Chalk Group Lithostratigraphy: Marlborough Downs/Berkshire Downs/Chilterns - Top Rock

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The Top Rock, first described at Goring, Berks, by Chatwin & Withers (1908), comprises a group of hardgrounds a short distance (c. 4m in the Chilterns; Wood, 1996) above the Chalk Rock, which fade laterally into nodular chalk (Bromley & Gale, 1982). It has been described at various localities between Dorset and Norfolk (Bromley & Gale, 1982). Near Hitchin, it consists of hard, fine grained chalkstone, without glauconite (Hopson et al., 1996), but Shephard-Thorn et al. (1994) recorded that it comprised 0.3-0.5 m of chalkstone capped by a glauconitised hardground strewn with glauconitised pebbles. Bromley & Gale (1982) defined the Top Rock as a lithified unit that terminates the nodular chalk interval overlying the Chalk Rock, and Mortimore (1983) showed that previous records of the Top Rock in southern England equated with Navigation and or overlying hardgrounds in the higher part part of the Lewes Nodular Chalk Member. Wood (in Murray, 1986) reported that the Top Rock in a borehole at Thundridge [TL 3810 1751], Hertfordshire, probably represented a condensation of the greater part of the M. cortestudinarium Zone. Fossils are less common in the Top Rock compared to the Chalk Rock, but locally include the inoceramid bivalves Cremnoceramus waltersdorfensis and C. rotundatus and the echinoid Micraster cortestudinarium (Wood, 1996).

Macrofossil Biozonation: lower M. cortestudinarium Zone

Correlation: see Correlation with other parts of the UK

References[edit]

BROMLEY, R G & GALE, A S. 1982. The lithostratigraphy of the English Chalk Rock. Cretaceous Research, Vol. 3, 273 - 306.

CHATWIN, C P & WITHERS, T H.1908. The zones of the Chalk in the Thames Valley between Goring and Shiplake. Proceedings of the Geologists’ Association. Vol. 20, 390-420.

HOPSON, P. M., ALDISS, D. T. & SMITH, A. 1996. Geology of the country around Hitchin. Memoir of the British Geological Survey, Sheet 221 (England & Wales).

MORTIMORE, R N.1983. The stratigraphy and sedimentation of the Turonian - Campanian in the Southern Province of England. Zitteliana, Vol. 10, 27-41.

SHEPHARD-THORN. E. R., MOORLOCK, B. S. P., COX, B. M., ALLSOP, J. M. & WOOD, C. J. 1994. Geology of the country around Leighton Buzzard. Memoir of the British Geological Survey, Sheet 220 (England & Wales).

WOOD, C. J. 1996. Upper Cretaceous: the Chalk Group. In SUMBLER, M. G., British Regional Geology: London and the Thames Valley. Fourth Edition. (London: HMSO for the British Geological Survey).

See: hardground, nodular chalk, Lewes Nodular Chalk Member (Southern Region), Navigation Hardground, Chalk Rock