The Collaborative International Dictionary
Weald \Weald\, n. [AS. See Wold.] A wood or forest; a wooded land or region; also, an open country; -- often used in place names.
Fled all night long by glimmering waste and weald,
And heard the spirits of the waste and weald
Moan as she fled.
--Tennyson.
Weald clay (Geol.), the uppermost member of the Wealden strata. See Wealden.
Wikipedia
Weald Clay or the Weald Clay Formation is a Lower Cretaceous sedimentary rock underlying areas of South East England. It is part of the Wealden Group of rocks. The clay is named after the Weald, an area of Sussex and Kent. It varies from orange and grey in colour and is used in brickmaking.
The un-weathered form is blue/grey, and the yellow/orange is the weathered form; they have quite 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.