The origin of the Blakeney Esker, Norfolk

The origin of the Blakeney Esker, Norfolk J. M. Gray GRAY, J. M. 1997. The origin of the Blakeney Esker, Norfolk, Proceedings of the Geologists' Association, 108, 177-182. Many theories have heen proposed to explain the origin of the distinctive ridge that runs for 3.5 km between Blakeney and Glandford in north Norfolk. These theories include an erosional remnant of a larger depositional body, an ice-marginal ridge, a crevasse filling, and a supraglacial, englacial or subglacial esker. Renewed quarrying in the Wiveton Downs Gravel Pit in the 1980s revealed that the ridge is underlain by a system of channels cut into the chalk-rich till. The stratigraphy of one channel infill indicates that meltwater streams flowed through these Nye channels and eventually overfilled them to build the ridge. This evidence, together with the undulating long profile, indicates that the ridge was formed as a subglacial esker. It is designated as a UK Site of Special Scientific Interest (SSSI) and steps are being taken to develop it as an important educational/visitor resource. Department of Geography, Queen Mary and Westfield College, University of London, Mile End Road, London E1 4NS.

1. INTRODUCTION

The feature discussed in this paper is a distinctive sinuous ridge (Fig. 1), extending NW-SE over a distance of some 3.5 km from the north Norfolk coast road between Blakeney and Morston [TG 017 438] to the village of Glandford in the Glaven valley [TG 045 414], around which it becomes disjointed. Over this distance the ridge crest rises and falls through c. 30 m, the highest part of the ridge reaching almost 50 m O.D. in the central section, known as Wiveton Downs (Fig. 2b). This location gives its name to the local Site of Special Scientific Interest (SSSI) whose boundary follows the ridge (see Fig. 8). The ridge is sinuous in plan, with several near 90° bends, particularly in the NW (Fig. 2b), though these are never acute enough to be comparable with the 'concertina eskers' associated by Knudsen (1995) with surging glaciers. The origin of the ridge has been hotly debated for many decades, with several theories proposed. These include: (i)

(ii)

(iii)

(iv)

(v)

the ridge is a linear remnant resulting from the erosion of a larger mass of sand and gravel (Straw, 1973, see Fig. 3.1) or from erosion of silty diamicton surrounding a linear body of sand and gravel; the ridge was formed along the edge of an ice-sheet as a moraine or ice-marginal river deposit (West, 1957, see Fig. 3.2); the ridge was formed in an open ice-crevasse extending down to the land surface (Sparks & West. 1964, see Fig. 3.3); the ridge was formed in a supraglacial channel on the ice surface, the ribbon of sediment subsequently being lowered onto the bed (see Fig. 3.4); the ridge is an englacial esker, i.e. formed by a river flowing through an ice-tunnel above the bed (Gale & Hoare, 1986, see Fig. 3.5);

Proceedings of the Geologists' Association, 108, 177-182.

(vi) the ridge is a subglacial esker, i.e, formed by a river flowing through a tunnel at the base of the ice (Gale & Hoare, 1986, see Fig. 3.6). From studies of the gravel fabric, stratification and particle sorting, Gale & Hoare (1986) argued that the sediments

Fig. 1. The Blakeney esker in 1964 looking southeast from the north Norfolk coast road. Pits A, B, D and E are labelled. The Wiveton Downs Pit discussed in this paper is Pit B. The village of Glandford (G) in the Glaven Valley is also indicated. Cambridge University Collection: copyright reserved. 0016--7878/97 $10·00 © 1997 Geologists' Association

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2. EVIDENCE FROM RENEWED QUARRYING Quarrying of the sands and graveLs comprising the ridge has taken place intermittently at [east from the time of World War II when an Interim Development Order was granted on the largest pit, at Wiveton Downs. The sands and gravels of the Blakeney ridge rest on a chalk-rich till (Marly Drift) that is very easily identified. The policy of the last operators of the Wiveton Downs Pit was to quarry all the available sand and gravel down to the level of this chalky till. In doing so, it became clear that the basal surface of the sand and gravel is not flat, but instead is characterized by a number of linear depressions or channels that are cut into the till (Gray, 1988). Figure 2c shows the location of the channels mapped by the author during quarrying operations in the late 1980s. Channel A, a long, deep, steep-sided depression was the most impressive. It could be traced over a distance of about 100 m, parallel to, and a few metres away from, the southern boundary of the ridge. It had steep-sided slopes

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5. ENGLACIAL , must have been transported under conditions of dispersive stress under a high hydraulic gradient, i.e. under pipeflow conditions in a glacial tunnel. They concluded that the ridge was a sub- and/or englacial esker. Rose (in Banham, Davies & Perrin, 1975) believed that the feature is an esker which might represent the course of a feeder stream flowing towards the outwash fan of Salthouse Heath to the southeast, now generally regarded as part of the Cromer Ridge end moraine formed during a limited late Anglian readvance (Hart & Boulton, 1991).

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179

ORtGIN O F THE BLAKENEY E S KER, N ORFOLK

(40-70°) and was up to 5 m deep and l5 m wide at the top, narrowing to less than 2 m in places on its floor. At its western end it merged with Channel B which was generally a wider, shallower feature (Fig. 2d). This channel could be traced eastwards through the centre of the quarry to link with Channel D (Fig. 2c). Channel C was exposed in the face immediately below the copse of trees by the present pit entrance. The base of the channel was not fully exposed but till was present in both banks and a broad channel infilling of sand and gravel was well exposed (Fig. 4). Figure 5 shows the stratigraphy of the sediments at this site. The channel was 20-25 m wide at the top.

3. INTERPRETATION The erosional interpretation (i) can be eliminated on several grounds, such as general consistency of width and composition of the ridge. Interpretations (ii) and (iii) are unlikely given the undulating long profile. Under icemarginal or open crevasse conditions , rivers do not flow uphill. That the ridge was formed subglacially (vi), rather than in a supragiacial channel (iv) or englacial tunnel (v) is demonstrated by two pieces of evidence. First, the presence of channel s in the till clearly demonstrates that the water must have been flowing on the land surface. Secondly, the sediments in Channel C are pinched out towards the margin and clearly thicken towards the channel centre (Fig. 4). This evidence, together with the lack of faulting, indicates that the sediments were not lowered by melting ice onto the land surface from a supraglacial or englacial position, but instead were deposited by water flowing in this channel. The fact that the channels underlie and are parallel to the ridge, even where there are right-angle bends, indicates a close association in terms of origin and it is suggested that we are dealing with a river flowing on the land surface under hydrostatic pressure, i.e. the ridge is a subglacial

esker. The channels are assumed to be Nye channels cut by meltwaters flowing NW-SE prior to infilling and overfilling by sands and gravels to produce the ridge form. Thus the processes changed from erosional to depositional, and the channel type changed from Nye to Rothlisberger as deposition progressed (Fig. 6). This origin is consistent with the sedimentological evidence presented by Gale & Hoare (1986).

4. EVIDENCE FROM SHALLOW GEOPHYSICS Several linear, till-margined, gravel pits occur about I km to the southwest of the Blakeney ridge around Bilsey Hill and Brecks Farm (TG 024 415] and it was thought that these might also represent channel fills and thus be relevant to the formation of the Blakeney ridge. In order to explore the morphology of the till/gravel contact in this area a ground conductivity survey was carried out using a portable Geonics EM3 1 conductivity meter which gives average conditions in the uppermost 6-7 m. Point or continuous recording is possible, and the method has proved valuable in tracing channel fills in other parts of East Anglia (e.g. Mathers & Zalasiewicz, 1986). The main point recordings are shown in Fig. 7, the location of which is shown on Fig. 2. The main object of the survey was to determine whether the Little Bilsey and Four Acre Plantation Pits (Fig. 7) are linked by a gravel-filled channel that is in tum linked to the Blakeney ridge. The point survey demonstrated that the Marly Drift has a conductivity value of 22- 24 millirnhos/m whereas the sand and gravels general have values of less than 10 millimhos/m. Figure 7 was supplemented by continuous transects in the crucial areas between the visible gravel bodies. From this evidence, it is clear that although the Little Bilsey gravels extend southeastwards to Brecks Farm, beyond the existing pit, there is no clear link with the Four Acre Plantation Pit.

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180

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than with surface channel infills. The narrow, continuous nature of the Wiveton Downs depressions is quite different to those at Bilsey Hill and a morphological or genetic link is unlikely.

Furthermore, exposures in the Little Bilsey Pit demonstrate clear thrust structures in the bounding till (Ehlers, Gibbard & Whiteman, 1991, fig. 146) and it seems likely that the gravel bodies around Little Bilsey and Brecks Farm are associated with glaciotectonic movements rather

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181

ORIGIN OF THE BLAKENEY ESKER, NORFOLK

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5. CONSERVATION AND RESTORATION Since the latest phase of quarrying ceased in 1990, the issue of how to restore the Wiveton Downs Pit has been discussed between the landowner, the pit operators, English Nature, Norfolk County Council and the author. Norfolk County Council own and manage a disused pit to the southeast of the Wiveton Downs Pit on the other side of a minor road (Fig. 2c), and they have developed this as a picnic site. There is an explanatory board at this site with a short section on the esker, but there are clearly educational/visitor opportunities in conserving the scientific value of this site and the Wiveton Downs Pit. The normal practice in restoring such a pit would be to level the site to match the surrounding gently rolling farmland. However, the author has suggested that restoration should attempt to retain the pattern of Nye channels as shown in Fig. 2c, though not the full relief of the channel morphology. Clearly it would be impractical to retain channel side slopes of up to 70° if the land is to be given over to agricultural use, and substantial infilling and grading has in fact already occurred. The principle of restoring the site in

this way has been agreed by all parties but has yet to be implemented. The potential still exists, however, for the position and pattern of the channels to be retained as part of the SSSI, thus greatly increasing the value of the site for educational purposes. Figure 8 shows that the esker survives in rather fragmented and depleted form. A number of roads cross it, but more importantly numerous pits have removed large parts of the landform, especially at A, B, C, D and E in Fig. 8. Future quarrying within the SSSI boundary will be very carefully controlled, with the priority given to conserving the remaining landform. However, it is an enduring dilemma of glaciofluvial geomorphology that it is often only by quarrying, and at least partly destroying a feature, that its origin can be better understood. This point is vividly illustrated by this example. The presence of the channels means that the gravel reserves of the esker are considerably higher than would be calculated on the assumption of a flat bed. At D in Fig. 8 quarrying has removed gravels to a level at least 5 m below that of the surrounding farmland, indicating that here too the gravels occupy a significant channel. Figure 2c shows that

182

1. M . G RAY

short distan ce to the west into the ex isting ridge. Such a scheme would partly satisfy the demand for more gravel from the ridge, but might also appeal to the scientific community and English Nature in revealing more of the structure of the ridge with minimal impact on the existing land form .

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6. CONCLUSION

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Recent qu arrying in the Blakeney Esker has revealed that it is underlain by a system of Nye channels cut into chalk-rich till. The ridge sediments appear to be cha nnel infills of the Nye channels and subsequently of a majo r subglacia l tunnel (Rothlisberger channel). This evidence indicates that the ridge was formed as a subglacial esker. Agree ment has been reached on retaining the channel pattern during restoration of the pit, thus conserving part of the scientific interest of the site, which together with the ridge morphol ogy anti stratigraphy could form an important educat ional/visitor resourc e acces sed from an adjacent Norfolk Count y Council picnic site .

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ACKNOWLEDGEMENTS Fig. 8. Map showing the extent of the Wiveton Downs SSSI, the fragmented nature of the esker, the dissection by roads and the depletion by gravel quarrying.

immediately to the west of the Wiveton Down s Pit, part of the esker has been remo ved by quarrying and subsequently levelled . The channel pattern in the pit shows that channels A and B merge to the east of the road, and it may be predicted that a major channel exists to the west of the road, underlying the levelled area . The gravel infill of this channel would be worth excav ating and, in terms of revealing the structure of the esker, could be extended a

Durin g his career at Queen Mary & Westfield Colle ge, Frank Middlemiss taught geology as a subsidiary subject to hundr eds of physical geography students who bene fitted greatly from his enthusiasm, knowledge and skills. Thi s pape r is writt en to acknowledge the debt of gratitude owed by the Department of Geograph y. I am very grateful to Dr S. Campbell and Dr M. Bennett of the Nature Conservancy CouncillEnglish Nature for successively supporti ng the propo sed restoration of the site. The quarry mana ger, Mr R. Powell pro vided much useful informati on. The diagram s were kindl y drawn by Mr E. Oliver of the Department of Geography at Queen Mary & Westfield Coll ege.

REFERENCES BANHAM, P. H., DAVIES, H. & PERRIN, R. M. S. 1975. Short field meeting in North Norfolk. Proceedings of the Geologists' Association, 86,251-258. EHLERS, J., GIBBARD, P. & WHITEMAN , C. A. 1991. The glacial deposits of northwestern Norfolk. In (Ehlers, J., Gibbard, P. L. & Rose, J.; eds) Glacial deposits of Great Britain and Ireland. Balkema, Rotterdam, 223-232. GALE, S. J. & HOARE, P. G. 1986. Blakeney ridge sands and gravels. In (West, R. G. & Whiteman, C. A.; eds) The Nar Valley and North Norfolk. Quaternary Research Association Field Guide, Cambridge, 94-95. GRAY, J. M. 1988. Glaciofluvial channels below the Blakeney esker. Norfolk. Quaternary Newsletter, 55, 8- 12. HART, J. K. & BOULTON, G. S. 1991. The glacial drifts of northeastern Norfolk. In (Ehlers. J., Gibbard, P. L. & Rose, J.; eds) Glacial deposits of Great Britain and Ireland. Balkema, Rotterdam, 233-243.

KNUDSEN, O. 1995. Concertina eskers, Bniarj okull, Iceland: an indicator of surge-type glacier behaviour. Quaternary Science Reviews, 14,487-493. MATHERS, S. J. & ZALASIEWICZ, J. A. 1986. A sedimentation pattern in Anglian marginal meltwater channels from Suffolk, England. Sedimentology, 33,559-573. SPARKS. B. W. & WEST, R. G. 1964. The drift landforms around Holt , Norfolk . Transactions of the Institute of British Geographers. 35, 27-35. STRAW, A. 1973. The glacial geomorphology of central and north Norfolk. East Midland Geographer, 5, 333-354. WEST, R. G. 1957. Notes on a preliminary map of some features of the drift topography around Holt and Cromer, Norfolk. Transa ctions of the Norfolk & Norwich Naturalist Society. 18, 24-29.

Manuscript recei ved 2 December 1996; rev ised typescript acce pted 3 March 1997