Weald Clay or the Weald Clay Formation is a Lower Cretaceous sedimentary rock underlying areas of South East England, between the North and South Downs, in an area called the Weald Basin It is the uppermost unit of the Wealden Group of rocks within the Weald Basin, and the upper portion of the unit is equivalent in age to the exposed portion of the Wessex Formation on the Isle of Wight. It predominantly consists of thinly bedded mudstone. The un-weathered form is blue/grey, and the yellow/orange is the weathered form, it is used in brickmaking.The formation was deposited in lagoonal, lacustrine and alluvial conditions that varied from freshwater to brackish. 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.