Weald Clay or the Weald Clay Formation is a Lower Cretaceous sedimentary rock unit 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.^[1] The un-weathered form is blue/grey, and the yellow/orange is the weathered form, it is used in brickmaking.

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Weald Clay
Stratigraphic range: Hauterivian-Barremian, 136–125 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Weald Clay exposed at Clock House Brickworks
Type	Geological formation
Unit of	Wealden Group
Sub-units	Horsham Stone Member
Underlies	Atherfield Clay Formation
Overlies	Tunbridge Wells Sand Formation
Thickness	up to 460 m
Lithology
Primary	Shale, Mudstone
Other	Siltstone, Sandstone, Limestone, Ironstone
Location
Region	England
Country	UK
Type section
Named for	Weald
Extent of the Weald Clay within the Weald Basin, shown with horizontal lines

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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.

The weathered and unweathered forms of the Weald Clay have different physical properties. Blue looks superficially like a soft slate, is quite dry and hard and will support the weight of buildings quite easily. Because it is quite impermeable, and so dry, it does not get broken by tree roots. It is typically found at 750mm down below a layer of yellow clay. Yellow, found on the surface, absorbs water quite readily so becomes very soft in the winter. The two different types make quite different bricks.

• Wealden District
• London Clay
• Oxford Clay
• List of dinosaur-bearing rock formations