Editing River Thames (section)

== Geology ==
{{See also|Ancestral Thames}}
[[File:Europe20000ya.png|thumb|European [[Last Glacial Maximum|LGM]] refuges, 20,000 years ago. The Thames was a minor river that joined the [[Rhine]], in the southern [[North Sea Basin]] at this time.<br />{{legend|#c54b00|[[w:Solutrean|Solutrean]] and Proto Solutrean Cultures}}{{legend|#ca00b0|Epi [[w:Gravettian|Gravettian]] Culture}}]]

Researchers have identified the River Thames as a discrete drainage line flowing as early as 58&nbsp;million years ago, in the [[Thanetian]] stage of the late [[Palaeocene]] epoch.<ref name=terriv>{{cite web |url=http://www.qpg.geog.cam.ac.uk/research/projects/tertiaryrivers/ |title=History of the major rivers of southern Britain during the Tertiary |publisher=Quaternary Palaeoenvironments Group |year=2006 |access-date=28 November 2007 |archive-date=11 October 2007 |archive-url=https://web.archive.org/web/20071011112249/http://www.qpg.geog.cam.ac.uk/research/projects/tertiaryrivers/ |url-status=dead}}</ref> Until around 500,000 years ago, the Thames flowed on its existing course through what is now [[Oxfordshire]], before turning to the north-east through [[Hertfordshire]] and [[East Anglia]] and reaching the [[North Sea]] near present-day [[Ipswich]].<ref>{{Cite web |title=The early Ice Age |url=http://www.geoessex.org.uk/the_early_ice_age.html |website=www.geoessex.org.uk |access-date=7 February 2016 |archive-url=https://web.archive.org/web/20160119080111/http://geoessex.org.uk/the_early_ice_age.html |archive-date=19 January 2016 |url-status=dead}}</ref>

At this time the river-system headwaters lay in the English [[West Midlands (region)|West Midlands]] and may, at times, have received drainage from the [[Berwyn Mountains]] in [[North Wales]].

=== Ice age ===
About 450,000 years ago, in the most extreme [[Last Glacial Period|Ice Age]] of the [[Pleistocene]], the [[Anglian (stage)|Anglian]], the furthest southern extent of the ice sheet reached [[Hornchurch]]<ref>{{cite web |url=http://www.essexwt.org.uk/geology/geology3.htm |title=Essex Wildlife Trust, The Geology of Essex |url-status=dead |archive-url=https://web.archive.org/web/20121021073553/http://www.essexwt.org.uk/geology/geology3.htm |archive-date=21 October 2012 }}</ref> in east London, the Vale of St Albans, and the [[Finchley Gap]].

It dammed the river in [[Hertfordshire]], resulting in the formation of large ice lakes, which eventually burst their banks and caused the river to divert onto its present course through the area of present-day London.

The ice lobe which stopped at present-day [[Finchley]] deposited about 14 metres of [[boulder clay]] there.<ref>Ellison RA (2004), ''Geology of London'', British Geological Survey, p58.</ref> Its torrent of [[meltwater]] gushed through the [[Finchley Gap]] and south towards the new course of the Thames, and proceeded to carve out the [[River Brent|Brent Valley]] in the process.<ref>{{cite web |url=https://www.mylondon.news/news/local-news/retro-a-river-worth-preserving-5989210 |title=Retro: A river worth preserving |publisher=Ealing Gazette |date=18 February 2011 |access-date=27 August 2020 |archive-date=3 December 2020 |archive-url=https://web.archive.org/web/20201203173255/https://www.mylondon.news/news/local-news/retro-a-river-worth-preserving-5989210 |url-status=live }}</ref>

The Anglian ice advance resulted in a new course for the Thames through [[Berkshire]] and on into London, after which the river rejoined its original course in southern [[Essex]], near the present [[River Blackwater (Essex)|River Blackwater]] estuary. Here it entered a substantial freshwater lake in the southern North Sea basin, south of what is called [[Doggerland]]. The overspill of this lake caused the formation of the [[Channel River]] and later the [[Strait of Dover|Dover Strait]] gap between present-day [[Great Britain|Britain]] and France. Subsequent development led to the continuation of the course that the river follows at the present day.<ref name=nwriv />

Most of the [[bedrock]] of the [[Aylesbury Vale|Vale of Aylesbury]] comprises [[clay]] and [[chalk]] that formed at the end of the [[ice age]] and at one time was under the [[Ancestral Thames|Proto-Thames]]. At this time the vast underground reserves of water formed that make the [[water table]] higher than average in the Vale of Aylesbury.<ref>{{cite web |title=Buckingham Surface Water Management Plan |url=https://old.buckscc.gov.uk/media/2275771/Buckingham-phase1-swmp.pdf |publisher=Buckinghamshire County Council |access-date=27 August 2020 |archive-date=27 August 2020 |archive-url=https://web.archive.org/web/20200827085728/https://old.buckscc.gov.uk/media/2275771/Buckingham-phase1-swmp.pdf |url-status=live}}</ref>

[[File:Geological map of London Basin.jpg|thumb |A geological map of the [[London Basin]]; the London Clay is marked in dark brown.]]
[[File:Confluence of Rivers Thames and Brent - geograph.org.uk - 921332.jpg|thumb |The confluence of the Rivers Thames and Brent. The [[narrowboat]] is heading up the [[River Brent]]. From this point as far as [[Hanwell]] the Brent has been canalised and shares its course with the main line of the [[Grand Union Canal]]. From Hanwell the Brent can be traced to various sources in the [[Chipping Barnet|Barnet]] area.]]

At the height of the [[Last Glacial Period|last ice age]], around 20,000 BC, Britain was connected to mainland Europe by a large expanse of land known as [[Doggerland]] in the southern [[North Sea Basin]]. At this time, the Thames' course did not continue to Doggerland but flowed southwards from the eastern Essex coast where it met the waters of the proto-[[Rhine–Meuse–Scheldt delta]]<ref name=nwriv>{{cite web |url=http://www.qpg.geog.cam.ac.uk/research/projects/nweurorivers/ |title=History of the northwest European rivers during the past three million years |publisher=Quaternary Palaeoenvironments Group |year=2007 |access-date=28 November 2007 |archive-date=2 November 2007 |archive-url=https://web.archive.org/web/20071102213709/http://www.qpg.geog.cam.ac.uk/research/projects/nweurorivers/ |url-status=dead}}</ref> flowing from what are now the [[Netherlands]] and [[Belgium]]. These rivers formed a single river – the [[Channel River]] (''Fleuve Manche'') – that passed through the Dover Strait and drained into the Atlantic Ocean in the western [[English Channel]].

Upon the valley sides of the Thames and some of its tributaries can be seen other terraces of [[brickearth]], laid over and sometimes interlayered with the clays. These deposits were brought in by the winds during the [[periglacial]] periods, suggesting that wide, flat marshes were then part of the landscape, which the new rivers proceeded to cut into.

The steepness of some valley sides indicates very much lower [[mean sea level]]s caused by the glaciation locking up so much water upon the land masses, thus causing the river water to flow rapidly seaward and so erode its bed quickly downwards.

The original land surface was around {{cvt|350|to|400|ft|m|abbr=off}} above the current sea-level. The surface had sandy deposits from an ancient sea, laid over sedimentary clay (this is the blue [[London Clay]]). All the erosion down from this higher land surface, and the sorting action by these changes of water flow and direction, formed what is known as the Thames [[Fluvial terrace|River Gravel Terraces]]. Sand and gravel was deposited near Beaconsfield and other places by the [[Ancestral Thames| ancestral River Thames]] This sand and gravel is now being excavated near near Beaconsfield.<ref>{{Cite web |date=2003 |title=Mineral Resource Information in Support of National, Regional and Local Planning Buckinghamshire and Milton Keynes |url=https://nora.nerc.ac.uk/id/eprint/527334/1/CR03077N.pdf |access-date=November 3, 2024 |website=nora.nerc.ac.uk |publisher=British Geological Survey}}</ref>

Since Roman times and perhaps earlier, the [[isostasy|isostatic]] rebound from the weight of previous ice sheets, and its interplay with the [[eustatic]] change in sea level, have resulted in the old valley of the River Brent, together with that of the Thames, silting up again. Thus, along much of the Brent's present-day course, one can make out the [[water-meadow]]s of rich alluvium, which is augmented by frequent floods.