It is characterised by a wide, flat, clay floodplain of the upper reaches of the Bristol Avon river, which divides the Cotswolds to the west from the chalk downland of east and south Wiltshire.
The area is defined by a broad, flat expanse of clay soil along the upper Bristol Avon River, dividing the Cotswolds in the west from the chalky land of eastern and southern Wiltshire.