GEOSUFFOLK RIGS
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SIZEWELL ‘B’ CRAGS AN UNAVAILABLE GEOLOGICAL SITE (RUGS) BOB MARKHAM Initial shallow (and above O.D.) excavations for the Central Electricity Board’s (CEGB) Sizewell ‘B’ Power Station took place in 1970, exposing shelly sand (‘Norwich Crag’), but deep boreholes showed that fossiliferous Crag extended down to about -48 m O.D. A wonderful opportunity to see some of these deposits came in 1988 and 1989 when deep excavations (to -14 m O.D.) for a pump house took place after construction of the power station had been approved. It was of course not possible to see the ‘grey-green sand with shell fragments’ recorded from -48 m to -18 m O.D. (in boreholes) in situ, but construction of a concrete diaphragm wall down to about -50 m O.D. involved pumping up some of this deposit. The ground inside this surrounding wall was then pumped dry of underground water so that Sizewell ‘B’ could be built. Grey-coloured Crag was pumped up from the excavations; it rested on greycoloured London Clay into which the base of the diaphragm wall was embedded. Shells found which were distinctive of the grey-coloured Crag were the gastropod Admete viridula and the bivalves Astarte obliquata, Cyclocardia and Glycimeris. I used the working name ‘Magnox Crag’ (named after the Sizewell ‘A’ type of power station) for this material, the fauna reminding me somewhat of the Red Crag of the Butley-Bawdsey area. Excavations for the power station’s coolant water pump house, in the south-east corner of the site, reached to -14 m O.D. and exposed, from approximately O.D. downwards, ferruginous sands often with thin ferruginous bands with green clay interiors; a thin horizon of small phosphatic nodules at -7 m rested on a well-defined cross-bedded unit. Borehole records suggest that this red-coloured Crag extended a further 4 m below the base of the pit. Characteristic shells found at this horizon were the bivalves Spisula constricta, Diplodonta astartea and Mya pullus; trace fossils (vertical tubes) were very common. I used the working name ‘PWR Crag’ (named after the Pressurised Water Reactor of the Sizewell ‘B’ power station) for this material, the fauna reminding me somewhat of the Scrobicularia Crag of the Chillesford area. The 1988 excavations showed that the base of the ‘Norwich Crag’ at Sizewell ‘B’ was about -2 to -3 m on the west side of the site (where the ground level was about +4 or +5 m O.D.), but appeared to be at about O.D. in the pump house pit. This yellow-coloured Crag was characterised by the gastropods Hydrobia and Potamides icenicus and the rooted-tooth vole Mimomys. There was a stone bed (flint and quartz) at the base of this Crag, from which (particularly from the west side of the site) a number of pieces of deer antlers, whale vertebrae, and pieces of proboscidian tusks and bones were found, many by enthusiastic site workers. Of particular interest was the tooth of a mastodon, found by Security Guard David Heywood on a heap of sand taken from the excavation; it had been about 150 years since the previous such find at Sizewell! Many fossil molluscs, especially Macoma, Mytilus, Chlamys, Spisula, ‘Cardium’ and Nucella, were common in all the Crag horizons.
14 I thank Nigel Dixon (Site Administrator), Jim McFarlane (Site Manager) and Ian Kennedy (Information Officer) of the CEGB who gave all help in studying the excavation. The fossil bones and teeth found on the site excited a great deal of public interest, featuring in Sizewell ‘B’ News (March 1988), East Anglian Daily Times, Evening Star, and the Citizen, and on BBC Look East, About Anglia, Radio Norfolk and Radio 4 (‘The Week That Was’). Some of them were exhibited for a period in the Sizewell Information Centre; they are now preserved in the Ipswich Museum. The following two publications may be consulted for further information about the lower Crag deposits at Sizewell: Funnel, B. M. (1983). Preliminary note on the Foraminifera and stratigraphy of C.E.G.B. Sizewell boreholes L and S. Bull. Geol. Soc. Norfolk 33: 54– 62. Zalasiewicz, J. A., Mathers, S. J., Hughes, M. J., Gibbard, P. L., Peglar, S. M., Harland, R., Nicholson, R. A., Boulton, G. S., Cambridge, P. And Wealthall, G. P. (1988). Stratigraphy and palaeoenvironments of the Red Crag and Norwich Crag formations between Aldeburgh and Sizewell, Suffolk, England. Phil. Trans. Roy. Soc. London, B 322: 221–272. Although it is not possible to preserve a geological exposure in such deep sites as Sizewell ‘B’, every opportunity should be taken to photograph them and collect and preserve samples of strata and fossils so that they are at least still ‘available’ in part. Bob Markham 28 Balliol Close Woodbridge Suffolk IP12 4EQ.