Northern England – introduction to geology
| From: Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M and Lawrence, D J D. 2010. British regional geology: Northern England. Fifth edition. Keyworth, Nottingham: British Geological Survey. |
Introduction
This account describes the geology of northern England southwards from the Scottish border to the latitude of Morecambe Bay and Teesside (P916031). It covers England’s four northernmost counties: Cumbria, Durham, Tyne & Wear and Northumberland, with some overlap at the southern margin into Cleveland, Yorkshire and Lancashire. The region is underlain by a wide variety of rocks (P916032) with a geological history spanning almost 500 million years (P916110). It is this underlying diversity that imparts to the region its scenic range and high landscape value. Two of England’s National Parks, the Lake District and Northumberland, form substantial parts of the area described here whilst a third, the Yorkshire Dales, adjoins its southern margin. Recognised Areas of Outstanding Natural Beauty include the Northumberland Coast, the Solway Coast, and the North Pennines, the last of these areas also being designated a UNESCO European and Global Geopark.
At the centre of the Lake District (an inlier of Lower Palaeozoic rocks) is Scafell Pike which, at 977 m above sea level, is England’s highest mountain. The eleven principal lakes occupy deep glaciated valleys disposed in a broadly radial pattern around the high fells of the Scafell massif (P916031). Surrounding Scafell is craggy, mountainous terrain formed by Ordovician volcanic rocks and associated igneous intrusions, mostly granitic(P005227). To the north are smoother, more rounded hills underlain by Ordovician sandstone and mudstone; the highest of these hills is Skiddaw at 931 m (equivalent strata form the highest point in the Isle of Man, Snaefell, 621 m). Still farther north, the rolling countryside of the Caldbeck Fells encompasses more Ordovician volcanic rocks and a variety of igneous intrusions. The southern Lake District is characterised by undulating hills and rocky crags made up mostly from Silurian strata; the same rocks extend eastward to form the Howgill Fells massif, rising to over 700 m. Whilst these mountain landscapes have been largely sculpted by glacial erosion, the depositional legacy of the ice age can be seen in many lowland areas. Most spectacular are the major spreads of drumlins — elongate mounds of glacial detritus — that swing westward across the southern and northern margins of the Lake District and extend eastward through the Stainmore gap.
The mineral wealth of the Lake District has long been exploited. In the 16th century the area was one of the world’s principal copper producers and many other minerals have been worked over the ensuing years. All of the mines have now closed, but the most recent activity, at Carrock Fell, continued until 1981. Slate and aggregate quarrying are now the only extractive industries, and also have a lengthy history going back to perhaps the earliest exploitation of the Lake District’s geological resources: the Neolithic stone axe industry of Langdale.
Towards the coast of north-west England, the Lake District fells give way to undulating lowland as the ancient rocks disappear beneath a cover of younger, Carboniferous strata, which now crop out across much of west and south Cumbria. In the south, limestones give rise to areas of distinctive karst scenery (Frontispiece) whilst in the west the Carboniferous sequence includes coal and the limestone contains substantial deposits of haematite. Together, these resources fuelled the now largely historical heavy industries of Workington, Whitehaven and Barrow-in-Furness; deep mining for coal ceased in 1986 but some coal is still won from open-cast sites. On the north-west Cumbrian coast, at St Bees Head, spectacular cliffs are formed by red Triassic sandstone (P220633) and eastwards from there the Permo-Triassic strata extend beneath the gentle lowlands that border the Solway Firth. Around Carlisle the sedimentary sequence extends upwards into the Lower Jurassic.
Farther to the east, the fells of the Bewcastle and Liddesdale areas mark the outcrop of Carboniferous strata along the Scottish Border. From beneath these rocks in north Northumberland emerge the Devonian lava and granite of the Cheviot Hills. The geological features do not precisely respect the Anglo–Scottish border, which in this wild and remote area bisects the igneous massif. The summit of The Cheviot (815 m) lies just on the Northumberland side of the border.
Inland from the sandy bays and ruined castles of the Northumberland coast run a series of ridges formed by hard, resistant rocks. Most of the ridges testify to tough, Carboniferous sandstone, but the most prominent of them arises from the Whin Sill, an intrusive sheet of earliest Permian dolerite. The Whin Sill provides the imposing, defensive foundation of Hadrian’s Wall, frontier of the Roman Empire, for much of its seventy-mile course across the narrowest part of England; it is now a World Heritage site (P222329). South of Hadrian’s Wall is the high, flat moorland of the northern Pennines. The moors rise gently westward to the spectacular west-facing Pennine escarpment at Cross Fell (the highest point in the Pennines at 893 m) that overlooks the broad, fertile lowland of the Vale of Eden. Carboniferous rocks, mostly sandstone and limestone form the fault-controlled escarpment, at the foot of which they overlie Lower Palaeozoic rocks of Lake District character. The Vale of Eden, separating the Pennine escarpment from the main Lake District massif, is mostly floored by Permian and Triassic sandstone.
A distinctive feature of the northern Pennine landscape is the marked terrace featuring of the hillsides caused by the alternation of relatively softer and harder beds within the Carboniferous sedimentary succession (P689653); resistant sandstone or limestone usually forms the steep ‘steps’, and mudstone forms the intervening terraces. The harder rocks also host myriad veins of base metal ore that have been worked since Roman times, and many of the area’s small towns and villages owe their development to the once-prosperous mining industry. Whereas ores of lead, zinc and silver were mostly sought in the 19th century industrial heyday, working of baryte, witherite and fluorspar were of greater importance in the 20th century. All mining has now ceased but its effect remains a major influence on the northern Pennine landscape.
Towards the east coast, the limestone and sandstone of the Pennine uplands give way to the Carboniferous coal-bearing measures of the Northumberland and Durham Coalfield. This was one of Britain’s earliest worked coalfields and supported the steel and shipbuilding industries of Tyneside and Wearside. Coal mining has declined in importance in recent years and, with the closure of the last underground mine in 2005, coal working is now restricted to a handful of opencast operations (P548139). Within the coalfield lies the ancient cathedral city of Durham, one of Britain’s World Heritage Sites, built within a magnificent incised meander of the River Wear. To the east of Durham, the Carboniferous rocks of the coalfield are concealed beneath Permian limestone, the outcrop of which is marked by a distinctive belt of arable country terminated by the limestone cliffs of the Durham coast.
The geodiversity seen across northern England is striking. Throughout the region, the underlying geology has had a strong influence on the scenery and pattern of land use. That influence has been locally enhanced or moderated by the erosive and depositional effects of the Quaternary ice age. Some upland areas were scoured by ice, whilst some lower-lying districts were plastered with a thick layer of glacially transported debris — remember the drumlin swarms to the north and south of the Lake District. The human factor has added a further influence, whether through mineral exploitation or the use of local building materials. And still, of course, the landscape is evolving through both natural and anthropogenic processes. So why does northern England look the way it does? How did this collage of rocks come about? The region’s dynamic geological evolution is explored in the following account.
Regional tectonic framework
England’s northern geographical border coincides, more or less, with one of the most fundamental geological boundaries in Britain. This is the Iapetus Suture, the trace of a long- vanished, Early Palaeozoic ocean obliterated by the convergence and ultimate collision of the ancient continents that it once separated. The Iapetus Ocean (as a forerunner of the Atlantic Ocean it was named after the father of the eponymous Atlas) was initiated during late Neoproterozoic times and grew to a likely maximum width in excess of 1000 km by the end of the Cambrian Period (P916033)a. Thereafter, subduction at its margins wrought its eventual destruction and drove the series of collisional events that built up the Caledonian Orogen, a major tectonic zone that can be traced from Scandinavia, through Britain and Ireland, and on into Greenland and maritime North America.
To the north of the Iapetus Ocean, in sub-tropical latitudes, lay the continent of Laurentia. The Archaean and Proterozoic crystalline basement rocks of Scotland formed a part of this continent, and subduction of Iapetus oceanic crust beneath its margin led to the sequential accretion of oceanic terranes. The history of the ocean’s destruction at this northern margin is recorded in these: for example, in the Tremadoc to Arenig, Ballantrae Complex ophiolite, the obduction of which occurred at about 470 Ma during the collision of a volcanic arc complex with the continental margin. This collision provoked the Grampian Event of the polyphase Caledonian Orogeny. Thereafter, continued late Ordovician to mid Silurian subduction at the northern margin of the Iapetus Ocean is demonstrated by the Southern Uplands accretionary thrust belt.
At the southern margin of the Iapetus Ocean, at a latitude of about 60° south, lay the shores of the Gondwana continent from which a small fragment broke away early in Palaeozoic times. This fragment, known as Avalonia, drifted north, towards Laurentia, as the intervening Iapetus Ocean closed (P916033)b. In northern England, Lower Palaeozoic inliers (P916032) reveal parts of the northern margin of Avalonia and show how it developed in response to the changing geotectonic regime. The oldest rocks present in the region are likely to be those from the enigmatic, possibly Neoproterozoic, metasedimentary Ingleton Group. This unit is exposed a little to the south of the Lake District in small inliers around the Craven district. Within the main Lake District inlier, the oldest rocks seen are the Tremadoc to Llanvirn, turbiditic mudstone and sandstone of the Skiddaw Group (a broad correlative is the Manx Group in the Isle of Man) that were deposited in extensional basins along the continental margin of Avalonia as it rifted from Gondwana. As much as 5000 m of strata accumulated with much evidence for large-scale slumping of the unconsolidated sediment.
Along other parts of the Avalonian continental margin, and as close to the Lake District as north Wales, there is evidence from volcanic rocks that southward subduction of the Iapetus Ocean commenced during late Cambrian times, but in the Lake District inlier, the earliest subduction-related magmatism was late Ordovician in age. As a precursor to volcanic activity, the deep-marine strata of the Skiddaw Group were uplifted and eroded. Then, during a short-lived but violently active Caradoc volcanic interval (<5 million years culminating at about 450 Ma — see summary of age data in (P916034), the Borrowdale and Eycott volcanic groups were erupted. These groups now crop out, respectively, to the south and north of the Skiddaw Group, a disposition inherited from their having originally built up within opposing half-grabens, each originally 40 to 50 km wide. Several thousand metres of lava, ignimbrite and volcaniclastic sediment were preserved in a series of subsiding volcanic calderas. Beneath the volcanoes, granitic plutons were emplaced, coalescing to form the major part of the Lake District Batholith: the exposed intrusions of Eskdale, Ennerdale and Threlkeld are representative.
The relative brevity but great intensity of the Borrowdale–Eycott volcanic episode may have been the result of the subduction of the Iapetus Ocean spreading ridge at its Avalonian margin. Such a phenomenon would have two important outcomes. Firstly, it would allow the disruption of the subducting slab which might explain the abrupt cessation of volcanism at the Avalonian margin. Secondly, it would combine Avalonia with the oceanic plate that was being subducted northwards beneath the Laurentian margin of the ocean. These arguments follow from associating the Lake District volcanic sequences with subduction of Iapetus Ocean crust, but an alternative possibility should also be admitted. In this view, the Borrowdale and Eycott volcanicity was generated by subduction of Tornquist Sea crust during convergence of Avalonia with Baltica, and should be linked with the contemporaneous volcanic rocks (known mostly from boreholes) that occur along the eastern margin of Avalonia in what are now the English Midlands and parts of Belgium. In truth, it is probably wrong to regard these two possibilities as being discrete alternatives, since the processes involved must have been tectonically linked and spatially continuous.
Thermal subsidence followed the cessation of volcanic activity and granite intrusion, allowing marine transgression across the eroded remains of the Borrowdale and Eycott volcanoes during late Ordovician times. The Dent Group, the lowest part of the Windermere Supergroup and of Ashgill age, encompasses a range of shallow marine lithofacies with some volcanic rocks produced during the final throes of volcanicity (P916034). It was followed during the Llandovery by accumulation in deeper water of a thin condensed sequence of marine mudstone, the Stockdale Group. Meanwhile, the convergence of Laurentia and Avalonia continued, with the Southern Uplands accretionary thrust terrane testament to the Caradoc to Wenlock subduction of Iapetus Ocean crust beneath the margin of Laurentia. As the ocean narrowed, the fossil faunas preserved at its opposing margins became progressively more cosmopolitan.
The inevitable collision occurred at some time during the mid Silurian, but was something of a tectonic anticlimax. It was not a mountain-building event of orogenic proportions and its effects are hard to find in the tectonic record preserved in the rocks. There was instead something of a tectonic continuum, as the Southern Uplands accretionary thrust terrane overrode Avalonia and continued southwards as a foreland fold and thrust belt. A load-induced, flexural foreland basin advanced ahead of the thrust front and was an influential control on sedimentation during the accumulation of the mid to late Silurian parts of the Windermere Supergroup. Subsidence and sedimentation rates both reached their maximum during late Silurian (Ludlow) times with deposition of the Coniston Group, up to 2000 m or maybe more of turbidite sandstone that accumulated during the course of less than two graptolite biozones, probably no more than one million years. Thereafter, the later Ludlow and Pridoli succession reflects a slowing of the subsidence rate and a commensurate filling of the sedimentary basin. It would seem that convergence between Laurentia and Avalonia ceased, the foreland basin failed to migrate southwards, and isostatic adjustments reversed the earlier effects of loading.
The tectonic effects seen within the exposed rock sequence, though created by collision-related processes, give little indication of the deeper structure of the suture zone. This is more usefully modelled from geophysical data. A number of seismic lines have traversed the Iapetus Suture Zone and have generally been interpreted in terms of a north-west-dipping reflective zone projecting to the surface close to the northern coast of the Isle of Man and thence striking north-east beneath northern England (P916035). Regional interpretations of gravity and magnetic data present a rather more complicated picture in which Avalonian-type crust is caught up in a compound suture zone that extends well to the north beneath the Southern Uplands terrane (P916036).
The continental collision between Laurentia and Avalonia was not an orthogonal event. A wealth of evidence shows that sinistral strike-slip movement was important during the later stages of convergence, and indeed may have been the dominant final effect. One result was the establishment of a late Caledonian fault pattern involving conjugate systems trending north-west and east-north-east across both the Southern Uplands terrane and the Lower Palaeozoic outcrop in northern England. These faults were to have a profound structural influence during subsequent episodes of extensional tectonism when their reactivation controlled late Palaeozoic basin development and geometry. More immediately, in the transtensional tectonic regime pertaining to Early Devonian times, strike-slip basins opened across the region and were filled with the clastic, terrestrial sediments of the Old Red Sandstone lithofacies.
It is something of a geological paradox that the Lower Palaeozoic rocks of the Avalonian margin did not experience their maximum deformation as a result of the Laurentia–Avalonia collision. Instead, that event ground to a halt and was followed by a compressive hiatus; it was not until the Mid Devonian Acadian Orogeny that pervasive deformation and cleavage formation occurred. Stratigraphical constraints, the relationship of cleavage fabrics to dated granite intrusions, and dating of white mica formed within the cleavage planes, all combine to suggest an Emsian age, of about 400 Ma. The metamorphic grade attained then, albeit very low, still implies a substantial overburden, now eroded, of nonmarine Old Red Sandstone strata. A minimum 3500 m pretectonic thickness for this cover has been estimated in the southern Lake District, and tectonically driven subsidence of the underlying Avalonian crust is clearly required to accommodate such a thickness of nonmarine sediment.
One possible mechanism is flexure of the Avalonian footwall by continued shortening along the Iapetus Suture Zone. This has an immediate attraction in that it links the well-established tectonic models for the Southern Uplands — imbricate thrust belt — and southern Lake District — foreland basin — but sadly fails on several scores. In local terms, in the Windermere Supergroup foreland basin in the southern Lake District, a decelerating subsidence rate allowed the basin to fill during latest Silurian and earliest Devonian times; the implication is that the convergence rate declined rapidly well before the onset of Acadian deformation. Further, the age of that deformation is consistent across Britain and Ireland, which is incompatible with a prograding, flexural mechanism. A more likely explanation for Early Devonian subsidence derives from the sinistral strike-slip model, whereby the Old Red Sandstone accumulated in transtensional basins, formed during the hiatus in compression that separated the mid Silurian, final convergence of Laurentia and Avalonia, from the Early Devonian, Acadian Orogeny. The Acadian Orogeny itself was probably initiated by collision of another rifted Gondwanan fragment, Armorica, with the south side of Avalonia (P916033)c.
Accompanying or closely following the Acadian deformation was the intrusion of lamprophyre and felsic dyke swarms and a range of granitic plutons (P916034) with, in the case of the Cheviot Pluton, associated volcanicity. The subvolcanic batholith that had previously formed beneath the Lake District during the Ordovician magmatism was now enlarged by the intrusion of Acadian components at its margins: the Skiddaw granite in the north and the Shap granite in the south. In addition, the several granitic plutons of the substantial North Pennine Batholith were emplaced beneath the central part of northern England and now underpin the structural high of the Alston Block (P916037). These igneous phenomena are all more readily accommodated within a transtensional tectonic model than within one requiring convergence-driven crustal flexure.
The disposition of Acadian and earlier structures and intrusions then became the principal control on the pattern of renewed extension late in Devonian times and through the Carboniferous. A major structural control on sedimentation in the Nothumberland–Solway Basin is the Maryport–Stublick–Ninety Fathom (M–S–NF) fault system (P916037) that, together with most of the other large synsedimentary faults, appears to be inherited from reactivated, pre-existing basement structures. In particular, the east-north-east-trending M–S–NF faults formed by extensional reactivation in the hanging wall of the Iapetus Suture Zone. They define the northern margin of the Lake District and Alston blocks, structural horsts underpinned by buoyant granitic massifs. The southern margin of the Alston Block (and northern margin of the Stainmore Trough) is formed by the Closehouse–Lunedale–Butterknowe fault system. A broadly similar structural control extends farther south, beyond the confines of the area described in this account, to define the Askrigg Block (underpinned by the Wensleydale granite pluton) and the adjacent Craven/Bowland Basin. To the west, the Isle of Man is formed by a separate structural block above the Manx Pluton.
Throughout the region there is a probability that structural control inherited from the underlying Lower Palaeozoic and older basement continued from Carboniferous into Permian and later times. Separating the Alston and Lake District blocks is the Vale of Eden half-graben, a mostly Permian extensional feature controlled at its north-east margin by the north-westtrending Pennine Fault system. To the west of the Lake District Block, the north-west-trending Lake District Boundary Fault fulfils a similar structural role at the margin of the Irish Sea Basin. Basement control by structures within the Iapetus Suture Zone has also been invoked to explain aspects of Carboniferous volcanism and the intrusion of the early Permian Whin Sill-swarm at about 300 Ma. But consideration of these phenomena is to get a little ahead of the geological story.
Carboniferous sedimentation patterns across northern England were strongly controlled by the subsiding basins and the intervening, more stable blocks. All were developed within a much larger basin system that extended from Ireland, eastwards through north Wales, northern and central England — where it is known as the Pennine Basin — to the North Sea. Within northern England, the stable blocks underpinned by granite batholiths are characterised by thin and incomplete sedimentary sequences; the subsiding basins were infilled with sediment and contain thick and relatively complete sedimentary successions. By this time the region formed part of the southern margin of Laurussia, a ‘supercontinent’ created by the amalgamation of Avalonia, Laurentia, Baltica and other terranes of Asian Russia. The British sector drifted slowly north through equatorial latitudes. An initially arid climate became progressively more hot, humid and wet during that northward drift, and then reverted to arid conditions towards the end of the period. The tectonic regime was broadly extensional, with dextral strike-slip becoming progressively more important.
The Carboniferous sedimentary record is the result of a complex interplay between several factors: subsidence rates, changes in sea level, deposition of limestone in shallow water, and the progradation of major sandstone deltas into the subsiding basins where mudstone accumulated in the deeper water. Early Carboniferous sedimentation was in fluvial and lacustrine to paralic environments, building up the dominantly clastic Inverclyde and Ravenstonedale groups. With continuing subsidence, a greater marine influence is seen in the succeeding strata, the largely deltaic Border Group and the reef to detrital carbonates of the Great Scar Limestone Group. Through the middle part of the Carboniferous succession, a major delta system built out into the subsiding basin and the Yoredale Group accumulated. Cyclic sedimentation was a particular feature, with the sandy, alluvial and deltaic flats subject to intermittent marine incursions that allowed the development of limestone. The colonisation of the delta tops by lush swamp vegetation, and its subsequent burial and conversion to coal, has been a key economic factor in the modern history of the region. This phenomenon is a particular feature of the upper Carboniferous, Pennine Coal Measures Group. Finally, towards the end of the Carboniferous Period, deposition of the Warwickshire Group occurred in a fluvial to deltaic plain setting but under increasingly arid, oxidising climatic conditions; coal is largely absent and the strata are generally reddened.
By late Carboniferous times, the thermal subsidence that had largely controlled sedimentation patterns earlier in the period began to wane. Instead, the region experienced the peripheral effects of a major orogenic collision as, far to the south, Laurussia and the huge Gondwana continent came together to unite the Earth’s land areas into the single vast expanse of Pangaea (P916033)d. The compressive deformation associated with these events, the Variscan Orogeny, was most intense across mainland Europe, and the southern parts of England, Wales and Ireland, south of the so-called ‘Variscan Front’. Related deformation in the north of England was relatively weak and manifest largely by dextral transtension. It spanned an interval of approximately 15 Ma from the late Carboniferous to the early Permian, and was accompanied by the large-scale intrusion of basaltic magma to form the Whin Sill-swarm and the associated Northern England Tholeiitic Dyke-swarm (P916034).
Renewed extension in Permian times reactivated the broadly north-trending Caledonian structures to form the margins of depositional half-graben basins such as the Vale of Eden; pre-existing east–west faults experienced mainly strike-slip reactivation at this time. Global sea level during Permo-Triassic times was relatively low, and the northern British region was located far from the contemporary coastline, within the interior of Pangaea and a little to the north of the contemporary Equator (P916033)d: about 10°N at the beginning of the Permian, drifting to about 30°N by the end of the Triassic. Thus, the onshore sequences are largely the result of terrestrial sedimentation in an arid environment. There was also much weathering and erosion of the newly created upland areas, with as much as 500 m of Carboniferous strata worn away in some places. Several late Permian marine transgressions are evident, particularly in the North Sea sequence and its onshore continuation in northern England, but more general marine transgression did not reach the region until late in the Triassic.
The lowest Permian strata of northern England, comprising the Appleby and Rotliegendes groups, are mostly aeolian and fluvial sandstones with conglomerates derived locally from the sides of the fault-defined depositional basins. Later in the Permian, the limestone and evaporite deposits of the Cumbrian Coast and Zechstein groups were produced by the series of marine inundations and the subsequent evaporation of the trapped sea water. A broadly similar sedimentary pattern continued through much of the Triassic Period. Deposition was initially of sandstone and mudstone, the Sherwood Sandstone Group, effected by ephemeral rivers on braided alluvial plains and playa mudflats. Later in Triassic times, the marine influence was stronger, as seen in the sequences of mudstone, evaporitic gypsum and limestone, the Mercia Mudstone and Penarth groups, which are now preserved in the Solway/Carlisle Basin and to the south of the Lake District along the northern shores of Morecambe Bay.
It was the break-up of the Pangaea ‘supercontinent’ during Late Triassic times that brought an end to the prolonged period of mainly terrestrial conditions across northern England. Marine transgression extended across an ever-widening area until, by Early Jurassic times (P916033)e, global sea levels were relatively high and most parts of the region were submerged. Calcareous marine mudstone from this time, part of the Lias Group, is preserved in the centre of the Solway/Carlisle Basin. A period of uplift and erosion in Mid to Late Jurassic times is recorded by a widespread unconformity, with the maximum effect seen in North Sea basinal sequences. The commensurate fall in sea level continued through the Early Cretaceous interval and an extensive unconformity developed across the surrounding land areas, until rising sea levels brought renewed marine transgression through the later part of the Cretaceous. Continental drift had carried Britain to the latitude of about 35°N by the end of the Triassic and a slow northwards drift continued during the Jurassic and Cretaceous periods. The climate was strongly seasonal with warm, relatively dry summers and cool wet winters.
The later Mesozoic geological history is obscure, with no sedimentary rocks from that interval, or the succeeding Cenozoic, preserved in northern England. There is some evidence that normal and possibly strike-slip faulting continued, and in the Solway/Carlisle Basin sedimentation probably continued into Early Cretaceous times. Conversely, around that same interval, the Lake District and Alston blocks experienced further erosion during development of a widespread unconformity. Thereafter it is likely that renewed subsidence allowed deposition of the Upper Cretaceous Chalk Group across the region, with the maximum post-Variscan burial of northern England probably achieved towards the end of the Cretaceous Period.
At the end of the Cretaceous and into the early Palaeogene Period, regional uplift began as a precursor to the major magmatism associated with the initial opening of the Atlantic Ocean (P916033)f. This was driven by development of the proto-Icelandic mantle plume, which had its maximum impact in what is now the Hebridean province of western Scotland and Northern Ireland and in Greenland, areas that were then adjacent. There, from about 60 Ma to 55 Ma, immense volumes of basaltic magma were erupted with the coeval intrusion of plutons, sill-swarms and swarms of dykes. Some of the latter, emanating from a volcanic centre on Mull, run south-eastwards across northern England, more than 400 km from their source; examples include the Cleveland and Acklington dykes (P916034). Other dykes run from Northern Ireland across the Isle of Man and into the English Midlands.
And so to the present
Additional impetus was given to the regional uplift of northern England in Miocene times as a distant effect of the Alpine Orogeny. This resulted from collision between the European and African plates, but its main structural effects are not seen much beyond southern England and Wales. Around northern England, its influence was restricted to likely uplift of the Irish Sea and Solway/Carlisle basin sequences. Overall, the Palaeogene to Neogene uplift episodes created erosive conditions across northern England that have continued to the present day. Up to 2500 m of strata may have been removed in this time, including all of the Upper Jurassic and Cretaceous succession. Further erosion of Carboniferous rocks revealed the Caledonian, Lower Palaeozoic basement across the structural highs of the Lake District, the Isle of Man and, less extensively, the Alston Block; to the north, the Southern Uplands block was similarly exhumed.
Erosion was locally much more vigorous from about 2.6 million years ago as glacial conditions were established across northern England during the Quaternary ‘ice age’. In fact this was a period of alternating cold and more temperate interludes, with the most recent of the latter, commencing only about 10 000 years ago continuing to the present day. Ice sheets repeatedly built up on the higher ground and fed glaciers that eroded deep valleys, for example those now occupied by the lakes of the Lake District. Sometimes the ice sheets reached the sea, but at other times Irish Sea or North Sea ice encroached onto the land areas. This, and the multiphase nature of glacial advance and retreat led to complex variations in ice flow direction through time, resulting in a complicated pattern of glacial landforms. As the ice sheets waxed and waned so relative sea level rose and fell: short-term, ‘eustatic’ changes were directly related to the growth of the ice sheets, but longer term, ‘isostatic’ changes arose from the depression of the Earth’s crust by the enormous weight of ice, and its slow recovery once the ice had melted. These effects have led to a range of submerged and perched coastal features, the most prominent of which are raised beaches and cliff-lines now abandoned several metres above present sea-level.
Since the last retreat of the ice from northern England, the human species has become a significant geological agent, modifying landscapes and sedimentary patterns through deforestation, agriculture, mining and urban development, whilst driving many fellow species to extinction. Global warming is once again leading to a renewed rise in sea level, but this time accentuated by an anthropogenic contribution. How these changes will affect the future geological record remains to be seen. In the meantime, across northern England, we continue to rely on geological sources of many raw materials, particularly for construction and road building, whilst attempting to mitigate the effects of geologically induced hazards such as landslides and subsidence.
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