Editing Permian and Carboniferous rocks of Knaresborough - an excursion

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== Geological background ==
 
== Geological background ==
  
At the start of the last glacial advance (Devensian) the topography of the Knaresborough district was different to that seen today. The proto River Nidd ran to the north and east of the present town. It deviated from its present course at Nidd [SE 302 608] and ran through Brearton and past Farnham [SE 345 605] to the northern outskirts of Knaresborough [SE 363 580], before heading eastwards. During the advance of the Devensian ice-sheet a thick fan of sand and gravel was deposited in this valley, emanating from the front of the ice-sheet via glacial channels around Farnham [SE 352 606] and Occaney [SE 352 619]; this deposit was worked in the gravel pits north of Knaresborough [SE 356 587]. If Knaresborough is approached from the north via the B6166 from Boroughbridge the extent of this buried valley, and its associated sand and gravel deposits, can be appreciated from the road. As the ice advanced further to the south and west it over-rode the sand and gravel, completely blocking the proto-Nidd drainage and diverting the river westwards. Here the river exploited the lowest, softest rocks and incised the present Nidd Gorge. West of the Nidd Gorge the glacial deposits are generally thin and probably pre-Devensian in age; east of the gorge the Devensian deposits comprise thick hummocky glacial '''till''' with '''moraines''', '''eskers''' and late glacial lake deposits.
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At the start of the last glacial advance (Devensian) the topography of the Knaresborough district was different to that seen today. The proto River Nidd ran to the north and east of the present town. It deviated from its present course at Nidd [SE 302 608] and ran through Brearton and past Farnham [SE 345 605] to the northern outskirts of Knaresborough [SE 363 580], before heading eastwards. During the advance of the Devensian ice-sheet a thick fan of sand and gravel was deposited in this valley, emanating from the front of the ice-sheet via glacial channels around Farnham [SE 352 606] and Occaney [SE 352 619]; this deposit was worked in the gravel pits north of Knaresborough [SE 356 587]. If Knaresborough is approached from the north via the B6166 from Boroughbridge the extent of this buried valley, and its associated sand and gravel deposits, can be appreciated from the road. As the ice advanced further to the south and west it over-rode the sand and gravel, completely blocking the proto-Nidd drainage and diverting the river westwards. Here the river exploited the lowest, softest rocks and incised the present Nidd Gorge. West of the Nidd Gorge the glacial deposits are generally thin and probably pre-Devensian in age; east of the gorge the Devensian deposits comprise thick hummocky glacial till with moraines, eskers and late glacial lake deposits.
  
The solid rocks exposed in Knaresborough Gorge are of Carboniferous (Namurian) and Upper Permian age. Here the Permian strata '''overlap''' onto and submerge a surface of irregular relief eroded in the underlying Carboniferous sandstones and shales. The exposed Carboniferous sequence extends upwards from the Lower Follifoot Grit to the Upper Plompton Grit. At the northern end of Knaresborough the Carboniferous rocks are folded around the nose of the Harrogate '''Anticline''' and '''dip''' at up to 27° to the east and southeast. This anticline was formed during the '''Variscan Orogeny''', prior to the late Permian deposition.
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The solid rocks exposed in Knaresborough Gorge are of Carboniferous (Namurian) and Upper Permian age. Here the Permian strata overlap onto and submerge a surface of irregular relief eroded in the underlying Carboniferous sandstones and shales. The exposed Carboniferous sequence extends upwards from the Lower Follifoot Grit to the Upper Plompton Grit. At the northern end of Knaresborough the Carboniferous rocks are folded around the nose of the Harrogate Anticline and dip at up to 27° to the east and southeast. This anticline was formed during the Variscan Orogeny, prior to the late Permian deposition.
  
The overlying Permian strata dip gently eastwards and rest unconformably on the Carboniferous rocks (cross-section in [[:File:YGS_YORKROCK_FIG_13_01.jpg|Figure 13.1]]). The lowest Permian rocks seen here are the Cadeby Formation (formerly Lower Magnesian Limestone). This formation is over 40 m thick and subdivided into two members: the Wetherby Member (formerly Lower Subdivision) and the Sprotbrough Member (formerly Upper Subdivision). The Cadeby Formation (Sprotbrough Member) forms the ridge on which Knaresborough is built. The ridge consists largely of '''cross-bedded oolites''' and is capped by a small '''outlier''' of Edlington Formation (formerly Middle Marl). The Knaresborough ridge appears to be a primary depositional feature, for the limestone thins away from it in all directions and depositional dips mimic the form of the ridge. The thinning of the formation is best illustrated northwest of the town centre, near Conyngham Hall, where it thins to 1–3 m of even-bedded, sandy '''dolomite'''; the westward thinning is illustrated by the cross-section in [[:File:YGS_YORKROCK_FIG_13_01.jpg|Figure 13.1]]. The overlying Edlington Formation comprises red-brown calcareous mudstone with some '''gypsum''' and is generally poorly exposed. It overlaps the Cadeby Formation both northwards and westwards to rest directly on the Carboniferous rocks. The Edlington Formation is in turn overlapped by the Brotherton Formation (formerly Upper Magnesian Limestone), a sequence of dolomitic limestone 5–15 m thick, which rests directly on Carboniferous rocks at Scriven [SE 345 585] and Rudfarlington [SE 342 543].
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The overlying Permian strata dip gently eastwards and rest unconformably on the Carboniferous rocks (cross-section in [[:File:YGS_YORKROCK_FIG_13_01.jpg|Figure 13.1]]). The lowest Permian rocks seen here are the Cadeby Formation (formerly Lower Magnesian Limestone). This formation is over 40 m thick and subdivided into two members: the Wetherby Member (formerly Lower Subdivision) and the Sprotbrough Member (formerly Upper Subdivision). The Cadeby Formation (Sprotbrough Member) forms the ridge on which Knaresborough is built. The ridge consists largely of cross-bedded oolites and is capped by a small outlier of Edlington Formation (formerly Middle Marl). The Knaresborough ridge appears to be a primary depositional feature, for the limestone thins away from it in all directions and depositional dips mimic the form of the ridge. The thinning of the formation is best illustrated northwest of the town centre, near Conyngham Hall, where it thins to 1–3 m of even-bedded, sandy dolomite; the westward thinning is illustrated by the cross-section in [[:File:YGS_YORKROCK_FIG_13_01.jpg|Figure 13.1]]. The overlying Edlington Formation comprises red-brown calcareous mudstone with some gypsum and is generally poorly exposed. It overlaps the Cadeby Formation both northwards and westwards to rest directly on the Carboniferous rocks. The Edlington Formation is in turn overlapped by the Brotherton Formation (formerly Upper Magnesian Limestone), a sequence of dolomitic limestone 5–15 m thick, which rests directly on Carboniferous rocks at Scriven [SE 345 585] and Rudfarlington [SE 342 543].
  
 
== Excursion details ==
 
== Excursion details ==

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