In the Sussex stratotype succession, the basal sequence boundary is formed by the Hope Gap Hardground (Grant et al., 1999). At the basin margin (e.g. Charnage Down, Wiltshire) the Hope Gap Hardground locally merges with the highly condensed correlative units of the underlying sequence to form a composite hardground (=Top Rock). Above a few metres of lowstand systems tract deposits in Sussex, a return to more nodular chalk indicates the transgressive systems tract, the top of which is marked by the Beeding Hardgrounds (Grant et al., 1999). A thin highstand sequence tract, representing less than 2 m of sediment above the Beeding Hardgrounds in Sussex, caps the sequence (Grant et al., 1999).
GRANT, S F, COE, A L, ARMSTRONG, H A. 1999. Sequence stratigraphy of the Coniacian succession of the Anglo-Paris Basin. Geological Magazine, 136, 17–38.