The Seven Sister were described April 2023 by Nelson Pain :

“The cliffs that surround Ault beach are made up of chalk from the Upper Cretaceous, more precisely from the terminal Turonian and Coniacian (about 83 Ma to 90 Ma). They are dotted with nodules of flint which underline the sub-horizontal stratification planes … The relative proportion between chalk and flint is approximately 90-95% chalk to 5-10% flint.
Flint horizons are stratigraphic markers. On the cliffs around Ault, the regular band of flint named Seven Sisters is recognizable and characterizes the boundary between the Middle Coniacian and the Upper Coniacian (about 87 Ma). The boundary between Lower and Middle Coniacian is marked by the marly Shoreham 2 level, which is more difficult to identify. These lithostratigraphic markers allow in particular the correlation of distant deposits, such as between the Normandy and Picardy coasts, or even between the French and English coasts.”

In : Nelson Pain. La côte picarde – Les falaises d’Ault (Somme), une illustration des risques associés à l’érosion. Planet Terre https://planet-terre.ens-lyon.fr/ressource/falaises-Ault.xml (retrieved June 20, 2023)

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