Editing River Mole (section)

==Geology==

===Upper Mole <span style="display:none">(geology)</span>===

The Mole rises south of Rusper in West Sussex, where an outcrop of the [[Wealden Group|Hastings Beds sandstone]] dips below the impermeable [[Weald Clay]].<ref name=MAls_2013/> From the source to [[Dorking]], the river drains an area of {{convert|340|km2|sqmi|abbr=on}}, of which approximately 60% is on Wealden or Atherfield Clay, 20% is on Tunbridge Wells Sand and 20% is on [[greensand]].<ref name=physiography/> [[Brickearth]] deposits are common in the valley around Betchworth and east of Dorking.<ref>{{cite web |url= http://www.surreymuseums.org.uk/collections/geology/geosur.html |title= The geology of the county of Surrey |date= November 2006 |work= Geological Collections in Surrey Museums |publisher= Surrey Museums Consultative Committee |access-date= 6 February 2012 |url-status= dead |archive-url= https://web.archive.org/web/20120229212044/http://www.surreymuseums.org.uk/collections/geology/geosur.html |archive-date= 29 February 2012 |df= dmy-all }}</ref>

The upper Mole catchment is dominated by a single broad terrace, which runs continuously from [[Tilgate|Tilgate Forest]] to the entry to the Mole Gap. Above Meath Green (near [[Horley]]), the terrace corresponds to the [[flood plain]] of the river, however from this point downstream to Brockham, the river cuts a narrow trench no more than {{cvt|150|m|yd}} wide. A similar trench is runs up several of the Mole's tributaries, notably along Deanoak Brook which follows a local [[strike and dip|strike]]. The presence of the trench protects the land from flooding, rendering it suitable for [[agriculture]].<ref name=physiography/>

===Mole Gap <span style="display:none">(geology)</span>===
[[File:Dry River Mole, 20 July 2022.jpg|thumb|The dry channel of the River Mole in [[Norbury Park]] in July 2022. The main flow has ceased leaving isolated shallow pools.]]
Between Dorking and Leatherhead the Mole cuts a steep-sided valley through the North Downs, creating a {{convert|170|m|ft|adj=mid|-high}} river cliff on the western flank of Box Hill. The bedrock is permeable chalk and the [[water table]] lies permanently below the level of the riverbed, allowing water to drain out of the river through swallow holes in the bed and banks.<ref name=Edmunds>{{cite journal |author= Edmunds FH |year= 1943 |title= Swallow holes and openings in the chalk of the Mole Valley |journal= The London Naturalist |pages= 2–7 }}</ref> The amount of water lost from the river is significant and in very hot summers the channel can become dry between Mickleham and Thorncroft Manor; this was recorded most recently in 1949,<ref>{{cite web |url= http://geoscenic.bgs.ac.uk/asset-bank/action/viewAsset?id=65886&index=2&total=4&collection=Swallow%20hole&categoryId=1330&categoryTypeId=1&filterId=0&sortAttributeId=1050&sortDescending=true&movedBr=null |title= Photograph of swallow-hole in bed of River Mole |author= Rhodes J |year= 1949 |work= Geoscenic Digital Assets |publisher= British Geological Survey |access-date= 16 February 2012 |archive-url= https://web.archive.org/web/20160304230326/http://geoscenic.bgs.ac.uk/asset-bank/action/viewAsset?id=65886&index=2&total=4&collection=Swallow%20hole&categoryId=1330&categoryTypeId=1&filterId=0&sortAttributeId=1050&sortDescending=true&movedBr=null |archive-date= 4 March 2016 |url-status= live }}</ref><ref name=Chouler>{{cite book |last1= Chouler |first1= WH |title= Horley: Pageant of a Wealden Parish |year= 1975 |publisher= WH Chouler |location= Horley |isbn= 0-9502949-1-8 }}</ref> 1976<ref name=Currie>{{cite book |last1= Davison |first1= M |last2= Currie |first2= I |title= Surrey Weather Book |edition= 4th |year= 1996 |publisher= Frosted Earth |location= Coulsdon |isbn= 978-0-9516710-6-1}}</ref> and 2022.<ref name=ITV_News_2022/><ref>{{cite news |last= Seymour |first= Jenny |date= 19 August 2022 |title= Environment Agency rescue fish from River Mole in Surrey as water levels drop |work= Surrey Live |url= https://www.getsurrey.co.uk/news/surrey-news/fish-river-mole-rescued-drought-24797230 |access-date= 5 September 2022 |archive-date= 4 September 2022 |archive-url= https://web.archive.org/web/20220904162156/https://www.getsurrey.co.uk/news/surrey-news/fish-river-mole-rescued-drought-24797230 |url-status= live }}</ref><ref>{{cite news |last= Bradshaw |first= David |date= 19 August 2022 |title= In pictures: Environment Agency workers rescue fish from dried-out River Mole |work= Surrey Live |url= https://www.getsurrey.co.uk/news/surrey-news/gallery/pictures-fish-rescued-river-mole-24797394 |access-date= 5 September 2022 |archive-date= 5 September 2022 |archive-url= https://web.archive.org/web/20220905170650/https://www.getsurrey.co.uk/news/surrey-news/gallery/pictures-fish-rescued-river-mole-24797394 |url-status= live }}</ref> At [[Leatherhead]], the river leaves the chalk and flows across impermeable [[London Clay]]. At this point, the water table rises enough for the water to flow back into the main river channel.<ref name=Edmunds/>

In a survey in 1958, the geologist C.&nbsp;C. Fagg identified 25 active swallow holes between Dorking and [[Mickleham, Surrey|Mickleham]]; the majority were only a few centimetres in diameter and were located in the vertical banks of the river below the water line.<ref name=Fagg>{{cite journal |author= Fagg CC |year= 1957 |title= Swallow holes in the Mole Gap |journal= The South-eastern Naturalist and Antiquary |volume= 62 |pages= 1–13 }}</ref> Most holes were difficult to observe in times of normal or heavy flow and were susceptible to silting up as new holes were continually being formed.<ref name=Fagg/> A few much larger swallow holes were also observed separated from the main river by a channel{{clarify|date=November 2012}} of about a metre.<ref name=Fagg/><ref name=Sheppard>Shepperd R (1982) ''The Manor of Wistomble in the Parish of Mickleham: A local history'' chapter 9 page 85</ref> About six of these larger swallow holes were found to the west of the [[Burford Bridge Hotel]], along the course of the [[A24 road (Great Britain)|A24 Mickleham Bypass]] during its construction in 1936. Initially the surveyors tried to fill the holes with rubble to prevent the foundations of the new road subsiding. However this proved to be impractical and they were instead covered by concrete domes, up to 18 m in diameter, each fully supported by the surrounding chalk and provided with a manhole and access shaft to allow periodic inspection.<ref name=Sheppard/><ref name=Dumbleton>{{cite journal |vauthors=West G, Dumbleton MJ |year= 1972 |title= Some observations on swallow holes and mines in the Chalk |journal= Quarterly Journal of Engineering Geology |volume= 5 |issue=1–2 |pages= 171–177 |publisher= Geological Society of London |doi=10.1144/gsl.qjeg.1972.005.01.16|s2cid= 129120488 }}</ref> In the late 1960s the domes were reopened and inspected and the alluvium in the largest swallow hole was observed to have subsided by {{convert|1.5|m|0}} under the centre of one of the domes.<ref name=Dumbleton/> When the Dorking to Leatherhead railway was constructed in 1859, a fossilised swallow hole was discovered in the cutting at the south end of [[Box Hill & Westhumble railway station]], suggesting that even in its early history, the river had swallow holes.<ref name=Fagg/>

The author [[Daniel Defoe]], who attended school in [[Dorking]] and probably grew up in the village of [[Westhumble]],<ref>{{cite journal |author= Andrews JH |year= 1960 |title= Defoe and the Sources of His 'Tour' |journal= The Geographical Journal |volume= 126 |issue= 3 |pages= 268–277 |publisher= The Royal Geographical Society |doi=10.2307/1793630|jstor= 1793630 }}</ref><ref name=SAC_Defoe>{{cite journal |author= Bastian F |year= 1957 |title= Daniel Defoe and the Dorking District |journal= Surrey Archaeological Collections |volume= 55 |pages= 41–64 |url= https://archaeologydataservice.ac.uk/catalogue/adsdata/arch-379-1/dissemination/pdf/vol_55/surreyac055_041-064_bastian.pdf |access-date= 27 September 2020 |doi= 10.5284/1068897 |archive-date= 2 January 2021 |archive-url= https://web.archive.org/web/20210102102002/https://archaeologydataservice.ac.uk/archiveDS/archiveDownload?t=arch-379-1%2Fdissemination%2Fpdf%2Fvol_55%2Fsurreyac055_041-064_bastian.pdf |url-status= live }}</ref> described the swallow holes in the River Mole in his book ''[[A tour thro' the Whole Island of Great Britain]]'' (first published in 1724):
{{blockquote|...the current of the river being much obstructed by the interposition of those hills, called Box Hill ... it forces the waters as it were to find their way through as well as they can; and in order to do this, beginning, I say, where the river comes close to the foot of the precipice of Box-Hill, called the Stomacher, the waters sink insensibly away, and in some places are to be seen (and I have seen them) little channels which go out on the sides of the river, where the water in a stream not so big as would fill a pipe of a quarter of an inch diameter, trills away out of the river, and sinks insensibly into the ground. In this manner it goes away, lessening the stream for above a mile, near two, and these they call the Swallows.|<small>Daniel Defoe (1724)</small><ref>{{cite book |last= Defoe |first= Daniel |author-link=Daniel Defoe |year= 1724 |title= A tour thro' the whole island of Great Britain: divided into circuits or journies, giving a particular and diverting account of whatever is curious and worth observation ... |location= London |publisher= G. Strahan |page= 95 }}</ref>}}

Not all of the water removed from the river by the swallow holes is returned to the channel at [[Leatherhead]]. The chalk aquifer also feeds the springs at the southern end of [[Fetcham]] Mill Pond, which have never been known to run dry.<ref name=Edmunds/> A survey in March 1883 estimated that the Fetcham springs were producing about {{convert|3.6|e6impgal|m3}} every day.<ref>JW Grover (1887) Chalk springs in the London basin, illustrated by the Newbury, Wokingham, Leatherhead and Rickmansworth Water Works ''Proceedings of the Institution of Civil Engineers'' 90</ref> A second survey in 1948 estimated that the same springs were yielding about {{convert|5|e6impgal|m3}} a day.<ref>{{cite journal |author= Edmunds FH |year= 1948 |title= Correspondence on the movement of water in the middle and lower chalk of the River Dour catchment |journal= Journal of the Institution of Civil Engineers |volume= 29 |page= 73 |publisher= Institution of Civil Engineers }}</ref> The [[water table]] in the chalk of the [[River Wey|Wey Gap]] is significantly higher than might be expected from natural [[groundwater recharge|rainwater percolation]] alone. It has been suggested that a proportion of the excess water originates from the Mole Gap.<ref name=Dines>{{cite book |title= The geology of the country around Reigate and Dorking |last1= Dines |first1= HG |last2= Edmunds |first2= FH |year= 1933 |series= Memoirs of the Geological Survey, England and Wales |publisher= HM Stationery Office |location= London }}</ref>

===Lower Mole <span style="display:none">(geology)</span>===
At Leatherhead the river leaves the chalk bedrock, moving onto [[London Clay]].<ref>{{cite journal |author=Green JFN |year= 1936 |title= Field Meeting in the Region of the River Mole |journal= Proceedings of the Geologists' Association |volume= 47 |pages= 11–14 |doi= 10.1016/s0016-7878(36)80018-2}}</ref> The river meanders across an [[alluvial plain]] between {{cvt|400|and(-)|800|m|yd}} wide towards Cobham, where it begins to descend to a lower flood plain, which broadens as the river turns in an axehead meander. At the east end of Painshill Park, the flood plain narrows into a trench about {{cvt|60|m|yd}} wide, in which the river runs northwards for {{convert|6.5|km|mi|abbr=on}} towards Hersham, where the river enters the flood plain of the River Thames.<ref name=physiography/>

Between Cobham and Esher, the Mole's historic courses have deposited gravel on top of the London Clay.<ref>{{cite journal |author=Monckton HW |year=1904 |title=Excursions to the Farnham Gravel Pits on April 23rd and to the Brickfields and Gravel Pits at Dawley, between Hayes and West Drayton on April 30th 1904 |journal=Proceedings of the Geologists' Association |volume=18 |issue=8 |pages=409–414 |doi=10.1016/s0016-7878(04)80055-5 |url=https://zenodo.org/record/1428520 |access-date=5 July 2019 |archive-date=2 January 2021 |archive-url=https://web.archive.org/web/20210102110414/https://zenodo.org/record/1428520 |url-status=live }}</ref> The depth of the deposits generally varies from {{convert|2.5|to(-)|7|m|ft|0|abbr=on}};<ref name=ChertseyGeology>{{cite book |author=Dewey H |title=The Geology of the county around Windsor and Chertsey |series= Memoirs of the Geological Survey |year=1915 |publisher= British Geological Survey |location=London |pages=71–72}}</ref> the lower layers are generally highly compacted and cemented together with brick-red iron oxide, whereas the upper layers are loosely packed with angular flints and sand.<ref>{{cite journal |author= Hudleston WH |year= 1886 |title= On a recent Section through Walton Common exposing the London Clay, Bagshot Beds, and Plateau-gravel |journal= Quarterly Journal of the Geological Society |volume= 42 |issue= 1–4 |pages= 147–172 |doi= 10.1144/gsl.jgs.1886.042.01-04.18 |s2cid= 129839441 |url= https://zenodo.org/record/1448529 |access-date= 5 July 2019 |archive-date= 2 January 2021 |archive-url= https://web.archive.org/web/20210102110415/https://zenodo.org/record/1448529 |url-status= live }}</ref> Remains of a further gravel terrace, containing cherts and flints to a depth of {{convert|4|m|ft|0|abbr=on}}, line the east side of [[St George's Hill]].<ref name=ChertseyGeology/>