Ancient woodland on high ground fills most of the northern part of Harrow Weald on the border of Greater London (and the traditional county of Middlesex) with Hertfordshire. The area is one of the highest in Greater London; the highest point in Middlesex is near the woods at . The eastern part of the woods merges into those of Stanmore and encompasses Bentley Priory. The priory's grounds are now Bentley Priory Nature Reserve, a biological Site of Special Scientific Interest. Along the southern edge of the high ground runs the road Old Redding and a car park here gives views over parts of London. The western part of the woodland forms Harrow Weald Common, which adjoins Harrow Weald SSSI, a geological Site of Special Scientific Interest. Other smaller woods surround the Grim's Dyke Hotel, the former country house of W. S. Gilbert. Gilbert and Sullivan's comic operas are frequently performed here. The house is named after the nearby earthwork Grim's Ditch, a 3-mile long ancient monument that runs from Harrow Weald to Pinner Green. The monument remains largely mysterious but is known to have been named in the Saxon era. The ditch hints at a long history of habitation in the area and many artefacts have been found on the common to support this.