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SUTTON KNOLL ROGER DIXON Introduction This S.S.S.I. (Rockhall Wood of some authors) has been studied for over 170 years, long known as an island of Coralline Crag during Red Crag sea times, with its Coralline Crag cliffs and blocks ‘buried’ in Red Crag. The classic description was by Prestwich (1871a, b), some of the figures of which were reproduced by Boswell (1928), but Balson (1999) gives excellent recent description and discussion. Other recent descriptions include Balson & Long (1988) and Balson et al. (1990); Wood (2000) reproduces two of Prestwich’s diagrams and Balson et al. (1993) describe the stratigraphy of the Coralline Crag as a whole.

The locality has frequently been visited by field groups, including the Geologists’ Association, Quaternary Research Association, International Sedimentological Congress, Geological Society of Norfolk, Ipswich Geology Group, Harrow & Hillingdon Geological Society and, of course, the Suffolk Naturalists’ Society. It has frequently been used for educational purposes, and has often been used by undergraduates for project work.

Over the past five years there has been a resurgence of interest in this unique site. English Nature have cleared faces as part of their ‘Facelift’ programme and the local GeoSuffolk group have been actively maintaining sections. Furthermore, new research is underway to help interpret the site from a modern perspective.

Coralline Crag Suffolk’s Pliocene [c. 3·75 Ma] Coralline Crag outcrops as a 12 km inlier ridge running from the north of Aldeburgh south-westwards to the Butley River, with further small outliers, including Sutton, to the southwest, and rests unconformably on the London Clay. The bioclastic sands, about 20 m thick, can be broadly divided into three sedimentary and faunal units, two of which outcrop at Sutton: a lower Ramsholt Member and an overlying Sudbourne Member (seen also at Richmond Farm and Crag Farm S.S.S.I.s and elsewhere). Over 300 species of mollusc have been recorded from these beds, many extant forms having ‘southern’ (Mediterranean) distributions, and suggest summer water temperatures of 17–20°C. The beds are possibly best known for their bryozoan fossils (‘corallines’) and the common occurrence of one of Britain’s largest fossil brachiopods, Terebratula grandis.

The Sudbourne Member The best exposure is found on the northern side of the ‘island’ in the Quarry Pit. About 5 m of shelly sands are exposed, calcite-cemented to form a relatively durable limestone, with evident bioturbation and a variety of infaunal burrow types, including those of worms, crustaceans and spatangoid echinoids, and a good bryozoan fauna. The Member exhibits well-developed medium- and large-scale trough cross-bedding and are interpreted as storm and tidal sand-waves migrating in relatively shallow water (c. 30 m) in a generally south to south-west direction.

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The lower beds in the quarry (also better seen in the small section immediately to the east of the quarry) contain mud drapes, small scale ripples and laminations, possibly of tidal origin, and abundant infaunal burrows.

Although there is no stratigraphic implication nowadays, the term “Rock Bed” has been used for this Coralline Crag limestone. Post-depositional solution processes have dissolved aragonite shells. Re-precipitated carbonate in the form of calcite has led to cementation. The “Rock Bed” processes occurred before the deposition of the later Red Crag, as pebbles and boulders of Rock Bed are found in the Red Crag. It was often used locally as a building stone in farm walls and other small structures, including on the Sutton Estate, but can also be seen as major components in Chillesford and Wantisden churches, and Orford Castle.

Joint sets are well developed in the “Rock Bed” and can be seen in the pit walls; they are probably related to the flexuring of the North Sea Basin.

Occasional solution pipes can be seen extending down from the surface.

The Ramsholt Member Following the work by English Nature, the Bullock-yard Pit, on the eastern side of the ‘island’, currently exposes about 5 m of silty/muddy sands of the Ramsholt Member, making the section better than it has been since the days when this was a working pit. Exploratory excavations extended below the water-table almost to the London Clay basement.

Medium-scale current bedding is evident, although often indistinct, with mud-drapes, smaller ripples and extensive bioturbation, indicating weaker currents and slow depositional rates. Estimates by different authors of water depth vary between <20 m–>50 m.

An abundant benthic fauna of molluscs and bryozoans can be collected, and it is evident that differences occur in the faunas from different beds. A predominance of smaller, more fragile shells (eg. Lucina, Venus, Corbula, Pteromeris and Anomia) with abundant foraminifera can be seen in some beds, possibly having been deposited selectively by currents. Well-defined accumulations of larger, more robust shells (eg. Arctica, Cardita, Pecten and Terebratula), again often articulated, forming shell-beds occur elsewhere (currently exposed in a trench); these may represent in situ shell-gravel faunas from which smaller shells were removed by the winnowing action of currents.

Red Crag The Red Crag is of late Pliocene age, around 2·5 Ma, unconformably overlying the Coralline Crag and London Clay. It was formed in a high energy, shallowing sea dominated by strong tidal currents, with submarine sand waves piling up against the shoreline to the west, and the bedforms are considered classic of their type (Dixon, 1979). It is dominated by molluscan shell gravel assemblages which indicate climatic conditions similar to those of Britain today: Glycimeris-Venerupis assemblages at Walton-on-the-Naze and elsewhere in the south, and Spisula, Macoma, Mya and Mytilus assemblages in the Deben area (Dixon, 1976, 2005).

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Surprisingly little is known about the Red Crag at Sutton, and no modern (post-war) quantitative or detailed molluscan studies or sedimentological studies have been done. However, it is clear that very different localised molluscan communities were able to develop in the different ecological environments that must have existed around the ‘island’. It is this aspect that is the focus of current research.

The Red Crag abutting and unconformable on Coralline Crag is best seen on the east face of the ‘island’ at the southern end of the Bullockyard Pit, but is much overgrown. Prestwich recorded over 2 m of Red Crag and his sketch sections show Red Crag banked up against a cliff of Coralline Crag. It contains much derived Coralline Crag fauna, and even pebbles of that Crag encrusted with Red Crag barnacles, indicating that the Coralline Crag had already been cemented prior to the deposition of the Red Crag. Nevertheless, collecting yields an abundant fauna, including the little sea urchin Echinocyamus pusillus, and Prestwich records abundant articulated Mytilus edulis.

Recent ‘Facelift’ clearance work by English Nature has exposed over 13 m of the wave-cut platform cut by the Red Crag sea into the Coralline Crag It dips at c. 10°. Blocks of Coralline Crag can be seen resting on the platform, and clusters (lenses) of articulated Mytilus can be found. The precise boundary is not easy to see at first sight and it takes a well-practised eye to trace it laterally, but it is probably the first time it has been exposed for over 100 years!

The Red Crag/Coralline Crag junction can also be seen in the Chicken Pit (potential RIGS), adjacent and to the west of the S.S.S.I. Exposures here also show Red Crag banked up against Coralline Crag, and resting on the London Clay.

Unusual faunas have been recorded. One, occurring in pockets between Coralline Crag boulders, is dominated by articulated Venerupis, possibly in the life position and with adult and juvenile forms present, with articulated Ensis and Macoma, and common Capulus, Buccinum and Neptunea. Yet nearby, in the eastern end of the pit, a Glycimeris-rich fauna is evident.

The London Clay surface exposed in the base of the pit contains a densely packed and in situ community of burrows of the boring bivalve Zirfaea crispata, each individual being only a few centimetres apart.

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Basement (Pebble) Bed At the base of the Crags a discontinuous basal lag or remanié pebble bed is often found containing Cetacean remains, shark teeth, flints, Boxstones (rounded pebbles of Miocene sandstone), crustaceans and, importantly, phosphatic nodules (mistakenly called ‘coprolites’) derived from the London Clay. These ‘coprolites’ were excavated on a large scale from Sutton and elsewhere in the local area during the mid-Nineteenth Century for making phosphate fertiliser. The waste material of flints and Boxstones was used as a building material in the 19th century restoration of Sutton and Shottisham churches, and in some other local buildings, including barns on the Pettistree Estate.

Of particular interest was the recording by Prestwich in this layer of a ¼ ton porphyry block from a trial pit on the western flank of the knoll. It was the only known coprolite pit dug in the Coralline Crag (rather than Red Crag), but did not prove profitable and was filled in by the mid-1860s. It is not known what happened to the porphyry boulder – there is apparently no record of its whereabouts and it may well still be buried. The location of the pit has been discovered using old map records and by ‘walking the ground’, and it is hoped that geophysical methods may be able to detect the boulder at depth (if it is still there!).

Later deposits Quartz pebbles are commonly found on the summit of the Knoll. This may suggest the former presence of Kesgrave Sands & Gravels, deposited about 750,000 years ago (pre-Anglian) before the first major glaciation. The gravels, found in much of eastern Suffolk and Essex, mark the position of the former course of the proto-Thames before it was deflected southwards by ice sheets during the Ice Age. Endpiece This unique locality is an important and valuable part of Suffolk’s geological heritage with, thanks to English Nature and GeoSuffolk, enabled and enhanced access. Not only can the relationship between and nature of four of Suffolk’s main lithologies be seen, but the opportunity has arisen to develop a greater understanding of the site through further research. It is hoped that the results of the recording, sampling and analysis taking place over the next few months will be described in a future issue of these Transactions. GeoSuffolk would like to thank Guy Quilter for access permission and his generous support.

Further reading Balson, P. S. (1999). The Coralline Crag. In: British Tertiary Stratigraphy.

Geological Conservation Review Series No. 15, (B. Daley & P. Balson).

Joint Conservation Committee, Peterborough, pp.253–288.

Balson, P. S., Mathers, S. J. & Zalasiewicz, J. A. (1993). The lithostratigraphy of the Coralline Crag (Pliocene) of Suffolk. Proceedings of the Geologists’ Association 104: 59–70.

Balson, P. S., Humphreys, B. & Zalasiewicz, J. A. (1990). Field Guide No.3: Coralline and Red Crags of East Anglia. 13th International Sedimentological Congress UK.

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Balson, P. S. & Long, P. E. (1988). In: Gibbard, P. L. & Zalasiewicz, J. A. Op Cit. Boswell, P. G. H. (1928). The Geology of the country around Woodbridge, Felixstowe and Orford. Memoir of the Geological Survey of England and Wales.

Dixon, R. G. (1976). The occurrence of Scrobicularia plana in the Red Crag, Lower Pleistocene. Proceedings of the Geologists’ Association, 87: 409–412.

Dixon, R. G. (1979). Sedimentary facies in the Red Crag (Lower Pleistocene, East Anglia). Proceedings of the Geologists’ Association, 90: 117–132.

Dixon, R. G. (2005). Field Meeting: Coastal Suffolk Crag week-end, 23–25 April, 2004. Proceedings of the Geologists’ Association, 116: 149–160.

Gibbard, P. L. & Zalasiewicz, J. A. (eds.) (1988). Pliocene - Middle Pleistocene of East Anglia Field Guide. Quaternary Research Association: Cambridge.

Prestwich, J. (1871)a. On the structure of the Crag-beds of Suffolk and Norfolk with some observations on the organic remains. Part I: The Coralline Crag of Suffolk. Quarterly Journal of the Geological Society of London 27: 115–146.

Prestwich, J. (1871b). On the structure of the Crag-beds of Suffolk and Norfolk with some observations on the organic remains. Part II: The Red Crag of Essex and Suffolk. Quarterly Journal of the Geological Society of London 27: 325–356.

Wood, A. M. (2000). The stone tapestries of the Red Crag. In: Dixon, R.G. (ed): The Geological Society of Norfolk Jubilee Volume 41–49.

Roger Dixon The White House, 7 Chapel Street, Woodbridge, Suffolk IP12 4NF

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