Editing London Borough of Croydon (section)

===Geology===
The bedrock of the northern part of the borough is mostly London Clay Formation<ref name="BGS London Clay">{{cite web|url=https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=LC|title=London Clay Formation|website=British Geological Society|access-date=1 May 2025}}</ref> (blue-grey and grey-brown calcareous clay formed between 56 and 47.8 million years ago (mya) during the [[Palaeogene]] period) with a Claygate member<ref name="BGS Claygate">{{cite web|url=https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=CLGB|title=Claygate Member|website=British Geological Society|access-date=1 May 2025}}</ref> (dark grey clays with sand laminae and silt) of the same period making up the [[Norwood Ridge]]. A band of rocks of the Lambeth Group<ref name="BGS Lambeth">{{cite web|url=https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=LMBE|title=Lambeth Group|website=British Geological Society|access-date=1 May 2025}}</ref> (clay with sand/gravel/limestone/lignite, laid between 59.2 and 47.8 mya), Harwich Formation<ref name="BGS Harwich">{{cite web|url=https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=HWH|title=Harwich Formation|website=British Geological Society|access-date=1 May 2025}}</ref> (sand and gravel with [[glauconite]], laid between 56 and 47.8 mya) and Thanet Formation<ref name="BGS Thanet">{{cite web|url=https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=TAB|title=Thanet Formation|website=British Geological Society|access-date=1 May 2025}}</ref> (silty, fine-grained sand with glauconite, laid between 59.2 and 56 mya) crosses the borough from east to west under Waddon, Addiscombe and Shirley into Spring Park. In the south, most of the bedrock is of the Lewes Nodular/Seahaven Chalk/Newhaven Chalk Formation<ref name="BGS Lewes">{{cite web|url=https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=LSNCK|title=Lewes Nodular Chalk Formation, Seaford Chalk Formation And Newhaven Chalk Formation (Undifferentiated)|website=British Geological Society|access-date=1 May 2025}}</ref> (laid during the [[Cretaceous]] period, between 93.9 and 72.1 mya), from South Croydon and Addington down past Kenley and King's Wood with a small area of Holywell Nodular/New Pit Chalk Formation<ref name="BGS Holywell">{{cite web|url=https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=HNCK|title=Holywell Nodular Chalk Formation and New Pit Chalk Formation (undifferentiated)|website=British Geological Society|access-date=1 May 2025}}</ref> (also Cretaceous, laid between 100.5 and 89.8 mya) in lower areas between hills, beginning just east of Kenley station and followed by the railway line curving southwards.<ref name="BGS viewer">{{cite web|url=https://geologyviewer.bgs.ac.uk/|title=Geology Viewer|website=British Geological Society|access-date=14 April 2025}}</ref>

There are five types of overlying superficial rock, all of the [[Quaternary]] period. In the very north, an unnamed sand-and-gravel member<ref name="BGS sand and gravel uncertain">{{cite web|url=https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=SGAO|title=Sand and gravel of uncertain age and origin|website=British Geological Society|access-date=1 May 2025}}</ref> of the [[Quaternary]] period overlies the Claygate member of the Norwood Ridge along Church Road and Crystal Palace Parade, with another area along Crown Lane to the east. An area of Lynch Hill Gravel<ref name="BGS Lynch Hill">{{cite web|url=https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=LHGR|title=Lynch Hill Gravel Member|website=British Geological Society|access-date=1 May 2025}}</ref> (deposited between 362 and 126 thousand years ago (tya)) extends from Norbury down to under West Croydon and East Croydon stations. A narrow Hackney Gravel Member <ref name="BGS Hackney">{{cite web|url=https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=HAGR|title=Hackney Gravel Member|website=British Geological Society|access-date=1 May 2025}}</ref> (laid between 362 and 126 tya) curves down through lower elevations from the west of Norbury and broadly followed by the railway line from South Croydon to south of Purley Station. Extending from this and other lower elevation areas are narrower deposits of Head<ref name="BGS Head">{{cite web|url=https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=HEAD|title=Head|website=British Geological Society|access-date=1 May 2025}}</ref> (poorly sorted and stratified angular rock debris and slow glacial hillwash, deposited between 2.588 mya and the present), related to the downward movement of water, e.g. under Coulsdon Town, Coulsdon South and Kenley stations and by Riddlesdown and Whyteleafe stations. Higher chalk areas in the south are overlain with Clay-with-flints Formation<ref name="BGS Clay-with-flints">{{cite web|url=https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=CWF|title=Clay-with-flints Formation|website=British Geological Society|access-date=1 May 2025}}</ref> (orange- or red-brown clay with nodules of flint, laid between 23.03 mya and 11.8 tya), e.g. at Netherne-on-the-Hill, Old Coulsdon and Kenley.

Elevations range from 32 metres in the north on London Clay (west of London Road ([[A23 road|A23]]} south-southeast of Norbury Station and west of Thornton Heath station) to 175 metres in the south on a small area of Lambeth Group rock (at [[Sanderstead]] Plantation on [[A2022 road#West Wickham to Purley|Addington Road]]).<ref name="BGS viewer"/>