Atherfield Clay Formation

The Lower Greensand Group is a geological unit, which forms part of the underlying geological structure of southeast England. South of London in the counties of West Sussex, East Sussex and Kent, which together form the wider Weald, the Lower Greensand can usually be subdivided to formational levels with varying properties into the Atherfield Clay Formation, the Hythe Formation, the Sandgate Formation, Bargate Formation and the Folkestone Formation. In areas north and west of London, including Cambridgeshire, Bedfordshire and Buckinghamshire the Lower Greensand is referred to as the Woburn Sand Formation.

The Lower Greensand typically comprises loose, unconsolidated sandstone (termed rubblestone/rubble in construction) and sands of varying grain size with subordinate amounts of siltstones, mudstones (containing smectites and similar) and limestones.

In the Weald of East Sussex the lowermost part of the group is recognised by green glauconitic clays with a basal bed of phosphate nodules. These clays are overlain by green sandy clays and silts and finally homogeneous fine grained sands. The sediments are noted to become increasingly fine grained and glauconitic to the east.

The Lower Greensand Group and the Woburn Sands Formation were deposited during the Early Cretaceous Period, which lasted for approximately 40 million years from 140 to 100 million years ago, also described as "of Aptian to Early Albian". The Group is the lowermost of two geological units that take their name from their colouration due the presence of the mineral glauconite, the other being the Upper Greensand.

...
Wikipedia