Unlike going to the gym, or other forms of exercise, most of which offer about as much mental stimulation as a tin of paint with the lid off, dry stone walling combines the kind of puzzle-solving one might find in games like Sudoku and Tetris, together with a respectable amount of physical work.

When you consider one metre of an average dry stone wall contains about a ton of stone, it starts to put things into perspective.

Terry Bollen, 66, a former engineer, who took up dry stone walling in 1992 and is chairman of the Otley and Yorkshire Dales branch of the Dry Stone Walling Association of Great Britain, said: "Part of me taking this up came about because I came out of hospital after a major heart operation and basically I had to do more exercise. I just didn't fancy swimming, or riding a bike or going to the gym, so I started this.

"People today find themselves emotionally drained at the end of the week. They get to Friday and from the neck up they are shattered, but the body is still full of energy. There needs to be a balance, and dry stone walling offers you that.

"We have members whose age ranges from the late 20s right up to their 70s. What I find good about it is that you can produce something which will last for hundreds of years and people will see what you have done.

"Every wall is different. It's like a big puzzle. A lot more people are taking this up as a means of exercise. Some people call it the green gym."

Tracey Blackwell, 47, worked as a gardener for years but for the last nine she's been a professional dry stone waller. The 47-year-old mother-of-two is one of only a handful of women in the country who have become instructors in professional dry stone walling. She works mainly in North and West Yorkshire but has led courses further afield even in the Cevennes Mountains in the South of France.

She also runs her own company, ATA Dry Stone Walling (www.atadrystone.co.uk), with Andy Couldwell and Andy Hudson.

She loves the outdoors and wouldn't want to do anything else.

She said: "I used to be a gardener and I did some dry stone walling in that but gradually I found myself doing more and more of it and it began to take over.

"You might be surprised but there is a big demand for dry stone walling, from farmers and private land owners.

"What's nice is that people will still pay for dry stone walls rather than put up a cemented wall. They are part of our heritage and they last for hundreds of years. It's a tried and tested method of building and that's why we are still doing it.

"Eight years on, it still fascinates me.