The Norwich Crag Formation is a stratigraphic unit of the British Pleistocene Epoch. It is the second youngest unit of the Crag Group, a sequence of four geological formations spanning the Pliocene to Lower Pleistocene transition in East Anglia. It was deposited between approximately 2.4 and 1.8 million years ago, during the Gelasian Stage.
The Norwich Crag is a marginal facies of the thicker, much better developed sedimentary sequence in the southern North Sea basin. It outcrops in the eastern half of the counties of Norfolk and Suffolk, and is also represented in Essex and Hertfordshire. It was deposited in a near-shore environment, and comprises a range of sands, silty clays and flint-rich gravels representing various transgressive and regressive marine episodes. It rests in some places on the Red Crag Formation and in others unconformably on Coralline Crag, Palaeogene formations and Chalk Group bedrock. It is overlain by the Wroxham Crag Formation, and unconformably by the Kesgrave Catchment Subgroup (part of the Dunwich Group) and various Mid Pleistocene glacigenic deposits.
Norwich Crag fossil fauna and flora have been studied since the 19th century for information about environmental conditions during the early Pleistocene. They provide evidence for a general climatic cooling trend from the Pliocene to the Pleistocene.
History of research The term Crag was first used in a geological sense by R.C. Taylor in 1823, a word commonly used in Suffolk to designate a deposit of fossil sea shellsForby, F (1830). The Vocabulary of East Anglia. JB Nichols & Son, London, Volume I, p.81 or any shelly sand or gravel.Woodward, HB (1881). The Geology of the Country around Norwich. Memoirs of the Geological Survey of the United Kingdom. London: HMSO.
The Norwich Crag was first identified in the early 19th century as a predominantly marine geological formation, then thought to be of Pliocene age, outcropping from Aldeburgh to the valley of the River Bure. It was variously termed the Mammaliferous Crag (Charlesworth 1836), Norwich Crag (Lyell 1839), Norfolk Crag (Phillips 1863) and Upper Crag (Godwin-Austen 1868). Five horizons were initially recognised: the Mammaliferous Stone-Bed; the Fluvio-marine Crag; the Chillesford Crag; the Chillesford Clay; the Bure Valley Crag, including the Westleton Beds. Additional units were identified in the Norwich Crag Series, the Weybourne CragPrestwich, J (1890). On the relation of the Westleton Beds or Pebbly sands of Suffolk to those of Norfolk and on their extension inland; with some observations on the period of the final elevation and denudation of the Weald and the Thames valley, etc. Part 1. Quarterly Journal of the Geological Society, vol.46, pp.84-119.Reid, Clement (1882). The Geology of the Country around Cromer. Memoirs of the Geological Survey of the United Kingdom. London:HMSO, p.9. and the Bure Valley Beds.Woodward, HB (1876). The Geology of England and Wales. Longmans, Green & Co, London; pp.286-291. This stratigraphy was later formalised by FW Harmer, who grouped them all into an Icenian faunal zone of the Pliocene.Harmer, FW 1900. The Pliocene Deposits of the East of England - Part II : The Crag of Essex (Waltonian) and its relations to that of Suffolk and Norfolk. Quarterly Journal of the Geological Society, vol.56; pp.705-744. Research into sediments from the Ludham research borehole (1959) allowed a sequence of vegetational and faunal episodes to be recognised in the Norwich Crag, based on fossil foraminifera, pollen and mollusca.Funnell, BM (1961). The Palaeogene and early Pleistocene of Norfolk. Transactions of the Norfolk and Norwich Naturalists’ Society, vol.19, pt.6, pp.340-364West, RG (1962). Vegetational history of the Early Pleistocene of the Royal Society Borehole at Ludham, Norfolk. Proceedings of the Royal Society of London, B155, pp.437-453.Norton, PEP (1967). Marine Molluscan Assemblages in the Early Pleistocene of Sidestrand, Bramerton and the Royal Society Borehole at Ludham, Norfolk. Philosophic Transactions of the Royal Society of London, Series B, vol.253, no.784. A revised lithostratigraphical and biostratigraphical framework was published by Funnell and West (1977).Funnell, BM & West, RG (1977). Preglacial Pleistocene deposits of East Anglia. In: Shotton, FW (ed). British Quaternary Studies: recent advances. Oxford University Press, 1977. . Research into the lithostratigraphy of the Norwich Crag was carried out by the British Geological Survey between 1975 and 2006 as part of work to remap the geology of Norfolk and Suffolk; new techniques allowed improved understanding of local detail,Zalasiewicz, JA & Mathers, SJ (1985). Lithostratigraphy of the Red and Norwich Crags of the Aldeburgh-Orford Area, south-east Suffolk. Geological Magazine, vol.122, pt.3; pp.287-296. and it became possible to distinguish the Norwich Crag from the succeeding Wroxham Crag (now subsuming the Weybourne Crag and Bure Valley Beds). Following international revisions to the Pliocene/Pleistocene boundary, the Norwich Crag is now placed in the Lower Pleistocene.Gibbard, PL, Zalasiewicz, JA & Mathers, SJ (1998). Stratigraphy of the marine Plio-Pleistocene crag deposits of East Anglia. 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; pp.239-262. .
Six lithostratigraphic members were recognised by the Geological Society of London:Bowen, DQ (ed) (1999). A revised correlation of Quaternary deposits in the British Isles. Special Report no.23, The Geological Society, p.22. . the Chillesford Church Member (a basal deposit of marine sand, formerly the Chillesford Sand Member); the Chillesford Member (micaceous, silty clays overlying the Church Member, formerly the Chillesford Clay Member); the Creeting Member (micaceous, inter-tidal sands); the College Farm Member (silty clay of mud flats associated with the Creeting Member); the Easton Bavents Member (clay with sand laminae); the Westleton Member (flint-rich gravels overlying the Easton Bavents Member).
The type site of the Formation is at Bramerton Pits SSSI, near Norwich.Reid, Clement (1890). The Pliocene Deposits of Britain Memoirs of the Geological Survey of the United Kingdom, London: HMSO
The sedimentary record
The Norwich Crag Formation is a marginal facies of the thicker, much better developed sedimentary sequence in the southern North Sea basin. It outcrops in the eastern half of the counties of Norfolk and Suffolk, and is also represented in Essex and Hertfordshire.Mathers, SJ & Zalasiewicz, JA (1988a). The Red Crag and Norwich Crag Formations of southern East Anglia. Proceedings of the Geologists’ Association vol.99, pt.4; pp.261-278 Geological field relationships show that it rests with discontinuity on the Red Crag Formation and oversteps in other places onto the Coralline Crag, the Palaeogene formations and Chalk Group bedrock. It is overlain disconformably by the Wroxham Crag Formation, and unconformably by the Kesgrave Catchment Subgroup and Mid Pleistocene glacigenic deposits.The BGS Lexicon of Named Rock Units. British Geological Survey. Online at http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=NCG. Accessed October 2017.
The Norwich Crag comprises a widespread sheet of well sorted, fine- to medium-grained micaceous, glauconitic, locally shelly sands (e.g. the Chillesford Sand Member), with localized beds of laminated silty clays (particularly the Chillesford Clay and Easton Bavents Clay members) and well sorted fine to medium sands with beds of rounded flint gravels (notably the Westleton Beds Member). The pebbles are predominantly composed of flint, typically more than 95% local material, either chattermarked (beach-abraded) or angular or sub-angular flint; the remaining 5% is typically white vein quartz and white quartzite.Rose, J (2007). Palaeogeography of Eastern England during the Early & Middle Pleistocene. In: Candy, I et al. The Quaternary of Northern East Anglia Field Guide. Quaternary Research Association, 2007. The heavy minerals in the sand-sized fraction of the sediment are characterised by high concentrations of garnet, amphibole and epidote, which suggests that the sands originated from eastern (continental) rather than western (British) fluvial sources.Riches, P (2012). The palaeoenvironmental and neotectonic history of the Early Pleistocene Crag basin in East Anglia. Ph.D thesis, Royal Holloway, University of London. The sandy sediments are near-shore marine deposits, with the clay members being estuarine or lagoonal, and the flint gravels are also near-shore marine, interpreted as channel infills, the gravels having been reworked from gravel beach deposits.Mottram, HB, (2017). Tidal-inlets and gravel deposition during the late Norwich Crag (Lower Pleistocene) of north-eastern Suffolk, U.K. Proceedings of the Geologists’ Association 128; pp.547–557. These deposits represent environments fluctuating between marine transgressive and regressive episodes on the western margins of the North Sea basin.Moorlock, BSP, Hamblin, RJO, Booth, SJ & Morigi, AN (2000). Geology of the country around Lowestoft and Saxmundham. Memoir for 1:50,000 Geological Sheets 176 and 191. British Geological Survey. . The source of the distinctive flints, which are often rounded in the coarser sizes in the Westleton Beds Member, is as yet to be conclusively determined but it has been argued that the flints were obtained from local rather than distant Chalk .Mottram, HB, (2022). On the supply of clasts to the Westleton Beds: a review. Bulletin of the Geological Society of Norfolk 72; pp.11–28. Comparison with better preserved sedimentary sequences of similar age in the Netherlands has shown that the Norwich Crag sequence is highly incomplete.Jones, RL & Keen, DH (1993). Pleistocene Environments of the British Isles. Chapman & Hall. . Interpretation is hampered by difficulties with distinguishing major non-sequences from minor, local erosional discontinuities.
For surface mapping purposes, the British Geological Survey has recognised five members: the Chillesford Sand, Chillesford Clay, Easton Bavents Clay, Westleton Beds and Sidestrand Members.Lee, J. R.; Woods, M. A.; Moorlock, B. S. P., eds. (2015). British Regional Geology: East Anglia (5th ed.). British Geological Survey. . The uppermost beds of the Crag Group (formerly the Bure Valley Beds and Weybourne Crag) are now attributed to the Wroxham Crag Formation, based on their high proportion of fluvially-derived, far-travelled lithological components.Rose, J, Moorlock, BSP & Hamblin, RJO (2000). Lithostratigraphy and palaeoenvironments of the pre-Anglian sands and gravels of East Anglia. In: Lewis, SG, Whiteman, CA & Preece, RC (eds). The Quaternary of Norfolk and Suffolk Field Guide. Quaternary Research Association, 2000.
The Westleton Member may be regarded as a sedimentologically coherent and a lithologically and stratigraphically consistent unit stretching from central Norfolk to the Suffolk coast.Sinclair, J (1999). Plio-Pleistocene relative sea-level changes in East Anglian: evidence from the Westleton Member and related deposits. Proceedings of the Geologists' Association 110, pp.149-162. Beds of fine-grained sediment may also serve as marker horizons in the Crag Basin. Considering the lithostratigraphy to include subsurface sediments evidence by borehole, Riches (2012) proposed three major units for the Basin; most recognisable where they are separated by regressive marine marker beds of silty clay, with the uppermost equated with the Chillesford Clay Member.
Palaeogeography