for the parkland at Houghton remains intact. His "twisting wilderness paths" were cleared in the early 18th century; and they have been maintained since then. The park, which extends into Bircham and Harpley parishes, was designated Grade I on the National Register of Historic Parks and Gardens in 1987. Bridgeman replaced the formal geometry of intersecting avenues with blocks of woodland and parkland which, as he saw it, was better able to complement the Hall's compelling architectural statement. Tree-lined vistas in four directions are aligned on the Hall. The ha-ha barriers at Houghton were an innovative feature credited to Bridgeman. In his 1780 "Essay upon Modern Gardening," Horace Walpole explained: "The contiguous ground of the park without the sunk fence was to be harmonized with the lawn within; and the garden in its turn was to be set free from its prim regularity, that it might assort with the wilder country without." Sir Robert Walpole constructed a watertower (1731–1732) with the appearance of an architectural folly which was designed by Henry Herbert, 9th Earl of Pembroke. It was restored in 1982 and is a Grade I listed building. In this well-established context, a number of contemporary outdoor sculptures have been commissioned in recent years by David Cholmondeley, 7th Marquess of Cholmondeley. To the west of the house is a circle of Cornish slate at the end of a path mown through the grass. This land art feature was designed by the British sculptor Richard Long. Two modern follies lie in a wooded area to the side of the west front. American artist James Turrell contrived "Skyspace" for Houghton. Turrell's construction presents itself from the exterior as an oak-clad building raised on stilts. From the inside of the structure, the viewer's point of view is focused upwards and inevitably lured into contemplating the sky as framed by the open roof. "The Sybil Hedge" is another folly in this vicinity. It is based on the signature of the current marquis' grandmother, Sybil Sassoon. Scottish artist Anya Gallaccio has created a sarcophagus-like marble structure which is sited at the end of a path; and nearby is a copper-beech hedge which is planted in lines mirroring Sybil’s signature. A walled kitchen garden lies beyond the stables. Over the course of time, the productive area was reduced in size, and the enclosure was mostly grassed over. In 1996, the fallow enclosure was redesigned and replanted.