Chalk Group Lithostratigraphy: Marlborough Downs/Berkshire Downs/Chilterns - Chalk Rock
The Chalk Rock, typically c. 4-5 m thick, comprises a variably developed succession of intensely hard chalkstones and associated hardgrounds, typically iron-stained, glauconitised and sometimes phosphatised, and represents a much condensed equivalent of strata spanning the interval from the middle part of the Middle Chalk to the lower Upper Chalk in the Southern Region. At its fullest development, at Ogbourne Maizey, near Marlborough, the Chalk Rock comprises up to three hardground suites, however, the Chalk Rock of different areas does not always comprise the full complement of hardgrounds (Bromley & Gale, 1982). While all three hardground suites are present in the Marlborough Downs, only the upper hardground suite is present in the north-eastern Chilterns (Bromley & Gale, 1982; Gale, 1996), thus making the base of the Chalk Rock, and therefore the base of the Upper Chalk, strongly diachronous.
Bromley & Gale (1982) assigned the Chalk Rock formational status (not followed herein), and detailed the stratotype succession at Ogbourne Maizey [SU 180 716] as follows:
DIAGRAM TO COME
BROMLEY, R G & GALE, A S. 1982. The lithostratigraphy of the English Chalk Rock. Cretaceous Research, Vol. 3, 273 - 306.
GALE, A S. 1996. Turonian correlation and sequence stratigraphy of the Chalk in southern England. In HESSELBO, S P & PARKINSON, D N (eds), Sequence Stratigraphy in British Geology, Geological Society Special Publication, No. 103, pp. 177-195.