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Geology of the Cromer district — a brief explanation of the geological map Sheet 131 Cromer

B S P Moorlock, R J O Hamblin, S J Booth, H Kessler, M A Woods and P R N Hobbs

Bibliographic reference: Moorlock, B S P, Hamblin, R J O, Booth, S J, Kessler, H, Woods, M A, and Hobbs, P R N. 2002. Geology of the Cromer district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 131 Cromer (England and Wales).

Keyworth, Nottingham: British Geological Survey © NERC 2003 All rights reserved.

Copyright in materials derived from the British Geological Survey's work is owned by the Natural Environment Research Council (NERC) and/or the authority that commissioned the work. You may not copy or adapt this publication without first obtaining NERC permission. Contact the BGS Intellectual Property Rights Manager, British Geological Survey, Keyworth. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract.

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Geology of the Cromer district (summary from rear cover)

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Modern development requires accurate geological information in order, for example, to identify resources and ensure that foundations are adequate. Modern agricultural practices also require knowledge of the underlying geology. This Sheet Explanation and the newly surveyed geological map that it describes provide valuable information on a wide range of earth science issues. A substantial list of references is provided for those who may wish to be directed to further geological information about the district.

The coastal town of Cromer, famous for its crabs, is situated on the northern side of the Cromer Ridge, a major topographical feature that attains a maximum height of about 100 m OD, and is the highest point in Norfolk. It is comprised of a thick sequence of Pleistocene sediments deposited by several ice-sheets that extended into the region. Some of the smaller

coastal settlements are associated with local fishing industries. Away from the coast the district is predominantly rural; fertile soils developed on the glacial tills and cover loams are well suited to a wide range of arable crops that include, wheat, barley, sugar beet, oil-seed rape and potatoes. In the west of the district, much of the coast is fringed by marshland, which attracts large numbers of birds, particularly migrants visiting during the winter months.

Extensive sea-cliffs to the west and south-east of Cromer provide a striking insight into the geology that lies beneath the district. The origin of the glaciogenic deposits exposed in these sections has been, and continues to be, strongly debated by researchers. In addition, these cliffs provide excellent exposures of some of the youngest Chalk strata in Britain, and also show some of the best examples of coastal landsliding in Britain.

Notes

The word 'district' refers to the area of Sheet 131 Cromer. National grid references are given in the form [1234 1234] or [123 123]. Unless otherwise stated all such references fall within the grid square TG. Numbers at the end of photograph descriptions refer to the official collection of the British Geological Survey.

Acknowledgements

We would like to thank all landowners who gave access to their land during the field survey, and the many companies and individuals who provided logs of boreholes. We would also like to acknowledge collaboration with Jim Rose, Jonathan Lee and Ian Candy, Department of Geography, Royal Holloway, University of London, and Colin Whiteman and Paul Fish, School of the Environment, University of Brighton who have investigated the composition and provenance of the glaciofluvial sands and gravels and tills.

The grid, where it is used on figures, is the National Grid taken from Ordnance Survey mapping.

© Crown copyright reserved Ordnance survey licence number GD272191/2002

Chapter 1 Introduction

The Cromer district extends from the village of Gunthorpe in the south-west, to the coastal settlements of Morston and Blakeney in the north-west and Overstrand and Sidestrand in the east. In addition to the coastal resorts of Cromer and Sheringham, the district also includes the small inland Georgian market town of Holt. The most prominent topographical feature in Norfolk, the Cromer Ridge (Figure 1) extends east–west across much of the northern part of the district, locally reaching heights just in excess of 100 m OD.

The underlying geology continues to have a big influence on the district. Much of the higher ground associated with the Cromer Ridge, and underlain by acid soils developed on sand and gravel, remains uncultivated and supports heathland and woodland habitats. Much of the wealth of the district stems from the fertile soils developed on the extensive tills and cover loams, where wheat, barley, potatoes and sugar beet are the main crops.

The geology is closely linked to two major issues within the district — coastal erosion and flooding. From the village of Weybourne eastwards, the alternation of poorly consolidated sands and clays with perched water tables within the cliffs leads to numerous small falls and larger slips with the subsequent removal of the slipped material by the sea during high tides and storms. To the west of Weybourne, extensive areas of saltmarsh and reclaimed salt-marsh are protected from the sea by a gravel ridge. The latter is currently being maintained artificially, but the gravel is diminishing and in the near future it may not be feasible to maintain the ridge in the current manner.

Regional structural setting

The district lies on the northern margin of a concealed platform of Palaeozoic rocks, the London–Brabant Massif (Wills, 1978), which acted as a positive structural feature through late Palaeozoic and Mesozoic times and was the dominant feature in the geological evolution of the region.

The interaction of the relatively stable platform with the adjacent subsiding Southern North Sea Basin resulted in attenuation or nondeposition of Carboniferous, Permian, Triassic and Jurassic strata with major erosion within the region during the Late Cimmerian (late Jurassic–early Cretaceous). The overlying marine Lower Cretaceous mudstones rest with marked unconformity on the underlying rocks. Their deposition marks the commencement of a period of sea level rise that extended throughout most of the Upper Cretaceous, culminating in the deposition of the micritic limestones of the Chalk Group.

A phase of regional uplift and erosion during the late Cretaceous was followed by sustained subsidence of the Southern North Sea Basin, which continued through the Palaeogene and Quaternary, with, perhaps, a period of uplift in mid to late Miocene times. Uplift followed, and no sediments were deposited until the early Pleistocene, when the marine Crag Group and the associated freshwater Cromer Forest-bed Formation were deposited.

During the late Pleistocene, ice-sheets originating in northern Britain and Scandinavia reached the district and deposited tills and outwash sands and gravels.

A eustatic rise in sea level during the ensuing Flandrian led to the accumulation of estuarine deposits. At the same time the coastline suffered major erosion and retreat.

Chapter 2 Geological description

The deposits beneath the Upper Chalk have not been proved in any borings within the district, but they have been encountered in the Saxthorpe Borehole [TG 1226 3013] about 5 km to the south of the district. The descriptions below are based entirely on cuttings, apart from a 3 m length of core taken within the strata that is possibly Silurian in age.

Silurian to Jurassic

?Silurian

The core of the Saxthorpe Borehole consists of medium grey, hard, indurated and cleaved siliceous siltstone intercalated with dark grey indurated mudstone. Some siltstone is slightly calcareous. Cuttings from higher in the sequence indicate the presence of dolomitic silty sandstone, and pyrite is locally common. A total of 170 m of probable Silurian strata were penetrated in the borehole.

Permo-Triassic

Unconformably overlying the probable Silurian strata are 75.6 m of interbedded red sandstone, mudstone and siltstone. These are of presumed Permo-Triassic age, but have not been assigned to any formation. Overlying these are some 44.5 m of white to pale green, poorly sorted, fine to medium-grained sandstone, provisionally correlated with the Sherwood Sandstone Group. The sandstone is overlain by 104.2 m of mudstone and siltstone assigned to the Mercia Mudstone Group. These are predominantly red with grey and green interbeds. Anhydrite is common at some levels. The Mercia Mudstone is overlain by 25.6 m of red, red-brown, light green and grey, locally very calcareous mudstone and siltstone assigned to the Penarth Group.

Jurassic

The 101.6 m of the overlying Lias Group have not been divided into formations. The sediments are described as grey and grey-green 'clay' and siltstone, locally with limonitic ooids.

Cretaceous

The Lias Group is overlain unconformably by 93.9 m of Lower Cretaceous sandstone, siltstone and mudstone. These have not been assigned to a formation.

Upper Cretaceous Chalk group

This is the oldest unit exposed within the district although it is largely masked by younger strata (Figure 2), however, its detailed stratigraphy is known from coastal exposures at Weybourne, Sheringham, Overstrand and Sidestrand. The BGS Trunch Borehole [TG 2933 3455] (Appendix in Arthurton et al., 1994), just east of the district, cored a total thickness of 468 m ranging from Cenomanian to Lower Maastrichtian, and represents the most complete onshore Chalk succession in the UK. Also, 342.9 m of undivided Chalk were proved in the Saxthorpe Borehole just south of the district.

Chalk is typically a very fine-grained, relatively soft, white limestone formed in a marine environment, and predominantly comprised of the disaggregated skeletal remains of tiny planktonic algae (coccoliths). Flints, clay-rich horizons (marls), and beds of indurated, mineralised chalk (hardgrounds) also occur, and some of these are geographically extensive marker-horizons. The exposed succession in the Cromer district, belonging to the Upper Campanian and Lower Maastrichtian, represents some of the youngest chalk preserved in the UK. It has traditionally been referred to as Upper Chalk, but more recent work has shown that a more refined classification is possible (Wood, 1988; Johansen and Surlyk, 1990) and this is described below.

The oldest part of the exposed succession belongs to the Pre-Weybourne Chalk of

Wood (1988). It occurs in limited exposures at Cley-next-the-Sea [TG 0540 4400] (Pitchford, 1990a) and in the coastal succession eastwards from Weybourne Hope [TG 1100 4370] where about 5.4 m is exposed (Peake and Hancock, 1970). Above this, 22 to 25 m of Weybourne Chalk occur in the stratotype section between Weybourne Hope and Old Butts Gap [TG 1236 4362], and at Weybourne Hope East [TG 1290 4360] to [TG 1390 4350] (Peake and Hancock, 1970; Pitchford, 1990a). Flinty, soft, white chalk, is followed in the middle and higher part of the Weybourne Hope–Old Butts Gap section, by harder, nodular chalk, with regular flints (Pitchford, 1990a). Intercalated horizons of hard and soft, flinty chalk appear to characterise the highest part of the Weybourne Chalk exposed at Weybourne Hope East, where the succession includes a pair of omission surfaces (Pitchford, 1990a). The Weybourne Chalk is terminated by the Catton Sponge Bed (sensu Wood, 1988), represented by the lowest of three sponge beds on the foreshore at Sheringham [TG 1565 4325].

Beeston Chalk is about 40 m thick (Pitchford, 1990a), and overlies the Catton Sponge Bed; it is exposed on the foreshore east of Sheringham [TG 165 435], below Beeston Hill. This chalk is characterised by common inoceramid bivalve shell fragments and ring-shaped flints (only apparent in plan view on bedding-plane surfaces) up to 1.8 m in diameter. The top is marked by a hardground exposed on the foreshore 180 m east of West Runton [TG 1795 4283], and characterised by accumulations of echinoids (Echinocorys) and saurian vertebrae (Peake and Hancock, 1970; Pitchford, 1990a).

The Paramoudra Chalk, at the top of the Campanian, is exposed on the coast between East Runton [TG 2010 4210] and Cromer [TG 2200 4220] (Peake and Hancock, 1970; Pitchford, 1990a), and is estimated to be 22 to 25 m thick. The name of this unit refers to vertical flint columns, formed around the burrow Bathichnus paramoudrae. The rather poorly fossiliferous coastal exposures contrast with the rich fauna recorded from this unit in the Norwich district (Wood, 1988). The top of the Paramoudra Chalk is the Overstrand Pyramidata Hardground, named because of the dominance of the echinoid Echinocorys pyramidata in a mass of fossils and hard chalk exposed at low water mark at Overstrand [TG 2490 4100].

The youngest exposed chalk in the district, belonging to the Lower Maastrichtian, forms a mass enclosed in glacial drift on the coast at Overstrand beneath the site of the old Overstrand Hotel [TG 255 406], and is referred to as the 'Overstrand Hotel Upper Mass'. The hard, yellow, flinty chalk belongs to the Sidestrand Chalk Member of Johansen (1990).

Traditionally, this succession has been subdivided into Lower, Middle and Upper Chalk formations, but recent work has shown that a more detailed lithostratigraphical classification is possible (Wood and Smith, 1978; Mortimore, 1986; Whitham,1991, 1993; Bristow et al., 1997; Rawson et al., 2001). However, none of the recently revised lithostratigraphical classifications of the Chalk Group have produced a single integrated scheme that is applicable to East Anglia. Thus, while many aspects of the Cenomanian, Turonian and Coniacian stratigraphy of the Trunch Borehole resemble the Chalk Group of northern England as described by Whitham (1991, 1993), including correlatives of marker-beds in the Ferriby, Welton and Burnham Chalk formations, there is no direct equivalent of the flintless Flamborough Chalk of northern England; in the Trunch borehole this part of the succession is more akin to coeval strata in southern England, as described by Mortimore (1986).

Quaternary

Crag Group

During the late Neogene, the district became submerged as a part of the marine North Sea Basin, with a series of prograding deltas developing around the margins (Cameron et al., 1992). During the Pliocene and early Quaternary, the district lay near to the western coast of this basin, and three Crag formations represent the local infilling of the basin. These are, from base to top, the Red Crag, Norwich Crag and Wroxham Crag formations (Figure 3). In general, the formations indicate an upward shallowing, with beach-face and estuarine facies significant in the upper part of the Norwich Crag and some freshwater beds in the Wroxham Crag. Within the district, the Wroxham Crag can be seen in discontinuous coastal sections, to just west of Weybourne, commonly within glacially tectonised thrust blocks. The best sections are in the coastal cliffs which extend eastwards from Weybourne beach, in which horizontal Wroxham Crag rests on Upper Chalk (Plate 1) and is overlain by contorted Overstrand Formation. Inland there are no exposures within the district; it is exposed at Aldborough [TG 18 34], 3 km to the south. The Crag is up to about 15 m thick in the district; the western limit runs from west of Weybourne, somewhat east of south to pass east of Plumstead [TG 13 34]. It is likely that only the Wroxham Crag is present, but some Norwich or Red Crag may also underlie this in the south-east of the district.

The three Crag formations are distinguished by virtue of their being separated by disconformities that represent west-ward transgressions and eastward regressions of the North Sea. The Red Crag, of Pre-Ludhamian, Ludhamian and Thurnian age, and the Norwich Crag, of Antian/Bramertonian and Baventian age, were redefined by Hamblin et al. (1997) on this basis. The Wroxham Crag is a newly named formation that represents the deposits of a further transgression post-dating the regression at the end of Norwich Crag formation. The deposits formerly known as Weybourne Crag (Reid, 1882) and Bure Valley Beds are included within the Wroxham Crag Formation. While dips are low throughout the sequence, the Red, Norwich and Wroxham crags dip north-eastward at successively lower angles, such that the Norwich and Wroxham crags overlap the underlying crag formation westwards.

Following traditional Geological Survey practice, the crags are considered to be 'Solid' (bedrock), and the overlying Cromer Forest-bed Formation to be 'Drift' (superficial deposits). The Crag Group comprises fine to coarse-grained micaceous sands, clays and gravels. Below the watertable, unweathered sands are dark green to black owing to their high glauconite content, but they weather to a yellowish or reddish brown colour. The weathered sands contain beds of 'iron pan' (ferruginous concretions) formed from the iron oxides and hydroxides released by weathering of the glauconite.

Gravels in the Crag Group are generally well sorted, and are dominated by well-rounded to subangular, chattermarked high-sphericity flint pebbles and cobbles. These are normally grey or brown both inside and out, indicating a long history of weathering; many second-cycle material may be derived from Palaeogene strata. Of the far-travelled components, quartz and colourless or yellow quartzite are the commonest, but are significantly more common in the Wroxham Crag than in the lower formations. Clays in the Crag Group are generally pale to medium grey or buff, and very silty, with ochreous staining and scattered laminae of silt and sand. Some of the clays in the Red Crag may have formed in relatively deep water offshore, while those in the Norwich and Wroxham crags, which are commonly interbedded with beach-face gravels, are likely to be intertidal. It is suggested that these clays formed in an estuarine environment protected on the seaward side by banks of beach-face gravel.

Red Crag Formation

Is preserved in a series of north-easterly trending basins, of which one is known to pass beneath Ludham in the North Walsham district to the south-east where about 30.8 m of Red Crag were recorded (Funnell, 1961; West, 1961a). Its north-westward extent is unknown, and the nearest that undoubted Red Crag has been recorded to this district is in the Happisburgh Borehole [TG 3832 3110] (West, 1980). Hamblin et al. (1997) suggest that the basins were formed by folding and faulting contemporaneous with sedimentation.

Norwich Crag Formation

Comprises a tabular sheet, about 30 m thick, which dips gently north-eastward. It rests disconformably upon the Red Crag in the Ludham Trough, and oversteps the Palaeogene strata to rest upon Upper Chalk. Its north-westward extent is not known, but it is present in the Happisburgh Borehole. Shelly sands are rare in the Norwich Crag.

Wroxham Crag Formation

Includes the Bure Valley Beds of the Wroxham area, traditionally included in the Norwich Crag, and, on the coast, strata of Pre-Pastonian and Pastonian age previously referred to as Weybourne Crag, and also further strata previously included in the Cromer Forest-bed Formation. The formation is a near-shore, estuarine and freshwater complex. It is lithologically similar to the Norwich Crag in that it includes sand, gravel and clay, but

differs in containing a higher proportion of gravel and quartz, and quartzite pebbles form a significant proportion of the gravel. Typically, they are characterised by about 30 to 40 per cent quartzose lithologies, small percentages of Carboniferous chert and persistent traces of Greensand chert and Rhaxella chert (Rose et al., 2001). Very low percentages of Spilsby Sandstone, acid and basic volcanic rocks are recorded, and chalk occurs locally in the lowest unit.

Working at inland sites in the North Walsham and Aylsham districts, Rose et al. (2001) distinguished two members. The basal Dobb's Plantation Member is defined at Dobb's Plantation Pit [TG 273 158], where it overlies Norwich Crag. It is characterised by gravels containing about 10 per cent far-travelled materials, mainly white or colourless quartz and quartzite, Carboniferous and Rhaxella chert. At the type site, examination of clast lithologies indicated a marked change from the flint-dominated Norwich Crag assemblage to quartzose Wroxham Crag lithologies some 0.8 to 1.0 m above the Chalk, with Carboniferous chert appearing persistently about 1.0 m above the Chalk. Cambridge (1978a, b) recorded the incoming of the cold-water marine bivalve Macomabaltica about 1.5 m above the Chalk, indicating that this earliest member of the Wroxham Crag is of Pre-Pastonian age.

The type site for the overlying and more extensive How Hill Member is at How Hill [TG 377 199] in the North Walsham district (Rose et al., 1995). It comprises a complex of sands and gravels deposited in tidal current flow. Provenance-indicators suggest sources to the south and north-west, from the proto-Thames and 'Northern rivers'. The gravels are composed dominantly of angular flint (57–43%), quartz and quartzite (39–26%), chattermarked flints (9–6%) and Carboniferous chert (8–5%). The quartz and quartzite, which are dominantly white or colourless, and also small quantities of Lower Greensand chert and acid volcanic rocks of Welsh provenance, demonstrate an input from the proto-Thames, while the Carboniferous cherts and small quantities of Rhaxella chert (from the Howardian Hills of Yorkshire) and glauconitic Spilsby Sandstone indicate the Ancaster River draining from the southern Pennines. Carboniferous chert and Spilsby Sandstones could also be carried by the Bytham River known to have flowed from the Midlands to Suffolk, but the absence of reddish brown and brown quartzites indicates that this was not a major contributor to this deposit.

On the coast, there has been confusion between the Crag Group and the Cromer Forest-bed Formation. West (1980) did not recognise the presence of true Crag, but raised seven members within the 'Cromer Forest Bed Series'. These are, from the base upwards, the Weybourne Crag and the Sheringham, Paston, Runton, West Runton, Mundesley and Bacton members. The Weybourne Crag was subsequently named the Sidestrand Member (West, 1980), and was placed in the Norwich Crag by Lewis (in Bowen, 1999). Under the new BGS classification of the Crag Group, which places all the quartz/quartzite-rich marine units in the Wroxham Crag Formation, the Sidestrand, Paston and Mundesley members are included in the latter, and the Sheringham, Runton, West Runton and Bacton members in the Cromer Forest-bed Formation. The two formations are thus accepted as interdigitating in this district.

The Sidestrand Member (stratotype at Sidestrand, [TG 255 405]) comprises marine laminated clays, silts, sands and shelly gravels up to about 13 m thick. Marine bivalves, including Macoma baltica, commonly form dense shell banks within the gravels. The member is of Pre-Pastonian 'a' age (West, 1980) and correlates with the Dobb's Plantation Member inland. The base of the Crag generally comprises a bed of flint gravel, up to around 0.5 m thick, resting upon Upper Chalk. The flints are commonly fresh, up to and over 20 cm long, with black cortices and white patinas, mostly rather worn, with their horns broken off. At West Runton this basal bed is moderately cemented; it forms the lower part of the foreshore and is exposed over wide areas at low tide when not covered with sand.

The Paston Member (stratotype at Paston [TG 330 355]) indicates a marine transgression following freshwater deposition of the Sheringham Member of the Cromer Forest-bed Formation, and comprises about 3 m of tidally laminated silts and clays, marine shelly sands, cross-bedded sands, clay conglomerates and gravels. It was subdivided by Funnell (1977) and West (1980) to include the Paston Bed, West Runton Sand and West Runton Clay, the latter comprising 1.5 m of tidal silty clay. These should not be confused with the West Runton Member of the Cromer Forest-bed Formation, also raised by Funnell (1977). The Paston member is of Pastonian age, and may correlate with the How Hill Member inland.

The Mundesley Member (stratotype at Mundesley [TG 319 363]) comprises up to about

2.5 m of tidally laminated silty clays and sands and beach-face sands and gravels (West, 1980). It contains a marine molluscan fauna and a larger proportion of far-travelled rocks than the How Hill Member (Rose et al., 2001). The unit includes the Mundesley Clay at Mundesley and Paston, and the Yoldia (Leda) Myalis Bed of Reid (1882) at West Runton.

Pre-Anglian Freshwater and Mass-Movement Deposits

Before the onset of glaciation in the Middle Pleistocene, eastern England was drained by a series of major eastward-flowing rivers, three of which crossed East Anglia (Rose et al., 2001). These were the Palaeo-Thames, which flowed from Wales to enter the sea in what is now Suffolk, the Bytham River, which flowed from the Midlands to enter the sea on the borders of Suffolk and Norfolk, and the Ancaster River (Clayton, 2000) which flowed from the Pennines to enter the sea in North Norfolk.

Until recently it had been generally accepted that the Palaeo-Thames and Bytham rivers joined and flowed into the sea in North Norfolk. Green and McGregor (1990) recorded gravels of Pre-Pastonian age at West Runton which they inferred to indicate the close proximity of the trunk stream of the proto-Thames system. Certainly gravels with Kesgrave-derived clasts of Welsh provenance are widespread in Norfolk, but most of these are marine and hence fall within the Wroxham Crag Formation. Mapping of the Lowestoft and Saxmunudham districts (Hamblin and Moorlock, 1995) cast doubt on the Palaeo-Thames ever having flowed north of Suffolk, and it appears likely that transport of Palaeo-Thames and Bytham-derived detritus into North Norfolk is entirely a result of marine transport within the Wroxham Crag. Those freshwater deposits which do occur in North Norfolk, and which are ascribed to the Cromer Forest-bed Formation, are believed to have been formed by the Ancaster River, and indeed the bulk of the pre-glacial gravels examined by Green and McGregor (1990) indicated transport from the north or north-west.

Cromer Forest-Bed Formation

The term Cromer Forest Bed Series was given by Reid (1882) to a sequence of marine, brackish and freshwater sediments deposited in the coastal region of northern and north-eastern East Anglia. The Cromer Forest Bed Formation (Funnell and West, 1977) was included in the Kesgrave Group by Arthurton et al. (1994), but this practice has not been adopted elsewhere. It is current BGS practice to hyphenate Forest-bed because of the unfortunate juxtaposition of the terms 'bed' and 'formation'. West and Wilson (1966) and West (1980) extended the scope of the formation somewhat beyond that envisaged by Reid, to encompass strata of Pre-Pastonian to Cromerian age (see discussion on Wroxham Crag, above). Current BGS practice is to include only the dominantly nonmarine members in the Cromer Forest-bed Formation, that is the Sheringham, Runton, West Runton and Bacton members, although this results in interdigitation of the Wroxham Crag and Cromer Forest-bed formations in this district. The deposits are exposed discontinuously along the coast from Weybourne to the east of the district, commonly caught up in glacial thrust masses, but have not been recorded inland.

The deposits may be observed along the coast depending on the state of coastal erosion and protection; they have also been studied by borings (West, 1980). The Sheringham Member, of Pre-Pastonian 'b' to 'd' age (West, 1980), comprises impersistent horizons of freshwater organic mud, clay and sand, indicating regression of the sea. At the stratotype at Beeston [TG 169 434], up to 2.5 m of freshwater clay, sand and mud directly overlie Chalk (Lewis, in Bowen, 1999). The pollen evidence suggests severe climatic conditions, with a herb-dominated assemblage (West, 1980).

The Sheringham Member is succeeded by the marine Paston Member of the Wroxham Crag Formation, followed by the Runton Member of the Cromer Forest-bed Formation. Funnell (1977) and West (1980) recognised a number of beds within the Runton Formation, including laminated freshwater silty clay known as the Woman Hythe Clay. Pollen assemblages and evidence of contemporaneous periglacial activity indicate a cold climate, with herbaceous vegetation (West, 1980).

The succeeding West Runton Member of Cromerian I–II age comprises layers of alluvial clay and organic freshwater mud up to about 2.0 m thick, including the 'West Runton Freshwater Bed' (Reid, 1882) at West Runton (Plate 2). The climate was temperate woodland, with areas of grass and herbs (West, 1980), and the stratotype at West Runton [TG 189 431] is also the stratotype for the Cromerian interglacial temperate stage of the Pleistocene. The age of the deposit is estimated as between 600 and 700 ka (Stuart, 2000). In 2000, the bed, 0.5–2.0 m thick, was exposed in the sea cliff 100 to 350 m east of West Runton Gap (Woman Hythe) and also 300 m west of the gap (Stuart, 2000).

Excavations in 1992 and 1995 resulted in the recovery of most of the skeleton of a mammoth Mammuthus trogontherii from the Freshwater Bed (Stuart, 1992; Ashwin and Stuart, 1996; Stuart, 1997). This has been interpreted as a male, which at its death was around 4 m high, 10 tonnes in weight, and 40 years old. Tooth marks and coprolites indicate extensive hyaena scavenging, and many small bones were thus missing.

The Mundesley Member of the Wroxham Crag Formation marks a further marine transgression. It is succeeded by the Bacton Member, of Cromerian IV age, which comprises a further 1.5 m of clay and organic mud. This is the 'Arctic Freshwater Bed' of Reid (1882). It occurs between Trimingham and Bacton (in the North Walsham and Mundesley district), and the stratotype is at Mundesley [TG 319 363]. The climate was cold, with evidence for periglacial conditions (West, 1980).

Mass-movement deposits

In the sea cliffs east of Weybourne [TG 112 438], a solifluction deposit is preserved overlying the Upper Chalk and underlying the Wroxham Crag. The deposit is formed almost wholly from the Upper Chalk; it is clearly of very local derivation with rounded clasts of chalk up to about 20 cm across and a few shattered flint set in a chalk paste matrix. Since the deposit is overlain by the Wroxham Crag of Pre-Pastonian age, it is likely to date from the preceding Baventian stage, which was characterised by a very cold climate (West, 1961b; Hamblin et al., 1997). Hence the deposit is correlated with the Norwich Crag Formation of the marine sequence.

Glacial Deposits

Traditionally, the glaciogenic sediments of northern East Anglia have been divided into the North Sea Drift Formation and the overlying Lowestoft Formation (Zalasiewicz et al., 1988; Ehlers et al., 1991), both of Anglian age, and the younger Hunstanton Formation of Devensian age. Unfortunately the term Hunstanton Formation has also been used for the Red Chalk at Hunstanton, which has precedence over its use for the glaciogenic deposits. We have, therefore, included the Devensian glaciogenic deposits of north

Norfolk within the Holderness Formation (see Lewis in Bowen, 1999).

The North Sea Drift Formation included a complex sequence of outwash sands and gravels together with three diamictons, named the First, Second and Third Cromer tills by Banham (1988); later renamed the Happisburgh, Walcott and Cromer diamictons (Lunkka, 1994). Lunkka also identified a fourth diamicton, the Mundesley Diamicton which he considered to be a water lain variety of the Cromer Diamicton. The North Sea Drift Formation was considered to have been deposited by ice sheets emanating from Scandinavia (Lunkka, 1994).

The Lowestoft Formation, which includes the well known Lowestoft till or 'chalky boulder clay' (Baden-Powell, 1948), was deposited by the British Eastern Ice Sheet that originated in northern Britain. Traced eastwards into north-east Norfolk the Lowestoft till becomes increasingly chalky, where it is often referred to as 'the marly drift' (Banham, 1975; Fish et al., 2000). Some authors have suggested that a 'marly drift' facies may also be present within the North Sea Drift Formation (Ehlers et al., 1987).

Recent work (Moorlock et al., 2000a) has demonstrated that the Walcott Diamicton has more features in common with the Lowestoft till, than with either of the other diamictons in the North Sea Drift Formation. From extensive field work these authors concluded that the Walcott Diamicton is the lateral equivalent of the Lowestoft till in north-east Norfolk (Figure 4). Such a correlation renders the North Sea Drift Formation an invalid lithostratigraphical unit.

The recent survey has confirmed the view that 'marly drift facies' occur in both in the Lowestoft Formation and the Beeston Regis Formation. The chalk-rich diamicton in the Lowestoft Formation is characterised by the presence of palynomorphs derived from Jurassic and Carboniferous strata over which the ice sheet passed, whereas the chalk-rich diamicton in the Beeston Regis Formation contains only local palynomorphs, derived from the nearby Upper Cretaceous Chalk (Riding et al., 1999). It is these latter diamictons that occur high in the cliffs to the south-east of Cromer which have been misidentified as Lowestoft till (Lunkka, 1991, 1994), and have led to the incorrect conclusion that the Lowestoft till overlies the North Sea Drift Formation.

Corton Formation

This formation comprises the Happisburgh Till Member and associated sorted sediments. It is exposed in cliff sections, where it commonly forms the lowermost glaciogenic unit (Plate 3). The till, which is a grey to grey-brown in colour, is generally massive, although a faint stratification is commonly present. Particle size analysis reveals that the till is composed of silty clayey sand (Figure 5). Lunkka (1991) reports the carbonate content of five samples as being within the range 7.2 to 10.2 per cent. The till contains clasts of flint, and chalk, and less abundant igneous and metamorphic rocks. Although clasts of rhomb porphyry, derived from the Oslofjord area of Norway, have been reported from this till, it is perhaps significant that none were found during the field survey or during an extensive clast lithological analysis study (J R Lee, oral communication, 2001).

The till contains palynomorphs derived from Carboniferous, Jurassic and Cretaceous strata, although Jurassic palynomorphs are relatively scarce. The derived palynomorphs demonstrate that the ice sheet from which the till was deposited crossed and eroded strata of these ages, during its journey to north-east Norfolk. As the only available source of such strata would have been in north-eastern Britain the ice sheet must have travelled across this region before reaching north-east Norfolk.

The Happisburgh Till is overlain by the laminated Happisburgh Clay in many coastal sections, but its extent inland is unknown. Elsewhere along the coast, sands intervene between the Happisburgh Till and the overlying Lowestoft Formation.

Lowestoft Formation

This formation consists of till and associated sorted sediments. The till crops out extensively in the western part of the district where it caps much of the higher ground in the Field Dalling and Sharrington areas, and in the central part of the district in areas where the overlying Briton's Lane Sand and Gravel is absent. In the eastern part of the district, the Lowestoft till is generally overlain by younger glaciogenic deposits. The till is a uniform, tenacious, stony, sandy, silty, grey clay. Chalk is present both as clasts and as fine-grained 'flour' within the matrix. Flints and chalk are the most abundant clasts, but others include Jurassic mudstone and fossils, sandstone, quartz, quartzite, igneous and metamorphic rocks. Near the surface the till is commonly decalcified and weathered to an olive-brown.

Over much of the central part of the district, the Lowestoft till is very chalk-rich and very pale in colour, and has often been referred to as the 'marly drift'. The presence of derived Jurassic and Carboniferous palynomorphs within this chalk-rich till are consistent with it being a variety of the Lowestoft till. The chalk-rich till is well-exposed in the Weybourne Town Pit [TG 114 431] where it contains reworked lenses of the underlying Happisburgh Till. The maximum thickness of the Lowestoft till in the district is unknown, but recent boreholes on the side of the Glaven valley suggest that over 40 m are present locally. In the east of the district the till thins to only a few metres. Outwash sands and gravels form a subordinate part of the Lowestoft Formation. They have been mapped in the extreme west of the district to the north-east of Field Dalling, and are also exposed in the lowermost part of the Glaven Valley Pit at Glandford [TG 055 416] beneath presumed Lowestoft till. In the latter pit, they comprise up to 4 m of well-bedded and relatively well-sorted sandy, chalky, flint gravel that contain small clasts of dark grey mudstone derived from the Kimmeridge Clay Formation. The base of the gravel is not exposed. Sand and gravel may also be present as lenses within the till.

Beeston Regis Formation

This term has been introduced in the Cromer district to include the third till of the old North Sea Drift Formation, the Cromer Diamicton of Lunkka (1994), and associated outwash and lacustrine deposits. Since the term 'Cromer' is pre-empted by the Cromer Forest-bed Formation, this till is here re-named the Bacton Green Till Member (Figure 4). The till is generally a clayey, silty sand, with a small content of pebbles, but this sandy till is interbedded with 'marly drift' formed from reconstituted chalk. The 'marly drift' varies from stringers only centimetres thick interbedded with the till, to incorporated masses hundreds of metres across. Rafts of Upper Chalk of very local derivation and even larger size are also known, for instance at West Runton (Figure 6). The Bacton Green Till is typically very highly tectonised, with the interbedded sandy till and 'marly drift' commonly occurring as isoclinal folds, forming the 'Contorted Drift' of Reid (1882). To the south of the Cromer Ridge, the Hanworth Till Member comprises unbedded clayey, silty sand, incorporating masses of 'marly drift', but differs from the Bacton Green Till in not being contorted.

Moorlock et al. (2000b) originally included the Bacton Green and Hanworth members in the Overstrand Formation. Sand within the upper part of the Beeston Regis Formation crops out very locally near Trimingham in the extreme east of the district. The sand has been named the Trimingham Sand Member.

Beeston Regis Formation (undivided)

The 'undivided' category has been used to define the outcrop where the Beeston Regis Formation has been highly deformed by glaciogenic processes and loading, so that individual members cannot be mapped separately at the surface. Within the outcrop, rapid lateral changes in lithology occur from sands and gravels to chalk-rich diamictons and possible chalk rafts. These deformed strata have formerly been described as the 'contorted drift'.

Between Overstrand and Sheringham, large basin-and-dome-shaped folds occur. The limbs of the folds are typically steep and commonly overturned with the formation of mushroom-type structures. The basins, which are up to several tens of metres deep and wide, are filled with Gimingham Sands. The folding ceases abruptly downward at a plane of décollement which usually lies above the Happisburgh Till Member, although locally the deformation extends down into the Cromer Forest-bed Formation (Banham, 1975). Within the contorted strata, the more sandy units are boudinaged, whereas the tills have become folded into flat, tight, commonly sheared minor folds. The basin-and-dome deformation appears to have been initiated by loading, when the sediments were in a soft and wet condition.

Chalk rafts within the Beeston Regis Formation

Small rafts of chalk occur within the 'contorted drift' to the west of Cromer. Large rafts are restricted to three localities, Trimingham (just in the adjacent Mundesley district), Sidestrand (Plate 4), and Wood Hill between East and West Runton. The large rafts are about 10 m thick and up to several hundred metres long. The fauna within the rafts indicates that they have been derived locally (Peake and Hancock, 1970; Riding, 1999). The rafts at Sidestrand and Wood Hill are much thrust and folded. Their steep dips do not reflect the original angle of thrusting but result from the later diapiric dome-and-basin folding (Banham, 1975). During the diapiric flow some of the chalk rafts have become fragmented and overturned.

Overstrand Formation

This term was introduced by Moorlock et al. (2000b), and includes the Briton's Lane Sand and Gravel Member, a thick sequence of coarse gravels and sands containing erratics of rhomb porphyry from the Oslofjord region of Scandinavia. In many places the sand and gravel is 'draped' over a pre-existing topography which suggests that the sediment was deposited directly from ice rather than in a glaciofluvial environment.

The sand and gravel has an extensive outcrop within the district where it occupies much of the higher ground on, and adjacent to, the Cromer Ridge. It takes its name from the locality on the northern slopes of the Ridge near Beeston Regis where, up to about 40 m of sand and gravel are exposed in a large working quarry [TG 168 145] (Plate 5). Lithological clast analysis of the 8–16 and 16–32 gravel fraction in the quarry has revealed a high content of far-travelled exotic clasts (Figure 6). An examination of larger clasts collected from the quarry revealed that about 70 per cent of the exotic clasts are igneous in origin, including rhomb porphyries of Scandinavian origin.

Metamorphic rocks make up about 20 per cent of the exotic clasts and include schists and gneisses. The schists are fine to coarse grained, and mainly of psammitic composition. A large proportion of these may be deformed granitic rocks. A small proportion are pelitic (garnet-mica schist). The gneisses consist principally of coarse quartz, feldspar and biotite, usually with augen of quartz and/or feldspar. They probably have an igneous origin (orthogneiss).

Sedimentary rocks make up the remaining 10 per cent (excludes quartz and quartzite). Most samples are of dark purple-brown (red-bed type), medium to fine-grained sandstone, with, some thinly laminated sandstone.

The high proportion of exotic clasts makes it distinct from any of the other gravels within the district.

Blakeney esker

A distinctive sinuous, linear, topographical feature from around Glandford [TG 045 414] in the River Glaven valley trending north-westwards for some 3.5 km through Wiveton Downs [TG 031 423] and Blakeney Downs [TG 018 435] to the coast at Morston Downs [TG 017 437] has been the subject of much speculative debate in terms of its age and origin. Although now generally regarded as a glacial esker, its origin has been variously ascribed to:

• a ridge resulting from erosion of a larger mass of sand and gravel (Straw, 1973)
• a morainic or ice-marginal deposit (West, 1957)
• an infilled formerly open-crevasse (Sparks and West, 1964)
• a ridge formed in a supraglacial channel (Gray, 1997)
• a ridge formed as an englacial channel (Gale and Hoare, 1986)
• The esker consists predominantly of rounded, cobble-size, clast-supported, flint gravel devoid of any exotic pebbles (Rose, personal communication, 2000) (Plate 6), which suggests, a local source and deposition in a high-energy environment. Gale and Hoare (1986) concluded that the direction of water flow was from north-west to south-east. Rose (in Banham et al., 1975) had earlier suggested that the esker resulted from a subglacial stream flowing towards the outwash fan of the Salthouse Heath area to the south-east.
• Local, temporary quarry sections, at Wiveton Downs [TG 031 423] and near Kettlehill Plantation [TG 023 433], have shown that the esker gravels rest on, or are cut into, chalk-rich Lowestoft till (Gray, 1988, 1997; J Rose personal communication), which suggests that the esker formed during the Anglian glaciation. However, the recent regional mapping favours correlation of the esker with the younger Briton's Lane Sand and Gravel Member of the Overstrand Formation. Others consider that the esker formed during the Devensian glaciation.

Kame-like features

Within the Glaven valley and adjacent area in the northern central part of the district are rounded 'knolls' composed predominantly of gravel. The knolls are typically very steep sided

and have generally not been cultivated. Some workers have identified the knolls as kames and kame terraces, but their internal structure is unknown. Others, most notably Straw (1973), have argued that the features are erosional rather than constructional, and result from processes such as spring sapping. Certainly spring sapping has been responsible for the steep slopes and other features along the northern side of the Cromer Ridge. Several boreholes through the gravels within the Glaven valley demonstrated that the gravels all overlie very chalky till and are not interbedded with the till. It would thus appear that whatever the origin of the kame-like features the gravels represent a relatively late infill of a pre-existing valley.

Glaciofluvial sands and gravels (undifferentiated)

Several outcrops of sand and gravel (including [TG 140 340] and [TG 106 340]) in the extreme south of the district cannot be assigned with any certainty to either the Lowestoft Formation or the underlying Corton Formation. In the absence of any diagnostic criteria, these deposits have been designated as Glaciofluvial sands and gravels (undifferentiated).

Holderness Formation

During the Devensian glacial maximum, some 22 000 to 18 000 years BP (Rose, 1989; Ehlers et al., 1991; Bowen, 1999), it is estimated that around 5 per cent of the global water budget was locked up in the form of ice; sea level as a consequence fell to as much as 100 to 120 m below its present level (Lamb, 1977, 1982; Lambeck, 1995). At its maximum extent the Devensian ice sheet extended southwards from Holderness and Lincolnshire into north Norfolk where its position is marked by the occurrence of drab red-brown to purple, sandy clay tills. The ice reached as far east as Cley next the Sea, and probably beyond Salthouse.

During this glaciation, ice from the north impinged on the northern part the Cromer district, but appears not to have penetrated far inland. Lewis (in Bowen, 1999), includes the Devensian tills and associated sorted outwash deposits of north-west Norfolk within the Hunstanton Formation. However, the term 'Hunstanton Formation' has previously been assigned to the Red Chalk at Hunstanton, and, therefore, cannot be used for the Devensian deposits. Lewis (in Bowen, 1999), uses the term Holderness Formation for Devensian deposits in Holderness and Lincolnshire; the use of this formational name has been extended here to include the Devensian deposits of north Norfolk. The Holderness Formation includes the two local members, the Ringstead Sand and Gravel Member and the Holkham Till Member.

The Ringstead Sand and Gravel Member comprises glaciofluvial sands and gravels associated with the Holkham Till Member (Lewis in Bowen, 1999). Within the district, the landforms identified with this meltwater outwash comprise a number of low (generally below 10 m but up to 19 m OD), coast-parallel, principally gravel-rich mounds extending from Morston Chase [TF 994 435] to the Salthouse Marshes [TG 086 441]. The seven gravel mounds from Blakeney Eye [TG 040 452] eastwards to Granborough Hill [TG 086 441] are distinctive in that they crop out just above marsh level; they are known locally as 'Eyes'. The gravel component of the deposits ranges from rounded cobbles, to subangular and angular clasts, consisting mainly of flint.

The Holkham Till Member comprises a dull reddish brown, sandy clay, commonly containing chalk clasts and flint pebbles, together with Carboniferous and Triassic

material, and a variety of igneous and metamorphic rocks (Lewis in Gallois, 1978; Bowen, 1999). The stratotype section lies some 9 km west of the district where it is several metres thick. Within the district the deposit is restricted to a 0.1 km2 area (below 10 m OD), just east of Stiffkey Sluices [TF 992 439]. A further very localized outcrop was exposed in a temporary exposure on the foreshore near Cley next the Sea (at [TG 051 453]). Augering in the marshland adjacent to Cley Eye [TG 051 451] has also proved stiff reddish brown clay with chalk clasts.

Confirmation that this sandy clay relates to the Holderness ('Hunstanton') Till of Holderness and Lincolnshire is provided by analysis of sediments from the Morston section [TF 987 441] by Gale et al. (1988) and by magnetic analysis work by B M Funnell and W M Corbett at the Cley next the Sea site (B M Funnell, personal communication 1999).

Interglacial deposits

Interglacial deposits of Ipswichian and possible Hoxnian age have been identified within the district, but are not depicted on the map. Pollen obtained from organic silt and clay from a cliff-top site at Sidestrand is indicative of a late glacial to early interglacial environment (Hart and Peglar, 1990). The interglacial deposits, now lost to the sea, overlay sands and diamictons believed to be within the Beeston Regis Formation (Banham et al., 2001). The pollen spectra obtained are dominated by non-tree pollen, including an abundance of Hippophae (sea buckthorn), which may indicate a correlation with the very early part of the Hoxnian interglacial (Hart and Peglar, 1990; Banham et al., 2001) that followed the Anglian cold stage, but the evidence is inconclusive.

During the more recent Ipswichian interglacial, some 132 000 to 122 000 years BP, maximum sea levels are believed to have been some 2 m higher than at present. The remarkably straight southern marshland edge that extends from Salthouse [TG 077 435] westwards to beyond Morston [TG 008 438] possibly represents a former Ipswichian shoreline, although the feature may reflect a fault within the underlying Chalk bedrock.

Between Stiffkey and Morston, Solomon (1931, 1932) recorded the existence of 'an ancient shingle beach', composed mainly of flint, extending east–west along the edge of the saltmarsh for approximately 5 km. Gale et al. (1988), described a section [TF 987 441] which they interpreted as a complex sequence of former beach deposits laid down under warm, marginal marine, interglacial conditions. The beach complex is underlain by calcareous till (probably a chalk-rich facies of the Lowestoft till) and overlain by brown sandy till, interpreted as Devensian, Holkham Till.

Postglacial events

At the start of the Holocene (around 10 000 years BP), the North Sea coastline lay well to the north of its current position (Lambeck, 1995, fig. 3), and the present-day marshland and river valleys of the district had a significantly different topography. The original valley floors of the River Glaven and minor streams draining northwards to the Salthouse, Cley and Morston marshland lie well below the present-day surface.

Early in the Holocene, the seaward outfall for these drainage channels was some 6 km north of the present-day coastline (Andrews et al., 2000). Farther east, the presumed former cliff line from Weybourne to Cromer and beyond to Happisburgh, extended sea-wards linking up with a contemporary coastline and River Yare estuary that lay some 7 km offshore, to the east of Great Yarmouth (Arthurton et al., 1994)

As relative sea level rose, the marine incursion recycled glacial deposits lying 'offshore'. The recycled material, and the coastline, migrated southwards, eventually merging into and perhaps anchored by the low morainic mounds ('Eyes'), and culminating in the present-day barrier ridge, marshland and cliffed coastline.

The onshore Holocene deposits comprise alluvial and marine/estuarine deposits. East of gridline easting 614, and south of an arcuate line from Edgefield [TG 097 345] to Sidestrand [TG 263 395] (see (Figure 1)), drainage is towards the south and south-east linking up with the rivers Bure and Ant, which drain to the River Yare outfall at Great Yarmouth. Elsewhere, drainage is predominantly northwards, either within head-filled 'misfit' valleys along the northern flank of the 'Cromer Ridge' or as the larger River Glaven that drains to the reclaimed marsh-land behind Blakeney Spit. Freshwater sediments, characterised by sand, gravel, silt and clay with localised surface and interbedded peat, pass down-valley into marshland comprising estuarine clay, silt and sand, which in turn, give way to sand dunes, a gravel-dominated ridge (or spit) and shoreface deposits of the coastal fringe.

The section of coastline west of Weybourne to beyond Blakeney Point, includes part of the coast often referred to as a 'barrier coast-line'. This refers to the coast-parallel, gravel-dominated ridge that forms the Blakeney Spit. Towards the western end of the spit, the offshore slope is gently inclined resulting in wave-influenced sand flats while behind the spit lies a sediment-choked estuary (see schematic section on map face). Offshore, modern marine sediments (mud, sand and pebbly sand) overlie chalk. Foreshore deposits also directly overlie Upper Chalk from the western margin of the district to Overstrand [TG257 406]; east-wards from here these sediments overlie Pleistocene Wroxham Crag in the higher reaches of the beach.

River terrace deposits

Sand and gravel, assigned to River Terrace Deposits, have a very restricted outcrop. They are likely to have a maximum thickness of between 1 and 3 m. Because of their isolated occur- rences it has not been possible to assign the deposits to any particular terrace sequence.

Head

Head is present within all of the valleys and is notably present along the northern flank of the 'Cromer Ridge'. It comprises poorly sorted and poorly stratified deposits formed by mass movement of superficial materials on sloping ground. This mass movement process includes hill-wash and soil creep as well as solifluction, an important mode of sediment transport in periglacial conditions which would have been prevalent during the Devensian period extending into the Flandrian (Holocene). Head occurs as a veneer generally less than a metre thick draping the valley slopes, but is up to several metres thick towards the base of steep slopes and lining valley floors. Locally, it is overlapped by peat; elsewhere, it passes into or is intercalated with alluvium. Shear planes may be present within, and at the base of, the head deposits.

Gravelly Head

This comprises sandy gravel and gravelly sand, the gravel component being dominated by angular, shattered flint. It has been mapped in several broad valleys on the northern side of the Cromer Ridge between Weybourne and Overstrand, and also less extensively elsewhere within the district. It is believed to have formed during periglacial periods,

by sheet-flood and stream-flood processes, rather than by solifluction. There is little information on the thickness of Gravelly Head deposits, but they are likely to be less than 3 m thick.

Cover loam

A veneer of Cover Loam was deposited across much of north-east Norfolk under periglacial conditions during the late Devensian. The Cover Loam is not depicted on the map as it would mask the underlying geology. Fieldwork has confirmed that Cover Loam is present through- out much of the district, but is generally absent to the north and west of the Cromer Ridge. The loam is generally less than 1 m thick but can reach thicknesses of several metres. A section near Gibbet Plantation [TG 1570 4950] showed almost 4 m of interbedded, beige and brown silt and fine sand without having reached the base of the deposit. A hard gravelly, clayey layer is commonly encountered at the base of the Cover Loam; this is interpeted as a deflationary product. The Cover Loam itself is usually slightly stony as a result of both solifluction or cryoturbation.

Due to its aeolian origin and the wind direction being predominantly westerly the deposits are generally thicker on east-facing slopes. This is well demonstrated in the asymmetric valleys near Roughton [TG 2100 3800] (Figure 7) where the shallow eastern slopes contain up to 2 m of silty Cover Loam merging into head towards the foot of the slope; the westerly facing slopes are steep and have little or no Cover Loam.

Storm beach deposits

The most striking feature of the coastline is Blakeney Spit, a storm beach spit that extends from its landward attachment just east of Kelling Hard [TG 097 439] westwards for some 15.5 km to Blakeney Point and beyond (Plate 7). It comprises a shingle bank some 200 m wide and 9 to 10 m high, containing an estimated '82 million cubic feet' of cobbles (Hardy, 1964). These cobbles consist of at least 97 per cent flint (Steers in Allison and Morley, 1989). Alluvial and intertidal sediments enclosed behind the spit form a series of interlinked marshes that are open to the sea at the western end.

The feature is entirely postglacial. Devensian and possibly older glacial deposits lying off the present-day shore are thought by Hardy (1964) to have provided the source for the beach bar accretion. The shape of the spit and the occurrence of a number of landward-curved shingle banks (such as Watch House, The Hood and Long Hills) are probably earlier positions of the end of the spit and tend to imply a west-wards growth of the spit fed by erosion from the cliffs between Weybourne and Sheringham and possibly the 'Eyes' at Cley and Salthouse. However, there are few cobble-size clasts in the cliff sediments and there is no clear evidence of shingle grading westwards along the spit.

This westwards longshore drift model is partly supported by sediment movement measurements made by Steers (1969). However, the model does not fully account for the predominance of sand at the distal end of the spit and the prevalence of storm currents from the north-west. It is suggested by the Environment Agency (1996) that the most likely source of sand lies beyond the western end of the spit. A study of aerial photos, taken over 40 years, indicates that sand bars move eastwards across the Blakeney Channel ultimately merging into the distal end of the spit. Thus, probably storm events from the north and north-west rather than longshore drift, together with a sediment source from the west rather than from the east, provide an alternative mechanism for extending the spit and may account for the landward-curved banks, for example the Hood, as noted above. Another possibility is that sediment is derived from offshore (Shih-Chiao and Evans, 1992).

Cozens-Hardy (1927) refers to a map of 1586 which indicates that the Blakeney Spit then terminated around the position of the Watch House. If this position is correct, the Cambridge Research Unit (1997) calculated that the spit has extended by over 2 km between 1580 and the present-day, giving an average extension of 5 m per annum. By contrast, based on spit growth between 1886 and 1904, Steers (1927) estimated the extension rate at 86.3 m per annum and between 1904 and 1925 at 45.7 m per annum.

There is also much evidence to show that the shingle ridge has moved, and is moving, landward. From 1649 to 1924, the ridge was recorded as moving landward at 1 m per annum, and this migration is reportedly continuing (Hardy, 1964). By reference to old maps such as Barringer (1989), it is clear that the shingle bar has subsumed parts of the marshland, such as Weybourne Marsh at the eastern end of the marsh/shingle complex. Today, the glacial 'Eyes', which include Blakeney Eye, Cley Eye, Little Eye and Gramborough Hill, all show signs of being overwhelmed by the shingle. During particularly stormy weather, material from the foreshore is carried up over the berm and redeposited on the landward sides as 'washover' deposits or 'washover' fans. This redistribution of the barrier sediments eventually leads to a critical reduction in the effectiveness of the barrier which in some winter storms, such as that of 1997, has allowed the sea to break through and inundate the marshland.

Moreston Meals

There are a number of other coast-parallel ridges within the marshland tidal flat deposits, which as they restrict direct tidal inundation, often mark the boundary between upper and lower marsh environments. These are low ridges about 1.5 m high and up to 10 m wide, which may extend for up to 3 km although interrupted by major channels (Boomer and Woodcock, 1999). Funnell and Pearson (1989), believed the ridges were entirely aeolian, but Boomer and Woodcock (1999) have shown that although the ridges may act as nuclei for dune sands, they have cores composed of sand, pebble and well sorted gravel layers containing well rounded cobbles. These ridges overlie marshland sediments.

These ridges, known locally as 'Meals' or 'Meols', are natural in origin, although there is no direct evidence to suggest how they developed. However, studies of the Stiffkey Meals (adjacent to the district) by Boomer and Woodcock (1999), indicate that the ridges accumulated initially as more northerly, seaward features. Their migration landwards over tidal flat marshland may have occurred as a result of an increase in storminess, and associated rapid rise of sea-level, drowning and redistributing the offshore barrier. It is probable that the present ridges are much-diminished versions of the parent barrier(s) and that climate change initiated the mechanism.

Shoreface deposits

The 29.34 km-long foreshore generally comprises sand, pebbly sand and gravel; while locally towards the 'top' of the beach, there may be ephemeral shingle-rich 'cusps' associated with storm deposition. A typical beach profile comprises shingle (mostly 10 to 15 mm diameter) of well-rounded clasts of flint and subordinate quartz and quartzite, within a matrix of medium-grained sand.

It has been a generally held view, that the foreshore sediments were subject to a wave-induced westward migration, or long-shore drift, that resulted in the formation of the Blakeney Spit. However, studies of the hydrodynamic setting for north Norfolk

(Environment Agency, 1996) have indicated a tidally influenced net flow eastwards and offshore along most of the coast. Studies of the foreshore (Clayton, 1976, 1977) indicate that sediment movement comprises localised net gains with net losses. For example, along the uncliffed shoreline from around Blakeney Point to Kelling Hard, the 1977 survey recorded beach losses in the order of 15 per cent by volume. This loss is partially accounted for by the landward migration of the spit, and human intervention in maintaining the spit. Along the cliffed coastline, there is a net gain. The main gains are from Weybourne to Sheringham whereas beaches from Cromer to Happisburgh mainly reveal losses (Clayton, 1977).

The 'beach' varies in width from 30 to 160 m, while the shoreface and beach zone extends from the subtidal limit to 3 m above OD, the highest points being on the landward side. From Trimingham [TG 2767 3872] in the east to Kelling Hard [TG 096 439] in the west, shoreface deposits are banked up against cliffs; west of Kelling Hard, they drape and intercalate with Storm Beach, Blown Sand, or Tidal Flat Deposits.

Beach sediments are naturally ephemeral, with periods of sediment accretion inter- rupted by storm events that result in a net seaward removal of material. The deposits generally form a gently inclined (5° to 7°) trapezoid wedge that, after storm scours, at low tide may be seen to rest on a wave-cut bench of Upper Chalk (for example at Robin's Friend [TG 145 436] or glacially emplaced chalk rafts at [TG 262 401]). Where beach deposits are being eroded, the beach wedge narrows and the profile steepens.

Tidal flat deposits

These deposits are confined to the area behind the Blakeney Spit, and include the sediments referred to as 'Upper' and 'Lower' saltmarshes and 'salt-pans' (see schematic section on map face). Saltpans or 'saltings' are small flat-bottomed, shallow pools, which are remnant old creeks. Saltpans may also be present as man-made hollows dug for a former sea salt cottage industry.

Tidal flat deposits comprise modern marine and estuarine-derived, layered mud and silt with interbedded sand lenses. The surface distribution of these deposits is dynamic and ever changing as each new tidal flood brings with it new sediment or redistributes earlier deposits. In some places the slope of the flat allows the development of vegetation which aids the entrapment of mud and silt. Elsewhere, especially where the ebb tide runoff is fast, the surface remains bare.

Boreholes reveal that the sequence of Tidal Flat Deposits includes a complex intercalation of marine/estuarine mud, silt and sand with freshwater peat layers. These differing lithologies reflect relative movements in sea level (mostly positive upward movements) and changes (some man-induced) in the coastal morphology and tidal dynamics.

The thickness of the marsh sediments is determined by a pre-Holocene topography. A distinctive feature is an east–west, coast-parallel trough defined by the surface of the chalk and interpreted as a palaeovalley (Chroston et al., 1999) (Figure 8). The trough, now infilled with Holocene sediments up to 12 m thick, possibly drained eastwards towards Granborough Hill, and is thought to reflect the position of a Devensian ice-marginal stream. Transects across the trough, based on seismic, bore-hole and auger data, provide evidence of a rapidly changing climate and environment during the early Holocene. The basal deposits that directly overlie Chalk bedrock or remnant glacial gravels are generally peat formed as the watertable rose in advance of a marine transgression. The peats are overlain by 'back-barrier' silt and mud, locally interrupted by discontinous peat layers, which progress upwards (at Holkham and Burnham Overy just outside of the district) into sand with gravels, as the marine influence increased, and the 'shoreline' migrated southwards. A cross-section for the Cley next the Sea area, based on the work of Andrews et al. (2000, fig. 10a, 10b), is shown in (Figure 9).

Tidal river or creek deposits

Deposits of sand, silt and clay are being deposited along the present-day open tidal channels or creeks that cross the marshland. These channels usually develop as a result of vegetation growth on marsh affected by tidal flooding; the vegetation impedes water movement, which gradually leads to preferential flow routes and subsequent channel erosion.

Channel sections observed at low tide show well laminated silt which may be charged with plant debris. A few auger holes drilled to a metre or so on these channel banks variously proved plant-rich clayey deposits, peat layers or sand lenses.

Sheet deposits

An extensive area of Sheet Deposits occurs offshore in the extreme north-west of the district. The deposits, which directly overly Upper Chalk, are composed predominantly of medium- and coarse-grained sand which may contain broken shell debris. The Sheet Deposits have a maximum thickness of several metres.

Bank deposits

A small elongate coast-parallel sand bank is present offshore to the north of Weybourne. The bank is comprised of fine to medium-grained sand. The bank partially overlies remnant offshore Tidal Flat Deposits.

Alluvium

This is mapped along the floodplains of the main drainage courses, notably the River Glaven and the Stiffkey River which drain northwards, and tributaries of the River Bure and River Ant which flow southwards. The alluvium consists mainly of unconsolidated layers of sand and silt but includes sediments that range from organic clay through to coarse gravel, derived from nearby sources. Downstream, alluvium may be interbedded with, or merge into, the marshland sequence (for example alluvium of the River Glaven at Cley next the Sea), while upstream the alluvium is typically intercalated with, or replaced by, head. Locally, where drainage has been impeded, alluvium may be associated with peat. The

alluvium is estimated to have a maximum thickness of about 2 m.

Peat

Peat occurs as localised spreads within the confines of narrow valleys, commonly near the headwaters, where drainage is, or has been, impeded. Notable occurrences are mapped at The Lowes [TG 077 408], Manor Yards, Kelling Hall [TG 092 417], Snipes Marsh [TG 061 440], Beeston Regis Common [TG 163 420] and [TG 165 419] and at Southrepps Common [TG 263 350]. These 'surface' peats are generally less than 1 m thick, but may locally exceed 2 m, for example at The Lowes.

Peat also occurs as discontinuous, buried layers within the Salthouse to Stiffkey marshland sequence. These peats, proved in boreholes, comprise black to dark brown peat and organic mud formed in low-lying open ground, possibly adjacent to the sea or an estuary, but in a predominantly freshwater environment that ranged from swamp to Alder carr. Peat, or traces of peat, are present at various levels within the marsh-land deposits reflecting sea-level changes; the thickest and most continuous of these is a basal layer dated at between 11–10 cal. ka BP with peat formation continuing until at least 7 cal. ka. BP (Andrews et al., 2000).

Blown sand

Wind-blown sand occurs as localised dunes or dune belts along, and on the landward side of, the Blakeney Spit, particularly adjacent to Stanley's Cockle Bight [TF 994 460]. The dunes overlie remnant, generally arcuate, storm beach ridges that probably mark former, more westerly positions of the Blakeney Spit. Dunes tend to be ephemeral features, especially where they form thin drapes.

The dune deposits are characterised by well-developed laminar bedding, ripple marks and plant root structures. The dunes typically comprise yellow-buff, fine-grained quartz sand with larger shell fragments. It is assumed that most of the material is locally derived; however, very little sand is present on the nearby beaches except for the sea-facing sand flats just west of the district and on the inner flats by Pits Point [TG 004 456]. This

may explain the limited extent of dune development on the spit. Blown Sand ranges in thickness from centimetre-thick drapes up to 6 m.

Elsewhere, Blown Sand is present overlying a small number of coast-parallel sand and gravel ridges (or Meals) — see Storm Beach Deposits.

Artificially modified ground

Worked ground

Such areas include the present relatively large-scale sand and gravel workings, but also many former small pits associated with brick making, lime-manufacture and 'marling'. In addition there are numerous small pits excavated in the various tills that appear to have been dug as watering holes for livestock. Many of these have subsequently been backfilled during the period when many of the hedgerows were removed to make larger fields. Road and rail cuttings and very small pits are generally omitted from the 1:50 000 Series map, but are recorded on the larger scale maps used to compile the 1:50 000 map.

Made ground

This category has been used where man-made deposits have been placed on the original land surface. It includes areas such as screening mounds and dams. Road and rail embankments have generally been omitted from the 1:50 000 map, but are depicted on larger scale maps.

inFilled ground The infilled ground category has been used where areas of worked ground have been back-filled or partially back-filled. Where the infilling has been raised above original ground level it may be impossible to determine the extent of the former workings without detailed borehole information or a geophysical study.

Reclaimed ground

In the north of the district, areas of marshland fringing the coast that have been reclaimed for agriculture by a combination of drains and embankments have been depicted separately.

Chapter 3 Applied geology

Resources

Sand and gravel aggregate

The largest gravel resources are associated with the Briton's Lane Sand and Gravel Member of the Overstrand Formation, which crops out along much of the higher ground of the Cromer Ridge. These sands and gravels extend southwards, but become increasingly finer grained in the adjoining Aylsham and North Walsham districts.

At the time of survey, sand and gravel was being dug commercially from four quarries (Holt [TG 059 379], Edgefield [TG 085 356], Briton's Lane, Beeston Regis [TG 168 415], and Glandford [TG 055 416]) within the Briton's Lane Sand and Gravel Member of the Overstrand Formation. The workings at Glandford extend down into sand and gravel of the Lowestoft Formation.

Sand and gravel have also been dug on a small scale for individual estate or farm use.

Brick Clay

Historically clay, for brick-making, has been dug on a small scale from the Lowestoft till and the Hanworth Till.

Chalk

Chalk has been dug in the past as a source of lime for marling the more acid soils. The more chalk-rich parts of Lowestoft till and diamicton in the overlying Beeston Regis Formation have also been dug for this purpose.

Building stone

Flint has been used extensively as a building stone within the district. The flint cobbles may be used whole, or knapped to produce fresh flat surfaces. In the past the flint cobbles were probably obtained from beach deposits, but in recent times cobble-grade material has been screened by gravel operators from the Briton's Lane Sand and Gravel.

Soils

These are an important natural resource in the Cromer district. Most of the soils are developed on Quaternary sediments. Over much of the district, the soils have formed on Cover Loam. These soils tend to be Brown Earths with base-rich brown weathered subsoils. Where there is evidence of downward clay translocation such as dense reddish clay accumulation, these soils are known as 'sol lessivé'. These good arable soils are exposed overlying till on the cliff [TG 1750 4534] near Sheringham and inland have been proved in augerholes [TG 2090 3570] overlying Cover Loam near Hanworth. In some areas [TG 2240 3970] the Cover Loam is so silty that water is retained against gravity and a special variety of surface water gley soil has formed. The water capillary forces are so strong, that the ground is waterlogged for long periods throughout the year without the existence of a dense subsoil horizon. In areas of outwash sands and gravels where there is little or no Cover Loam, Podzols are developed. These have a leached topsoil due to the translocation of ferro/manganese oxides. They precipitate in the subsoil to form an orange-brown horizon rich in sesquioxides and are very dry and poor in nutrients. On calcareous tills along the coastal plain, Rendzinas and calcareous Brown Earths are developed. On thick, silty colluvium Brown Earths of the Sheringham Series form some of the best soils in the area, usually unaffected by drought. On floodplains and other low-lying ground Gley and Organic soils with grey water-logged subsoils have formed due to the constantly high watertable.

The modern marshland surface is the result of centuries of draining and agriculture.

Most of the ground comprises heavy, clayey soils derived from the weathering of estuarine, locally shelly deposits (Upper Clay). A distinctive feature in the uppermost metre of these soils is a stiff to indurated profile characterised by strong orange mottling. This soil 'ripening' (as it is commonly described) is a consequence of drainage with irreversible shrinkage and leads to the development of a strong prismatic clay structure, grading to blocky as ripening proceeds. This tough upper surface is commonly underlain by a soft thixotropic sediment; if heavy agricultural vehicles break through the crust they inevitably founder.

Ripening may be accompanied by the development of acid sulphate conditions characterised by a yellow mottling of jarosite (KFe₃(SO₄)₂(OH)₆). The acidification is due to the oxidation of pyrite in the former estuarine sediments. Where pyrite is accompanied by calcium carbonate (from shell fragments in the clay), the sulphuric acid reacts to form gypsum. Aeration and resulting acidification of pyrite-rich sediments often leads to the bacterial precipitation of ochre within drainage networks causing problems. A decrease of salinity of estuarine-derived soils with time is common within the drained marshland as the salts are leached out. The drainage of chemically unstable sodium-rich clay soils leads to deflocculation, movement and redeposition of clay particles, another cause of drainage blockages and surface ponding. In contrast, soils developed on peaty soils beyond the intrusion of saline waters contain little pyrite and thus rarely acidify once drained, unless underlain by estuarine clays turned up by deep ploughing.

Landfill gas

Gas produced from a landfill site at Edgefield [TG 085 356] is collected and used to generate electricity that is fed into the National Grid.

Water supply and hydrogeology

The water resources of the district are regulated by the Anglian Region of the Environment Agency. The major aquifers in the district are the Upper Chalk and the Wroxham Crag. They are in hydraulic continuity throughout the district. The sorted sediments within the glaciogenic formations may also form minor aquifers.

Chalk

Despite its high porosity, unfissured chalk is rather impermeable. Boreholes penetrating the Chalk in interfluve areas tend to give unsatisfactory yields, whereas boreholes under, and in the vicinity of major valleys, tend to provide copious supplies. Groundwater in the interfluves is relatively old, in a reduced state, and low in nitrates, whereas that under or near the valleys is typically modern and high in both nitrate and dissolved oxygen.

The quality of water abstracted from the Chalk is normally good. Where the aquifer is confined the chloride ion concentration tends to be higher due to old saline water being trapped in the Chalk.

Crag and overlying deposits

The Crag aquifer is an important source of ground-water in the east of the district, serving local, mostly agricultural demand. Yields are generally less dependable than those of the Chalk, due in part to the presence of low permeability layers of clay within the sands of the formation. Such layers may produce perched aquifers. Throughout the district, the Crag is in hydraulic continuity with the underlying Chalk.

Groundwater from the Crag is characterised by a high total hardness at outcrop, of which over half is usually attributable to carbonates. High chloride values occur in wells on low ground near the coast, and near tidal rivers. Water abstracted from the Crag shows a high concentration of dissolved and suspended iron (Forbes, 1952; Wood, 1961; Brereton, 1978). Methods of removing this in pumped supplies have been discussed by Clarke and Phillips (1984). The nitrate content of Crag water is high, as the shallow well supplies are vulnerable to leaching of nitrate from adjacent agricultural land.

Pumped drainage of the marshes near the coast has reduced groundwater levels to around or slightly below sea level, and allowed intrusion of high-chloride water.

Flooding and flood protection

The land along the north coast from Weybourne westwards lies at or below Mean High Water, and is vulnerable to seasonal flooding. Marine flooding is also a significant hazard to the low-lying hinterland. Marine incursions through breaches in the coastal barrier have occurred on numerous occasions, the earliest recorded event was in 1287. More recently, two severe floods occurred in 1938 and 1953, both occasioned by tidal surges, gale force winds and a high rainfall.

Coastal landslides

There are many landslides along the coast, principally affecting the tills. While coastal recession may be considered a gradual process on the large physical and temporal scales, it is episodic on the small scale. Occasionally, a single large landslide event may cause tens of metres of recession at the cliff-top in a matter of minutes or hours, seriously affecting villages, roads, and farmland. The direct initiating factor may be excessive rainfall or erosion of the cliff toe caused directly by wave action during storms. In the case of dominantly fine-grained lithologies, landsliding usually takes the form of rotational slumps. These are accompanied, particularly on the larger scale, by debris flows and mudflows. The depth and volume of the landslides, and hence the amount of cliff-top recession, is usually proportional to the cliff height, unless wide variations in lithology restrict the landsliding to certain zones of the cliff, as for example where chalk rafts are present within the till. Rotational landslides in fine-grained lithologies (e.g. clay and silt) tend to produce symmetrical embayments separated by un-slipped buttresses. The more uniform the lithology, then the more uniform and regularly spaced are the landslides.

Large rotational landslides produce a deep-seated shear surface, which can sometimes be seen emerging at beach level or higher up within the cliff. The slipping mass rotates on this surface causing the rear to drop, thus creating a scarp at the cliff-top, and causing the front to move forward and upward, creating a backtilt in the lowermost third of the cliff. In the case of coarse-grained lithologies (e.g. sand and gravel) landslides tend to be shallower, more rapid, and more likely to break down into flows, particularly when the ground is fully saturated. Large landslides often undergo further, smaller landslides within their boundaries. The greater the amount of initial movement, then the more likelihood of this happening. Coastal 'soft rock' landslides are typically regressive in the upper part and progressive in the lower part, but as the toe is removed rapidly by the sea, the support provided by it is removed. For this reason, the lower part tends to remain active for longer than a comparable slide inland where little erosion occurs. It is unusual for coastal landslides to occur entirely within previously unslipped material. Landslides do not respond immediately to environmental factors such as rainfall and coastal erosion. There tends to be a time-lag between the onset of causal factor and slope failure, particularly in the case of fine-grained, soft rocks. This may result in all year round landslide activity, irrespective of season or sea and weather conditions.

The change in curvature of the coastline at Cromer results in an increase in longshore currents, although currents are predominantly offshore/onshore in nature. The longshore currents that exist are mainly southerly. Cliff recession is up to 2 m per year. The section between Overstrand (south) and Trimingham (north) is a geological SSSI, part of which is concerned with the preservation of some of the best examples of deep-seated rotational soft- rock landsliding in England.

Information sources

The Geological Data Index (GDI) is now available on the internet at http://www.bgs.ac.uk. This provides a searchable database of index level information on data and other products available from BGS; this includes access to the BGS Lexicon of named rock units, and to part of the extensive collection of BGS photographs. Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth. Information on BGS products is listed in the current Catalogue of geological maps and books, available on request.

Maps

• Geological maps
• 1:1 500 000
• Tectonic map of Britain, Ireland and adjacent areas, 1996
• 1:1 000 000
• Pre-Permian geology of the United Kingdom, 1985
• 1:625 000
• Solid geology map UK South Sheet 2001; Quaternary geology 1977
• 1:250 000
• 52N 00 East Anglia, Solid Geology, 1991
• 1:50 000
• Sheet 132 and 148 North Walsham and Mundesley
• Sheet 161 Norwich, 1975
• Sheet 162 Great Yarmouth, 1990
• 1:10 000
• The 1:10 000 maps are not published but are available for public reference in the libraries of the British Geological Survey at Keyworth and Edinburgh and the BGS London Information Office in the Natural History Museum, South Kensington, London. Uncoloured dyeline sheets or photographic copies are available for purchase from the BGS Sales Desk.
• Geophysical maps
• 1:1 500 000
• Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1996 Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1996
• 1:250 000
• 52N 00 East Anglia, Bouguer gravity anomaly, 1991
• 52N 00 East Anglia, Aeromagnetic anomaly, 1991
• 1:50 000
• A geophysical information map (GIM) at a scale of 1:50 000 is available for this district. This shows information held in BGS digital databases, including Bouguer gravity and aeromagnetic anomalies and locations of data points, selected boreholes and detailed geophysical surveys.
• Sea bed sediments map
• 1:250 000
• 52N 00 East Anglia, 1988
• Hydrogeological map
• 1:250 000
• Northern East Anglia, 1976

Books

British Regional Geology

East Anglia, fourth edition, 1961

Documentary collections

Boreholes

Borehole data for the district are catalogued in the BGS archives (National Geological Records Centre) at Keyworth on individual 1:10 000 scale sheets. For further information contact: The Manager, National Geological Records Centre, BGS, Keyworth.

Other relevant collections

Requests for access to the palaeontological, petrological and materials collections should be made to the Chief Curator, Keyworth.

References

Most of the references listed below are held in the Library of the British Geological Survey at Keyworth, Nottingham. Copies of the references can be purchased subject to the current copyright legislation.

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Index to the 1:50 000 Series maps of the British Geological Survey

The map below shows the sheet boundaries and numbers of the 1:50 000 Series geological maps. The maps are numbered in three sequences, covering England and Wales, Northern Ireland, and Scotland. The west and east halves of most Scottish 1:50 000 maps are published separately. Almost all BGS maps are available flat or folded and cased.

(Index map)

The area described in this sheet explanation is indicated by a solid block.

British geological maps can be obtained from sales desks in the Survey's principal offices, through the BGS London Information Office at the Natural History Museum Earth Galleries, and from BGS-approved stockists and agents.

Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland.

Figures and plates

Figures

(Figure 1) Relief and drainage (based on Ordnance Survey 1:50k DTM). Red line indicates drainage divide.

(Figure 2) Lithostratigraphy and chronostratigraphy of the Chalk Group.

(Figure 3) Chronostratigraphy and lithostratigraphy of the preglacial deposits.

(Figure 4) Correlation of old and new stratigraphy for pre-Devensian glacial deposits.

(Figure 5) Comparison between the Happisburgh, Walcott and Lowestoft tills. Data from Perrin et al. (1979) and Lunkka (1994).

(Figure 6) Lithological content of the gravels at Briton's Lane Gravel Pit, Beeston Regis, north Norfolk, expressed as percentage of total sample number (n). After Moorlock et al (2000).

(Figure 7) Schematic cross-section through valley illustrating extent of Cover Loam.

(Figure 8) Map showing the position of the deepest part of the Quaternary trough along the north Norfolk coast.

(Figure 9a) Section through the Holocene sediments at Cley.

(Figure 9b) Sedimentological and biofacies log of the Holocene sequence.

Plates

(Geological succession) Geological succession of the Cromer district.

(Front cover) Coast between Sheringham (fore- ground) and Cromer (distance). Coastal protection at Sheringham has resulted in lower rates of erosion than in adjacent areas. (Photograph by Derek Edwards, Archaeology & Environment Division, Norfolk Museums Service).

(Rear cover) Geology of the Cromer district

(Index map) Index to the 1:50 000 Series maps of the British Geological Survey

(Plate 1) Wroxham Crag on Upper Chalk, Weybourne [TG 113 437]. Fifty-pence piece for scale (GS1166).

(Plate 2) Freshwater Beds, Cromer Forest-bed Formation, West Runton, site of the 'West Runton elephant' Mammuthus trogontherii excavated during 1992 and 1995

[TG 188 432]. Height of section about 1–2 m (GS1167).

(Plate 3) Happisburgh Till overlying Wroxham Crag, Trimingham [TG 276 393]. Height of section about 2 m (GS1169).

(Plate 4) Tectonically emplaced raft of chalk within glaciogenic deposits, Sidestrand [TG 2557 4042] (GS1170).

(Plate 5) Briton's Lane Sand and Gravel Member, Overstrand Formation, Beeston Regis [TG 168 415] (GS1168).

(Plate 6) Close up of clast-supported gravels within Blakeney Esker, almost entirely comprised of flint [TG 016 437]. Scale in centimetres (GS1171).

(Plate 7) Artificially maintained gravel ridge, Cley next the Sea. Note erosion on the seaward side [TG 050 452] (GS1172).

Figures

(Geological succession) Geological succession of the Cromer district

(Figure 3) Chronostratigraphy and lithostratigraphy of the preglacial deposits

(Figure 4) Correlation of old and new stratigraphy for pre-Devensian glacial deposits

(Figure 5) Comparison between the Happisburgh, Walcott and Lowestoft tills. Data from Perrin et al. (1979) and Lunkka (1994)