# Crag Group

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{{Short description|Geological group in south-east England}}
{{Use dmy dates|date=April 2022}}
{{Infobox rockunit
| name           = Crag Group
| period         = Pliocene
| age            = [Plio](/source/Pliocene)-[Pleistocene](/source/Pleistocene)<br />~{{fossil range|4.4|0.78}}
| image          = Red_crag_cliff_at_bawdsey_cliff_suffolk_30-10-2013.jpg
| imagesize      = 
| caption        = [Red Crag Formation](/source/Red_Crag_Formation) at Bawdsey cliff, Suffolk
| type           = Geologic group
| prilithology   = [Sand](/source/Sand)
| otherlithology = [Gravel](/source/Gravel), [clay](/source/clay), [silt](/source/silt), [calcarenite](/source/calcarenite)
| unitof         = [Great Britain Superficial Deposits Supergroup](/source/Great_Britain_Superficial_Deposits_Supergroup) 
| subunits       = [Coralline](/source/Coralline_Crag_Formation), [Red](/source/Red_Crag_Formation), [Norwich](/source/Norwich_Crag_Formation) & [Wroxham Crag Formation](/source/Wroxham_Crag_Formation)s
| underlies      = [Dunwich](/source/Dunwich_Group), [Albion Glacigenic](/source/Albion_Glacigenic_Group), [Britannia Catchments](/source/Britannia_Catchments_Group) and [British Coastal Deposits Group](/source/British_Coastal_Deposits_Group)s
| overlies       = [Unconformity](/source/Unconformity) with [Chalk Group](/source/Chalk_Group) and [London Clay Formation](/source/London_Clay_Formation)
| thickness      = Up to {{convert|70|m|ft|abbr=on}} onshore
| namedby        = R.C. Taylor
| year_ts        = 1823
| area           = 
| map            = 
| map_caption    = 
| location       = 
| region         = [East Anglia](/source/East_Anglia)
| country        = England
| extent         = [East Anglia](/source/East_Anglia) and [North Sea](/source/North_Sea)
}}

The '''Crag Group''' is a [geological group](/source/Group_(geology)) outcropping in [East Anglia](/source/East_Anglia), England and adjacent areas of the [North Sea](/source/North_Sea). Its age ranges from approximately 4.4 to 0.478 million years [BP](/source/Before_Present), spanning the late [Pliocene](/source/Pliocene) and early to middle [Pleistocene](/source/Pleistocene) epochs.<ref>Head, MJ (1988). Marine environmental change in the Pliocene and early Pleistocene of eastern England: the dinoflagellate evidence reviewed. In: van Kolfschoten, T and Gibbard, PL (eds): The Dawn of the Quaternary - proceedings of the SEQS-EuroMam Symposium : Kerkrade, 16–21 June 1996. Netherlands Institute of Applied Geoscience, 1998; fig.2. {{ISBN|9072869613}}.</ref><ref>Mathers, SJ & Hamblin, RJO (2015). Late Pliocene and Pleistocene Marine Deposits. In: Lee, JR; Woods, MA; Moorlock, BSP, eds. British Regional Geology: East Anglia (5th Edition). British Geological Survey; fig.46. {{ISBN|978 085272 823 9}}</ref>

== Background ==
The Crag Group comprises a range of marine and estuarine sands, gravels, silts and clays deposited in a relatively shallow-water, tidally-dominated marine embayment on the western margins of the North Sea basin. The sands are characteristically dark green from [glauconite](/source/glauconite) but weather bright orange, with [haematite](/source/haematite) 'iron pans' forming. The [lithology](/source/lithology) of the lower part of the Group is almost entirely [flint](/source/flint). The highest formation in the Group, the [Wroxham Crag](/source/Wroxham_Crag_Formation), contains over 10% of far-travelled lithologies, notably [quartzite](/source/quartzite) and [vein quartz](/source/quartz) from the Midlands, [igneous rock](/source/igneous_rock)s from Wales, and [chert](/source/chert) from the [Upper Greensand](/source/Upper_Greensand) of southeastern England. This exotic rock component was introduced by rivers such as the [Bytham River](/source/Bytham_River) and [Proto-Thames](/source/River_Thames).<ref>The BGS Lexicon of Named Rock Units. British Geological Survey; online at http://www.bgs.ac.uk/Lexicon/lexicon.cfm?pub=CRAG. Accessed October 2017</ref><ref>Lee, JR; Woods, MA; Moorlock, BSP, eds. (2015). British Regional Geology: East Anglia (5th Edition). British Geological Survey; p.110. {{ISBN|978 085272 823 9}}.</ref>

The constituent formations of the Crag Group are the [Coralline Crag](/source/Coralline_Crag_Formation) (mid to late Pliocene); the [Red Crag](/source/Red_Crag_Formation) (late Pliocene / early Pleistocene); the [Norwich Crag](/source/Norwich_Crag_Formation) (early Pleistocene) and the [Wroxham Crag](/source/Wroxham_Crag_Formation) (early to Middle Pleistocene).<ref>Lee, JR; Woods, MA; Moorlock, BSP, eds. (2015). British Regional Geology: East Anglia (5th Edition). British Geological Survey; p.111. {{ISBN|978 085272 823 9}}.</ref> The sedimentary record is incomplete, leading to difficulties in correlating and dating sequences.<ref>Jones, RL & Keen, DH (1993). Pleistocene Environments of the British Isles. Chapman & Hall. {{ISBN|0 412 44190 X}}.</ref>

The Crag Group goes from East Anglia to [Hertfordshire](/source/Hertfordshire) in a south-westward formation. It is up to 70 metres thick onshore.<ref>{{Cite web |title=BGS Lexicon of Named Rock Units - Result Details |url=https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=CRAG |access-date=2025-06-24 |website=British Geological Survey}}</ref>

The term Crag was first used in a geological sense by R.C. Taylor in 1823, a word commonly used in Suffolk to designate any shelly sand or gravel.<ref>Woodward, HB (1881): The Geology of the Country around Norwich. Memoirs of the Geological Survey of the United Kingdom; p.32</ref>

== See also ==
* [List of fossiliferous stratigraphic units in England](/source/List_of_fossiliferous_stratigraphic_units_in_England)

== References ==
{{reflist}}

Category:Geological groups of the United Kingdom
Category:Geology of England
Category:Neogene England
Category:Geology of East Anglia

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Adapted from the Wikipedia article [Crag Group](https://en.wikipedia.org/wiki/Crag_Group) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Crag_Group?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.