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2 (see below), or alternatively is an unrelated hardground locally developed near the top of the
formation.
The higher part of the M. cortestudinarium Zone is poorly exposed across Suffolk, but where
locally seen west of Bury St Edmunds, for example at Dalham, showed hard, nodular chalk
above the Top Rock passing upwards into rather featureless, finer-grained, flinty chalk (Wood
& Bristow, 1990, fig. 9; Fig. 3B). The total thickness of Lewes Nodular Chalk (as defined
herein) seen in the Stowlangtoft Borehole is about 78 m.
Seaford Chalk Formation
Borehole geophysical logs near Sudbury (Pattison et al., 1993, fig. 13) suggest the presence of
both the Shoreham Marls and Belle Tout Marls; the former marking the base of the Seaford
Chalk Formation and the Micraster coranguinum Zone in Sussex, and the latter characterising
the lower part of the formation in that area (Fig. 3). Slightly higher in the succession, an old
chalk pit at Euston [TL 89453 777359 to TL 89528 77481] (Figs 3, 6), between Bury St
Edmunds and Thetford, exposes soft, smooth-textured, flint-rich chalk, with abundant large
shell fragments of inoceramid bivalves in the lower part. These mainly comprise
Platyceramus and Volviceramus, and are particularly concentrated below a semitabular flint
near the base of the section. This flint represents the Seven Sisters Flint, a marker-bed in the
lower part of the Seaford Chalk in the stratotype Sussex succession, which is likewise
underlain by inoceramid-rich Chalk and may correlate with the giant Eppleworth Flint in the
Northern Province Burnham Chalk Formation (Wood in Gaunt et al., 1992; Fig. 3). A similar
concentration of Volviceramus was recorded in the lower (Middle Coniacian ) part of the M.
coranguinum Zone in the Stowlangtoft Borehole (Wood & Bristow, 1990, fig. 7), as well as
in numerous localities in and around Bury St. Edmunds.