Editing Geological history of Northumbria

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Northumbria (Northumberland, Durham, Tyne & Wear and Cleveland north of the Tees) is dominated by rocks of Carboniferous age [[:File:YGS_NORTROCK_FIG_00_1.jpg|Figure 1]], [[:File:YGS_NORTROCK_FIG_00_2.jpg|Figure 2]]. On the northern margin of the area, they rest on a range of older rocks in the Scottish borders, principally Silurian sediments and the lavas of the early Devonian Cheviot volcano. Lower Carboniferous sediments, which crop out on the coast of north Northumberland, form a broad belt inland, skirting the ancient volcanic pile and striking southwest parallel to the Scottish border, before turning south-southeast along the Pennine front. Gentle easterly and southeasterly '''dips''' bring in the mid Carboniferous to the southeast, forming a narrow triangular outcrop with its apex on the mid Northumberland coast and its base forming the north Pennine peaks and the Durham dales. The upper Carboniferous Coal Measures in turn form the south Northumberland coast and extend inland as a north–south outcrop, widest at the Tyne and narrowing into south Durham. There, the Coal Measures are '''overstepped''' by the Permian, which rests '''unconformably''' upon them. These Permian rocks form the high ground in the east of County Durham and the distinctive buff coastal cliffs of Durham and Tyne & Wear. They pass up into Triassic rocks underlying the low ground of Tees-side. Younger Mesozoic and Tertiary rocks, apart from some small '''igneous''' intrusions, are not preserved in Northumbria, but the effects of the Pleistocene glaciation are apparent everywhere. '''Tills''' mantle the solid rocks, particularly thickly in central and south Durham.
 
Northumbria (Northumberland, Durham, Tyne & Wear and Cleveland north of the Tees) is dominated by rocks of Carboniferous age [[:File:YGS_NORTROCK_FIG_00_1.jpg|Figure 1]], [[:File:YGS_NORTROCK_FIG_00_2.jpg|Figure 2]]. On the northern margin of the area, they rest on a range of older rocks in the Scottish borders, principally Silurian sediments and the lavas of the early Devonian Cheviot volcano. Lower Carboniferous sediments, which crop out on the coast of north Northumberland, form a broad belt inland, skirting the ancient volcanic pile and striking southwest parallel to the Scottish border, before turning south-southeast along the Pennine front. Gentle easterly and southeasterly '''dips''' bring in the mid Carboniferous to the southeast, forming a narrow triangular outcrop with its apex on the mid Northumberland coast and its base forming the north Pennine peaks and the Durham dales. The upper Carboniferous Coal Measures in turn form the south Northumberland coast and extend inland as a north–south outcrop, widest at the Tyne and narrowing into south Durham. There, the Coal Measures are '''overstepped''' by the Permian, which rests '''unconformably''' upon them. These Permian rocks form the high ground in the east of County Durham and the distinctive buff coastal cliffs of Durham and Tyne & Wear. They pass up into Triassic rocks underlying the low ground of Tees-side. Younger Mesozoic and Tertiary rocks, apart from some small '''igneous''' intrusions, are not preserved in Northumbria, but the effects of the Pleistocene glaciation are apparent everywhere. '''Tills''' mantle the solid rocks, particularly thickly in central and south Durham.
  
As well as dominating Northumbria geologically, Carboniferous rocks, with their natural resources, have been a major socio-economic influence on the region. Mineral deposits have long been worked in the Pennine dales, and the rich coal resources of the Northumberland and Durham Coalfield underpinned the heavy industry of Tyneside, Wearside and Tees-side. The Permian has contributed also, with salt and '''anhydrite''' deposits formerly worked under Tees-side. The region is still in transition following the painful contraction of its powerful industrial base of the earlier 20th century.
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As well as dominating Northumbria geologically, Carboniferous rocks, with their natural resources, have been a major socio-economic influence on the region. Mineral deposits have long been worked in the Pennine dales, and the rich coal resources of the Northumberland and Durham Coalfield underpinned the heavy industry of Tyneside, Wearside and Tees-side. The Permian has contributed also, with salt and anhydrite deposits formerly worked under Tees-side. The region is still in transition following the painful contraction of its powerful industrial base of the earlier 20th century.
  
The earliest geological events recorded in the rocks of the region are the final stages in the closure of an ancient seaway, the Iapetus Ocean, which separated the northern and southern halves of the British Isles in the Lower Palaeozoic ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]a). England, Wales and southeast Ireland were part of the microcontinent of Eastern Avalonia. This, together with Western Avalonia, moved northwards towards the equator from high southern latitudes, rapidly during the Ordovician and more slowly during the Silurian, as the ocean closed. Sediments deposited on the northern margin of the microcontinent, together with '''subduction'''-related volcanic rocks, are exposed in the Lake District and the west face of the Pennines, but only a very small '''inlier''' in Teesdale reveals a part of this sequence east of the Pennines in Northumbria. However, borehole evidence in central Durham proves rocks of this type at 860 m, thus showing they extend beneath the county at depth.
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The earliest geological events recorded in the rocks of the region are the final stages in the closure of an ancient seaway, the Iapetus Ocean, which separated the northern and southern halves of the British Isles in the Lower Palaeozoic ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]a). England, Wales and southeast Ireland were part of the microcontinent of Eastern Avalonia. This, together with Western Avalonia, moved northwards towards the equator from high southern latitudes, rapidly during the Ordovician and more slowly during the Silurian, as the ocean closed. Sediments deposited on the northern margin of the microcontinent, together with subduction-related volcanic rocks, are exposed in the Lake District and the west face of the Pennines, but only a very small inlier in Teesdale reveals a part of this sequence east of the Pennines in Northumbria. However, borehole evidence in central Durham proves rocks of this type at 860 m, thus showing they extend beneath the county at depth.
  
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.
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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.
  
 
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.
 
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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