There are 132 known crosses within the Dartmoor National Park boundary but probably the most famous of them is Nun's or Siward's Cross. It stands at the junction of two of the ancient tracks that linked the abbeys of Tavistock, Buckland and Buckfast – the Abbots’ way and the Monks’ Path. It was first recorded in the 1240 Perambulation of the Forest of Dartmoor as a boundary mark of the King’s Forest although it was probably erected during Edward the Confessor's reign (1042–1066). Historically known as Siward's Cross, named after Earl Siward, a Saxon, who owned the lands around Tavistock prior to the Norman Conquest, the reasons for its more common name of Nun's Cross are unclear. It was first recorded as such in 1699 but there is no evidence linking either the cross or the area with any nuns. William Crossing gave a plausible explanation, in that it is a derivation of the Cornu-Celtic word ‘Nans’ meaning a valley, dale or ravine, however the local geography of its situation does not seem to give much weight to this theory. It was knocked down in 1846 and the shaft broken but it was restored in 1848 at the orders of Sir Ralph Lopes. It stands 6 feet 10 inches tall (2.08 metres) and is the largest recorded cross on the moor. Both faces of the cross have badly weathered inscriptions which have had various interpretations. Some say the eastern face bears the word ‘SIWARD’ or ‘SYWARD’ and the western face has the inscription ‘BOC LOND’.

The cross stands near Nuns Cross Farm which is about 2.5 miles from Princetown. The farm was abandoned in the 1950’s, was then used as a base for survival training by the Royal Navy before being leased by the Duchy of Cornwall for its current use as an Adventure Centre. Nearby the only underground section of Devonport Leat can be found in the form of a 500 yard long tunnel situated south of the farm which reappears by the ruins of an old Blacksmith’s Shop associated with former tin workings.