Editing River Severn (section)

==Geography and geology==
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[[File:Severnsource.jpg|thumb|right|upright|Post marked as the [[Source (river or stream)|source]] of the River Severn on [[Plynlimon]], Wales. The wording is in both English and Welsh.]]
The River Severn's current form is the result of a multi-million year history and complex underlying geology but is in part the result of glaciation during the last [[ice age]] in the [[Pleistocene]] [[Epoch (geology)|epoch]] of the [[Quaternary]] period.<ref>{{cite journal|issn= 1750-855X |journal=Proceedings of the Shropshire Geological Society|volume=13|pages= 92–99|publisher=Shropshire Geological Society|title=Glaciation and drainage evolution in the southern Welsh Borderland|citeseerx = 10.1.1.616.8048}}</ref> Within Wales, the river runs through a landscape formed in [[Ordovician]] and [[Silurian]] rocks. As it enters the Shropshire Plain, these lower [[Palaeozoic]] rocks are replaced by [[Permian]] and [[Triassic]] age strata though largely unseen beneath a thick cover of Quaternary deposits. Certain stretches also run across [[Carboniferous]] strata as at Shrewsbury and for much of the distance between Ironbridge and Bewdley. Permo-Triassic bedrock then continues until the Severn moves intermittently onto the [[Jurassic]] outcrop from Tewkesbury southwards. Only in the SSW-NNE aligned valley either side of Welshpool is there any obvious relationship to geological structure where the valley follows the lines of the Severn Valley Fault Belt.<ref>{{cite map |year=2008 |title=Welshpool |scale=1:50,000 |series=England and Wales Geology |location=Keyworth, Nottingham |publisher=British Geological Survey |isbn=9780751834710 }}</ref> For much of the rest of its course it runs directly across geological structures.

It was first proposed in the 1900s that the former northerly course of the upper Severn was disrupted during the course of the [[ice age]] by the blocking of its access to the [[Irish Sea]] through [[Cheshire]] causing a large lake to develop across much of Shropshire. It was supposed that this lake, named as [[Lake Lapworth]], overtopped its southern margin and rapidly cut down to form the [[Ironbridge Gorge]] providing the Severn with a southerly exit to the sea as remains the case today.<ref>{{cite journal|url=http://www.esci.keele.ac.uk/people/pgk/Dissertation/Pannett%202008.pdf|title=The Ice Age Legacy in North Shropshire|journal=Proceedings of the Shropshire Geological Society|volume=13|pages=86–91|year=2008|publisher=Shropshire Geological Society|access-date=16 February 2020|archive-date=13 June 2020|archive-url=https://web.archive.org/web/20200613185926/http://www.esci.keele.ac.uk/people/pgk/Dissertation/Pannett%202008.pdf|url-status=live}}</ref> An alternative theory which has gained favour in recent years does away with Lake Lapworth, suggesting that the upper Severn flowed beneath the icesheet in a bedrock hollow known as the Severn Trench eastwards from Melverley to the Ironbridge Gorge. It is possible that the trench and gorge were cut over successive ice ages.<ref>{{cite book |last1=Toghill |first1=Peter |title=Geology of Shropshire |date=2006 |publisher=The Crowood Press |location=Marlborough |isbn=1861268033 |pages=240–243 |edition=Second}}</ref>