On Anglesey, north Wales, it is called a Clawdd and 20 miles of it have been built along a new extension of the A55 leading to the Hollyhead ferry.

In fact, there are two Cloddiau (the plural of Clawdd), one on either side of the road, which, when it opened in March, became the first PFI road in Wales. The walls are turf-topped and 1.5m high, with each linear metre containing 1.8 tonnes of stone.

They, along with five miles of 2.5m high dry stone walling, were chosen to line the road extension to damped the sound of traffic using the road.

These and other environmental concerns played a big part in the development. Along the way nature reserves and habitats have been created for voles, badgers, otters and great crested newts. Nearly 700,000 plants, hedges and shrubs line the road and a new island has been created for the local tern colony.

The Cloddiau have been traditionally built by building up the two outside walls 1.8m apart. These are filled with stone rubble that is compacted. A final layer of small stones is topped off with turf.

Unfortunately, the art of building these walls had been lost in north Wales and the joint venture Carillion Laing, which built the £100million road, had to look further afield for the skills they needed.

Fortunately, Carillion (although as Tarmac at the time) had contracted just such a wall in Cornwall in 1995. The man they had given the job was Roger Clemens, of Wadebridge, Cornwall, and it was he they turned to for the Anglesey wall.

We call them Cornish hedges, says Clemens. I\'m going to put a slate plaque on it saying I have done the work.

We had as many as 20 people up there at a time, though the average was 12. Most would spend a fortnight there at a time. A lot of them stayed on a farm and I\'m going to put the plaque in a layby near there.

They used a local limestone from a quarry called Perry\'s on the island and built as much as 1,000m of the walling a week. One man on the project could lift 40 tonnes of stone a day, says Clemens, and most of them managed 25 tonnes a day.

The first thing Roger Clemens did when he went to look at the project was change the design because the road builders had not realised the walls needed to be convex.

The work has taken well over a year to complete, due in part to the incessant rain. We had 90 consecutive days of rain at one point, says Clemens.

In Cornwall, he says, he has built 105 miles of walling in this way in the past 22 years. There\'s lots of two-man bands in Cornwall doing this. I\'ve got two sons doing the work as well. It\'s very much a tradition in Cornwall, though not so much in the rest of the world.

It is a traditional kind of wall construction in north Wales, although Clemens says there is no evidence of a Clawdd having been built on Anglesey for 150 years.

There might be more in future because during the construction of the wall eight local people were trained in the skill by Clemens and his teams.

And if you want to see the stonework on these turf covered walls, now is the time to do it because they will quickly become colonised by vegetation.

There are 150 varieties of moss live in these walls, says Clemens. After five years you can\'t see the stone, it gets grown over.