The BGS Lexicon of Named Rock Units — Result Details

Crag Group

Computer Code: CRAG Preferred Map Code: Cg
Status Code: Full
Age range: Pliocene Epoch (NP) — Pleistocene Epoch (QP)
Lithological Description: Sands, gravels, silts and clays. The sands are characteristically dark green from glauconite but weather bright orange with haematite 'iron pans'. The gravels in the lower part of the group are almost entirely composed of flint. Those higher in the group include up to 10% of quartzite from the Midlands, igneous rocks from Wales, and chert from the Upper Greensand of south-eastern England.
Definition of Lower Boundary: A sharp, planar unconformity upon strata extending from the Chalk Group (Cretaceous) in the west to the London Clay Formation (Palaeogene) of the Thames Group in the east. Usually there is a glauconitic conglomerate of rounded flints at the base of the Crag Group.
Definition of Upper Boundary: Overlain by deposits of the Dunwich Group, Albion Glacigenic Group, Britannia Catchments Group or British Coastal Deposits Group. Also interdigitates with the Dunwich Group, from which it can be distinguished by the marine origin of the sediments (all Dunwich Group fromations are fluvial). May be distinguished from the Albion Glacigenic Group by the absence of northern British erratic clasts in the Crag Group.
Thickness: Up to 70 m onshore.
Geographical Limits: East Anglia, and extending south-westwards into Hertfordshire.
Parent Unit: Great Britain Superficial Deposits Supergroup (GBG)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  East Anglia 
Reference(s):
McMillan, A A, Hamblin, R J O and Merritt, J W. 2004. An overview of the lithostratigraphical framework for Quaternary and Neogene deposits of Great Britain (onshore). British Geological Survey Research Report. RR/04/04. 
Hamblin, R J O, Moorlock, B S P, Booth, S J, Jeffery, D H and Morigi, A N. 1997. The Red Crag and Norwich Crag formations in eastern Suffolk. Proceedings of the Geologists' Association, Vol.108, 11-23. 
Rose, J, Moorlock, B S P and Hamblin, R J O. 2001. Pre-Anglian fluvial and coastal deposits in Eastern England: lithostratigraphy and palaeoenvironments. Quaternary International, 79, 5-22. 
Rose J, Candy, I, Moorlock, B S P, Wilkins, H, Lee, J A, Hamblin, R J O, Lee, J R, Riding, J B and Morigi, A N. 2002. Early and Middle Pleistocene river, coastal and neotectonic processes, southeast Norfolk, England. Proceedings of the Geologists' Association, Vol.113, 47-67. 
Gibbard, P L and Zalasiewicz, J A (editors). 1988. The Pliocene-Middle Pleistocene of East Anglia, Field Guide. Quaternary Research Association, Cambridge. 
Riding, J B, Moorlock, B S P, Jeffrey, D H and Hamblin, R J O. 1997. Reworked and indigenous palynomorphs from the Norwich Crag Formation (Pleistocene) of eastern Suffolk: implications for provenance, palaeogeography and climate. Proceedings of the Geologists' Association, Vol.108(1), 25-38. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E176 E191 E190 E159 E161 E162 E132 E148 E189 E206 E222 E223