Subgroup The Grey Chalk Subgroup (formerly the Lower Chalk minus the Plenus Marls) is usually relatively soft and greyish in colour. It is also the most fossiliferous (especially for ammonite fossils). The strata of this subgroup usually begin with the 'Glauconitic Marl Member' (formerly known as the Glauconitic or Chloritic Marl), named after the grains of the green minerals glauconite and chlorite which it contains. The remainder of the subgroup is argillaceous in its lower part (the West Melbury Marly Chalk Formation (formerly the 'Chalk Marl') and becomes progressively purer in the 'Zig-zag Chalk Formation' (the former 'Grey Chalk'). In the central Chilterns the two parts are separated by the hard Totternhoe Stone, which forms a prominent scarp in some places. There are few, if any, flint nodules present. These two formations are not recognised within the northern province i.e. the outcrop north from East Anglia to Yorkshire, where the entire sequence is now referred to as the 'Ferriby Chalk Formation'. The thickness of the Grey Chalk Subgroup strata varies, averaging around , depending upon the location. They often contains fossils such as the ammonites Schloenbachia, Scaphites, and Mantelliceras, the belemnite Actinocamax, and the bivalves Inoceramus and Ostrea. White Chalk Subgroup The White Chalk Subgroup includes what were formerly designated the Middle Chalk and Upper Chalk Formations, together with the Plenus Marls (topmost part of the former Lower Chalk Formation). In the southern province it is divided in the following way (youngest/uppermost at top): Portsdown Chalk Formation (formerly part of 'Upper Chalk' and the equivalent of Rowe Chalk Formation, below) Culver Chalk Formation (formerly part of 'Upper Chalk') Spetisbury Chalk Member (formerly part of 'Upper Chalk') Tarrant Chalk Member (formerly part of 'Upper Chalk') Newhaven Chalk Formation (formerly part of 'Upper Chalk') Seaford Chalk Formation (formerly part of 'Upper Chalk') Lewes Nodular Chalk Formation (formerly part of 'Upper Chalk') New Pit Chalk Formation (formerly part of 'Middle Chalk') Holywell Nodular Chalk Formation (formerly part of 'Middle Chalk') Plenus Marls Member In the northern province the sequence is divided thus: Rowe Chalk Formation (formerly part of 'Upper Chalk' and the equivalent of Portsdown Chalk Formation, above) Flamborough Chalk Formation (formerly part of 'Upper Chalk') Burnham Chalk Formation (formerly part of 'Upper Chalk') Welton Chalk Formation (formerly 'Middle Chalk') Plenus Marls Member In the southern province, the former Middle Chalk, now the Holywell Nodular Chalk Formation and overlying New Pit Formation, averages