Grim's Ditch

'The largest relic of early man in the Chilterns is Grim's Ditch, which was described by Massingham as "the longest serpent of antiquity." It originally ran for about 50 miles from the River Thames near Wallingford, up past Nuffield towards Henley and then from near Marlow behind the scarp past Lacey Green, Great Hampden, Berkhamsted on on to the Ivinghoe Beacon...'
-- Duncan Unsworth, Walking in the Chilterns