Chalk Group Lithostratigraphy: East Anglia - Middle Chalk
The Middle Chalk thins from c. 68 m in Bedfordshire, to c. 60 m around Diss (south Norfolk) and to just over 30 m in the Trunch Borehole [TG 2933 3455] in north Norfolk (Chatwin, 1961; Mathers et al., 1993; Arthurton et al., 1994). In the southern part of East Anglia, around Saffron Waldon, the Middle Chalk comprises hard, nodular, shelly chalk, with sparse flints in the lower part, passing up into mostly soft, white, massive but occasionally nodular chalk, with conspicuous marl seams and moderately common flints (White, 1932). Northwards, the higher part of the Middle Chalk becomes more indurated. In the Bury St Edmunds district, where the Middle Chalk is c. 65 m thick, the higher part of the formation contains numerous irregular green-coated surfaces (Bristow, 1990), possibly representing hardgrounds or stylolites, and in north Norfolk Peake & Hancock (1970) recorded that as the Middle Chalk thins it becomes much more indurated, and capable of being used as a building stone locally. As in the adjacent Chilterns district, flint is more common at a relatively lower stratigraphical level in the Middle Chalk of East Anglia compared to most of the Southern Region. Thus, in the Cambridge district, where the Middle Chalk is about 70 m thick, flint first occurs c. 25 m above the base of the formation (Worssam & Taylor, 1969), and in north-west Norfolk Peake & Hancock (1970) recorded almost continuous bands of large mottled grey flints.
The base of the Middle Chalk is marked by the Melbourn Rock, a hard, feature-forming bed which was first described at a village at the edge of the adjacent Chilterns Region. The top of the Middle Chalk has been variously defined in the district. Historically, it was taken at the base of the Chalk Rock, but this horizon is poorly and inconsistently developed in East Anglia, and some recent surveys (Mathers et al., 1993; Bristow, 1990) have placed the top of the Middle Chalk slightly lower, at the base of a flint-rich succession (Brandon Flint Series) by analogy with traditional BGS practice in the North Downs where the Chalk Rock is undeveloped.
Macrofossil Biozonation: M. geslinianum (pars), N. juddii, Mytiloides spp. & T. lata (pars)
Correlation: see Correlation with other UK Chalk Group successions
References
ARTHURTON, R S, BOOTH, S J, MORIGI, A N, ABBOTT, M A W & WOOD, C J. 1994. Geology of the country around Great Yarmouth. Memoir of the British Geological Survey.
BRISTOW C R. 1990. Geology of the country around Bury St. Edmunds. Memoir of the British Geological Survey.
CHATWIN, C P. 1961. East Anglia and adjacent areas (4th Edn.). British Regional Geology (Her Majesty's Stationery Office, London.)
MATHERS, S J, HORTON, A & BRISTOW, C R. 1993. The geology of the country around Diss. Memoir of the British Geological Survey.
PEAKE, N B & HANCOCK, J M. 1970. The Upper Cretaceous of Norfolk [reprinted with corrigenda and addenda] In LARWOOD, G P & FUNNELL, B M (eds.), The Geology of Norfolk. London & Ashford.
WHITE, H J O.1932. The Geology of the country near Saffron Walden. Memoir of the Geological Survey England and Wales (Sheet 205).
WORSSAM, B C & TAYLOR, J H. 1969. Geology of the country around Cambridge. Memoir of the British Geological Survey.
See: Chalk Rock (Chilterns Region), Brandon Flint Series, Melbourn Rock, marl, hardground, stylolite, nodular chalk, flint