SUTTON KNOLL

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SUTTON KNOLL

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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.

Trans. Suffolk Nat. Soc. 42 (2006)


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