ENGLISH Heritage archaeologists using global positioning technology and old-fashioned leg work have found one of the country's largest hill forts in North Yorkshire.

The spectacular 40-acre site, located at Sutton Bank on the western boundary of the North York moors, near Thirsk, is believed to date back to 400BC.

Suspicions that a hill fort existed on the site date back to the mid-19th century, when an Ordnance Survey team mapped a short stretch of "tell-tale" Iron Age earth works.

However, later archaeologists failed to follow up on this discovery, confusing these earthworks with nearby boundary ditches of mediaeval origin.

English Heritage was prompted to investigate the mystery further following environmental work undertaken by the North York Moors national park.

During the summer, a team re-surveyed the area around Roulston Scar and the famous White Horse of Kilburn.

The promontory is buttressed by a sharp escarpment and commands breathtaking views over the Vale of York and further afield to the Yorkshire dales.

They found a series of ancient ditches, banks, humps and other features extending much further than anyone had previously thought, embracing much of the present day airfield of the Yorkshire gliding club. Some stretches of the defences survive to a height of nearly 3m.

Mr Alastair Oswald, English Heritage archaeological field investigator, said: "Hill forts are uncommon in Yorkshire, so we were obviously shocked to discover such a huge complex.

"Over the years there have been tantalising clues that such a monument existed, but no one had done a comprehensive survey to settle the question.

"It's possible the fort was constructed by the Brigantes or Parisi tribes, perhaps more as a statement of power than a defensive bastion.

"We didn't find much evidence of settlement within its boundaries, but it may have been a temporary refuge in times of trouble, or a giant corral for livestock.

"What is for certain, is that such a large fort would have taken a vast amount of timber and labour to build which poses many more intriguing questions."

The survey revealed that the fort was enclosed by a2m-deep trench and a 4m-high "box rampart" - fronted by a timber palisade and topped by a defended walkway. This has been traced to a distance of 1.3 miles.

Only two entrances were detected, adding to the site's impregnability. Individual features visible on the surface were carefully plotted on to a map using co-ordinates generated by the global positioning system, which uses signals from orbiting satellites to get a precise geographic fix.

As a result of the work, the area's scheduled ancient monument designation will be amended and the national park authority is considering how best to present this important prehistoric monument to the thousands of walkers who pass through on the Cleveland Way.