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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, 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.
 
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.
  
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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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 Igth century. Today the gangue minerals, fluorite and baryte, are the main resource; lead and zinc ores are produced as by-products.
  
Only part of the Upper Coal Measures and no Stephanian sediments are preserved in Northumbria. A period of uplift, tilting and foiding, followed by deep weathering and erosion, occurred in late Carboniferous-early Permian times due to the compressional effects of the '''Variscan Orogeny''' to the south, and a major '''eustatic''' sea-level fall as water was locked up in the glaciation of the southern continent of Gondwana. A regional unconformity exists below the earliest Permian deposits, which overstep folded Coal Measures in the south of County Durham to rest on Namurian and locally Dinantian beds. Climatic conditions had changed from the tropical hot, wet regime of the Westphalian to a hot, arid regime in the early Permian as the British Isles drifted northwards from the equator. These conditions led to the formation of a zone of reddening 5–10 m, exceptionally 300 m, thick below the Permian unconformity. On this continental landscape, east of higher ground in the region of the Pennines, the surface had been reduced to a vast, rolling peneplain extending out into the area of the North Sea, on which the earliest sediments preserved are '''lag''' breccias and breccio-conglomerates ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]e). The succeeding Yellow Sands, now preserved as a series of east-northeast–west-southwest ridges up to 60 m thick in County Durham, are dune sands containing large-scale cross-bedded units indicating derivation from the east and northeast ([[Carboniferous and Permian rocks between Tynemouth and Seaton Sluice - an excursion|Excursion 8]], [[Magnesian Limestone between South Shields and Seaham - an excursion|Excursion 13]]).
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Only part of the Upper Coal Measures and no Stephanian sediments are preserved in Northumbria. A period of uplift, tilting and foiding, followed by deep weathering and erosion, occurred in late Carboniferous-early Permian times due to the compressional effects of the Variscan Orogeny to the south, and a major eustatic sea-level fall as water was locked up in the glaciation of the southern continent of Gondwana. A regional unconformity exists below the earliest Permian deposits, which overstep folded Coal Measures in the south of County Durham to rest on Namurian and locally Dinantian beds. Climatic conditions had changed from the tropical hot, wet regime of the Westphalian to a hot, arid regime in the early Permian as the British Isles drifted northwards from the equator. These conditions led to the formation of a zone of reddening 5–10 m, exceptionally 300 m, thick below the Permian unconformity. On this continental landscape, east of higher ground in the region of the Pennines, the surface had been reduced to a vast, rolling peneplain extending out into the area of the North Sea, on which the earliest sediments preserved are lag breccias and breccio-conglomerates ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]e). The succeeding Yellow Sands, now preserved as a series of east-northeast – west-southwest ridges up to 60 m thick in County Durham, are dune sands containing large-scale cross-bedded units indicating derivation from the east and northeast ([[Carboniferous and Permian rocks between Tynemouth and Seaton Sluice - an excursion|Excursion 8]], [[Magnesian Limestone between South Shields and Seaham - an excursion|Excursion 13]]).
  
The beginning of the Upper Permian was marked by the rapid inundation of the low lying areas east and west of the Pennines ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]e). In the North Sea Basin, the Zechstein Sea reworked the upper part of the Yellow Sands. The deposits in the sea are strongly cyclic due to eustatic sea level rise and fall, resulting in the repeated expansion and contraction of the marine area under strongly evaporitic conditions. At the base of the first cycle is a very finely laminated deposit of alternating limestone or dolomite with organic rich layers, formed on the euxinic sea floor. This, the Marl Slate, is famous for the beautiful preservation of palaeoniscid fish, which swam in the oxygenated near surface waters, together with the remains of reptiles that fell or were transported into the sea. '''Euxinic''' conditions persisted throughout the first cycle in deeper parts of the basin but around the shallow oxygenated margins, '''oolitic carbonates''' developed, protected on their seaward side by a massive '''bryozoan-algal''' shelf edge reef, up to 100  m thick and more than 30 km long, that is well exposed in County Durham ([[Magnesian Limestone between South Shields and Seaham - an excursion|Excursion 13]]). The cycle is completed by the basin-ward development of the Hartlepool Anhydrite, probably as a primary precipitate on the basin floor during a fall in sea-level. Four further cycles of carbonate and/or marl deposition, of decreasing thickness, with overlying and basin-ward '''evaporites''' are known. The carbonates are often oolitic and stromatolitic, with restricted faunas. Those of the second cycle pass into deeper water slope carbonates showing signs of slumping, and in which a wide range of '''concretionary''' structures are developed. At the top of the third cycle, the Billingham Anhydrite and overlying Boulby Halite have been extensively mined beneath Tees-side where they formed the basis for the local chemical industry. Further south in east Yorkshire, the halite gives way in its upper part to the Boulby Potash, which is mined at depth northwest of Whitby. Thus the third cycle represents the most complete development of an evaporitic mineral sequence in the region.
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The beginning of the Upper Permian was marked by the rapid inundation of the low lying areas east and west of the Pennines ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]e). In the North Sea Basin, the Zechstein Sea reworked the upper part of the Yellow Sands. The deposits in the sea are strongly cyclic due to eustatic sea level rise and fall, resulting in the repeated expansion and contraction of the marine area under strongly evaporitic conditions. At the base of the first cycle is a very finely laminated deposit of alternating limestone or dolomite with organic rich layers, formed on the euxinic sea floor. This, the Marl Slate, is famous for the beautiful preservation of palaeoniscid fish, which swam in the oxygenated near surface waters, together with the remains of reptiles that fell or were transported into the sea. Euxinic conditions persisted throughout the first cycle in deeper parts of the basin but around the shallow oxygenated margins, oolitic carbonates developed, protected on their seaward side by a massive bryozoan-algal shelf edge reef, up to 100 m thick and more than 30 km long, that is well exposed in County Durham ([[Magnesian Limestone between South Shields and Seaham - an excursion|Excursion 13]]). The cycle is completed by the basin-ward development of the Hartlepool Anhydrite, probably as a primary precipitate on the basin floor during a fall in sea-level. Four further cycles of carbonate and/or marl deposition, of decreasing thickness, with overlying and basin-ward evaporites are known. The carbonates are often oolitic and stromatolitic, with restricted faunas. Those of the second cycle pass into deeper water slope carbonates showing signs of slumping, and in which a wide range of concretionary structures are developed. At the top of the third cycle, the Billingham Anhydrite and overlying Boulby Halite have been extensively mined beneath Tees-side where they formed the basis for the local chemical industry. Further south in east Yorkshire, the halite gives way in its upper part to the Boulby Potash, which is mined at depth northwest of Whitby. Thus the third cycle represents the most complete development of an evaporitic mineral sequence in the region.
  
 
The cyclic deposits of the Upper Permian grade upwards into red marls with thin lenses of anhydrite and ultimately thick, dominantly fluvial sandstones of the Triassic Sherwood Sandstone Group in southeast Durham and Tees-side. These deposits are almost completely unfossiliferous and the Permo-Triassic boundary is placed as a matter of convenience at a distinctive level within the marls. The Sherwood Sandstone is succeeded, after a short break, by the Mercia Mudstone Group, a sequence of vari-coloured sandstones and red-brown and green marls, with beds of dolomite, anhydrite and evaporite residues in the lower part. These were the deposits of an extensive coastal plain, intermittently flooded by shallow saline waters from the southeast. Triassic rocks are now known almost exclusively from borehole evidence around Tees-side.
 
The cyclic deposits of the Upper Permian grade upwards into red marls with thin lenses of anhydrite and ultimately thick, dominantly fluvial sandstones of the Triassic Sherwood Sandstone Group in southeast Durham and Tees-side. These deposits are almost completely unfossiliferous and the Permo-Triassic boundary is placed as a matter of convenience at a distinctive level within the marls. The Sherwood Sandstone is succeeded, after a short break, by the Mercia Mudstone Group, a sequence of vari-coloured sandstones and red-brown and green marls, with beds of dolomite, anhydrite and evaporite residues in the lower part. These were the deposits of an extensive coastal plain, intermittently flooded by shallow saline waters from the southeast. Triassic rocks are now known almost exclusively from borehole evidence around Tees-side.
  
Younger deposits, excepting the widespread '''glaciogenic''' sediments of the recent past, are unknown in Northumbria, although evidence from outside the area suggests that the major marine transgressions of the Lower Jurassic and Upper Cretaceous may have covered the area. Any sediments deposited were subsequently removed, particularly during the Tertiary, when the northwestern parts of the British Isles underwent rapid uplift following the northward extension of Atlantic '''seafloor spreading''' between Greenland and Scandinavia. Associated igneous activity in centres in western Scotland extended its influence into Northumbria in the form of tholeiitic dykes, the best known being the Armathwaite-Cleveland Dyke in south Durham ([[Geology and landscape of Upper Teesdale - an excursion|Excursion 16]], [[Carboniferous and Permian rocks in southern County Durham - an excursion|Excursion 17]]), and the Tynemouth and Acklington dykes in Northumberland. All have a similar late Palaeocene age and appear to be far-flung representatives of the Mull dyke swarm.
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Younger deposits, excepting the widespread glaciogenic sediments of the recent past, are unknown in Northumbria, although evidence from outside the area suggests that the major marine transgressions of the Lower Jurassic and Upper Cretaceous may have covered the area. Any sediments deposited were subsequently removed, particularly during the Tertiary, when the northwestern parts of the British Isles underwent rapid uplift following the northward extension of Atlantic seafloor spreading between Greenland and Scandinavia. Associated igneous activity in centres in western Scotland extended its influence into Northumbria in the form of tholeiitic dykes, the best known being the Armathwaite-Cleveland Dyke in south Durham ([[Geology and landscape of Upper Teesdale - an excursion|Excursion 16]], [[Carboniferous and Permian rocks in southern County Durham - an excursion|Excursion 17]]), and the Tynemouth and Acklington dykes in Northumberland. All have a similar late Palaeocene age and appear to be far-flung representatives of the Mull dyke swarm.
  
A general trend in global cooling begun in the early Tertiary culminated in the sequence of cold and temperate climates which have affected the British Isles over the last 2.6 '''Ma'''. Several advances of ice probably covered Northumbria but almost all the deposits now preserved relate to the last extensive ice sheet glaciation in the late Devensian, around 17 000 yr '''B.P.''' ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]f; [[The Quaternary of South Tynedale - an excursion|Excursion 12]]). In upland regions, most of the soils and unconsolidated deposits were stripped off and the hills and ridges moulded and streamlined. Ice, moving southwards from Scotland and the Cheviot, and extending into the westernmost part of the North Sea Basin, deflected to the southeast ice moving into the region from the Lake District and Galloway. Vast quantities of debris were deposited in the lowlands, as tills and water-laid deposits, smoothing out the pre-Quaternary relief of southeast Northumberland and eastern Durham. River valleys, some graded to cold stage sea levels down to −50 m O.D., were plugged by clays, sands and gravels. As the ice withdrew, most rivers re-established themselves close to their original courses. Ice melt resulted in the formation of widespread sheets of '''glaciofluvial''' sand and gravel, some now standing as terraces above the main rivers, and also areas of hummocky ice-contact deposits including '''kames, kame terraces''' and '''eskers''', with many '''kettle holes''' and dead-ice hollows. The persistence of coastal and sea ice dammed the eastward drainage of meltwaters leading to the formation of lakes in low-lying areas. In these, laminated clays and silts were deposited with sand and gravel deltas and fans at the margins. In upland areas, fine series of '''meltwater channels''', such as those around the Cheviots, were eroded mainly by water flowing beneath the ice during melting.
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A general trend in global cooling begun in the early Tertiary culminated in the sequence of cold and temperate climates which have affected the British Isles over the last 2.6 Ma. Several advances of ice probably covered Northumbria but almost all the deposits now preserved relate to the last extensive ice sheet glaciation in the late Devensian, around 17 000 yr B.P. ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]f; [[The Quaternary of South Tynedale - an excursion|Excursion 12]]). In upland regions, most of the soils and unconsolidated deposits were stripped off and the hills and ridges moulded and streamlined. Ice, moving southwards from Scotland and the Cheviot, and extending into the westernmost part of the North Sea Basin, deflected to the southeast ice moving into the region from the Lake District and Galloway. Vast quantities of debris were deposited in the lowlands, as tills and water-laid deposits, smoothing out the pre-Quaternary relief of southeast Northumberland and eastern Durham. River valleys, some graded to cold stage sea levels down to −50 m O.D., were plugged by clays, sands and gravels. As the ice withdrew, most rivers re-established themselves close to their original courses. Ice melt resulted in the formation of widespread sheets of glaciofluvial sand and gravel, some now standing as terraces above the main rivers, and also areas of hummocky ice-contact deposits including kames, kame terraces and eskers, with many kettle holes and dead-ice hollows. The persistence of coastal and sea ice dammed the eastward drainage of meltwaters leading to the formation of lakes in low-lying areas. In these, laminated clays and silts were deposited with sand and gravel deltas and fans at the margins. In upland areas, fine series of meltwater channels, such as those around the Cheviots, were eroded mainly by water flowing beneath the ice during melting.
  
The main event of the present interglacial has been the rise in sea level to give the coastal morphology we see today. Evidence of the rise is found in submerged forests and peats, extending to at least −5 m O.D., present on the coasts of Northumberland and Durham. With a low tide, and not too much beach sand, these features can be readily seen in several places, such as Hartlepool, Seaburn, Blyth Beach, Cresswell, and Hauxley south of Amble. In the uplands, extensive peat deposits have formed in the higher, wetter areas.
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The main event of the present interglacial has been the rise in sea level to give the coastal morphology we see today. Evidence of the rise is found in submerged forests and peats, extending to at least −5 m O.D., present on the coasts of Northumberland and Durham. With a low tide, and not too much beach sand, these features can be readily seen in several places, such as Hartlepool, Seaburn, Blyth Beach, Cresswell, and Hauxley south of Amble. In the uplands, extensive peat deposits have formed in the higher, wetter areas.
  
 
== [[Northumbrian rocks and landscape: a field guide#Glossary|Glossary]] ==
 
== [[Northumbrian rocks and landscape: a field guide#Glossary|Glossary]] ==

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