Norwich Crag Formation

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 shells or any shelly sand or gravel. 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). and the Bure Valley Beds. This stratigraphy was later formalised by FW Harmer, who grouped them all into an Icenian faunal zone of the Pliocene. 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. A revised lithostratigraphical and biostratigraphical framework was published by Funnell and West (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, 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. Six lithostratigraphic members were recognised by the Geological Society of London: 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. == 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. 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 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. 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. The sandy sediments are near-shore marine deposits, with the clay members being estuarine or lagoonal, These deposits represent environments fluctuating between marine transgressive and regressive episodes on the western margins of the North Sea basin. 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 . Comparison with better preserved sedimentary sequences of similar age in the Netherlands has shown that the Norwich Crag sequence is highly incomplete. Interpretation is hampered by difficulties with distinguishing major non-sequences from minor, local erosional discontinuities. 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. 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. 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 ==

Early Norwich Crag deposition in East Anglia took place within a topographical context established in the late Pliocene: an eastward dipping plain developed on Cretaceous Chalk interrupted by three SSW to NNE trending depressions partly filled with Red Crag sediments. The North Sea was a bight at this time, with its southern margin defined by the chalk hills of the Weald-Artois anticline where the Strait of Dover is now located. There is evidence for later marine transgression and extension of the sea as far west as the Stansted area of Essex, associated with deposition of the Chillesford Sand Member during a period of high sea levels around two million years ago correlated with the Antian/Bramertonian Stage. This was followed by marine regression during the Baventian/Pre-Pastonian a Stage. The coastline shifted north-eastwards to a position in north-east Norfolk by about 1.75 million years BP. This later period is associated with deposition of the Wroxham Crag Formation and marked fluvial input by the Proto-Thames, Bytham River and Ancaster River systems. == Tectonic context ==

Since its deposition, the Norwich Crag Formation has undergone tectonic uplift and tilting as part of regional processes operating on the margins of the North Sea basin. The uplift has, however, been uneven, with little change apparent in north-east Norfolk in the West Runton – Happisburgh area. Evidence for fluctuations in relative sea level in the Crag Basin during the Plio-Pleistocene have been attributed to a variety of mechanisms including glacio-isostatic adjustments, eustatic changes and basinal response to sedimentary loading. == Fauna and flora ==

The fossil fauna and flora of the Norwich Crag have been studied since the early 19th century for biostratigraphic and palaeoclimatic interpretation. Taxa used include plants, molluscs, foraminifers, mammals and dinoflagellates. They provide evidence for a general climatic cooling trend from the Pliocene into the Pleistocene. The historic chronostratigraphic correlations and palaeoenvironmental interpretations based on biostratigraphy (local and continental) have been criticised as poorly defined and unreliable by Riches (2012). Vertebrate fossils tend to be concentrated in the basement bed Species of warm and cold substages are represented, sometimes mixed together; this may pose problems for palaeoecological interpretation. Mammals Land mammal fauna From a biostratigraphic point of view, the terrestrial mammal fauna of the Norwich Crag belongs to the Late Villafranchian European Land Mammal Age and is correlated with European Neogene Mammal Zone MN17. The richest and best-preserved assemblage is found in deposits of Antian/Bramertonian age at Easton Wood, Covehithe. Taxa collected from this site include the mammoth Mammuthus meridionalis and the gomphothere Anancus arvernensis, horse Equus robustus, deer Eucladoceros falconeri and Eucladoceros sedgwicki, gazelle Gazella anglica, beaver Trogontherium sp., wolf Canis etruscus, voles Mimomys pliocaenicus, Mimomys reidi and Mimomys tigliensis, and lemming Lemmus kowalskii. Bramerton has yielded holotypes of the extinct otter Enhydra reevei and the gazelle Gazella daviesii. Vole fossils from the Norwich Crag contribute to the ‘vole clock’ used for biostratigraphic zonation of Pliocene and Pleistocene sediments. Marine mammal fauna Marine taxa from Easton Wood include Orca and the walrus Alachatherium cretsii. The albatross Phoebastria anglica has been recorded from Easton Wood, Covehithe. Reptiles and Amphibians No reptile and amphibian fossils have yet been recorded from the Norwich Crag. Fish Fossil marine fish from the Norwich Crag include genera Chrysophrys sp. (a snapper), Acipenser (sturgeon) and Notidanus (a cow shark), and species Platax woodwardi (a batfish), Gadus morhua (Atlantic cod), Rhinoptera woodwardi (a cownose ray) and Raja clavata (thornback ray). Teeth of the freshwater fish Esox lucius (northern pike) have been found at Bramerton. However they show evidence of considerable transportation, and consequent mixing of faunal assemblages from the earlier Red Crag Formation. The use of molluscan fossils for climatic reconstruction has proved problematic. For instance species found together at several levels in the Ludham borehole notably Serripes groenlandicus (Greenland cockle) and Calyptraea chinensis (Chinese hat snail) have notably different climatic tolerances today (arctic and lusitanic respectively). Further work on Norwich Crag sediments at Bramerton, Norfolk, allowed a Bramertonian stage to be identified, characterised by temperate forest with Quercus, Carpinus and Alnus. Correlation was made with the pollen assemblage from Chillesford. Specimens of fossil wood have occasionally been found, for example pyritised pine and oak at Holton. Foraminifers Fossil foraminifera provide important evidence for climatic and environmental interpretation and stratigraphic correlation in the Norwich Crag. The Ludham borehole has provided the most complete foraminiferal record, and has allowed the designation of seven biozones. Further biozones were identified at Bramerton and Easton Bavents, and assemblages identified at a number of other Norwich Crag sites, permitting correlation with the more complete sequence in the Netherlands. == Dating and correlations ==

The Norwich Crag Formation was deposited during the Gelasian Stage of the Pleistocene, between about 2.4 and 1.8 million years ago. The evidence from fossil plants, mammals, molluscs, foraminifers and dinoflagellates has been used to make biostratigraphic correlations between chronostratigraphic Stages recognised in East Anglia and the more complete sequence on the Continent. Wider correlation is made with Marine Isotope Stages (MIS) and magnetostratigraphy, despite uncertainties in interpretation. covering a time period of between 2.44 and 1.8 million years before present in the Matuyama Chron, corresponding to the magnetostratigraphic interval between the Reunion Sub-chron and the top of the Olduvai Sub-chron. == Eoliths and early humans ==

The discovery of chipped and flaked flints in the Norwich Crag and Red Crag basement beds in the late 19th and early 20th centuries was claimed as evidence for some of the earliest human settlement in Britain. A typologically diagnostic form of 'eolithic' beak-shaped instrument was proposed by E Ray Lankester, the ‘rostro-carinate’, based upon a ‘Norwich Test Specimen’ flint found in the basal Norwich Crag at Colman's Pit, Whitlingham. on the basis of a systematic analysis of flint fracture patterns and geological context. The fractures were identified by Warren (1923) as caused by sub-soil pressure flaking. == See also ==