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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.
+
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.
 
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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