A history of the plate tectonics in Britain (Part 2): When mountains fall – collapse, basins and the foundations of Britain’s lowlands

Jon Trevelyan (UK)

The collision that assembled Britain at the end of the Silurian (see A history of the plate tectonics of Britain (part 1): Britain assembled – oceans, collisions and the making of a geological patchwork) did not mark the end of tectonic influence on the landscape. Instead, it marked the beginning of a new phase – one that is often less intuitively grasped, but just as important. Mountain belts are temporary features. Once continental collision ceases, thickened crust becomes gravitationally unstable. Erosion strips material from the surface, while extension and faulting affect the deep roots of the orogen. The geological record that follows is not one of rising peaks, but of collapse, subsidence and redistribution.

In Britain, the decay of the Caledonian mountains shaped the foundations of much of the country’s later geology. Vast volumes of sediment were shed from eroding uplands into adjacent basins; new fault systems developed as the crust relaxed and stretched; and long-lived structural weaknesses were established that would be reactivated repeatedly during later tectonic episodes. Many of Britain’s most familiar lowland landscapes – and some of its most fossil-rich rocks – owe their existence to this period of tectonic collapse (Fig. 2.1).

Fig. 2.1. The Somerset Levels, southwest England. The low-lying floodplains of the Somerset Levels occupy a long-lived zone of subsidence controlled by ancient fault systems established during post-Caledonian tectonic collapse and repeatedly reactivated during later phases of extension. Although the modern landscape has been shaped by Holocene sea-level change, sedimentation and human drainage, its broad low relief reflects deep structural inheritance rooted in Devonian–Carboniferous basin development. (*Photo: Bill Nicholls, Wikimedia Commons, licensed under Creative Commons Attribution–ShareAlike 2.0 Generic (CC BY-SA 2.0).* *https://creativecommons.org/licenses/by-sa/2.0/*.)

After collision: collapse of the Caledonian mountain belt

By the early Devonian, continental collision between Laurentia and Avalonia had ended. Although the Caledonian mountains were still high, they were already beginning to collapse under their own weight. Thickened crust spread laterally, while ongoing erosion rapidly removed material from the surface. Compression gave way to extension across much of Britain.

Large, fault-bounded basins developed within and adjacent to the former mountain belt. These basins trapped immense quantities of sediment derived from the eroding uplands, producing the classic Old Red Sandstone successions of Scotland and parts of northern England and Wales (see box: The Old Red Sandstone Continent; Figs. 2.2 and 2.4). Deposition took place in river systems, alluvial fans and ephemeral lakes rather than marine environments, a change reflected in the fossil record by the scarcity of marine fossils and the presence of early freshwater fish and plant fragments (see, for example, Plants, fish and floodplains: The Heol Senni story).

Fig. 2.2. Llyn Fan Fach and the Carmarthen Fan escarpment, Black Mountain in the western Brecon Beacons (Bannau Brycheiniog), Powys, Wales. Broad escarpments and gently dipping strata in the Brecon Beacons are carved largely from Devonian Old Red Sandstone deposited in continental basins that developed following collapse of the Caledonian mountain belt. These red-bed successions record erosion of the former uplands and redistribution of sediment across fault-bounded basins, marking a shift from mountain building to landscape stabilisation and basin infill during the post-Caledonian period. (*Photo: SNappa2006 Wikimedia Commons, licensed under Creative Commons Attribution 2.0 Generic (CC BY 2.0).* *https://creativecommons.org/licenses/by/2.0/*.)

The Old Red Sandstone Continent
The term Old Red Sandstone does not refer to a single rock formation, but to a vast suite of predominantly continental sedimentary deposits laid down during the Devonian Period. These deposits accumulated across much of the newly assembled continent of Laurussia (sometimes called the Old Red Sandstone Continent), formed following the closure of the Iapetus Ocean and the Caledonian collision. As the Caledonian mountain belt began to collapse and erode, large fault-bounded basins developed across Laurussia (Fig. 2.3). Rivers draining the uplands transported enormous volumes of sand, gravel and mud into these basins, where they accumulated as river-channel deposits, floodplain sediments, alluvial fans, and ephemeral lake beds. The characteristic red coloration reflects oxidation of iron-bearing minerals in generally oxidising continental conditions. The fossil record of the Old Red Sandstone reflects these continental conditions. Marine fossils are rare or absent, while assemblages are dominated by early freshwater fish, arthropods, and fragmentary remains of early land plants. Together, these deposits record a critical interval in Earth history, when landscapes, ecosystems and sedimentary systems were increasingly shaped by life on land rather than in the sea. Although best known from Scotland (Fig. 2.4), the Old Red Sandstone represents a continent-scale geological episode, linking basins across what are now Britain, Scandinavia, Greenland and North America, and marking a fundamental shift in both sedimentation style and palaeoecology.

The Old Red Sandstone Continent

The term Old Red Sandstone does not refer to a single rock formation, but to a vast suite of predominantly continental sedimentary deposits laid down during the Devonian Period. These deposits accumulated across much of the newly assembled continent of Laurussia (sometimes called the Old Red Sandstone Continent), formed following the closure of the Iapetus Ocean and the Caledonian collision.

As the Caledonian mountain belt began to collapse and erode, large fault-bounded basins developed across Laurussia (Fig. 2.3). Rivers draining the uplands transported enormous volumes of sand, gravel and mud into these basins, where they accumulated as river-channel deposits, floodplain sediments, alluvial fans, and ephemeral lake beds. The characteristic red coloration reflects oxidation of iron-bearing minerals in generally oxidising continental conditions.

The fossil record of the Old Red Sandstone reflects these continental conditions. Marine fossils are rare or absent, while assemblages are dominated by early freshwater fish, arthropods, and fragmentary remains of early land plants. Together, these deposits record a critical interval in Earth history, when landscapes, ecosystems and sedimentary systems were increasingly shaped by life on land rather than in the sea.

Although best known from Scotland (Fig. 2.4), the Old Red Sandstone represents a continent-scale geological episode, linking basins across what are now Britain, Scandinavia, Greenland and North America, and marking a fundamental shift in both sedimentation style and palaeoecology.

Fig. 2.3. Post-orogenic collapse and basin formation. Schematic cross-section showing the collapse of a thickened mountain belt following continental collision. Extension and normal faulting thin the crust, creating fault-bounded basins that subside and fill with sediment. This process underpins the development of many Devonian and later sedimentary basins in Britain. (Diagram is schematic and not to scale.)

Fig. 2.4. The Old Man of Hoy, Orkney. The Old Man of Hoy is carved from thick, gently dipping Devonian Old Red Sandstone of the Orcadian Basin. These continental sandstones were deposited long after Caledonian mountain building, recording erosion of the former uplands and accumulation of sediment within a broad intracratonic basin. The dramatic sea stack is a product of later coastal erosion that exploits jointing and bedding within the basin fill, revealing the massive character of the Old Red Sandstone rather than any active tectonic deformation. (*Photo: Dave Wheeler, Wikimedia Commons, licensed under Creative Commons Attribution–ShareAlike 2.0 Generic (CC BY-SA 2.0).* *https://creativecommons.org/licenses/by-sa/2.0/*.)

Although often treated simply as a sedimentary package, the Old Red Sandstone represents a profound tectonic shift. It marks the transition from an active collisional mountain belt to a region dominated by erosion, subsidence and internal reorganisation.

Crucially, many of the fault systems and basin geometries established during Old Red Sandstone deposition did not disappear at the end of the Devonian, but provided the structural framework upon which Carboniferous basins later developed.

Inherited structure and the control of later basins

One of the most important consequences of post-Caledonian collapse was the creation of inherited weaknesses in the crust. Faults that formed during Devonian extension did not disappear once activity waned. Instead, they became zones of weakness repeatedly reactivated during later phases of tectonism.

One of the clearest signs that post-Caledonian collapse left a lasting imprint on Britain is the continued visibility of ancient fault systems in the modern landscape. Major structures such as the Great Glen Fault (Fig. 2.5) and the Highland Boundary Fault originated during Caledonian times, but were repeatedly reactivated during later episodes of extension and compression. These long-lived weaknesses helped control the development of sedimentary basins in Devonian and Carboniferous times and still influence topography today, with valleys, lochs and upland boundaries commonly following their traces.

Fig. 2.5. Loch Ness, Scottish Highlands. Loch Ness occupies a long, remarkably straight valley aligned with the Great Glen Fault, a major crustal fracture that has been repeatedly reactivated since Caledonian times. Although the present landscape was strongly modified by later glacial erosion, the linear form of the loch reflects the underlying tectonic structure, illustrating how ancient faults inherited from mountain building continued to control basin development, drainage and topography long after active collision had ceased. (*Photo:* Anne Burgess, Wikimedia Commons, licensed under Creative Commons Attribution–ShareAlike 2.0 Generic (CC BY-SA 2.0). *https://creativecommons.org/licenses/by-sa/2.0/*.)

Further south, inherited fault systems beneath northern England guided the formation of Carboniferous basins and later uplift of the Pennines, while subtler reactivation of deep-seated faults in southern Britain during much more recent Cenozoic Alpine-related compression (see A history of the plate tectonics of Britain (part 3): Britain breaks apart – rifting, volcanoes and the birth of the Atlantic) contributed to structures such as the Weald. Together, these examples show that faults created during post-orogenic collapse did not fade into geological obscurity, but remained fundamental organising elements in Britain’s tectonic and landscape evolution.

This inherited framework exerted a powerful control on Britain’s later sedimentary basins, particularly during the Carboniferous. Many basins developed along earlier fault systems, subsiding to receive thick successions of sandstone, shale, limestone and coal. As sea levels fluctuated and climates changed, these basins alternated between shallow tropical seas and low-lying coastal plains.

The fossil record reflects this tectonic rhythm. Warm, shallow seas produced the (Lower) Carboniferous limestones rich in corals, brachiopods and crinoids, while periods of terrestrial deposition preserved Upper Carboniferous coal measures dominated by fossil plants. The repeated alternation of marine limestones and coal-bearing strata is therefore best understood as a product of tectonically controlled subsidence, rather than simple global sea-level change (Fig. 2.6).

Fig. 2.6. Fault inheritance and Carboniferous basin development. Block diagram illustrating how inherited faults control differential uplift and subsidence during the Carboniferous. Some crustal blocks remain high, while adjacent basins repeatedly subside and accumulate sediment, including coal-bearing strata. This tectonic inheritance explains the patchy distribution of British coalfields and fossil-rich successions. (Diagram is schematic and not to scale.)

Southern Britain and the Variscan squeeze

While northern Britain was dominated by post-Caledonian collapse, southern Britain experienced a very different tectonic fate. During the late Carboniferous, the southern margin of Laurussia was compressed as Gondwana moved northwards, culminating in the Variscan (Hercynian) Orogeny. This episode affected southwest England, South Wales and parts of southern England, folding and faulting existing sedimentary successions (Fig. 2.7).

Fig. 2.7. Variscan compression in southern Britain. Simplified cross-section showing late Carboniferous horizontal compression during the Variscan Orogeny. Sedimentary rocks are tightly folded at the surface, while crustal thickening and partial melting at depth lead to granite intrusion beneath southwest Britain. Surface folds provide the structural context for classic coastal exposures such as Hartland Quay. (Diagram is schematic and not to scale.)

In contrast to the Caledonian Orogeny, Variscan mountain building was geographically restricted, but its effects were intense. Carboniferous sandstones, shales and limestones were shortened into tight folds and thrust structures, particularly in coastal southwest Britain. These structures provide some of the clearest surface expressions of continental compression anywhere in the UK.

One of the most striking examples occurs at Hartland Quay (Fig. 2.8), on the north Devon coast. Here, steeply inclined and tightly folded Carboniferous strata (approximately 320 million years old) are exposed in dramatic cliff sections and wave-cut platforms. The geometry of the folds records intense horizontal shortening during the Variscan orogeny, offering a rare opportunity to see plate-scale collision expressed directly at outcrop scale.

Fig. 2.8. Rocky shoreline at Hartland Quay, north Devon. Steeply inclined and tightly folded Carboniferous sandstones and mudstones exposed at Hartland Quay record sedimentation within a deep marine basin on the southern margin of Britain, followed by intense deformation during the Variscan (Hercynian) Orogeny. These rocks contrast sharply with the gently dipping Old Red Sandstone basin fills further north, illustrating how post-Caledonian Britain evolved into a patchwork of basins and fold belts shaped by successive phases of extension, sedimentation and later compression. (*Photo: Roger Kid*d, Wikimedia Commons, licensed under Creative Commons Attribution–ShareAlike 2.0 Generic (CC BY-SA 2.0). *https://creativecommons.org/licenses/by-sa/2.0/*.)

Granite intrusions and uplifted uplands

At depth, Variscan compression led to partial melting of the lower crust and the emplacement of large granitic bodies beneath southwest Britain. Although these granites crystallised several kilometres below the surface, later erosion has exposed them as prominent uplands.

The granite massifs of Dartmoor (Fig. 2.9) and Bodmin Moor are products of this process. Their resistance to erosion explains why they now stand high relative to surrounding sedimentary rocks, forming distinctive moorland landscapes. The heat associated with granite emplacement also drove mineralisation, underpinning the long mining history of southwest England.

Fig. 2.9. Laughter Tor, Dartmoor, Devon. Blocky, jointed granite exposed at Laughter Tor forms part of the Dartmoor granite batholith, emplaced during late Carboniferous Variscan tectonism. These intrusive igneous rocks record the thermal and magmatic consequences of continental collision in southern Britain, rather than deformation by folding. The characteristic tor landscape reflects deep chemical weathering of the granite followed by long-term erosion and stripping, linking late Palaeozoic tectonic processes to a distinctive modern landscape. (*Photo: Nigel Cox, Wikimedia Commons, licensed under Creative Commons Attribution–ShareAlike 2.0 Generic (CC BY-SA 2.0).* *https://creativecommons.org/licenses/by-sa/2.0/*.)

Here again, tectonics provides the key – without deep crustal melting and intrusion during the Variscan orogeny, these iconic landscapes would not exist.

Fig. 2.10. Disused tin mine near St Agnes, Cornwall. Ruined mine buildings on the north Cornish coast mark the surface expression of mineralisation associated with the Cornubian granite batholith, emplaced during late Carboniferous Variscan tectonism. Heat and fluids released during granite intrusion produced extensive tin and copper mineralisation in the surrounding country rocks, later exposed by prolonged uplift and erosion. The prominence of granite-related mining districts across Cornwall reflects both the deep magmatic processes linked to Variscan compression and the long-term unroofing of these intrusions to the surface. *(Photo: Wikimedia Commons, licensed under Creative Commons Attribution–ShareAlike 3.0 Unported (CC BY-SA 3.0).* https://creativecommons.org/licenses/by-sa/3.0/.)

Coal basins, subsidence and fossil-rich landscapes

The late Carboniferous also saw the development of extensive coal-forming environments across much of Britain. These environments were sustained by tectonically driven subsidence, which created accommodation space for repeated accumulation and burial of peat (see, for example, Reconstructing a Carboniferous forest from a handful of fossils).

The South Wales Coalfield provides a classic example. Subsidence allowed thick coal-bearing sequences to accumulate, later folded into a broad synclinal basin during Variscan compression. Fossil plants dominate the coal measures, recording lush, waterlogged lowlands that existed because the basin continued to sink even as sediment filled it (Fig. 2.11).

Fig. 2.11. Coal-measure plant assemblage from the South Wales Coalfield. Slab of Carboniferous roof shale preserving a mixed assemblage of fossil plants, including ribbed stems of *Calamites* sp. and pinnules of the seed fern *Neuropteris* sp. Such associations are typical of transported plant debris accumulating in waterlogged, low-energy floodplain and swamp environments. These fossil-rich coal-measure successions record sustained subsidence within the South Wales Coalfield, allowing thick peat-forming vegetation to accumulate even as sediment progressively infilled the basin, prior to later folding during Variscan compression.

Similar tectonic controls operated in central Scotland and northern England, producing the fragmented distribution of coalfields that later became the heartlands of Britain’s industrial development.

Fig. 2.12. View across the Rhondda valley at Cwmparc, illustrating the classic topography of the South Wales Coalfield. Steep-sided valleys are cut into resistant Pennant Sandstone of the Upper Carboniferous Coal Measures, forming broad upland ridges separated by narrow, deeply incised valleys. These rocks accumulated within the subsiding South Wales basin during the late Carboniferous and were later folded into a broad syncline during Variscan compression. Subsequent erosion, particularly during the Quaternary, carved the distinctive valley-and-ridge landscape that later became the focus of intensive coal mining and settlement along the valley floors. (Photo: Wikimedia Commons, licensed under Creative Commons Attribution-ShareAlike.)

From mountains to lowlands

By the early Permian, both the Caledonian and Variscan mountain belts were in terminal decline. Uplands were eroding, basins were filling, and Britain had entered a phase dominated by crustal relaxation rather than compression. Much of the country’s present lowland character owes its origin to this period of tectonic decay.

The contrast between granite uplands, folded coastal cliffs, subsiding coal basins and gently undulating lowlands reflects the uneven inheritance of past tectonism rather than any single episode of uplift or erosion.

Preparing the ground for breakup

The collapse of mountain belts did more than reshape Britain’s interior. By thinning and weakening the crust, it prepared the ground for later rifting. When continental breakup resumed during the Permian and then Mesozoic, it did so along lines of weakness established during this long period of post-orogenic collapse.

The next article follows that story – how Britain was stretched, fractured and flooded by magma as the Atlantic Ocean began to open, leaving behind the volcanic landscapes that dominate the western margins of the British Isles.

Other articles in this series
A history of the plate tectonics of Britain (Part 1): Britain assembled – oceans, collisions and the making of a geological patchwork
A history of the plate tectonics of Britain (Part 2): When mountains fall – collapse, basins and the foundations of Britain’s lowlands
A history of the plate tectonics of Britain (Part 3): Britain breaks apart – rifting, volcanoes and the birth of the Atlantic
A history of the plate tectonics of Britain (Part 4): A quiet crust with a long memory – tectonic inheritance in the modern British landscape