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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.
 
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.
 
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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