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Suffolk Natural History, Vol. 60
THE CHILLESFORD CLAY – MUDDYING AND UNMUDDYING THE WATERS HOWARD B MOTTRAM Introduction The Chillesford Clay, which is about two million years old, is a well known Member of the Norwich Crag Formation in Suffolk. The Chillesford Clay is usually a silty clay of dark grey colouration, but it can be light grey or brown. It is only up to about 5m thick and was not always deposited and, even when it was deposited, it didn’t always survive. Where it survived in eastern and southern Suffolk, it marks the top of the preserved Norwich Crag Formation. In north-eastern Suffolk the Chillesford Clay is overlain by the Westleton Beds Member of the Norwich Crag Formation and in this area of Suffolk the Chillesford Clay can be a useful marker horizon for separating the underlying Chillesford Sands from the overlying sands of the Westleton Beds. There have been several interpretations and reinterpretations on the recognition and mode of occurrence of the Chillesford Clay. In this article these are discussed with a view to clarifying the situation. The areal extent of the Chillesford Clay Eastern Suffolk (the area around Sudbourne) In 1849, Joseph Prestwich described sand overlain by mud that persisted in eastern Suffolk in the vicinity of Chillesford. He subsequently named the sand as the Chillesford Sand and the overlying mud as the Chillesford Clay (Prestwich, 1871). The Geological Survey of Great Britain identified the Chillesford Clay more widely in eastern Suffolk and showed this on geological maps of the area that it published between 1882 and 1884. Re-surveying carried out by the BGS (British Geological Survey) between 1982 and 1999 interpreted the outcrop slightly differently. This revised interpretation of the outcrop is used in Fig. 1. North-eastern Suffolk The original mapping by the Geological Survey identified Chillesford Clay between Westleton and Kessingland, see Fig. 2. Almost a century later, analyses of the mud in the cliffs of Easton Bavents and Covehithe revealed that the mud there had a heavy mineral content with a high proportion attributable to Scandinavia. This suggested a glacial supply (Funnell & West, 1962) and in the follow-up research, it was concluded that the pollen, molluscs and foraminifera reflected a cold environment (West et al., 1980). It was submitted that this mud was therefore of a different provenance and possibly of a different age to the mud around Chillesford. In consequence, the mud at Easton Bavents and Covehithe became referred to as the Baventian Clay (West et al., 1980) and then as the Easton Bavents Clay (Gibbard & Zalasiewicz, 1988). However, it was subsequently demonstrated that the flora and fauna of the Chillesford Clay in the Chillesford area also reflected a cold climate and that the heavy mineral contents of the Chillesford Clay and the Easton Bavents Clay were not dissimilar (Zalasiewicz et al., 1991). On the basis of these findings, it was proposed that the two muds were not totally different strata but fully or partly lateral equivalents of each other.
Trans. Suffolk Nat. Soc. 60 (2024)