Chalk Group Lithostratigraphy: Marlborough Downs/Berkshire Downs/Chilterns - Lower Chalk
The Lower Chalk is the oldest unit in the traditional tripartite subdivision of the Chalk Group adopted by the Geological Survey at the turn of the 20th Century (Jukes-Browne & Hill, 1903, 1904), and followed my most subsequent workers The Lower Chalk of the region is 50 - 60 m thick (Rawson et al., 1978), and comprises sandy, glauconitic strata at the base, passing up through rhythmically bedded marls and marly chalks, overlain by a calcarenitic chalk bed with an erosive base, overlain by massively bedded chalk, and capped by a 1-2 m succession of marls. It is subdivided into five members, in ascending order: the Glauconitic Marl / Cambridge Greensand, Chalk Marl, Totternhoe Stone, Grey Chalk and Plenus Marls (Wood, 1996). The base is the unconformable basal erosion surface of the Glauconitic Marl or Cambridge Greensand, and the top is the junction of the Plenus Marls and Melbourn Rock.
Macrofossil Biozonation: (see under Members)
Correlation: see Correlation with other parts of the UK
RAWSON, P F, CURRY, D, DILLEY, F C, HANCOCK, J M, KENNEDY, W J, NEALE, J W, WOOD, C J & WORSSAM, B C. 1978. A correlation of the Cretaceous rocks in the British Isles. Geological Society of London, Special Report No. 9, 70 pp.
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).