The formation was deposited in lagoonal, lacustrine and alluvial conditions that varied from freshwater to brackish.

[2] The clay alternates with other subordinate lithologies, notably hard red-weathering beds of ironstone, limestone (Sussex Marble) and sandstones, notably including the calcareous sandstone unit referred to as the Horsham Stone.

It has a gradual, conformable contact with the underlying Tunbridge Wells Sand Formation, and has a sharp, unconformable contact with the overlying Atherfield Clay Formation, a shallow marine unit deposited after marine transgression during the Aptian.

Blue looks superficially like a soft slate, is quite dry and hard and will support the weight of buildings quite easily.

Yellow, found on the surface, absorbs water quite readily so becomes very soft in the winter.