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Geology of the Mundesley and North Walsham district — a brief explanation of the geological map Sheet 132 Mundesley and Sheet 148 North Walsham

By B S P Moorlock, R J O Hamblin, S J Booth and M A Woods

Bibliographic reference: Moorlock, B S P, Hamblin, R J O, Booth, S J, and Woods, M A. 2002. Geology of the Mundesley and North Walsham district —A brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Series Sheets 132 and 148 Mundesley and North Walsham (England and Wales).

Keyworth, Nottingham: British Geological Survey, 2003.

© 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.

Notes

The word 'district' refers to the area of the geological 1:50 000 Series Sheets 132 and 148 Mundesley and North Walsham. National grid references are given in the form [1234 1234] or [123 123]. Unless otherwise stated all such references fall within the 100 km grid square TG. Numbers at the end of photograph descriptions refer to the official collection of the British Geological Survey. Symbols in brackets refer to symbols on the published 1:50 000 Series map.

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 the Department of Geography, Royal Holloway, University of London, with regard to research on the Quaternary sands and gravels of the district.

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

Crown copyright reserved Ordnance Survey licence no. GD272191/2002.

(Front cover) Windpump on the River Thurne [TG 399 157]. The pump is one of many that were formerly used to regulate water levels within Broadland (GS1173).

(Rear cover)

Geology of the Mundesley and North Walsham district (summary from rear cover)

(Rear cover)

(Geological succession) Geological succession of the Mundesley and North Walsham district

Modern development requires accurate geological information in order, for example, to identify resources and ensure that foundations are adequate. Modern agricultural practices also require a 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. This explanation is written for those who may have limited experience in the use of geological maps and for the professional user. A substantial list of references provides a guide to further geological information on the district.

The district is predominantly rural; fertile soils are developed on glacial till and cover silt, which is particularly suitable for a wide range of arable crops that include, wheat, barley, sugar beet, oilseed rape and potatoes. The small market town of North Walsham, situated in the north-west, services the local communities. In the south, much of the land is flat and very low lying; it forms the northern part of an area known as Broadland that comprises a number of small artificial lakes or 'broads' dug during the 13th and 14th centuries as a source of peat. The broads are linked by rivers and artificial waterways and have a thriving boating-holiday industry that attracts large numbers of tourists to the area each year.

The high sea-cliffs that extend north-westwards from Happisburgh provide excellent sections through the complex glaciogenic deposits of the region. However, care needs to be excercised when examining the cliffs as they are very prone to landslide; this results in large amounts of land being lost to the sea each year.

At Sea Palling, nine elongate artificial offshore reefs have recently been constructed from igneous rocks brought over by barge from Scandinavia, to arrest the southwards movement of nearshore sand and gravel.

The BGS Trunch Borehole proved the thickest onshore sequence of Chalk in the UK.

Chapter 1 Introduction

This Sheet Explanation describes the geological map Sheets 132 and 148 Mundesley and North Walsham. The district extends from Wroxham in the south-west, to the coastal settlements of Mundesley in the north and Scratby in the south-east. Much of the wealth of the district stems from the fertile soils developed on the underlying Quaternary sediments: wheat, barley, potatoes and sugar beet are the main crops. Cattle rearing and dairy farming are prevalent adjacent to the rivers, where stock can be pastured on the extensive alluvial tracts during the drier summer months. The now flooded medieval turbaries (peat diggings) have been linked to the rivers by artificial channels to form Broadland: the resulting network of navigable waterways attracts large, and increasing, numbers of tourists each year. The geology is also closely linked to two major issues within the district — coastal erosion and flooding.

Regional setting

The district lies on the northern margin of a concealed platform of Palaeozoic rocks known as the London–Brabant Massif, 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. This consists of folded and metamorphosed sedimentary and volcanic rocks with associated late Caledonian intrusions. The rocks proved in the upper parts of the massif, in and adjacent to the district, are metasediments of Silurian age which include cleaved mudstones, siltstones and sandstones. These are believed to belong to a turbiditic shelf succession which forms the concealed Caledonides (end-Silurian) fold belt of eastern England.

This structural high was flanked by sedimentary basins through the Upper Palaeozoic and Mesozoic. In early Permian times, the initiation of an east–west tensional stress regime, related to the development of the proto-Atlantic ocean, resulted in the first stages of development of the Southern North Sea Basin. On the relatively stable platform Carboniferous, Permian, Triassic and Jurassic strata are thin or absent. Major erosion occurred during the Late-Cimmerian (Late Jurassic–Early Cretaceous). The overlying marine Lower Cretaceous mudstones rest with marked unconformity on the underlying rocks. Their deposition mark the commencement of a period of sea level rise that extended throughout most of the Late Cretaceous, and culminated in the deposition of the thick limestones of the Chalk Group.

A period 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, Neogene and Quaternary with, perhaps, a phase of uplift in mid- to late Miocene times. During this period, clays and silts were deposited within the eastern part of the district. Uplift and non-deposition followed until the early Pleistocene, when the oldest formation of the Crag Group, the Red Crag Formation, was deposited locally in the Ludham area. Shallow marine and estuarine sedimentation ensued over much of the district with the deposition of the overlying Norwich Crag and Wroxham Crag formations, followed by the temperate and, in part, freshwater Cromer Forest-bed Formation.

Cold climatic conditions followed, with the deposition of a variety of glacial deposits, which include till, outwash sand and gravel, and lacustrine deposits of fine- grained sand, silt and varved clay. These were generally regarded as having formed during the Anglian Stage in Oxygen Isotope Stage 12, but recent work suggests that deposition of the glacigenic deposits may have occurred over a longer period of time, during several glaciations. These deposits form the heart of the Cromer Ridge, one of the major landforms in north Norfolk.

By the onset of the following Devensian cold period, the landforms of the district were probably very similar to those of today, although the coastline would have been several kilometres farther east. It was during this cold period that a veneer of wind-blown silt and fine sand settled over much of the district, masking the underlying geology.

A eustatic rise in sea level during the ensuing Flandrian led to the accumulation of estuarine deposits of the Breydon Formation. At the same time, the coastline suffered major erosion and retreat. Minor regressions were marked by the development of peats within the sequence.

Chapter 2 Geological description

The stratigraphy of the rocks buried beneath the district is known from several deep boreholes (Figure 1), (Figure 2), two of which, the Ormesby Borehole and the Trunch Borehole, were drilled for BGS specifically to elucidate the stratigraphy.

Silurian

The Somerton No.1 Borehole [TG 4607 2120] proved 31.4 m (1369.5 to 1400.9 m depth) of cleaved mudstones and siltstones of probable Silurian age overlain by 188.7 m (1180.8 to 1369.5 m) of interbedded mudstone, siltstone and sandstone originally assigned a Visean (late Dinantian) age by oil company geologists. Coring between 1390.5 and 1400.9 m depth yielded a sequence of cleaved, bluish grey to dark grey, highly compacted, slightly calcareous mudstones and siltstones which exhibit steep dips of between 30° and 70°. Microcrystallinity studies on white mica have indicated that the sequence has undergone greenschist facies metamorphism (Pharaoh et al., 1987).

A Visean age for the upper unit is not supported by the physical properties of the rock such as density and sonic velocity, and is also at odds with the structural evaluation of seismic data which indicates that this sequence is part of the Caledonide Lower Palaeozoic. Owens (1982) also noted that palynological determinations were inconclusive, because of contamination from higher levels in the borehole. This led Arthurton et al. (1994) to reinterpret the sequence as being of probable Silurian age. To the north-west, Silurian mudstone, siltstone, and sandstone, 412 m thick, were also proved in the East Ruston Borehole [TG 353 268].

Carboniferous

Dinantian limestones are present beneath much of the district but are absent in the south-west. The overlying Coal Measures have a more restricted subcrop in the north-east.

Dinantian limestones are present in both the Somerton [TG 4607 2120] and West Somerton [TG 4736 1935] boreholes. The latter terminated at a depth of 987.5 m after penetrating 8 m of limestone. In the former, 141 m of limestone was recorded below a depth of 1039.8 m (Figure 2), and overlain by Coal Measures sandstones. Coring between 1054.3 and 1083.4 m proved grey, brown and reddish brown, dolomitic limestone and interbedded pale grey to black mudstone. Galena, pyrite and calcite mineralisation in the form of veins, vugs, infilled fossil casts and geodes up to 12 cm across were present throughout. Foraminiferal assemblages suggest that the limestone ranges in age from Holkerian or early Asbian to late Asbian or Brigantian (Strank, 1982). Conodont determinations (Owens, 1982) indicate a Holkerian to Brigantian age. In the East Ruston Borehole, 114.3 m of Dinantian limestones was recorded.

Coal Measures were proved in both the West Somerton and Somerton boreholes. In the West Somerton Borehole, sandstone and mudstone were proved for 36.3 m (943.2 to 979.5 m) and in the Somerton Borehole for 60.9 m (978.9 to 1039.8 m), indicating a thickening of the sequence towards the north. Coal Measures were absent in the East Ruston Borehole.

Permian

Permian rocks were found in the cuttings of both the Somerton and West Somerton boreholes, 110 m and 89.1 m thick, respectively. The sequence can be divided into a Lower Permian Rotliegendes Group, comprised predominantly of sandstones, and an Upper Permian Zechstein Group, dominated by carbonate and evaporitic deposits.

The importance of the Rotliegendes sandstone as a major reservoir formation has resulted in many detailed studies related to specific gas fields (E G van Veen, 1975; France, 1975; Gray, 1975; Goodchild and Bryant, 1986).

In the Somerton Borehole (at 978.9 m depth) and West Somerton Borehole (943.2 m), the Rotliegendes basal Leman Sandstone Formation rests unconformably on Coal Measures sandstones. The top of the formation is taken below a thin muddy carbonate, which represents the first carbonate cycle of the Upper Permian Zechstein sequence.

The Leman Sandstone is red-brown to grey-green and contains subrounded to subangular grains, mainly of medium grain, but ranging from fine to coarse grain. The lower part of the sequence at Somerton comprises a red and yellow quartz-rich conglomerate. The sandstone is considered to have been deposited either in ephemeral fluvial– wadi conditions or in an aeolian environment, adjacent to the emergent and eroding London– Brabant Massif (1986; Marie, 1975). Erosion of the Variscan mountains to the south of Britain appears to have provided the main source of sediment to the basin, via northerly draining river systems (Maria, 1975). Some 28.1 m and 18.2 m of Rotliegendes were proved in the Somerton and West Somerton boreholes, respectively. In the East Ruston Borehole, 82.3 m of strata were proved, demonstrating a northerly thickening.

Zechstein Group

At the end of early Permian times, the topography of the sediment source area had been greatly reduced, and the rate of subsidence in the Southern North Sea Basin outstripped sedimentation with a resultant lowering of the basin floor.

A eustatic rise in sea level and the formation of the Zechstein Sea resulted in the rapid flooding of the basin in which alternating cycles of evaporation and recharge produced cycles of carbonate/evaporite sedimentation. Totals of 82.0, 70.9 and 70.1 m of Zechstein sedimentary strata were proved in the Somerton, West Somerton and East Ruston boreholes, respectively.

Triassic

The base of the local Triassic sequence is taken at the incoming of the Hewett Sandstone Member. The hot and arid conditions of the Permian persisted during Triassic times. Rejuvenation of Variscan massifs in early Triassic times was followed by gradual peneplanation. By late Triassic times this had resulted in the development of low-lying continental interior basins which underwent greater subsidence.

Bacton Group

This group is divided into the lower, dominantly argillaceous Bunter Shale Formation (equivalent to the Roxby Formation) and the upper, mainly arenaceous Bunter Sandstone Formation (equivalent to the Sherwood Sandstone Group). The group is 152.6, 146.9 and 111.3 m thick in the Somerton, West Somerton and East Ruston boreholes, respectively. The lower part of the Bunter Shale Formation is characterised by the Hewett Sandstone Member, of limited lateral extent. This distinctive sandstone is white to grey in colour, quartz-rich, poorly to moderately sorted, subrounded to rounded, fine to medium grained and contains traces of pyrite and anhydrite cement. It has a basal conglomerate. At Somerton and West Somerton the sandstone is 17.8 m and 15.5 m in thickness, respectively, and it thickens offshore. The sandstone is believed to have been derived from a moderately rejuvenated London–Brabant Massif (Cumming and Wyndham, 1975).

A 12 m-thick, gradational, fining-upwards sequence of red-brown, locally anhydritic, of laminated mudstone and interbedded argillaceous, slightly dolomitic, fine-grained sandstone and siltstone occurs between the Hewett Sandstone Member and the main body of the Bunter Shale Formation. This sequence may represent the Bröckelschiefer as described in the Hewett Gas Field by Cumming and Wyndham (1975).

The overlying Bunter Shale Formation comprises a sequence of red to brown, anhydritic mudstone and green fissile mudstone, interbedded with white to greenish white siltstone and fine-grained sandstone. At Somerton and West Somerton, 85.9 m and 81.4 m of Bunter Shale Formation were proved, respectively. The formation is thought to have been deposited in lacustrine or floodplain conditions within an inland sea or playa lake. Sandstones of the formation are grey, white or yellow in colour, subrounded to rounded, poorly sorted and medium to fine grained. Traces of pyrite, calcite, and dolomite have been recorded in cuttings; gypsum becomes more abundant towards the base of the formation. Interbedded mudstones are interpreted from geophysical logs, and coarser grained beds are represented in the cuttings. A change in the sonic velocity logs at both Somerton (735.4 m) and West Somerton (725.0 m) may reflect an increase in the degree of anhydrite cementation in the upper part of the sandstone (Arthurton et al., 1994). The thickness of the Bunter Sandstone is 36.8 m in the Somerton Borehole and 37.6 m in the West Somerton Borehole. This sandstone is considered to have formed under arid conditions of sheet flood and lacustrine environments.

Haisborough Group

This group is a dominantly fine-grained sequence that is equivalent to the Mercia Mudstone Group of the English Midlands.

At Somerton and West Somerton, the base of the group is marked by the appearance of fine-grained strata above the Bunter Sandstone. At the top of the group, a major unconformity, relates to a Late-Cimmerian (Late Jurassic–Early Cretaceous) post-extensional isostatic recovery and global sea level fall. This resulted in the removal of many of the Jurassic and late Triassic rocks from the district.

The group comprises a variable sequence of interbedded mudstone, siltstone, fine-grained sandstone and evaporite. The mudstone ranges in colour from red or red-brown to pale green or grey. It varies from noncalcareous to slightly calcareous, and locally contains abundant gypsum or anhydrite cement and thin beds of microcrystalline anhydrite. Thicker beds of white or clear microcrystalline or fibrous anhydrite or gypsum are also present, for example at about 670 m and about 630 m depth in the Somerton Borehole. Some 145 m of the Haisborough Group were proved in the Somerton Borehole and 116.7 m in the West Somerton Borehole. The group thickens significantly in the north of the district, where over 400 m are preserved.

The Haisborough Group was deposited during a period of greater tectonic stability than the underlying Bacton Group, within a low-lying basin subject to rapid transgressions related to minor rises of sea level. Sedimentation was mainly in distal floodplain environments alternating with coastal sabkha or shallow marine environments.

Jurassic

About 130 m of variably shelly grey mudstone and calcareous mudstone attributed to the Lias Group are present in the north of the district. The East Ruston Borehole proved 85.4 m of Lias. In the West Somerton and Somerton boreholes in the south of the district, sediments of Jurassic age are absent.

Cretaceous

Rocks of Lower Cretaceous age rest with marked unconformity on Triassic and Jurassic strata. The base of the Cretaceous sequence in the Somerton and West Somerton boreholes, based on cuttings and geophysical log characteristics is tentatively placed at 571.2 m and 590.5 m depth, respectively (Arthurton et al., 1994). The presence of abundant goethite ooids within pink and olive-green soft mudstone (8.2 m at Somerton and 9.8 m at West Somerton) is probably indicative of the presence of Barremian strata within the boreholes. However, comparable sedimentary rocks were not recognised in the nearby Trunch Borehole, and appear to be absent throughout much of north-east Norfolk. Because of poor sample recovery, the stratigraphy of the Lower Cretaceous in the East Ruston Borehole is poorly known.

Carstone

The Carstone (Lower Greensand) proved in the Somerton Borehole (545.1 to 563.0 m depth) and in the West Somerton Borehole (564.0 to 580.7 m) is of Albian age (Rawson et al., 1978; Gallois and Morter, 1982). The sequence comprises a coarsening-upwards sequence of poorly cemented, green or white, goethitic, medium- to coarse-grained sandstone, with some interbeds of mudstone. It was deposited in a relatively highenergy, shallow marine environment to the north of the London–Brabant Massif.

Gault Formation

Gallois and Morter (1982) have described the detailed stratigraphy of the Gault Formation in northern East Anglia. It was proved in the Somerton (539.5 to 545.1 m) and West Somerton (555.7 to 564.0 m) boreholes. Northwards, it passes laterally into the Red Chalk in the Trunch Borehole: a 0.63 m-thick, extremely condensed Upper Albian sequence. The Gault comprises fining-upwards cycles of pale to dark grey mudstone and sandy mudstone, which contain numerous phosphate- and glauconite-rich erosion surfaces (Gallois and Morter, 1982) and is thought to have formed between the London–Brabant Massif and a shoal (of Red Chalk) to the north.

Chalk Group

The Chalk of the district is almost entirely masked by younger strata. However, its detailed stratigraphy is known from the BGS Trunch Borehole [TG 2933 3455] (Figure 3), which cored a total thickness of 468 m ranging from Cenomanian to Lower Maastrichtian, the most complete onshore Chalk succession in the UK. The youngest chalk in onshore Britain occurs within the district, in a series of chalk masses enclosed in glacial drift on the foreshore east of Trimingham [TG 293 383] to [TG 298 379], and in a borehole at Mundesley [TG 317 364].

Chalk is typically a very fine-grained, relatively soft, white limestone formed in a marine environment, and consists predominantly 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 form geographically extensive marker-horizons. The Chalk Group has traditionally 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 proposed chalk lithostratigraphical classifications is wholly applicable to East Anglia, which appears to have features in common with both northern and southern England Chalk Group successions. A biozonal scheme enables correlation with successions elsewhere.

The following stage by stage account of the Upper Cretaceous in the Trunch Borehole [TG 2933 3455] is based on the detailed description given by Wood et al. (1994).

Cenomanian

The chalk assigned to this stage represents the condensed equivalent of the Ferriby Chalk Formation (sensu Whitham, 1991) and basal Welton Chalk Formation of northern England. A bed of hardened chalk at the base of the succession equates with the Paradoxica Bed seen at Hunstanton and in Humberside and north Lincolnshire. Above this, gritty, shell-rich chalks represent the Lower Inoceramus Bed of northern England, and calcarenitic chalk with phosphatised clasts above an erosion surface at 506.3 m equates with the Totternhoe Stone. A bed of hard, nodular chalk slightly higher in the succession represents the Nettleton Stone of northern England, and a much attenuated correlative of the Plenus Marls overlies an erosion surface at 501.1 m, marking the top of the traditional Lower Chalk.

Turonian

The chalk of this stage represents the condensed equivalent of the Welton Chalk Formation and lower part of the Burnham Chalk Formation of northern England. A planar hardground at 500.07 m depth is overlain by gritty chalk with shell fragments of the bivalve Mytiloides, resembling the Holywell Nodular Chalk Formation of southern England (Bristow et al., 1997; Rawson et al., 2001) The remainder of the Welton Chalk is stylolitic and flintless up to 473 m (Morter and Gallois, 1979), and then hard with tabular flints and conspicuous marl seams. An horizon of stylolitic marly partings at 469.32 m marks the base of the Burnham Chalk. The remainder of the Turonian succession is hard, white, massive bedded chalk (Morter and Gallois, 1979).

Coniacian

This chalk equates with the middle and upper parts of the Burnham Chalk Formation of northern England. The base of the Coniacian is a hardground with the bivalves Didymotis sp. and Cremnoceramus spp., correlating with the Navigation Hardground of southern England (Mortimore, 1986). The chalk is hard, grey-white and yellowish white, with stylolitic surfaces, thin marl seams, hardgrounds and medium-sized nodular and tabular flints (Morter and Gallois, 1979). Conspicuous beds of inoceramid shells occur in the higher part of the Coniacian, including Volviceramus aff. involutus and V. koeneni, indicative of the lower M. coranguinum Zone. The Seven Sisters Flint of Sussex (Mortimore, 1986), a widespread marker across the Anglo–Paris Basin, is possibly represented by a 0.09 m-thick flint at 400 m depth. Cladoceramus undulatoplicatus, characteristic of the basal Santonian (Bailey et al., 1984), possibly occurs at 366.5 m depth in the borehole.

Santonian

About 60 m of chalk (massive, white and poorly flinty) above 335.26 m depth, belong to this stage. In northern England, coeval chalk forms the lower part of the Flamborough Chalk Formation, which is typically devoid of flint (Whitham, 1993). The top of the Santonian is a burrowed junction between coarse-grained, bioclastic chalk above non-bioclastic white chalk, and is a possible non-sequence.

Campanian

Over half the total thickness of the Chalk Group in the Trunch Borehole, 245.72 m, belongs to the Campanian Stage. The basal part, below 300 m, comprises coarse-grained bioclastic chalk, overlain by massive white chalk with a few tabular flints (Morter and Gallois, 1979), equating with the upper part of the Flamborough Chalk Formation (Whitham, 1993). The top of the Lower Campanian is grey-white marly chalk and creamy white chalk with large and small nodular flints, overlain by about 143 m of Upper Campanian strata, representing the Belemnella mucronata Zone sensu lato. The succession contains correlatives of units described by Wood (1988) from the chalk of the Norwich area, and named (in ascending stratigraphical order): Pre-Weybourne Chalk, Weybourne Chalk, Beeston Chalk and Paramoudra Chalk.

Maastrichtian

Soft, marly chalk with large flints, belonging to the Belemnella lanceolata Zone sensu lato, forms the youngest part of the Upper Cretaceous in the Trunch succession. Due to poor core recovery in the top of the main (No. 1) Trunch Borehole, this part of the succession is better known from the top of a nearby borehole (Trunch No. 2 [TG 2939 3455]). In the latter borehole, at least 16 m are assigned to the Sidestrand Chalk Member, the oldest of the four sub-divisions of the Lower Maastrichtian chalk of north Norfolk described by Johansen and Surlyk (1990).

Fossiliferous, grey, calcarenitic chalk, belonging to the Lower Maastrichtian Beacon Hill Grey Chalk Member (see below) of Johansen and Surlyk (1990), occurred 1.7 m below the top of the Chalk in the bottom of a cored borehole at Mundesley [TG 317 364].

On the foreshore east of Trimingham [TG 293 383] to [TG 298 379], at the western margin of the Mundesley district, Brydone (1908) recorded three main foreshore exposures of chalk masses enclosed in glacial drift. Brydone (1908) named these the Western (or 'C'), Central (or 'A') and Eastern (or 'B') masses, but coastal recession has meant that the greater part of these is now below low tide level. The succession belongs to the Lower Maastrichtian, and includes (in ascending stratigraphical order) the higher part of the Sidestrand Chalk Member, the Trimingham Sponge Beds Member, the Little Marl Point Member and the Beacon Hill Grey Chalk Member of Johansen and Surlyk (1990) (Peake and Hancock, 1970).

Palaeogene

The Palaeogene strata are restricted to the east of the district (see inset on 1:50 000 Series Sheets 132 and 148 map) where they rest unconformably on the Upper Chalk and are concealed by a cover of Quaternary deposits. Just under 8 m of Palaeogene strata were recorded in the East Ruston Borehole which just lies west of the limit shown on the inset map. It is thought that these deposits may be preserved in a small faulted depression.

The beds were laid down in environments ranging from open marine shelf to shallow brackish water, adjacent to the margin of the then slowly subsiding Southern North Sea Basin.

An unconformity separates the Harwich Member of the Thames Group from the underlying Ormesby Clay Formation. There is no intervening representative of the Lambeth Group (Woolwich and Reading Beds).

Ormesby Clay Formation

This formation is known from its type locality in the Ormesby Borehole [TG 5145 1425] in the south-east of the district, where it is 27 m thick. The base of the Ormesby Clay is marked by a thin bed (Bullhead Bed), less than 2 m thick, of green, glauconite-coated flint pebbles and cobbles. This is succeeded by a pale olive-grey, poorly bedded mudstone that is slightly to highly glauconitic, with many thin poorly defined layers of pale green mudstone; glauconite granules and small phosphatic nodules are scattered throughout. There are several thin layers of altered volcanic ash, belonging to Phase 1 of the North Atlantic (Faeroe–Greenland province) early Palaeogene pyroclastic activity (Knox and Morton, 1988). The top of this unit is penetrated by burrows including Chondrites. The succeeding unit comprises a distinctive, pale reddish brown, poorly bedded mudstone which passes upwards into a pale grey, bioturbated, slightly calcareous and glauconitic mudstone. This unit is sharply overlain by greyish brown, poorly bedded, calcareous, highly glauconitic mudstone. This passes up into variegated mudstone.

Thames Group–Harwich Formation

The detailed local stratigraphy of this group is known from the Ormesby Borehole. Within the Harwich Formation a basal member, the Hales Clay Member is 7.1 m thick, and comprises pyritic and glauconitic siltstone with some layers of tuff. Above, there is some 26.5 m of Harwich Formation, an olive-grey to greyish brown, sandy siltstone with numerous basaltic ash layers (at least 86 were identified in the borehole). Younger deposits of the Thames Group may be present offshore as a consequence of observed eastwards thickening.

Quaternary

At the beginning of the Quaternary the district was situated near the western margin of the Southern North Sea Basin. Shallow marine and estuarine deposits of the Crag Group, accumulated in the district, fed by rivers transporting sediment from the west. At first, the sediment was derived relatively locally with flint being the main clast type. Later the supply included quartz and quartzite from the Kidderminster Formation of the English Midlands, cherts from the Carboniferous of northern England, and small quantities of volcanic rocks thought to have been transported from north Wales by the early River Thames.

During the Anglian glaciation the district was covered by ice which deposited several tills and associated outwash sands and gravels of the Corton, Lowestoft and Overstrand formations.

During the following Devensian glacial maximum, some 18 000 years BP, 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 120 m below its present level. With the progressive but irregular amelioration of climate during the ensuing Holocene Epoch (from about 11 000 years BP), melting of the ice sheets resulted in worldwide (eustatic) rise in sea level.

Thus, at the start of the Holocene Epoch, the present-day marshland and river valleys of the district had a significantly different topography. The original valley floors of rivers now draining to marshland lie well below the present-day surface; for example the pre-Holocene valley of the River Bure lies at least 7.5 m below OD under the marshland [TG 390 145]. The outfall for these valleys was east of Great Yarmouth where the contemporary coastline and estuary lay some 7 km offshore.

As sea level rose, the lower reaches at least would have been subject to tidal influence, but as drainage base level rose, flow rates moderated and there was a corresponding broadening of the floodplains. During this time reed-swamp and fen vegetation developed, producing organic debris now preserved in the Lower Peat.

Between 8700 to 4500 years BP, local sea level rose by between 22 and 26 m, the change being marked by the intrusion of tidal influence much farther inland up the valleys. The fen vegetation gradually gave way to estuarine communities, while estuarine clay and silt that forms the diachronous Lower Clay were deposited. During this period the coastline would have receded westwards but the drainage would still have been open towards Great Yarmouth.

Around 4500 to 5000 years BP, local sea levels had risen to about 7 m below OD, and freshwater conditions came to dominate. In the upstream parts of valleys, where little or no Lower Clay sediments were deposited, reed beds gave way to alder-dominated woodland whereas, in the lower reaches, floodplain swamp became established and extensive spreads of the Middle Peat accumulated. Some of the environmental change at this time was the result of wetter climatic conditions and locally, in the case of Broadland, was due also to the growth of a sand barrier across the mouth of the estuary near Great Yarmouth.

Towards the end of the Middle Peat episode about 2200 years BP, a second global increase in the rate of sea level change occurred. Peat accumulation gave way once more to marine-dominated mudflats and tidal channels, and the Upper Clay was laid down.

This second marine transgression, possibly accompanied by increased storminess and coastline erosion, penetrated deep into the hinterland. By this time, it is likely that the Rivers Bure, Thurne and Ant and Hundred Stream of today were open to the sea not only at Great Yarmouth but also through coastal breaches between Winterton and Horsey, between Horsey and Waxham and again, just north of Sea Palling. Cartographical information is scant, but contemporary maps imply there was a navigable waterway around the Isle of Flegg during the first few centuries AD.

Where the mudflats were not continuously submerged, they were gradually colonised by salt-tolerant plants, and farther up-valley the saltmarshes were succeeded by reed beds as the influence and salinity declined. Fringing fen or swamp vegetation thrived near to upland freshwater sources and locally, where groundwater seepage occurred well into the marshland, spring-fed islands of fen vegetation developed. In time, peat growth spread out and down-valley, coalescing to form the Upper Peat. The seaward extent of peat is unknown as the deposits suffered oxidation, weathering and wastage after human intervention (from about 1000 years BP).

The development of the present drainage network is equivocal. It is thought that the River Thurne/Hundred Stream originally drained westwards into the River Bure while the palaeocoastline lay some kilometres east of the present coast. As the coast receded westwards, the River Thurne/ Hundred Stream interfluve would have been breached probably resulting in an eastwards-dominated flow. Indeed, there are historical records of an outfall of the Hundred Stream at the coast near Bramble Hill [TG 467 225]. What caused a second possible reversal of the River Thurne/ Hundred Stream back into the River Bure is unknown. The most likely explanation is that it occurred as a result of longshore drift and beach accretion blocking the eastern outfall; this natural occurrence may have been encouraged in order to protect the hinterland from marine flooding. It is known that the River Ant once flowed into the River Thurne/Hundred Stream network but was artificially diverted into the River Bure between the 11th and 14th centuries.

The onshore Holocene deposits consist of alluvial deposits which become increasingly marine in nature southwards towards Great Yarmouth and eastwards towards the coast at Bramble Hill. Thus, freshwater sediments dominated by peat with silt and clay pass down-valley into marshland comprising estuarine clay, silt and sand, which, in turn, give way to sand dunes and shoreface deposits of the coastal fringe.

Offshore, modern marine sediments (mud, sand and pebbly sand) overlie Plio– Pleistocene Crag. Locally, adjacent to the northernmost part of the coast, marine sediments overlie chalk.

Crag Group

During the late Neogene the district became submerged as a part of the marine North Sea Basin, around the margins of which there developed a series of prograding deltas (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 infill of the basin. These are the Red Crag (lowest), Norwich Crag and Wroxham Crag. 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. The greatest thickness of the group recorded in the district is around 58 m in the Ormesby Borehole [TG 5145 1425]. Deposits of the Crag Group (probably all belonging to the Wroxham Crag Formation) are exposed over large areas of the sea floor in the north-east of the district.

The three Crag formations are separated by disconformities representing westward 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 postdating the regression at the end of Norwich Crag deposition. The deposits formerly known as the Weybourne Crag and the 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, while the Norwich and Wroxham crags both overstep the underlying crag formation westwards. Most of the Crag at surface within the district belongs to the Wroxham Crag Formation.

Following traditional Geological Survey practice, the crags are considered to be 'Solid' (bedrock), and the overlying pre-Anglian fluvial deposits and Cromer Forest-bed Formation to be 'Drift' (superficial deposits). The Crag Group comprises fine- to coarse-grained micaceous sands. Below the water-table, 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, chatter-marked, high-sphericity flint pebbles and cobbles. These are normally grey or brown both inside and out, indicating a long history of weathering, and are believed to be second-cycle material 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. Clay bodies 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 clay bodies in the Red Crag are considered to 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 possible that these clays formed in an estuarine environment protected on the seaward side by banks of beach-face gravel.

Red Crag Formation

The Red Crag is preserved in a series of north-easterly trending basins, of which one is known to pass beneath Ludham in this district. Two structures with similar trend shown on the contour diagram of the base of the Upper Chalk (inset map on 1:50 000 Series Sheets 132 and 148) may also be filled with Red Crag, since it is present in the Happisburgh Borehole [TG 3832 3110] (West, 1980); apart from this, its northward extent is unknown. The sediments differ between basins, but generally include deeper water facies than the higher Crags. Hamblin et al. (1997) suggested that the basins were formed by syndepositional folding and faulting, but alternative processes have been proposed (see Hamblin et al., 1997 for references).

The Red Crag in this district is overlain by Norwich Crag and hence is nowhere exposed, but it has been proved in the Ludham Royal Society Borehole [TG 385 199] (Funnell, 1961; West, 1961) and in the Ormesby Borehole [TG 5145 1425] (Arthurton et al., 1994; Harland et al., 1991). At Ludham, about 30.8 m of Red Crag were recorded between 18.9 and 49.7 m below OD. The basal 1.8 m of strata consists of grey shelly sand with black flints up to 8 cm in diameter; this bed represents a transgressive beach deposit. It is overlain by 22.0 m of grey, shelly sand with scattered thin clay seams and 1.8 m of grey, shelly, silty clay at the top, overlain, in turn, by 7.0 m of grey silty clay with subordinate shelly sands.

In the Ormesby Borehole, 24.7 m of Red Crag strata were penetrated. The lower 10.4 m were interpreted as intertidal or subtidal, and comprise bioturbated, glauconitic, coarse-grained sand with abundant comminuted shell debris, overlain by olive-grey, finely laminated clay interbedded with cross-bedded, glauconitic shelly sand. At the base is a transgressive gravel beach deposit. The upper 14.3 m are interpreted as tidal mud-flat sediments, and comprise finely interlayered sand and mud, and flaser- and lenticular-bedded clay, silt and fine-grained sand, with glauconite and mica. The clays were burrowed and sand-filled escape burrows were present at the top.

Norwich Crag Formation

This formation comprises a tabular sheet of strata up to 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. It is believed to extend just beyond the western edge of the district at Wroxham, but northwards it is not known beyond the Happisburgh Borehole (West, 1980).

In the Ludham Borehole, the Norwich Crag rests upon the Red Crag at 18.9 m below OD. At the base, 4.1 m of grey shelly sand are overlain by 4.7 m of grey silty clay with sandy partings and shell fragments. These are succeeded by further sands, which include a bed of black rounded flints at 9.5 m below OD, but it is unclear which crag is represented. The same problem arises at Ormesby (Arthurton et al., 1994), where the Norwich Crag overlies the Red Crag at 40.4 m below OD. From this depth to 20.7 m below OD, pale olive-grey, well-sorted, medium-grained sand with thin clay drapes over ripples are interpreted as the deposits of a high-energy tidal flat. Mica, glauconite and shell debris are recorded, and thick beds of flaser- and lenticular-bedded, micaceous clay occurs at intervals. This sand passes up into a further 11.6 m of sand with very little clay content, which may be the Norwich Crag and/or Wroxham Crag.

Shelly sands are rare in the Norwich Crag; it has been suggested that this is due to decalcification, but this is questionable since trace fossils as well as body fossils are rare, and shells do occur locally in weathered sands at outcrop. The marine (beach, estuarine or deeper water) character of the Norwich Crag sand has been confirmed by electron-microscope examination of the grains (Krinsley and Funnell, 1965), although they record an aeolian episode for some of the grains, which presumably have been blown into the marine environment.

Wroxham Crag Formation

This crag is a near-shore, estuarine and freshwater complex lithologically similar to the Norwich Crag. It includes sand, gravel and clay, but differs from the Norwich Crag in containing a higher proportion of gravels that contain significant quartz and quartzite pebbles. In this district, it includes the former Bure Valley Beds of the Wroxham area, and also certain strata of Pre-Pastonian and Pastonian age previously included in the Cromer Forest-bed Formation. The Wroxham Crag probably has a maximum thickness of about 20 m.

The Wroxham Crag occurs at outcrop over large areas of the district but, except in artificial excavations, the outcrop is generally obscured by a wash of gravel. This is derived both from the Crag itself and from the overlying Corton Formation, and may lead to the impression that the Crag is more gravelly than is genuinely the case. As a result of this gravel wash, it has not been possible to map individual gravel or clay bodies, although the gravel, clay and sand may be too intimately related to separate on this scale. The gravel wash also precludes the precise delineation of the Wroxham Crag–Corton Formation boundary.

The base of the Wroxham Crag was seen in a pit [TG 2657 1675] in the Aylsham district to the west. Here a 0.4 m basal bed of flint gravel rested upon Upper Chalk. The flints were fresh and up to over 20 cm long, with black cortices and white patinas, mostly rather worn, and with their horns broken off; they were set in a matrix of orange-brown sand. This gravel was overlain by 0.5 m of horizontally bedded pale grey, brown-stained clay and silt, and then at least 2.0 m of clast-supported flint gravel. This was poorly sorted and structureless except for some horizontal stringers of yellow-orange coarse-grained sands. Flints ranged in all sizes up to 20 cm long, the smaller ones well-rounded, chatter-marked and not commonly shattered, while the larger were less well rounded and more commonly shattered.

Similar gravels were seen in the North Walsham district in an active gravel pit [TG 2872 1724] (Plate 1) south of Belaugh. These are also clast-supported and pale yellow to deep orange-brown in colour. Most clasts consist of flints, many of which are chatter-marked, but there are significant quantities of quartz and quartzite; clasts are mostly about 4 cm long, but some flint, quartz and quartzite pebbles range up to 15 cm. Only a few of the clasts are shattered. The matrix is orange to grey, coarse-grained sand. Some cross-bedding in the gravel is picked out by seams of deep orange to yellow sand. The base of this pit is around a metre above the Upper Chalk. Similar gravels can be observed in a pit [TG 3448 2995] on Crostwight Heath below the Corton Formation.

Three trial pits excavated at How Hill [TG 377 199] (Rose et al., 1996) revealed a coastal complex of sand and gravel with north-north-west-trending tidal current flow, believed to be parallel with the contemporary coastline. Provenance-indicator lithologies in the gravels indicate dominant sources to the south and north-west, from the proto-Thames and 'Northern rivers' (see below). A gravel unit, 1.1 m thick, with an interbedded silty fine sand, is underlain by cross-bedded sand and gravel and overlain by well-sorted cross-bedded sand and horizontally bedded sand with silty clay laminations and clay drapes over ripple laminations (flasers). The gravel is composed dominantly of angular flint (57–43 per cent), quartz and quartzite (39–26 per cent), chattermarked flints (9–6 per cent) and Carboniferous chert (8–5 per cent). 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 'Northern rivers' draining from the southern Pennines and north-east Yorkshire. 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.

The formation is nowhere exposed along the coast, but has been detailed in boreholes by West (1980), and three members are recognised (Funnell and West, 1977; Gibbard and Zalasiewicz, 1988). The Sidestrand Member (Pre-Pastonian a age) at Happisburgh (West, 1980) comprises 12.6 m of marine grey sand with laminae of grey silty clay, including a 30 cm bed of grey silty clay with thin seams of grey sand. The Sheringham Member (Pre-Pastonian b to d) comprises impersistent horizons of freshwater organic mud and sand up to 2.5 m thick, indicating regression of the sea. The Paston Member (Pastonian) indicates a marine transgression and comprises up to 3 m of tidally laminated silt and clay, marine shelly sand, cross-bedded sand, clay, conglomerate and gravel. The deposits at How Hill may belong to this member.

Pre-Anglian fluvial deposits

None has been recorded from the district. However, trial pits at How Hill [TG 377 198], revealed the presence of marine Wroxham Crag containing clasts of Carboniferous rocks and Jurassic Rhaxella chert transported by a fluvial system from northern Britain to the western margin of the Crag Basin (Rose et al., 1996). These authors gave the name 'Northern rivers' to the transporting medium, but more recently, Clayton (2000) has called it the 'Ancaster River'. It is possible that fluvial deposits attributable to this river may be preserved locally within the district beneath the glaciogenic formations.

Cromer Forest-bed Formation

This term was given by Reid (1882) to a series of marine, brackish and freshwater sediments deposited in the coastal region of northern and north-eastern East Anglia. West and Wilson (1966) and West (1980) included strata of Pastonian, Beestonian and Cromerian age but, in this survey, only Cromerian strata have been included in the formation, that is the West Runton, Mundesley and Bacton members of Funnell and West (1977). The deposits are exposed discontinuously along the coast from Weybourne (Norfolk) to Kessingland (Suffolk), but they are not known between Happisburgh (in this district) and Corton (Suffolk), so possibly they relate to two separate estuaries, the Norfolk deposits relating to the 'Northern rivers' mentioned above.

The deposits may be observed along the coast depending on the state of exposure, but they have been studied by borings (West, 1980). The West Runton Member comprises about 2 m of clay and organic mud, including the West Runton Freshwater Bed and the Corton Rootlet Bed (Reid, 1882). The Mundesley Member comprises about 2.5 m of tidal silty clays and beach-face gravel and sand, including the Yoldia (Leda) myalis Bed of Reid (1882) and the Mundesley Clay, which is a bed of tidally laminated silty clay cropping out at the base of the cliffs at Mundesley and Paston. The Bacton Member comprises a further 1.5 m of clay and organic mud, including Reid's Arctic Freshwater Bed.

In the early part of the 19th century, fossiliferous freshwater deposits were recorded by Charles Green at Ostend, near Bacton. The fossils included the vole Arvicola t. cantiana (Stuart and West, 1976; Stuart, 1996). By the time of Reid's work (1882) the beds had been removed or obscured. West (1980) recorded organic sediments filling a channel beneath till at Ostend. Pollen from the site suggested accumulation during the post-temperate substage (Cr1V).

Corton Formation

This formation (Arthurton et al., 1994) was named after the coast section at Corton [TG 5451 9722] in the adjacent Great Yarmouth district, where it forms the lower part of the type section for the Anglian Stage (Banham, 1971; Mitchell et al., 1973). In the type section a basal till is overlain by outwash sand. The till is considered to correlate with the 'Norwich Brickearth' and Starston Till inland, and with the Happisburgh Till Member in this district, farther north along the coast.

In the North Walsham and Mundesley district all the deposits between the Cromer Forest-bed Formation and the overlying Lowestoft Formation were assigned to the Corton Formation.

At the time of the survey it was thought that the Corton Formation was equivalent to the deposits commonly referred to as the 'North Sea Drift' in the Cromer district to the north-west. The 'North Sea Drift' included the First, Second and Third Cromer Tills and their associated sorted sediments. It was also believed that the Lowestoft Till overlies the 'North Sea Drift'. More recent work in the Cromer district (Moorlock et al., 2000; Hamblin, 2000; Hamblin et al., 2000) has revealed that the Second Cromer Till is the lateral equivalent of the Lowestoft Till and that the Corton Formation should include only the First Cromer Till and its associated sorted sediments (Figure 4). The Third Cromer Till and the overlying sorted sediments have been assigned to the newly named Beeston Regis and Overstrand formations.

This new interpretation affects only the northern part of the North Walsham and Mundesley district, where deposits on the higher ground are now regarded as within the Lowestoft, Beeston Regis and Overstrand formations rather than the Corton Formation. Because the deposits are predominantly sand in this area, the boundaries between the individual formations are difficult to determine.

The Corton Formation is best exposed in the cliff sections at Trimingham, just north of the district, which have been extensively studied by Eyles et al. (1989), Lunkka (1991, 1994), Hart (1990, 1992) and Roberts (1995). Originally, Eyles et al. (1989) argued that part of the sequence at Trimingham was glaciomarine, but the subsequent work of Hart, Lunkka and Roberts favours a glaciolacustrine origin. Hart (1992) divided the sequence at Trimingham into the Happisburgh Diamict and the Trimingham Member; the lower part of the former originated as a subglacially deformed bed. This grades upwards into a series of glaciolacustrine sediment gravity flows. The Trimingham Member is a suite of interbedded proglacial glaciolacustrine sediments.

The Happisburgh Till (Plate 2), where fresh, is a dark grey diamicton containing scattered clasts of chalk, flint and metamorphic and igneous rocks. Rhomb porphyries, derived from the Oslo Fjord region of Norway, were recorded by early workers (Reid, 1882) but none was found during the recent study. The diamicton commonly exhibits a weak lamination characterised by subhorizontal laminae of material rich in calcium carbonate. In places this lamination is believed to be sedimentary in origin. Elsewhere it formed from shearing of chalk clasts (Roberts, 1995). The Happisburgh Till is typically several metres thick.

Inland, in the south of the district, the lowest unit of the Corton Formation is commonly the Happisburgh Till that rests directly on sand or pebbly sand of the Crag Group. However, heavy mineral analysis indicates that locally some sands directly beneath the till have a greater affinity with the Corton Formation than with the underlying Crag Group, and should, therefore, be included with the former rather than the latter.

To the north-east of Ludham, the lowest diamicton, with a maximum thickness of several metres, forms an extensive topographical 'flat' commonly capped by a veneer of cover silt, locally over a metre thick. Nearby, overlying the diamicton is buff fine-grained sand, up to several metres thick, which crops out along a gentle topographical rise. The sand is overlain on some of the higher hills (for example [TG 396 185]; [TG 430 167]; [TG 432 162]) in the south by a further sandy diamicton, of similar lithology to the lower diamicton.

Within the district there is little evidence to indicate that the deposits are greatly deformed as in the 'contorted drift' of the Cromer district to the north. However, at Little Marl Point to the north of Mundesley, there is a large raft of Upper Chalk overlying Wroxham Crag (Plate 3) which has been 'thrust' to a higher level and is now partly enclosed by glaciogenic deposits. It is thought that the deformation was associated with the ice that deposited the Overstrand Formation. Mapping of the area around Lessingham [TG 39 28] also suggested the presence of local deformation within the glaciogenic deposits, but any interpretation was hindered by a veneer of overlying cover silt.

Lowestoft Till Formation

Deposits mapped as Lowestoft Till Formation are restricted to the southern part of the district, where they overlie the Corton Formation. The formation is mainly till, but also includes outwash sand and gravel. The deposits cap some of the higher ground, but also occur low down within the major valleys demonstrating that the valleys were in existence prior to the deposition of the till. The linear nature of some of the outcrops of till suggests deposition within ice marginal channels.

The till is up to about 4 m thick, and consists of a stiff, bluish grey, chalky, flinty, variably silty and sandy clay that weathers yellowish brown. Other pebbles within the clay include quartzite, vein quartz, iron pan and Jurassic clasts such as limestone, mudstone, cementstone and derived fossils. In addition to the chalk clasts, much fine-grained chalk 'flour' is present within the clay matrix. Within the basal metre or so, the till is commonly brown and is either chalk-free or contains only scattered pellets of chalk. The proportion of clay in the matrix within the basal part is also typically lower than in the succeeding beds, while the amount of sand and gravel is proportionally increased, probably reflecting incorporation of the underlying deposits. Crude lamination is locally present in the lowermost metre or so, which suggests deposition in a viscous or fluid state, possibly as a flow till.

At Bacton Green, the till (Plate 4) overlying the Wroxham Crag is characterised by a higher carbonate and lower sand content compared with the Happisburgh Till. This till was named the Walcott Diamicton by Lunkka (1994) who included it within the 'North Sea Drift' and correlated it with the Second Cromer Till of Banham (1968, 1988). More recent work (Moorlock et al., 2000) has shown that this diamicton contains Jurassic macrofossils and abundant derived Jurassic palynomorphs. This together with data on calcimetry, particle size and opaque heavy minerals demonstrate that the diamicton has a much greater affinity with the Lowestoft Till than with the 'North Sea Drift' (Figure 5). It follows that the sands in the cliff above the Walcott Diamicton (here formally renamed the Walcott Till Member of the Lowestoft Formation) must belong either to the Lowestoft Formation or the succeeding Overstrand Formation, although they are depicted as Corton Formation on the 1:50 000 Series Sheets 132 and 148.

The clay mineralogy of the till is predominantly mica and kaolinite with variable smectite and locally a little chlorite.

Outwash sand and gravel have been mapped to the north of Ludham where they are poorly sorted sandy gravel composed predominantly of angular to rounded flints together with subordinate quartz and quartzite, and locally clasts of chalk. Although the soils suggest that gravel is dominant, augering indicates that sand and pebbly sand are also present. The estimated maximum thickness of the sand and gravel is about 3 m.

Sand and gravel of unknown age and origin

Relatively small patches of sand and gravel have been thus designated on the 1:50 000 Series Sheets 132 and 148 and everywhere overlie the Corton Formation. Following recent work in the Cromer district, it is probable that these gravels are associated with the Lowestoft Formation or the overlying Overstrand Formation. In the north around Gimingham, they form the most easterly part of the Cromer Ridge. Here it is almost certain that they are part of the Overstrand Formation. In the south of the district, the sand and gravel are probably less that 3 m thick, but in the north accumulations of more than 10 m may be present. The gravel consists predominantly of angular to rounded flint with smaller quantities of quartz, quartzite and other clasts.

River Terrace Deposits

Deposits that can be assigned to river terraces are present at only a few localities within the district. Because of the isolated occurrence of outcrops it has not been possible to number the terraces sequentally, and the deposits are, therefore, shown as 'undifferentiated' on Sheets 132 and 148. The deposits comprise gravel and sandy gravel; the gravel fraction comprises angular to well-rounded flints with subordinate rounded vein quartz and quartzite clasts. The sand fraction ranges from fine to coarse grained. They are thought to be nowhere greater than 3 m thick.

Yare Valley Formation

The buried pre-Holocene surface underlying the marshland cuts through Anglian strata into the Crag. Based on the available data, this valley floor 'drains' southwards around the Isle of Flegg towards Great Yarmouth. The base of this buried valley, is lined by gravels of the Yare Valley Formation, which underlies the Breydon Formation. This formation was defined by Arthurton et al. (1994) in the Great Yarmouth district. It occupies the floor of a buried valley system eroded into the underlying solid formations (mostly Crag) and underlies the Breydon Formation, in most instances the Basal Peat. It is proved in boreholes only, and is restricted to the valleys occupied by Broadland rivers. Within this district the extent of the formation is imperfectly known. The deposits comprise mainly flint gravel ranging from fine to coarse, with variable amounts of fine- to coarse-grained sand, up to several metres thick.

There is no direct evidence for the age of the Yare Valley Formation. Coxon (1979) referred to them as Devensian, while Cox (1989) suggested that deposition may have begun in the Devensian. It is likely that at least some of the formation consists of fluvial sediments of late Devensian and early Flandrian age, deposited by rivers flowing within the now buried valley system. The maximum age for the formation is more speculative. Funnell (unpublished, 1990) argued that the general characteristics of the deposits imply a late Anglian age, but Arthurton et al. (1994) maintained that the formation demonstrably postdates much of the Anglian succession, while admitting that glaciofluvial deposits of late Anglian age may be included. Recent work in the adjoining Cromer district suggests that some of the gravels could have been deposited at the time of deposition of the Briton's Lane Member of the Overstrand Formation.

Head

It is generally impossible to distinguish head deposits, formed by the mass movement of material downslope under periglacial conditions, from those deposits resulting from hillwash and soil creep, processes which are still active at the present time. However, most of the deposits shown on the map are thicker than 1 m and were probably deposited immediately prior to the Holocene.

The lithology of the head deposits tends to closely reflect that of their upslope source. For example, Head derived from till is clay-rich, although any chalk present in the parent material has usually been leached. Slope deposits, where particularly pebbly, have been mapped as Gravelly Head.

Shoreface and beach deposits

The foreshore to the district generally comprises sand, pebbly sand and gravel, while locally, there are shingle-rich 'cusps' associated with storm deposition. A typical profile comprises shingle (mostly 10 to 15 mm diameter) of well-rounded flint with subordinate quartz and quartzite, in a matrix of medium-grained sand. The storm beach varies in width from 50 to 120 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 where the deposits intercalate with blown sand and the coastal barrier deposits.

Beach sediments are naturally ephemeral, periods of sediment accretion are interrupted by storm events resulting in a net seaward removal of material. The deposits form a trapezoidal wedge, often resting on a wave-cut bench occasionally exposed by storm scour. The sediments are subject to an overall wave-induced southward migration or longshore drift. In historical times this has been responsible for the shingle accumulations (capped by blown sand), forming and modifying Winterton Ness and the beach bar down the coast to Winterton.

Coastal barrier deposits

Behind the modern dune ridge is a relatively low-lying, uneven area of sand, derived principally from an older (pre-1880) dune belt and former beach. Overlying, and in part intercalated with, these deposits are waterlogged, organic to peaty areas (former dune slacks), washover deposits (remnants of earlier marine breaches) and relatively recent blown sand. Locally, the ground has been modified by sea defence repairs after storm breaches.

Tidal river and creek deposits

Modern deposits of organic silt and clay are present within all tidal water courses. Two localities, around [TG 415 204] where the deposits are periodically exposed, and near the coast [TG 457 213] to [TG 472 220], are probable remnant creek deposits of the former Hundred Stream.

Bank deposits

Offshore linear sandbanks, up to about 10 m thick and broadly parallel to the coast are present in the south-east of the district, where they overlie deposits of the Crag Group. These include the named larger features of Winterton Overfalls, Winterton Shoal and Haisborough Sand and the northern parts of the Caistor Shoal and North Scroby banks that extend from the Great Yarmouth district to the south. The sediments are largely fine- to medium-grained sand. The nearshore banks are slightly asymmetrical with steeper western slopes of about 2° to 3°, and eastern slopes of about 0.5°. The banks farther offshore, such as the Haisborough Sand, may have slopes of up to about 7°.

Tabular or sheet deposits

Several small north-west-trending offshore outcrops are present, mainly in the south-east of the district. These consist predominantly of medium- to coarse-grained sand and gravelly sand. The deposits are generally less than 3 m thick.

Breydon Formation

This formation was defined by Arthurton et al. (1994) to include Holocene freshwater and estuarine sediments filling the buried valley system (see above), beneath the marshland of Broadland. The term is not intended to include fluvial deposits formed inland and away from estuarine influence, or deposits formed in recent times after the sea was artificially excluded from the area and the marshland drained. The formation forms the marshland that occupies the floodplains of the Rivers Ant, Bure and Thurne. The few boreholes that penetrate the formation show that it is up to 9 m thick. Near Great Yarmouth, the formation is up to 22 m thick. The Basal Peat comprises an entirely buried, thin impersistent layer, comprising woody peat passing up into fen and reed-swamp peat through to salt-marsh vegetation. The impersistent nature of this layer may be a consequence of subsequent erosion.

The overlying Lower Clay rarely exceeds 3 m thick and thins towards the buried valley margins (at about 6 m below OD). It is a soft grey-black clay which becomes firm with depth.

The Middle Peat is generally extensive, and is usually well defined between the Upper and Lower clays. The upper surface often shows evidence of erosion. The Middle Peat is difficult to differentiate from the Upper and Basal peats in the upper valleys where they coalesce. Within the middle reaches of the River Bure around [TG 365 150], the Middle Peat exceeds 5 m in thickness, while downstream the recorded thicknesses are less (2 to 3 m), probably due to dewatering and consolidation by an increasingly thick cover of Upper Clay sediments. Auger holes show an upward succession through salt-marsh, reed-swamp and fen peats into an alder-Salix carr.

The Upper Clay wedges out against Upper Peat in the upper valleys; it thickens southwards to a maximum of about 6 m and comprises the bulk of the Breydon Formation. Where exposed, the Upper Clay has a weathered upper layer, generally less than one metre thick. This 'ripened soil' crust comprises silty to very silty clay, firm to very stiff, and pale grey in colour with a distinctive tan mottling. It includes concretions of brown iron oxide and, commonly, traces of gypsum, plant fragments and rootlets.

Beneath the weathered layer of the Upper Clay, two conspicuously different sedimentary facies are present. One is a mainly soft silty clay, pale to medium grey, and rich in plant material which occurs as roots of the reed Phragmites in growth position, as scattered spongy fragments, or as comminuted debris imparting a bedding-parallel lamination to the sediments. This facies also includes sparse bivalve and gastropod shells. The other facies (commonly underlying the first) comprises a soft to very soft (often liquid) silty clay, dark bluish or brownish grey to black in colour; it may be bioturbated and include interlaminated silts. The black colouration is due to finely disseminated iron pyrite in diffuse layers, mottles and flecks, formed diagenetically by the reduction of marine-derived dissolved sulphates (Price, 1980). Shells are locally common (comprising gastropods and thin-shelled bivalves) but disseminated plant material is rare.

The Upper Peat occurs as discontinuous outcrops fringing the marshland and as extensive spreads infilling the upper reaches of major and most minor valleys. It comprises mostly compact reed (Phragmites) and sedge (Carex and Cladium) peat, with some brushwood peat. Peat supplied with artesian water tends to be raised slightly above the marshland surface and is characterised by Sphagnum.

Within some of The Broads, for example Hickling Broad, Heigham Sound and Horsey Mere, peat has grown out from the retaining walls into the body of the lakes. This recent growth is controlled by dredging. The peat is shown on the map as Breydon Formation even though it does not come within the strict definition of the formation, because in many areas (such as Ormesby Broad) it is impossible to distinguish between the two peat developments.

Alluvium

This comprises unconsolidated layers of sand, silt, clay and organic material overlying or interbedded with gravel, mainly derived from Pleistocene deposits. Thickness ranges up to 3 m. For the most part the valleys are floored by glacigenic deposits while upstream and upslope, the alluvial sediments commonly imperceptibly merge and interdigitate with head and gravelly head. A small area of gravelly Older Alluvium, of uncertain age, has been mapped near Walcott [TG 347 320].

Peat

Much of the peat within the district is associated with, and has been included within, the Breydon Formation, but small isolated areas of peat are also present, for example, near the coast [TG 487 205], [TG 460 245].

Blown Sand

Sand of wind-blown origin occurs towards the landward side of the shoreface and beach deposits, both as thin drapes but also as marram grass-stabilised, near-continuous linear dunes. These dunes partly overlie sediments of the coastal barrier deposits and are backed by low-lying marshland or locally 'islands' of older sediment. The sand is up to or over 7 m thick in the dune belt. It is generally of fine-grained quartz, typically yellow-buff, and derived locally from beach material.

Dunes tend to be ephemeral features. The dune complex near Winterton is the most stable, dated at about 100 years old, while locally, along the coast northwards, the dune belt has been repeatedly breached by sea storms and modified by man as part of the sea defence strategy.

Cover silt

An unmapped veneer of variably pebbly, sandy silt, generally less than 1.5 m thick, covers much of the district. It is particularly prevalent on east-facing slopes. The base of the deposit is commonly associated with a hard-packed gravel, a few centimetres thick, believed to be deflationary in origin. The deposit is important agriculturally, as it tends to retain moisture even where the underlying deposits are sand and gravel. The cover silt represents a wind-blown deposit probably laid down under Devensian periglacial conditions.

Chapter 3 Applied geology

Bulk minerals

Sand and gravel aggregate

Sand and gravel have been dug on only a small local scale within the district, generally for individual estate or farm use. The largest gravel resources are probably associated with the Wroxham Crag in the west of this district near Wroxham and the east of the adjoining Aylsham district. Hereabouts, several estate pits for example at [TG 2657 1675] have exploited the flint gravels which are up to about 8 m thick.

Glaciofluvial gravels within the Corton Formation have also been dug locally, for example at Crostwright Common [TG 345 300], together with the underlying gravels in the Wroxham Crag. The glaciofluvial gravels are typically more poorly sorted than the Crag gravels, with lenses and interbeds of fine- to coarse-grained sand. The glaciofluvial gravels may attain thickness of up to about 5 m.

Large resources of sand are available within the Crag deposits. These are typically fine grained, but range up to coarse grained; they are usually very ferruginous and commonly interbedded with thin silts and clays. Many of the sands within the Corton Formation are very fine grained and of limited economic use. These sands also contain abundant granular chalk, which further restricts their commercial use.

Brick clay

Clay, for brick-making, has been dug historically on a small scale from till in both the Lowestoft and Corton formations.

Chalk

Locally dug chalk was used historically as a source of lime for marling the more acid soils within the district. Chalky till, from within both the Lowestoft and Corton formations, was also dug for this purpose.

Building stone

Flint is an important building stone within the district. The flint cobbles may be used whole, or knapped to produce fresh flat surfaces. In the past, they were probably collected from the shore, whereas nowadays flint from quarries outside of the district is used.

Peat

Peat in the Breydon Formation was dug in several parts of the marshland for fuel between the 12th and 15th centuries AD (Smith, 1960) to meet a presumed substantial demand for fuel in medieval Norfolk, which, according to the Domesday Book, had been stripped of forest. Towards the 16th century, peat lost favour to imported coal as the main fuel source. The peat diggings, or turbaries, subsequently became flooded to produce the extensive water bodies known as broads (Lambert and Jennings, 1960). The excavations were mostly sited laterally adjacent to, or upstream of, estuarine clays. These localities provided the peat diggers with some 3 to 4 m thick peat, with little or no intervening clay. Uncut baulks of peat and/or clay (commonly forming 'islands' in The Broads) may have been left intentionally in order to compartmentalise the excavations, an aid to dewatering the workings. The sides of the excavations were usually steep, vertical, or stepped and the floors were generally horizontal except where the workings abutted the valley sides. It is believed that Horsey Mere is one of the few Broads that does not owe its existence to peat digging.

An examination of different editions of the Ordnance Survey map reveals the recent re-growth of much peat around the margins of many of the old workings.

Hydrogeology

The water resources of the district are regulated by the Anglian Region of the Environment Agency. The southerly flowing River Ant and its tributaries drain much of the central part of the district. This river joins the easterly flowing River Bure in the extreme south. The River Thurne flows west-south-westwards through the south-eastern part of the district before also joining the River Bure.

The town of Great Yarmouth obtains part of its public supply from within the district. Surface water is taken from the River Bure at Belaugh and from the Ormesby Broad, and groundwater is abstracted from two boreholes at Belaugh and transferred to Ormesby for treatment. The major aquifers in the district are the Upper Chalk and the Crag. The Corton Formation is also a minor aquifer.

Chalk

Despite its high porosity, unfissured chalk is rather impermeable. Boreholes penetrating the Chalk in interfluve areas tend to give unsatisfactory yields, whereas boreholes in the vicinity of major valleys, such as that of the River Bure may provide copious supplies. Groundwater in the interfluves is relatively old, in a reduced state, and low in nitrate, whereas that near 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 by Palaeogene strata 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 groundwater 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. Such layers may produce perched aquifers. In the west and north of the district, the Crag is in hydraulic continuity with the underlying Chalk but, in the south-east, Palaeogene clays intervene and form an aquiclude. 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. A feature of water abstracted from the Crag is its high concentration of dissolved and suspended iron. 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.

The sands and gravels of the overlying Corton and Lowestoft formations are highly permeable though recharge may be restricted by the presence of tills.

Particular aquifer conditions may occur near the coast where Holocene deposits are liable to saline intrusion. Pumped drainage of the marshes has reduced groundwater levels to around or slightly below sea level, and allowed intrusion of high-chloride water. A study by Downing (1966) in the Hickling–Horsey area found fresh groundwater at the surface near higher ground, but with saline water at depth.

Geotechnical considerations

Flooding and Flood Protection

Some 40 per cent of the district lies below 3 m OD, and is at risk from both marine and riverine flooding. Marine incursions through breaches in the coastal barrier have occurred on numerous occasions, the earliest recorded was in 1287. More recently, severe floods occurred in 1938 and again in 1953, both occasioned by tidal surges, gale force winds and atypical rainfall. The drained marshland has long been subject to riverine floods caused by exceptional tides surging upstream into and overtopping The Broads and confined marshland waterways. Aside from the flooding of agricultural land, these surges result in a rapid increase in salinity that severely affects freshwater fish stocks within The Broads.

Cliff landslides and erosion

The sea cliffs of the district are comprised of an alternating sequence of clay, sand and locally gravel. After prolonged or heavy rain these are particularly prone to landsliding and collapse, followed by erosion of the slipped material by the sea. Areas of active erosion vary with time, and stretches of cliffline that were being severely eroded a few years ago have become stabilised with the growth of marram grass on the slipped masses. The cliffline between Bacton Green and Mundesley is a good example of where there has been minimal recent cliff recession. The reader is referred to the work of Hutchinson (1978) who studied the cliffs just north of the district.

Earlier coastal defence relied on maintaining the dune belt and impeding sediment movement by regularly spaced groynes. After the 1953 sea breach, the defence structures were radically upgraded to include an extensive concrete sea-wall and sheet piling. Recently, the Environment Agency has commissioned defence work from Waxham northwards towards Happisburgh which involved dumping massive blocks of Scandinavian-imported rock both along the backshore (adjacent to the dune belt) and just offshore, as nine discrete 'reefs' (Plate 5). In addition beach sediments, removed by recent winter storms along the Sea Palling to Waxham stretch, have been replenished by offshore-dredged material pumped ashore.

Marshland soils

The Breydon Formation and other marshland soils present particular geotechnical constraints. The estuarine clay and silt forming the bulk of the deposits are inherently weak while interbedded layers of peat tend to have high moisture content and are highly compressible. Locally, the ground may be saline. Weathering of these sediments leads to acid sulphate soils, and the contained peats are a potential source of methane.

The modern marshland surface is the result of centuries of draining and agriculture. Much 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 orange mottling. This soil 'ripening' 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 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 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.

The induced drainage of marshland generally leads to the progressive decrease in salinity of the underlying clay and silt as the salts are leached out. However, in this district, water levels in the marshland ditches are for the most part maintained below sea level. Adjacent to the coast, marshland ditch bases may penetrate below the impervious Upper Clay into the underlying peats and locally into the salinity 'envelope'.

Ground disturbed by human activity

Most of the near-surface Holocene deposits have been modified by man through artificial drainage schemes, agriculture, sea defence and flood limitation schemes. Large areas of open water within Broadland owe their existence in greater part to turbaries (peat diggings) of the 12th to 15th centuries. The latter are shown as worked ground on the 1:50 000 Series Sheets 132 and 148 Mundesley and North Walsham.

Information sources

Further geological information held by the British Geological Survey relevant to the district is listed below. It includes published maps, memoirs and reports. Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth. Geological advice for this area should be sought from the Geological Enquiry Service, BGS, Keyworth.

Information on BGS products is listed in the current Catalogue of geological maps and books and other data is available at the BGS web site (address on back cover).

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
• United Kingdom (South Sheet Solid Geology 1979; Quaternary geology 1977)
• 1:250 000
• East Anglia 52N 00, Solid Geology, 1991
• 1: 50 000
• 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.

Surveyors R S Arthurton, S J Booth, F C Cox, D H Jeffery, R J O Hamblin, B S P Moorlock, and A N Morigi.

• 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
• East Anglia 52N 00, Bouguer gravity anomaly, 1991
• East Anglia 52N 02W, Aeromagnetic anomaly, 1991
• 1:50 000
• Sea bed sediments map
• 1:250 000
• East Anglia 52N 00, 1988
• Hydrogeological map
• 1:250 000
• Northern East Anglia, 1976

Books

• General geology
• British Regional Geology East Anglia, fourth edition, 1961
• Memoirs
• Geology of the country around Norwich (Sheet 161), 1989
• Geology of the country around Great Yarmouth (Sheet 162), 1994

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.

BGS Lexicon of named rock unit definitions

Definitions of the named rock units shown on BGS maps, including those shown on the 1:50 000 Series Sheets 132 and 148 Mundesley and North Walsham are held in the Lexicon database. This is available on Web Site http:// www.bgs.ac.uk. Further information on the database can be obtained from the Lexicon Manager at BGS, Keyworth.

BGS photographs

BGS holds a number of photographs from the district; these are held in the National Archive of Geological Photographs. The photographs may be viewed at BGS Libraries in Keyworth and Edinburgh. Part of the collection has been digitised and can be accessed on our web site. Copies of the photographs are available at a fixed tariff.

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 may be purchased from the library subject to the current copyright legislation.

Arthurton, R S, Booth, S J, Morigi, A N, Abbott, M A W, and Wood, C J. 1994. Geology of the country around Great Yarmouth. Memoir of the British Geological Survey, Sheet 162 (England and Wales).

Bailey, H W, Gale, A S, Mortimore, R N,Swieciki, A, and Wood, C J. 1983. The Coniacian — Maastrichtian stages of the United Kingdom, with particular reference to southern England. Newsletter of Stratigraphy, Vol. 12, 29–42.

Banham, P H. 1968. A preliminary note on the Pleistocene stratigraphy of northeast Norfolk. Proceedings of the Geologists' Association, Vol. 79, 507–512.

Banham, P H. 1988. Polyphase glaciotectonic deformation in the contorted drift of Norfolk. 27–32 in Glaciotectonics: Forms and Processes. Croot, D (editor). (Rotterdam: Balkema.)

Banham, P H. 1971. Pleistocene beds at Corton, Suffolk. Geological Magazine, Vol. 108, 281–285.

Bristow, C R, Mortimore, R N, and Wood, C J. 1997. Lithostratigraphy for mapping the Chalk of southern England. Proceedings of the Geologists' Association, Vol. 109, 293–315.

Brydone, R M. 1906. Further notes on the stratigraphy and fauna of the Trimmingham Chalk. Geological Magazine, Vol. 43, 13–22, 72–78, 124–131, 289–300.

Brydone, R M. 1908. On the subdivision of the Chalk at Trimmingham. Geological Magazine, Vol. 5, 134.

Cameron, T J D, Crosby, A, Balson, P S,Jeffery, D H, Lott, G K, Bulat, J, and Harrison, D J. 1992. U K offshore regional report: The geology of the southern North Sea. (Keyworth: British Geological Survey.)

Clarke, K B, and Phillips, J H. 1984. Experiences in the use of East Anglian sands and gravels ('Crags') as a source of water supply. Journal of the Institution of Water Engineers and Scientists, 38, 543–549.

Clayton, K M. 2000. Glacial erosion of the Wash and Fen basin and the deposition of the chalky till of eastern England. Quaternary Science Reviews, 19, 811–822.

Coxon, P. 1979. Pleistocene environmental history in central East Anglia. Unpublished PhD thesis, University of Cambridge.

Cumming, A D, and Wyndham, C L. 1975. The geology and development of the Hewlett Gas-field. 313–326 in Petroleum and the continental shelf of North-west Europe, Volume 1. Woodland, A W (editor). (Barking: Applied Science, for the Institute of Petroleum.)

Downing, R A. 1966. Hydrogeology ofnorthern East Anglia with special reference to the Chalk. Water Supply Paper of the Geological Survey of Great Britain.

Eyles, N, Eyles, C H, and McCabe, A M. 1989. Sedimentation in an ice-contact subaqueous setting — the Mid-Pleistocene North-Sea drifts of Norfolk, U K. Quaternary Science Reviews, Vol. 8, 57–74.

France, D S. 1975. The geology of the Indefatigable Gas-field. 233–240 in Petroleum and the continental shelf of North-west Europe, Volume 1. Woodland, A C (editor). (Barking: Applied Science, for the Institute of Petroleum.)

Funnell, B M, and West, R G. 1977. Preglacial Pleistocene deposits of East Anglia. 247–265 in British Quaternary Studies, recent advances. Shotton, F W (editor). (Oxford: Clarendon Press.)

Gale, A S. 1989. Field meeting at Folkestone Warren, 29th November, 1987. Proceedings of the Geologists' Association, Vol. 100, 73–82.

Morter, A A, and Gallois, R W. 1979. Provisional report on the I GS borehole drilled at Trunch, Norfolk. British Geological Survey Technical Report (un-numbered).

Gallois, R W, and Morter, A A. 1982. The stratigraphy of the Gault of East Anglia. Proceedings of the Geologists' Association, Vol. 93, 351–368.

Gibbard, P L, and Zalaziewicz, J. 1988. Pliocene–Middle Pleistocene of East Anglia field guide. (Cambridge: Quaternary Research Association.)

Goodchild, M W, and Bryant, P. 1986. The geology of the Rough Gas Field. 223–235 in Habitat of Palaeozoic gas in N W Europe. Brooks, J, Goff, J C, and Hoorn, B V (editors). Geological Society of London Special Publication, No. 23.

Gray, I. 1975. Viking Gas-field. 241–248 in Petroleum and the continental shelf of North-west Europe, Vol. 1. Woodland, A W (editor). (Barking: Applied Science, for the Institute of Petroleum.).

Hamblin, R J O. 2000. A new glacial stratigraphy for East Anglia. Mercian Geologist, Vol. 5, 59–62.

Hamblin, R J O, Moorlock, B S P, Booth, S J, Jeffery, D H, and Morigi, A N. 1997. The Red Crag and Norwich Crag formations in eastern Suffolk. Proceedings of the Geologists' Association, Vol. 108, 11–23.

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Harland, R, Bonny, A P, Hughes, M J, and Morigi, A N. 1991. The Lower Pleistocene stratigraphy of the Ormesby Borehole, Norfolk, England. Geological Magazine, Vol. 128, 647–660.

Hart, J K. 1992. Sedimentary environments associated with Glacial Lake Trimingham, Norfolk, U K. Boreas, Vol. 21, 119–136.

Hart, J K. 1990. Proglacial glaciotectonic deformation and the origin of the Cromer Ridge push moraine, north Norfolk, England. Boreas, Vol. 19, 165–180.

Hart, J K, Hindmarsh, R C A, and Boulton, G S. 1990. Styles of subglacial glaciotectonic deformation within the context of the Anglian ice sheet. Earth Surface Processes and Landforms, Vol. 5, 227–241.

Hutchinson, J N. 1978. Coastal landslides in cliffs of Pleistocene deposits between Cromer and Overstrand, Norfolk, England (C P 71/76). 166–193 in Foundations and soil technology. (Lancaster: Construct. Press.)

Johansen, M B, and Surlyk, F. 1990. Brachiopods and the stratigraphy of the upper Campanian and lower Maastrichtian Chalk of Norfolk, England. Palaeontology, Vol. 33, 823–872.

Knox, R WO B, and Morton, A C. 1988. The record of early Tertiary North Atlantic volcanism in sediments of the North Sea Basin. 407–419 in Early Tertiary volcanism and the opening of the N E Atlantic. Morton, A C, and Parson, L M (editors). Special Publication of the Geological Society of London, No. 39.

Krinsley, D H, and Funnell, B M. 1965. Environmental history of quartz sand grains from the Lower and Middle Pleistocene of Norfolk, England. Quarterly Journal of the Geological Society of London, Vol. 121, 36–43.

Lambert, J M, and Jennings, J N. 1969. Stratigraphical and associated evidence. 1–66 in The making of the Broads. Lambert, J M, Jennings, J N, Smith, C T, Green, C, and Hutchinson, J N (editors). Memoir of the Royal Geographical Society, No. 3.

Lunkka, J P. 1991. Sedimentology of the Anglian glacial deposits in northeast Norfolk, England. Unpublished PhD thesis, University of Cambridge.

Lunkka, J P. 1994. Sedimentation andlithostratigraphy of the North Sea Drift and Lowestoft Till formations in the coastal cliffs of northeast Norfolk. Journal of Quaternary Science, Vol. 9, 209–233.

Marie, J P P. 1975. Rotliegendes stratigraphy and diagenesis. 205–212 in Petroleum and the continental shelf of north-west Europe, Vol. 1. Woodland, A W (editor). (Barking: Applied Science, for the Institute of Petroleum.)

Mitchell, G F, Penny, L F, Shotton, F W, and West, R G. 1973. A correlation of Quaternary deposits in the British Isles. Special Report of the Geological Society of London, No. 4.

Moorlock, B S P, Hamblin, R J O, Morigi, A N, Booth, S J, and Jeffery, D H. 2000. The geology of the country around Lowestoft and Saxmundham. Memoir of the British Geological Survey, Sheets 176 and 191. (England and Wales).

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Strank, A R E. 1982. Foraminifera from the Somerton Borehole, Norfolk. Report of the Palaeontology Research Group, British Geological Survey, P DL/82/191.

Stuart, A J. 1996. Vertebrate faunas from the early Middle Pleistocene of East Anglia. 9–24 in The early Middle Pleistocene of Europe. Turner, C (editor). (Rotterdam: Balkema.)

Stuart, A J, and West, R G. 1976. Late Cromerian fauna and flora at Ostend, Norfolk. Geological Magazine, Vol. 113, 469–473.

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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.

Figures and plates

Figures

(Figure 1) Locations of principal boreholes around the district which penetrate Palaeozoic or Mesozoic rocks.

(Figure 2) Sequence through the Somerton No. 1 Borehole.

(Figure 3) Stratigraphy of the Chalk Group.

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

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

Plates

(Plate 1) Wroxham Crag in quarry on Trafford Estate [TG 2872 1724]. Gravel containing well-rounded clasts of flint, quartz and quartzite. The abundant clasts of quartz and quartzite are characteristic of the Wroxham Crag Formation and distinguish it from gravel within the Norwich Crag Formation (GS1174).

(Plate 2) Sandy Happisburgh Till with deformed inclusion of chalk. Micropalaeontological analysis of the chalk inclusions show that they are derived from local Chalk bedrock [TG 379 314] (GS1175).

(Plate 3) Raft of Upper Chalk and Crag within glacial deposits at Little Marl Point, Mundesley [TG 298 378] (GS1176).

(Plate 4) Walcott Till in cliffs at Bacton Green [TG 325 355] .The matrix of grey silty clay together with the abundant clasts of chalk and the common presence of Jurassic and Carboniferous palynomorphs suggests that this till should be correlated with the Lowestoft Till rather than with tills derived from Scandinavia. A thin bed of sand is present within the till (GS1177). Scale: handle about 10 cm.

(Plate 5) Offshore artificial reefs. These reefs are constructed from large blocks of igneous rock brought by barge from Scandinavian 'super quarries'. They are placed in order to restrict sediment movement along the coast (Photograph Derek Edwards, Archaeology and Environment Division, Norfolk Museums Service).

(Front cover) Windpump on the River Thurne [TG 399 157]. The pump is one of many that were formerly used to regulate water levels within Broadland (GS1173).

(Rear cover)

(Geological succession) Geological succession of the Mundesley and North Walsham district

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

Figures

(Geological succession) Geological succession of the Mundesley and North Walsham district

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

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