Weald–Artois Anticline: Difference between revisions
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[[File:Geologic map SE England & Channel EN.svg|thumb|[[Geologic map]] of southeast [[England]] and the region around the [[English Channel]], showing the Weald-Artois anticline in its regional context.]] |
{{Short description|Geologic structure in southern England and northern France}} |
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[[File:Geologic map SE England & Channel EN.svg|thumb|upright=1.75|[[Geologic map]] of southeast [[England]] and the region around the [[English Channel]], showing the Weald-Artois anticline in its regional context.]] |
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[[File:WealdBasinCrossSection.svg|thumb|upright=1.75|Cross-section over the Wealden anticline]] |
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The '''Weald–Artois |
The '''Weald–Artois Anticline''', or '''Wealden Anticline''', is a large [[anticline]], a geological structure running between the regions of the [[Weald]] in southern [[England]] and [[Artois]] in northern [[France]]. The fold formed during the [[Alpine orogeny]], from the late [[Oligocene]] to middle [[Miocene]] as an uplifted form of the [[Weald basin]] through [[Inversion (geology)|inversion]] of the basin. The [[fold (geology)|folding]] resulted in [[tectonic uplift|uplift]] of about {{convert|180|m}},<ref>[http://www-qpg.geog.cam.ac.uk/research/projects/tertiaryrivers/neog.html Tertiary Rivers: Neogene (Miocene and Pliocene)], [[Cambridge University]]</ref> though concurrent [[erosion]] may have substantially reduced the actual height of the resulting [[chalk]] ridges. |
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As is the case with all anticlines, older rock strata are found in the core of the structure. These are in this case [[Upper Jurassic]] and [[Lower Cretaceous]] strata. The |
As is the case with all anticlines, older rock strata are found in the core of the structure. These are in this case [[Upper Jurassic]] and [[Lower Cretaceous]] strata. The western part of the ridge (the Weald of [[Kent]], [[Sussex]] and [[Surrey]] in England) has been greatly eroded, with the presumed chalk surface removed to expose older, Lower Cretaceous rocks ([[Wealden Group]]) and a small area of Upper Jurassic [[Purbeck Beds]].<ref>Gallois R.W. & Edmunds M.A. (4th Ed 1965), ''The Wealden District'', British Regional Geology series, British Geological Survey, {{ISBN|0-11-884078-9}}</ref> On the French side of the English Channel more [[Upper Jurassic]] rocks crop out in a small area around [[Boulogne-sur-Mer]] and [[Desvres]].<ref>''Carte Géologique de la France'', BRGM, Orléans</ref> At the flanks of the anticline outcrops of the (younger) Upper Cretaceous [[Chalk Group|Chalk]] occur. The chalk survives as a rim of inward-facing [[escarpment]]s, forming the [[North Downs]] and [[South Downs]]. The Chalk forms characteristic white [[cliff]]s on both sides of the English Channel, an example being the [[White Cliffs of Dover]]. |
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==Formation of the Strait of Dover== |
==Formation of the Strait of Dover== |
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{{see also|Geology of the southern North Sea}} |
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[[File:France manche vue dover.JPG|thumb|The [[white cliffs of Dover]] as seen from [[Cap Gris-Nez]]. The [[Straits of Dover]] were carved out of the anticline surface layer, presumably chalk, by [[glacial lake outburst flood]]s]] |
[[File:France manche vue dover.JPG|thumb|The [[white cliffs of Dover]] as seen from [[Cap Gris-Nez]]. The [[Straits of Dover]] were carved out of the anticline surface layer, presumably chalk, by [[glacial lake outburst flood]]s]] |
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The Strait of Dover is geologically speaking a very young feature, |
The [[Strait of Dover]] is geologically speaking a very young feature, which cuts through the much older Weald–Artois Anticline. The anticline continues uninterrupted in the subsurface of the Strait of Dover and English Channel. |
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Gupta ''et al.'' argue that the section of the anticline between Britain and France was destroyed by two megafloods caused by the breaching of [[proglacial lake]]s. The first occurred around 450,000 years ago during the [[Anglian Glaciation]] ([[Marine isotope stage|MIS]] 12). It did not fully destroy the anticline although there was sporadic connection between the English Channel and the North Sea during periods of high sea level. The second megaflood occurred around 160,000 years ago during the [[Wolstonian Stage|Wolstonian]]/[[Saale glaciation|Saalian]] glaciation (MIS 6), after which Britain would be an island during periods of high sea level.<ref>{{cite journal|url= |journal=Nature Communications|last=Gupta|first= Sanjeev|display-authors=etal|title=Two-stage opening of the Dover Strait and the origin of island Britain|date=4 April 2017|volume=8|pages=15101|doi=10.1038/ncomms15101|pmid=28375202|pmc=5382280}}</ref> |
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During the [[Pleistocene]] an ice sheet blocked the flow of rivers including the [[Thames]] and [[Rhine]] to the [[North Sea]], resulting in a large [[proglacial lake]] to the northeast of the ridge. To the southwest, low-lying land connected the island that is now [[Great Britain]] to continental [[Europe]]. Two overtoppings of this ridge severed this peninsular connection. |
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* The first [[glacial lake outburst flood]] occurred approximately 425,000 years ago, resulting in a waterfall of up to a million cubic meters per second that gouged out the [[Straits of Dover]] and caused much sudden outwash flood scouring (as shown by [[braided channel]]s and diamond-shaped islands in sonar surveys) in low-lying land where the [[English Channel]] is now. |
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* The second, which may have been larger than the first, occurred approximately 225,000 years ago and finally severed the slender peninsula.<ref>{{cite web |url=http://www.nature.com/news/2007/070716/full/070716-11.html |title=The megaflood that made Britain an island |author=Schiermeier, Quirin |work=[[Nature (journal)|Nature]] |date=2007-07-18|accessdate=2008-01-31}}</ref><ref>{{cite web |url=http://environment.newscientist.com/article/dn12289-dambusting-megaflood-made-britain-an-island.html |title=Dam-busting "megaflood" made Britain an island |date=2007-07-18 |author=Marks, Paul|work=[[New Scientist]]|accessdate=2008-01-31}}</ref> |
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During the |
During the [[Last glacial period|most recent glacial period]], lowering of sea levels joined the British Isles once more to the continental mainland of [[Europe]] via [[Doggerland]], until about 6500–6200 BCE. |
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== References == |
== References == |
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{{DEFAULTSORT:Weald-Artois Anticline}} |
{{DEFAULTSORT:Weald-Artois Anticline}} |
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[[Category: |
[[Category:Anticlines]] |
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[[Category:Landforms of England]] |
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[[Category:Landforms of France]] |
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[[Category:English Channel]] |
[[Category:English Channel]] |
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[[Category:Geology of England]] |
[[Category:Geology of England]] |
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Latest revision as of 10:18, 22 April 2023
The Weald–Artois Anticline, or Wealden Anticline, is a large anticline, a geological structure running between the regions of the Weald in southern England and Artois in northern France. The fold formed during the Alpine orogeny, from the late Oligocene to middle Miocene as an uplifted form of the Weald basin through inversion of the basin. The folding resulted in uplift of about 180 metres (590 ft),[1] though concurrent erosion may have substantially reduced the actual height of the resulting chalk ridges.
As is the case with all anticlines, older rock strata are found in the core of the structure. These are in this case Upper Jurassic and Lower Cretaceous strata. The western part of the ridge (the Weald of Kent, Sussex and Surrey in England) has been greatly eroded, with the presumed chalk surface removed to expose older, Lower Cretaceous rocks (Wealden Group) and a small area of Upper Jurassic Purbeck Beds.[2] On the French side of the English Channel more Upper Jurassic rocks crop out in a small area around Boulogne-sur-Mer and Desvres.[3] At the flanks of the anticline outcrops of the (younger) Upper Cretaceous Chalk occur. The chalk survives as a rim of inward-facing escarpments, forming the North Downs and South Downs. The Chalk forms characteristic white cliffs on both sides of the English Channel, an example being the White Cliffs of Dover.
Formation of the Strait of Dover
[edit]
The Strait of Dover is geologically speaking a very young feature, which cuts through the much older Weald–Artois Anticline. The anticline continues uninterrupted in the subsurface of the Strait of Dover and English Channel.
Gupta et al. argue that the section of the anticline between Britain and France was destroyed by two megafloods caused by the breaching of proglacial lakes. The first occurred around 450,000 years ago during the Anglian Glaciation (MIS 12). It did not fully destroy the anticline although there was sporadic connection between the English Channel and the North Sea during periods of high sea level. The second megaflood occurred around 160,000 years ago during the Wolstonian/Saalian glaciation (MIS 6), after which Britain would be an island during periods of high sea level.[4]
During the most recent glacial period, lowering of sea levels joined the British Isles once more to the continental mainland of Europe via Doggerland, until about 6500–6200 BCE.
References
[edit]- ^ Tertiary Rivers: Neogene (Miocene and Pliocene), Cambridge University
- ^ Gallois R.W. & Edmunds M.A. (4th Ed 1965), The Wealden District, British Regional Geology series, British Geological Survey, ISBN 0-11-884078-9
- ^ Carte Géologique de la France, BRGM, Orléans
- ^ Gupta, Sanjeev; et al. (4 April 2017). "Two-stage opening of the Dover Strait and the origin of island Britain". Nature Communications. 8: 15101. doi:10.1038/ncomms15101. PMC 5382280. PMID 28375202.