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Scotland and northwest Ireland in the Lower Palaeozoic existed as parts of the margin of the Laurentian '''plate'''. During the later stages of closure and immediately afterwards, slices of the Laurentian plate margin were shuffled together along major '''strike-slip faults''', producing the pattern of outcrops we see today. The Southern Uplands forms one of these slices, and along its southeastern margin Silurian rocks are exposed ([[Geology of Siccar Point and Pease Bay - an excursion|Excursion 1]], [[Geology of Eyemouth and Burnmouth - an excursion|Excursion 2]], [[Cheviot — early Devonian volcanic rocks, granite and basement - an excursion|Excursion 4]], [[Geology of the North Tyne and Saughtree - an excursion|Excursion 10]]). These are thick '''turbidites''', predominantly of sand-grade, with some shales, locally containing '''graptolites''', sourced from rising land to the north and deposited in a narrow seaway, the remnant of the former ocean. Graptolites indicate a Llandovery age for outcrops on the coast between Siccar Point and St Abb's Head. Between Coldingham and Eyemouth, '''acritarchs''' suggest an early Wenlock age for some beds, but otherwise here and south to Burnmouth no diagnostic fossils have been found. Inland along the southeast margin of the Southern Uplands, scattered graptolite records indicate a Wenlock age. There are no records of younger Silurian sediments. Uplift, compression and deformation resulting from the collision of Eastern Avalonia and Laurentia affected the Southern Uplands area in late Silurian times and the Lake District–Teesdale area in the early Devonian. This '''orogenic''' episode concluded the long and complex '''Caledonian Orogenic Cycle''', which resulted in a belt of fold mountains and uplands '''striking''' across the newly welded continental mass of Laurentia, Baltica and Avalonia, following the line of the former seaway. The area of the British Isles affected was from the Northern Highlands to North Wales.
 
Scotland and northwest Ireland in the Lower Palaeozoic existed as parts of the margin of the Laurentian '''plate'''. During the later stages of closure and immediately afterwards, slices of the Laurentian plate margin were shuffled together along major '''strike-slip faults''', producing the pattern of outcrops we see today. The Southern Uplands forms one of these slices, and along its southeastern margin Silurian rocks are exposed ([[Geology of Siccar Point and Pease Bay - an excursion|Excursion 1]], [[Geology of Eyemouth and Burnmouth - an excursion|Excursion 2]], [[Cheviot — early Devonian volcanic rocks, granite and basement - an excursion|Excursion 4]], [[Geology of the North Tyne and Saughtree - an excursion|Excursion 10]]). These are thick '''turbidites''', predominantly of sand-grade, with some shales, locally containing '''graptolites''', sourced from rising land to the north and deposited in a narrow seaway, the remnant of the former ocean. Graptolites indicate a Llandovery age for outcrops on the coast between Siccar Point and St Abb's Head. Between Coldingham and Eyemouth, '''acritarchs''' suggest an early Wenlock age for some beds, but otherwise here and south to Burnmouth no diagnostic fossils have been found. Inland along the southeast margin of the Southern Uplands, scattered graptolite records indicate a Wenlock age. There are no records of younger Silurian sediments. Uplift, compression and deformation resulting from the collision of Eastern Avalonia and Laurentia affected the Southern Uplands area in late Silurian times and the Lake District–Teesdale area in the early Devonian. This '''orogenic''' episode concluded the long and complex '''Caledonian Orogenic Cycle''', which resulted in a belt of fold mountains and uplands '''striking''' across the newly welded continental mass of Laurentia, Baltica and Avalonia, following the line of the former seaway. The area of the British Isles affected was from the Northern Highlands to North Wales.
  
As the Caledonian mountains rose, weathering under hot, arid conditions provided masses of debris which accumulated in alluvial fans in '''intermontane basins'''. These deposits constitute the Old Red Sandstone of Devonian age ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]b). In addition, the orogenic event caused melting within the crust which gave rise to early Devonian volcanic activity at the surface. The Cheviot area was one such volcanic centre, surrounded by thick sequences of '''pyroclastic''' rocks and lava flows, mainly of '''andesitic''' composition, and possibly exceeding 1000 m thick ([[Cheviot — early Devonian volcanic rocks, '''granite''' and basement - an excursion|Excursion 4]]). Erosion deep into the volcanic pile has revealed a slightly younger granite intrusion into the core of the complex which now crops out at its centre. Other late Caledonian granites were emplaced in the Lower Palaeozoic rocks of the Alston and Askrigg Blocks (now covered by a few hundred metres of Carboniferous sediments but detected geophysically and proved by boreholes), the Lake District, Southern Uplands, and under what is now the North Sea ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]b). North of Cheviot, early Old Red Sandstone '''breccias, conglomerates''', red sandstones, '''marls''' and '''calcretes''' up to 600 m thick are associated with the volcanic rocks and rest with strong unconformity on folded Silurian sediments. West and north of Cheviot, these in turn are overlain by a second cycle of similar sediments, with common calcretes towards the top, unconformable on the Lower Old Red Sandstone, Cheviot volcanics and Silurian '''greywackes''' ([[Geology of Siccar Point and Pease Bay - an excursion|Excursion 1]], [[Geology of Eyemouth and Burnmouth - an excursion|Excursion 2]], [[Geology of the North Tyne and Saughtree - an excursion|Excursion 10]]). In places, this sequence contains evidence of a late Devonian age and is thus referred to the Upper Old Red Sandstone. Elsewhere it passes conformably upwards into early Carboniferous fluvial and '''lacustrine''' sediments. This second pulse of coarse debris reflects a phase of '''tectonic''' activity in the mid Devonian that rejuvenated the upland source areas.
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As the Caledonian mountains rose, weathering under hot, arid conditions provided masses of debris which accumulated in alluvial fans in intermontane basins. These deposits constitute the Old Red Sandstone of Devonian age ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]b). In addition, the orogenic event caused melting within the crust which gave rise to early Devonian volcanic activity at the surface. The Cheviot area was one such volcanic centre, surrounded by thick sequences of pyroclastic rocks and lava flows, mainly of andesitic composition, and possibly exceeding 1000 m thick ([[Cheviot — early Devonian volcanic rocks, granite and basement - an excursion|Excursion 4]]). Erosion deep into the volcanic pile has revealed a slightly younger granite intrusion into the core of the complex which now crops out at its centre. Other late Caledonian granites were emplaced in the Lower Palaeozoic rocks of the Alston and Askrigg Blocks (now covered by a few hundred metres of Carboniferous sediments but detected geophysically and proved by boreholes), the Lake District, Southern Uplands, and under what is now the North Sea ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]b). North of Cheviot, early Old Red Sandstone breccias, conglomerates, red sandstones, marls and calcretes up to 600 m thick are associated with the volcanic rocks and rest with strong unconformity on folded Silurian sediments. West and north of Cheviot, these in turn are overlain by a second cycle of similar sediments, with common calcretes towards the top, unconformable on the Lower Old Red Sandstone, Cheviot volcanics and Silurian greywackes ([[Geology of Siccar Point and Pease Bay - an excursion|Excursion 1]], [[Geology of Eyemouth and Burnmouth - an excursion|Excursion 2]], [[Geology of the North Tyne and Saughtree - an excursion|Excursion 10]]). In places, this sequence contains evidence of a late Devonian age and is thus referred to the Upper Old Red Sandstone. Elsewhere it passes conformably upwards into early Carboniferous fluvial and lacustrine sediments. This second pulse of coarse debris reflects a phase of tectonic activity in the mid Devonian that rejuvenated the upland source areas.
  
By the early Carboniferous, relief on the Caledonian mountains had been somewhat reduced. A period of crustal extension followed the end of the orogenic cycle and broad, fault-bounded '''half-graben''' basins began to develop to the north and south of the Cheviot Block. Locally, conglomerates accumulated at the base of the Carboniferous sequence flanking the Cheviot ([[Lower Carboniferous at Bowden Doors, Roddam Dene and the Coquet Gorge - an excursion|Excursion 9]]). The largest of these basins, the Northumberland Trough, developed along the line of the Iapetus '''suture''', bounded to the northwest by the Southern Uplands and to the south by the Ninety Fathom–Stublick–Maryport Fault system, and the Alston and Manx–Cumbria Blocks. It was itself split into an easterly Northumberland Basin and a westerly Solway Basin by a basement ridge in the Bewcastle area ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]c). Extension appears to have been marked by the localized outpouring of '''basaltic''' lavas in the early Dinantian (Lower Carboniferous), cropping out along the northwestern margin of the Northumberland Trough ([[Geology of the North Tyne and Saughtree - an excursion|Excursion 10]]). Lower Carboniferous successions are much thicker in the more rapidly subsiding basins, and thinner and less complete on the intervening blocks.
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By the early Carboniferous, relief on the Caledonian mountains had been somewhat reduced. A period of crustal extension followed the end of the orogenic cycle and broad, fault-bounded half-graben basins began to develop to the north and south of the Cheviot Block. Locally, conglomerates accumulated at the base of the Carboniferous sequence flanking the Cheviot ([[Lower Carboniferous at Bowden Doors, Roddam Dene and the Coquet Gorge - an excursion|Excursion 9]]). The largest of these basins, the Northumberland Trough, developed along the line of the Iapetus suture, bounded to the northwest by the Southern Uplands and to the south by the Ninety Fathom-Stublick-Maryport Fault system, and the Alston and Manx-Cumbria Blocks. It was itself split into an easterly Northumberland Basin and a westerly Solway Basin by a basement ridge in the Bewcastle area ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]c). Extension appears to have been marked by the localized outpouring of basaltic lavas in the early Dinantian (Lower Carboniferous), cropping out along the northwestern margin of the Northumberland Trough ([[Geology of the North Tyne and Saughtree - an excursion|Excursion 10]]). Lower Carboniferous successions are much thicker in the more rapidly subsiding basins, and thinner and less complete on the intervening blocks.
  
The Tweed Basin, north of the Cheviot Block, with 1300 m of sediments, and the Northumberland Basin, with c.5000 m, have similar depositional histories. In the early Carboniferous, sediments derived from the north and east fed a broad coastal plain of channel sandstones and floodplain siltstones with frequent thin bands cemented by '''dolomite (cementstones)''' in the lower part of the sequence. Conditions remained arid and ephemeral lake and flood-plain deposits contain crystals of '''gypsum''', anhydrite and '''halite''', now as '''pseudomorphs'''. These form the Cementstone Group ([[Geology of Siccar Point and Pease Bay - an excursion|Excursion 1]], [[Geology of Eyemouth and Burnmouth - an excursion|Excursion 2]], [[Lower Carboniferous at Bowden Doors, Roddam Dene and the Coquet Gorge - an excursion|Excursion 9]]), which in places in the Tweed Basin transitionally succeeds the Upper Old Red Sandstone. The climate became warmer and more humid during the Dinantian. Uplift of the source area in wetter conditions caused the '''progradation''' of a braided river system across the Northumberland Basin depositing the Fell Sandstone Group, a sequence dominated by planar and trough '''cross-bedded''' sandstones ([[Geology of Eyemouth and Burnmouth - an excursion|Excursion 2]], [[Lower Carboniferous at Bowden Doors, Roddam Dene and the Coquet Gorge - an excursion|Excursion 9]]). These rocks now form significant high ground across mid Northumberland. Crossing into the Solway Basin, with about 7000 m of Carboniferous deposits, the Lower Border Group consists of interfingering sandstones, shales and thin limestones, the result of deltas prograding from the northeast and northwest into a shallow marine gulf ([[Lower Carboniferous of Bewcastle and Gilsland - an excursion|Excursion 5]]). Fossils in some of the limestones reflect close to normal marine conditions, but others contain '''stromatolites''' and mounds of vermiform ''''gastropods'''' indicating fluctuating salinity. Thick sandstone bodies in the upper part of the Lower Border Group and the Middle Border Group result from westward progradation of the Fell Sandstone delta.
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The Tweed Basin, north of the Cheviot Block, with 1300 m of sediments, and the Northumberland Basin, with c.5000 m, have similar depositional histories. In the early Carboniferous, sediments derived from the north and east fed a broad coastal plain of channel sandstones and floodplain siltstones with frequent thin bands cemented by dolomite (cementstones) in the lower part of the sequence. Conditions remained arid and ephemeral lake and flood-plain deposits contain crystals of gypsum, anhydrite and halite, now as pseudomorphs. These form the Cementstone Group ([[Geology of Siccar Point and Pease Bay - an excursion|Excursion 1]], [[Geology of Eyemouth and Burnmouth - an excursion|Excursion 2]], [[Lower Carboniferous at Bowden Doors, Roddam Dene and the Coquet Gorge - an excursion|Excursion 9]]), which in places in the Tweed Basin transitionally succeeds the Upper Old Red Sandstone. The climate became warmer and more humid during the Dinantian. Uplift of the source area in wetter conditions caused the progradation of a braided river system across the Northumberland Basin depositing the Fell Sandstone Group, a sequence dominated by planar and trough cross-bedded sandstones ([[Geology of Eyemouth and Burnmouth - an excursion|Excursion 2]], [[Lower Carboniferous at Bowden Doors, Roddam Dene and the Coquet Gorge - an excursion|Excursion 9]]). These rocks now form significant high ground across mid Northumberland. Crossing into the Solway Basin, with about 7000 m of Carboniferous deposits, the Lower Border Group consists of interfingering sandstones, shales and thin limestones, the result of deltas prograding from the northeast and northwest into a shallow marine gulf ([[Lower Carboniferous of Bewcastle and Gilsland - an excursion|Excursion 5]]). Fossils in some of the limestones reflect close to normal marine conditions, but others contain stromatolites and mounds of vermiform 'gastropods' indicating fluctuating salinity. Thick sandstone bodies in the upper part of the Lower Border Group and the Middle Border Group result from westward progradation of the Fell Sandstone delta.
  
Later in the Dinantian, the marine influence from the southwest increased as the '''clastic''' supply from the north and east diminished. Repeated cycles of marine limestone, shale and sandstone in the Upper Border Group of the Solway Basin '''transgressed''' across the Northumberland Basin, where in addition, thick coals developed at the top of many cycles, and into the Tweed Basin, where proximity to the shore line is reflected in thinner limestones and more persistent coals. These sediments form the Scremerston Coal Group of the Northumberland and Tweed Basins ([[Carboniferous rocks around Berwick-upon-Tweed - an excursion|Excursion 3]], [[Geology of the North Tyne and Saughtree - an excursion|Excursion 10]]), whose coals have been widely worked. In the succeeding Lower and Middle Limestone Groups ([[Carboniferous rocks around Berwick-upon-Tweed - an excursion|Excursion 3]], [[Geology and landscape of Holy Island and Bamburgh - an excursion|Excursion 6]], [[Carboniferous rocks of the Howick shore section - an excursion|Excursion 7]], [[Carboniferous rocks of the Roman Wall and Haltwhistle Burn - an excursion|Excursion 11]]), which are equivalent to the Liddesdale Group in the Solway Basin, the marine influence is further enhanced, with limestones thicker and coals thinner or absent. These are classic '''Yoredale''' cycles. By the late Dinantian, conditions had become increasingly uniform across Northumberland and the differentiation into basin and block less marked. At this time, the sea began to transgress across the Alston Block to the south and in the latest Dinantian, uniform Yoredale '''facies''' extended, from the still emergent but reduced land mass of the Southern Uplands, right across the whole of the Northumbrian area ([[Carboniferous of the Wear Valley and Derwent Gorge, County Durham - an excursion|Excursion 15]], [[Geology and landscape of Upper Teesdale - an excursion|Excursion 16]], [[Carboniferous and Permian rocks in southern County Durham - an excursion|Excursion 17]]). These changes mark the beginning of a gradual transition from extensional, fault-bounded basinal subsidence to a phase of much broader subsidence caused by cooling and contraction of lower crustal rocks which affected the whole of Northern England. However, despite uniformity of facies, thickness differences between block and basin areas persisted through much of the Namurian.
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Later in the Dinantian, the marine influence from the southwest increased as the elastic supply from the north and east diminished. Repeated cycles of marine limestone, shale and sandstone in the Upper Border Group of the Solway Basin transgressed across the Northumberland Basin, where in addition, thick coals developed at the top of many cycles, and into the Tweed Basin, where proximity to the shore line is reflected in thinner limestones and more persistent coals. These sediments form the Scremerston Coal Group of the Northumberland and Tweed Basins ([[Carboniferous rocks around Berwick-upon-Tweed - an excursion|Excursion 3]], [[Geology of the North Tyne and Saughtree - an excursion|Excursion 10]]), whose coals have been widely worked. In the succeeding Lower and Middle Limestone Groups ([[Carboniferous rocks around Berwick-upon-Tweed - an excursion|Excursion 3]], [[Geology and landscape of Holy Island and Bamburgh - an excursion|Excursion 6]], [[Carboniferous rocks of the Howick shore section - an excursion|Excursion 7]], [[Carboniferous rocks of the Roman Wall and Haltwhistle Burn - an excursion|Excursion 11]]), which are equivalent to the Liddesdale Group in the Solway Basin, the marine influence is further enhanced, with limestones thicker and coals thinner or absent. These are classic Yoredale cycles. By the late Dinantian, conditions had become increasingly uniform across Northumberland and the differentiation into basin and block less marked. At this time, the sea began to transgress across the Alston Block to the south and in the latest Dinantian, uniform Yoredale facies extended, from the still emergent but reduced land mass of the Southern Uplands, right across the whole of the Northumbrian area ([[Carboniferous of the Wear Valley and Derwent Gorge, County Durham - an excursion|Excursion 15]], [[Geology and landscape of Upper Teesdale - an excursion|Excursion 16]], [[Carboniferous and Permian rocks in southern County Durham - an excursion|Excursion 17]]). These changes mark the beginning of a gradual transition from extensional, fault-bounded basinal subsidence to a phase of much broader subsidence caused by cooling and contraction of lower crustal rocks which affected the whole of Northern England. However, despite uniformity of facies, thickness differences between block and basin areas persisted through much of the Namurian.
  
 
In Northumbria, the Yoredale cycles extend up into the Namurian as the Upper Limestone Group ([[Carboniferous rocks of the Howick shore section - an excursion|Excursion 7]], [[Carboniferous rocks of the Roman Wall and Haltwhistle Burn - an excursion|Excursion 11]], [[Carboniferous of the Wear Valley and Derwent Gorge, County Durham - an excursion|Excursion 15]], [[Geology and landscape of Upper Teesdale - an excursion|Excursion 16]], [[Carboniferous and Permian rocks in southern County Durham - an excursion|Excursion 17]]). Although the thickest of all the Yoredale limestones, the Great, marks the base of the Namurian, upwards the limestones thin and the sandstones increase in thickness. Towards the top of the Namurian, marine influences and the limestones die out and the succession is dominated by cycles of erosive, coarse-grained, fluvial sandstones with interbedded fine sandstones, siltstones and mudstones. This is a thin northern equivalent of the Millstone Grit of the Central Pennines (Excursion 7, Excursion 15). There, the grit facies is developed throughout the Namurian and exceeds 2000 m in thickness. In Northumbria, the Namurian thins from just over 500 m in the subsurface near Newcastle to about 270 m on the Northumberland coast, of which only the top 50 m or so is of Millstone Grit facies.
 
In Northumbria, the Yoredale cycles extend up into the Namurian as the Upper Limestone Group ([[Carboniferous rocks of the Howick shore section - an excursion|Excursion 7]], [[Carboniferous rocks of the Roman Wall and Haltwhistle Burn - an excursion|Excursion 11]], [[Carboniferous of the Wear Valley and Derwent Gorge, County Durham - an excursion|Excursion 15]], [[Geology and landscape of Upper Teesdale - an excursion|Excursion 16]], [[Carboniferous and Permian rocks in southern County Durham - an excursion|Excursion 17]]). Although the thickest of all the Yoredale limestones, the Great, marks the base of the Namurian, upwards the limestones thin and the sandstones increase in thickness. Towards the top of the Namurian, marine influences and the limestones die out and the succession is dominated by cycles of erosive, coarse-grained, fluvial sandstones with interbedded fine sandstones, siltstones and mudstones. This is a thin northern equivalent of the Millstone Grit of the Central Pennines (Excursion 7, Excursion 15). There, the grit facies is developed throughout the Namurian and exceeds 2000 m in thickness. In Northumbria, the Namurian thins from just over 500 m in the subsurface near Newcastle to about 270 m on the Northumberland coast, of which only the top 50 m or so is of Millstone Grit facies.
  
By the beginning of the Westphalian, Northumbria was part of a broad Central Pennine Basin, in which maximum subsidence was in the Manchester area ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]d). There, the Coal Measures are over 3000 m thick, compared to some 875 m in the Northumberland and Durham Coalfield ([[Carboniferous and Permian rocks between Tynemouth and Seaton Sluice - an excursion|Excursion 8]], [[Carboniferous of the Wear Valley and Derwent Gorge, County Durham - an excursion|Excursion 15]], [[Carboniferous and Permian rocks in southern County Durham - an excursion|Excursion 17]]). Cyclic sedimentation of shale/mudstone–sandstone–'''seatearth'''–coal, continued under the influence of deltaic processes, often as small fluvially dominated deltas prograding into fresh to brackish water flood plain lakes. Major distributaries are marked by ribbons of erosive, cross-bedded sandstones cutting down into the coal-bearing sequences. Marine bands are most common in the lower part of the succession, and where present can be correlated over large areas. There is a gradual shift to more fluvial dominated conditions higher in the Westphalian. The thickest and most productive coals occur in the lower half of the Middle Coal Measures. Coal mining in Northumbria was at its peak in the 19th and early 20th centuries. The last 30–40 years have seen a drastic reduction in underground working as first the inland mines became exhausted or uneconomic and most recently the larger coastal pits have been closed.
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By the beginning of the Westphalian, Northumbria was part of a broad Central Pennine Basin, in which maximum subsidence was in the Manchester area ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]d). There, the Coal Measures are over 3000 m thick, compared to some 875 m in the Northumberland and Durham Coalfield ([[Carboniferous and Permian rocks between Tynemouth and Seaton Sluice - an excursion|Excursion 8]], [[Carboniferous of the Wear Valley and Derwent Gorge, County Durham - an excursion|Excursion 15]], [[Carboniferous and Permian rocks in southern County Durham - an excursion|Excursion 17]]). Cyclic sedimentation of shale/mudstone–sandstone–seatearth–coal, continued under the influence of deltaic processes, often as small fluvially dominated deltas prograding into fresh to brackish water flood plain lakes. Major distributaries are marked by ribbons of erosive, cross-bedded sandstones cutting down into the coal-bearing sequences. Marine bands are most common in the lower part of the succession, and where present can be correlated over large areas. There is a gradual shift to more fluvial dominated conditions higher in the Westphalian. The thickest and most productive coals occur in the lower half of the Middle Coal Measures. Coal mining in Northumbria was at its peak in the igth and early loth centuries. The last 30–40 years have seen a drastic reduction in underground working as first the inland mines became exhausted or uneconomic and most recently the larger coastal pits have been closed.
  
The volcanic and intrusive activity that had continued throughout much of the Carboniferous further south in England was absent in the north. Northumbria, however, was distinguished by a single, Permo–Carboniferous, intrusive event of considerable volume. This is the Whin '''Sill''' complex, a '''tholeiitic dolerite''' fed by '''dykes''' emplaced along approximately east–west extensional fractures formed at this time ([[Geology and landscape of Holy Island and Bamburgh - an excursion|Excursion 6]], [[Geology of the North Tyne and Saughtree - an excursion|Excursion 10]], [[Carboniferous rocks of the Roman Wall and Haltwhistle Burn - an excursion|Excursion 11]], [[Geology and landscape of Upper Teesdale - an excursion|Excursion 16]]). The sill, in composite form, reaches 100 m thick and extends from Teesdale to the Scottish borders, abruptly changing its stratigraphical level via faults and '''joints''' from the mid Dinantian to the Lower Coal Measures. It is lowest in the sequence at its northern and southern extremities and highest around Alnwick (up to Namurian) and in the Midgeholme Coalfield ([[:File:YGS_NORTROCK_FIG_00_1.jpg|Figure 1]]). The term 'sill' originated locally to describe any persistent hard bed (e.g. the Firestone Sill — a Namurian sandstone) and only subsequently took on its modern restricted meaning as an igneous rock.
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The volcanic and intrusive activity that had continued throughout much of the Carboniferous further south in England was absent in the north. Northumbria, however, was distinguished by a single, PermoCarboniferous, intrusive event of considerable volume. This is the Whin Sill complex, a tholeiitic dolerite fed by dykes emplaced along approximately east–west extensional fractures formed at this time ([[Geology and landscape of Holy Island and Bamburgh - an excursion|Excursion 6]], [[Geology of the North Tyne and Saughtree - an excursion|Excursion 10]], [[Carboniferous rocks of the Roman Wall and Haltwhistle Burn - an excursion|Excursion 11]], [[Geology and landscape of Upper Teesdale - an excursion|Excursion 16]]). The sill, in composite form, reaches 100 m thick and extends from Teesdale to the Scottish borders, abruptly changing its stratigraphical level via faults and joints from the mid Dinantian to the Lower Coal Measures. It is lowest in the sequence at its northern and southern extremities and highest around Alnwick (up to Namurian) and in the Midgeholme Coalfield ([[:File:YGS_NORTROCK_FIG_00_1.jpg|Figure 1]]). The term 'sill' originated locally to describe any persistent hard bed (e.g. the Firestone Sill — a Namurian sandstone) and only subsequently took on its modern restricted meaning as an igneous rock.
  
 
Carboniferous, particularly Lower Carboniferous sediments of the Alston Block, also host the many '''mineral veins''' and '''flats''' of the North Pennine Orefield ([[Northern Pennine Orefield: Weardale and Nenthead - an excursion|Excursion 14]]). '''Galena''' was the main mineral, extracted for lead, but '''sphalerite''' was also common together with, in the central area, minor amounts of '''pyrite''', '''marcasite''' and occasionally '''chalcopyrite''' and '''pyrrhotite'''. The '''gangue''' minerals are zoned, with '''fluorite''' predominating in the central part of the Alston Block, surrounded by '''baryte''', '''witherite''' or '''calcite'''. The zoning was temperature related, with fluids reaching 220°C in the fluorite area, dropping to as low as 60°C on the margins of the orefield. Ore-bearing fluids are thought to have originated as brines forced out of the thick surrounding sedimentary basins, stripping out metals as they migrated through the Carboniferous, Lower Palaeozoic and granite rocks, and channelled towards the block along its bounding faults. High heatflow from the Weardale Granite, a Caledonian intrusion in the Lower Palaeozoic basement of the Alston Block, set up a convection cell with the hottest brines rising through the granite and the Dinantian sediments of the block at its centre. The main phase of mineralization was most likely Permian, although potentially mineralizing fluids are still circulating in the area. The deposits have been worked probably since Roman times and extraction peaked in the 19th century. Today the gangue minerals, fluorite and baryte, are the main resource; lead and zinc ores are produced as by-products.
 
Carboniferous, particularly Lower Carboniferous sediments of the Alston Block, also host the many '''mineral veins''' and '''flats''' of the North Pennine Orefield ([[Northern Pennine Orefield: Weardale and Nenthead - an excursion|Excursion 14]]). '''Galena''' was the main mineral, extracted for lead, but '''sphalerite''' was also common together with, in the central area, minor amounts of '''pyrite''', '''marcasite''' and occasionally '''chalcopyrite''' and '''pyrrhotite'''. The '''gangue''' minerals are zoned, with '''fluorite''' predominating in the central part of the Alston Block, surrounded by '''baryte''', '''witherite''' or '''calcite'''. The zoning was temperature related, with fluids reaching 220°C in the fluorite area, dropping to as low as 60°C on the margins of the orefield. Ore-bearing fluids are thought to have originated as brines forced out of the thick surrounding sedimentary basins, stripping out metals as they migrated through the Carboniferous, Lower Palaeozoic and granite rocks, and channelled towards the block along its bounding faults. High heatflow from the Weardale Granite, a Caledonian intrusion in the Lower Palaeozoic basement of the Alston Block, set up a convection cell with the hottest brines rising through the granite and the Dinantian sediments of the block at its centre. The main phase of mineralization was most likely Permian, although potentially mineralizing fluids are still circulating in the area. The deposits have been worked probably since Roman times and extraction peaked in the 19th century. Today the gangue minerals, fluorite and baryte, are the main resource; lead and zinc ores are produced as by-products.

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