The last deformation to affect Britain was the distant Alpine orogeny, caused by the collision of Africa with Europe that raised the Alps in the Cenozoic. Its effects on Britain were mild because Britain lay far from the collision: it gently folded and uplifted southern England, producing broad structures such as the Weald-Artois anticline (whose erosion formed the scenery of the Weald) and the shallow London and Hampshire basins. The contrast between the gentle Alpine folding of the young cover rocks and the intense Caledonian and Variscan deformation of the older rocks is itself instructive.

The finishing touches came in the Quaternary, when the ice ages of the previous theme sculpted the present landscape, deposited till and outwash, and, through the erosion of the last glaciation and the post-glacial rise of sea level, cut Britain off from the continent to form an island. Read as a whole, the geology of Britain is a continuous narrative of drift across climate belts punctuated by orogenies and, finally, ice — a narrative recoverable entirely from the rocks by the methods of this course, which is why it makes such a fitting synoptic theme.

Worked example

Reconstructing Britain's drift from its rocks

Britain's rocks pass from Devonian Old Red Sandstone deserts, through Carboniferous tropical coals, to Permo-Triassic desert red beds. Deduce how Britain's latitude changed and justify it.

Devonian deserts
The arid Old Red Sandstone indicates Britain lay in a hot, dry belt south of the Equator in the Devonian.
Carboniferous tropics
The tropical limestones and equatorial coal swamps indicate Britain had drifted onto the Equator by the Carboniferous.
Permo-Triassic deserts
The New Red Sandstone deserts indicate Britain had drifted into the arid belt north of the Equator by the Permian and Triassic.
Conclude
The succession of desert, tropical, then desert climates records Britain drifting steadily northward across the climate belts.

Result: The desert-tropical-desert succession records Britain drifting northward from south of the Equator, across it, and into the northern arid belt.

Typical mistakes

Expecting intense folding from the Alpine orogeny in Britain; its distant effects were gentle folds and basins in the south, not mountains.
Assuming the Chalk is a hard crystalline limestone; it is a soft, pure limestone of microscopic plankton laid down in a warm, clear sea.

Active revision

Using the rock record, explain how Britain's climate changed from the Devonian to the Cretaceous and what this implies about its latitude.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — Mesozoic and Cenozoic Britain (British Geological Survey)

References & sources

Sources

WJEC / Eduqas

WJEC/Eduqas A level Geology specification

British Geological Survey

British Geological Survey — the geology of Britain

Geological evolution of Britain

4 sections~14 min reading time3 competenciesLevel Standard 4

Geological evolution of Britain

Key points

Evidence for separate origins

Key points

Reading the Caledonian record

Key points

Reconstructing the Carboniferous

Key points

Reconstructing Britain's drift from its rocks

Sources

Geological evolution of Britain

Key points

Evidence for separate origins

Key points

Reading the Caledonian record

Key points

Reconstructing the Carboniferous

Key points

Reconstructing Britain's drift from its rocks

Sources