IT was a hot July morning and the bracken was chest high. Among the handful of ramblers and dog walkers out for their early morning stroll, one particular figure was wading purposefully through the ferns, completing a circuit of the hill. Less than an hour later, Alastair Oswald had seen all he needed and packed up and headed off for an appointment.

His walk meant he was late for the English Heritage regional open day, being held in the North Yorkshire market town of Helmsley that morning, but at least he had a good response to the raised eyebrows. He had spent the morning discovering one of the largest prehistoric hill forts in Britain.

And the remarkable thing about the 40-acre fort, on the top of Sutton Bank on the western edge of the North York Moors, is that it has been staring everyone in the face ever since it was built, about 2,400 years ago. Alastair's discovery technique did not involve painstaking archaeological digs, or sophisticated satellite technology, but relied on more basic methods.

"It is about looking at what you can see with the naked eye," he says. "People get really hung up either on digging as the only form of archaeology, or they need to see some kind of hi-tech involvement, and that is everything we don't do.

"We walk around and look at what is on the surface very carefully. With that technique, we cover the whole gamut of human evolution, from deepest prehistory - the first monuments that were built in this country - right through to the Cold War."

Alastair's find at Roulston Scar may have been made through simple observation, but what made it even more startling was that part of the fort had already been discovered, almost 150 years earlier, when the first Ordnance Survey maps were drawn up. The only problem was, it had been incorrectly identified.

"Part of the fort was mapped in the 19th Century, but they thought it was a boundary earthwork," says Alastair, an investigator for English Heritage. In fact, the bank and ditch marked as part of a land boundary, was part of the fort perimeter. Had the original cartographers looked around the other side of Roulston Scar, they would have seen the rest of the fort.

The boundary theory prevailed more or less intact for the next 150 years, even though excavations carried out in the 1970s by the Yorkshire Gliding Club, whose airfield occupies part of the fort's interior, should have shown otherwise. The remains of holes for wooden posts were discovered, a sign of box ramparts, typical of the early Iron Age, where a timber framework was filled with rubble to create a high fence. Remains found during these excavations, including charred timbers, have never been analysed, even though they could shed light on the way of life for people inside the fort.

The fort finally gave up its secret in July, when a North York Moors National Park archaeologist, inspecting some conservation work on Sutton Bank, noticed a series of large, ditch-like structures, and asked Alastair to take a look. The ditches turned out to be medieval trackways, but in the course of inspecting them, Alastair discovered traces of earlier tracks.

In some places they had been damaged by ploughing during the Middle Ages, but where they were still reasonably visible, they made up a series of impressive banks and ditches, part of the Iron Age fort's defences.

"The path goes along the edge, and if you go to the edge and look down you can see a track, but you have to recognise it as something other than a natural ledge," he says. "We looked at the landscape as a whole. Unlike digging, where you are looking at a very small area, what we do is very rapid and enables us to look at very large areas.

"The skill is being able to recognise what you are looking at, and knowing whether it is artificial or natural. Some people can't see the humps and bumps, but when you have been doing it for so long, you know what these things look like and you know when they are in front of you.

"The extraordinary thing for me is that so many people go there, it must be tens of thousands each year, and not one of them has looked over the edge and thought 'that looks a bit funny'. It is one of the exciting things about our job, in that you show people something completely new, and wonder how everybody missed it. In one sense, I was very excited, but in another sense it was quite routine for the kind of work we do."

This work is carried out by a small team at English Heritage's York offices, also responsible for finding a castle pre-dating the stone keep at Scarborough, and now working on Iron Age forts in the Cheviots in Northumberland. And senior investigator Stewart Ainsworth has brought the technique to a larger audience through the Channel Four programme Time Team.

"One of the basic principles of the type of work we're involved in is that a lot of it is fairly straightforward common sense and observation," Stewart says. "We look at all the elements that make up the landscape, it can be everything from hedgerows to humps and bumps, and everything we see on the surface is an archaeological artefact. At some point in history, that thing has been built, and we're trying to work out the sequence.

"We get used to looking for signatures and patterns. In your mind, you are carrying all these templates of what things look like, and you are looking at the landscape and seeing if it fits those templates. It is a question of filtering out the things that are non-archaeological, and seeing what you are left with."

Although this technique does not provide the same detail as an archaeological excavation, it offers the advantage of covering a larger area in a much shorter time, a cheaper method which does not run the risk of wasting money if the dig is in the wrong place. It also ensures whatever is found remains intact, without the destruction involved in a dig.

And there are no plans to excavate Roulston Scar. Instead, Alastair is hoping to rediscover the artefacts unearthed in the 1970s, to see what they say about the fort's inhabitants.

There is also the possibility of carrying out a geophysical survey, which could pick up traces of hearths, for example, pointing to the location of any houses within the fort's perimeter.

"What we do doesn't give you the final story," Alastair says. "There are different techniques for looking at the same monument, and each one will give you a different angle. What we do is the oldest form of archaeology there is, but it is only recently that we started calling it archaeology."

Stewart adds: "The basic understanding of a site is embraced in a walk around. The rest is the detail. You get a big picture in your head and the skill is very much in recognising what you see from the evidence in front of you. You would aim to understand 70 per cent of what is going on from a walk around.

"You don't need technology for this - it doesn't take any technology to have a good look around."